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  • 1.
    Aydin, Alhun
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Nano Energy Research Group, Energy Institute, Istanbul Technical University, 34469 Istanbul, Turkey.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sisman, Altug
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Nano Energy Research Group, Energy Institute, Istanbul Technical University, 34469 Istanbul, Turkey.
    Thermosize voltage induced in a ballistic graphene nanoribbon junction2019In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 126, no 10, article id 104302Article in journal (Refereed)
    Abstract [en]

    A thermoelectric voltage is induced in a junction, constituted of two dissimilar materials under a temperature gradient. Similarly, a thermosize voltage is expected to be induced in a junction made by the same material but having differentsizes, so-called thermosize junction. This is a consequence of dissimilarity in Seebeck coefficients due to differencesin classical and/or quantum size effects in the same materials with different sizes. The studies on thermosize effectsin literature are mainly based on semi-classical models under relaxation time approximation or even simpler localequilibrium ones where only very general ideas and results have been discussed without considering quantum transport approaches and specific materials. To make more realistic predictions for a possible experimental verification, here,we consider ballistic thermosize junctions made by narrow and wide (n-w) pristine graphene nanoribbons with perfectarmchair edges and calculate the electronic contribution to the thermosize voltage, at room temperature, by using the Landauer formalism. The results show that the maximum thermosize voltage can be achieved for semiconducting nanoribbons and it is about an order of magnitude larger than that of metallic nanoribbons. In the semiconducting case, the thermosize voltage forms a characteristic plateau for a finite range of gating conditions. We demonstrate, throughnumerical calculations, that the induced thermosize voltage per temperature difference can be in the scale of mV/K,which is high enough for experimental measurements. Owing to their high and persistent thermosize voltage values,graphene nanoribbons are expected to be good candidate for device applications of thermosize effects.

  • 2.
    Banerjee, S.
    et al.
    Los Alamos Natl Lab, Inst Mat Sci, Los Alamos, NM 87545 USA.;KTH Royal Inst Technol, Ctr Quantum Mat, Nordita, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Royal Inst Technol, Div Theoret Chem & Biol, SE-10691 Stockholm, Sweden..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Black-Schaffer, Annica M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ågren, H.
    Royal Inst Technol, Div Theoret Chem & Biol, SE-10691 Stockholm, Sweden..
    Balatsky, A. V.
    Los Alamos Natl Lab, Inst Mat Sci, Los Alamos, NM 87545 USA.;KTH Royal Inst Technol, Ctr Quantum Mat, Nordita, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Granular superconductor in a honeycomb lattice as a realization of bosonic Dirac material2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 13, article id 134502Article in journal (Refereed)
    Abstract [en]

    We examine the low-energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in a honeycomb lattice structure. Using the example of graphene, we present evidence for the engineered Dirac nodes in the bosonic excitations: the spectra of the collective bosonic modes cross at the K and K' points in the Brillouin zone and form Dirac nodes. We show how two different types of collective phase oscillations are obtained and that they are analogous to the Leggett and the Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. We show that the Dirac node is preserved in the presence of an intergrain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with fermionic Dirac materials. The Dirac node dispersion of bosonic excitations is thus expanding the discussion of the conventional Dirac cone excitations to the case of bosons. We call this case as a representative of bosonic Dirac materials (BDM), similar to the case of Fermionic Dirac materials extensively discussed in the literature.

  • 3.
    Berggren, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Electron Paramagnetic Resonance of Single Magnetic Moment on a Surface2016In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 25584Article in journal (Refereed)
    Abstract [en]

    We address electron spin resonance of single magnetic moments in a tunnel junction using time-dependent electric fields and spin-polarized current. We show that the tunneling current directly depends on the local magnetic moment and that the frequency of the external electric field mixes with the characteristic Larmor frequency of the local spin. The importance of the spin-polarized current induced anisotropy fields acting on the local spin moment is, moreover, demonstrated. Our proposed model thus explains the absence of an electron spin resonance for a half integer spin, in contrast with the strong signal observed for an integer spin.

  • 4.
    Berggren, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Spin inelastic electron tunneling spectroscopy on local magnetic moment embedded in Josephson junction2014In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 108, no 6, article id 67009Article in journal (Refereed)
    Abstract [en]

    Recent experimental conductance measurements performed on paramagnetic molecular adsorbates on a superconducting surface, using superconducting scanning tunneling microscopy techniques, are theoretically investigated. For low temperatures, we demonstrate that tunneling current assisted excitations of the local magnetic moment cannot occur for voltage biases smaller than the superconducting gap of the scanning tunneling microscope. The magnetic moment is only excited for voltages corresponding to the sum of the superconducting gap and the spin excitation energies. In excellent agreement with experiment, we show that pumping into higher excitations gives additional current signatures by accumulation of density in the lower ones. Using external magnetic fields, we Zeeman-split the possible degeneracy and thereby resolve all excitations comprised in the magnetic moment.

  • 5.
    Berggren, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Stability and chaos of a driven nanoelectromechanical Josephson junction2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 19, p. 195439-Article in journal (Refereed)
    Abstract [en]

    We consider the motion of and Josephson current through a mechanically oscillating superconducting island asymmetrically embedded in a Josephson junction. The electromechanical coupling is provided by distance-dependent tunneling rates between the electrodes and the island. The system asymmetry, resulting from the geometrical configuration, leads, for weak coupling, to an equation of the mechanical motion that reduces to the well-known Duffing equation. At zero bias voltage the island motion is determined by the homogenous Duffing equation that opens up two separate regions of solutions depending on the superconducting phases. The island either moves under influence of an anharmonic single-well potential or is governed by a double-well potential that allows for off-center oscillations. Under applied bias voltage the island equation of motion turns into a modified Duffing equation, with time-dependent coefficients, that demonstrate both quasiperiodic and chaotic behavior.

  • 6.
    Berggren, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Theory of spin inelastic tunneling spectroscopy for superconductor-superconductor and superconductor-metal junctions2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 20, article id 205438Article in journal (Refereed)
    Abstract [en]

    We address the tunneling conductance and spin inelastic tunneling spectroscopy of localized paramagnetic moments in a superconducting environment, pertaining to recent measurements on Fe-octaethylporphyrin-chloride using superconducting scanning tunneling microscopy. We demonstrate that the Cooper pair correlations in the tip and substrate generate a finite uniaxial anisotropy field acting on the local spin moment, and we argue that this field may be a source for the observed changes in the conductance spectrum for decreasing distance between the scanning tunneling tip and the local magnetic moment. We make a side-by-side comparison between the superconductor-superconductor junction and normal-metal-superconductor junction, and find qualitative agreement between the two setups while quantitative differences become explicit. When simulating the effects of electron pumping, we obtain additional peaks in the conductance spectrum that can be attributed to excitations between higher-energy spin states. The transverse anisotropy field couples basis states of the local spin which opens for transitions between spin states that are otherwise forbidden by conservation of angular momentum. Finally, we explore the influences of temperature, which tend to enable in-gap transitions, and an external magnetic field, which enables deeper studies of the spin excitation spectrum. We especially notice the appearance of a low and high excitation peak on each side of the main coherence peak as an imprint of transitions between the Zeeman split ground states.

  • 7.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Penazzi, Gabriele
    Univ Bremen, BCCMS, D-28359 Bremen, Germany..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Frauenheim, Thomas
    Univ Bremen, BCCMS, D-28359 Bremen, Germany..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Controlling Electronic Structure and Transport Properties of Zigzag Graphene Nanoribbons by Edge Functionalization with Fluorine2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 36, p. 21227-21233Article in journal (Refereed)
    Abstract [en]

    In this work, we report a detailed study of the electronic structure and transport properties of mono- and difluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0 K indicate that the stability of the nanoribbons increases with the increase in the concentration of difluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by density functional tight binding method. Using the nonequilibrium Green functional method, we have calculated the transmission coefficients of several mono- and difluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and difluorinated C atoms at the edges.

  • 8.
    Bhandary, Sumanta
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Penazzi, Gabriele
    BCCMS, Universitat Bremen.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Frauenheim, Thomas
    BCCMS, Universitat Bremen.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Controlling electronic structure and transport properties of zigzag graphene nanoribbons by mono- and difluorinated edge functionalizationManuscript (preprint) (Other academic)
    Abstract [en]

    In this work, we report a detailed study of the electronic structure and transport properties of mono- and di-fluorinated edges of zigzag graphene nanoribbons (ZGNR) using density functional theory (DFT). The calculated formation energies at 0K indicate that the stability of the nanoribbons increases with the increase in the concentration of di-fluorinated edge C atoms along with an interesting variation of the energy gaps between 0.0 to 0.66 eV depending on the concentration. This gives a possibility of tuning the band gaps by controlling the concentration of F for terminating the edges of the nanoribbons. The DFT results have been reproduced by single band tight binding as well as density functional tight binding methods. Using non-equilibrium Green functional method, we have calculated the transmission coecients of several mono and di-fluorinated ZGNR as a function of unit cell size and degree of homogeneous disorder caused by the random placement of mono and di-fuorinated C atoms at the edges.

  • 9.
    Bhattacharjee, Satadeep
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Atomistic Spin Dynamic Method with both Damping and Moment of Inertia Effects Included from First Principles2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 5, p. 057204-Article in journal (Refereed)
    Abstract [en]

    We consider spin dynamics for implementation in an atomistic framework and we address the feasibility of capturing processes in the femtosecond regime by inclusion of moment of inertia. In the spirit of an s-d-like interaction between the magnetization and electron spin, we derive a generalized equation of motion for the magnetization dynamics in the semiclassical limit, which is nonlocal in both space and time. Using this result we retain a generalized Landau-Lifshitz-Gilbert equation, also including the moment of inertia, and demonstrate how the exchange interaction, damping, and moment of inertia, all can be calculated from first principles.

  • 10.
    Black-Schaffer, Annica M.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Balatsky, A. V.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Filling of magnetic-impurity-induced gap in topological insulators by potential scattering2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 20, article id 201411Article in journal (Refereed)
    Abstract [en]

    We show that the energy gap induced by ferromagnetically aligned magnetic impurities on the surface of a topological insulator can be filled, due to scattering off the nonmagnetic potential of the impurities. In both a continuum surface model and a three-dimensional tight-binding lattice model, we find that the energy gap disappears already at weak potential scattering as impurity resonances add spectral weight at the Dirac point. This can help explain seemingly contradictory experimental results as to the existence of a gap.

  • 11.
    Borlenghi, Simone
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Iubini, Stefano
    Lepri, Stefano
    Bergqvist, Lars
    Delin, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Coherent energy transport in classical nonlinear oscillators: An analogy with the Josephson effect2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, no 4, article id 040102Article in journal (Refereed)
    Abstract [en]

    By means of a simple theoretical model and numerical simulations, we demonstrate the presence of persistent energy currents in a lattice of classical nonlinear oscillators with uniform temperature and chemical potential. In analogy with the well-known Josephson effect, the currents are proportional to the sine of the phase differences between the oscillators. Our results elucidate general aspects of nonequilibrium thermodynamics and point towards a way to practically control transport phenomena in a large class of systems. We apply the model to describe the phase-controlled spin-wave current in a bilayer nanopillar.

  • 12.
    Borlenghi, Simone
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Iubini, Stefano
    Lepri, Stefano
    Chico, Jonathan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bergqvist, Lars
    Delin, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Energy and magnetization transport in nonequilibrium macrospin systems2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 92, no 1, article id 012116Article in journal (Refereed)
    Abstract [en]

    We investigate numerically the magnetization dynamics of an array of nanodisks interacting through the magnetodipolar coupling. In the presence of a temperature gradient, the chain reaches a nonequilibrium steady state where energy and magnetization currents propagate. This effect can be described as the flow of energy and particle currents in an off-equilibrium discrete nonlinear Schrodinger (DNLS) equation. This model makes transparent the transport properties of the system and allows for a precise definition of temperature and chemical potential for a precessing spin. The present study proposes a setup for the spin-Seebeck effect, and shows that its qualitative features can be captured by a general oscillator-chain model.

  • 13.
    Borlenghi, Simone
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Iubini, Stefano
    Univ Firenze, Dipartimento Fis & Astron, Via G Sansone 1, I-50019 Sesto Fiorentino, Italy.;Ist Nazl Fis Nucl, Sez Firenze, Via G Sansone 1, I-50019 Sesto Fiorentino, Italy..
    Lepri, Stefano
    Ist Nazl Fis Nucl, Sez Firenze, Via G Sansone 1, I-50019 Sesto Fiorentino, Italy.;CNR, Ist Sistemi Complessi, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Entropy production for complex Langevin equations2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 1, article id 012150Article in journal (Refereed)
    Abstract [en]

    We study irreversible processes for nonlinear oscillators networks described by complex-valued Langevin equations that account for coupling to different thermochemical baths. Dissipation is introduced via non-Hermitian terms in the Hamiltonian of the model. We apply the stochastic thermodynamics formalism to compute explicit expressions for the entropy production rates. We discuss in particular the nonequilibrium steady states of the network characterized by a constant production rate of entropy and flows of energy and particle currents. For two specific examples, a one-dimensional chain and a dimer, numerical calculations are presented. The role of asymmetric coupling among the oscillators on the entropy production is illustrated.

  • 14.
    Borlenghi, Simone
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mahani, M. R.
    KTH Royal Inst Technol, Sch Engn Sci, Dept Appl Phys, Electrum 229, SE-16440 Kista, Sweden..
    Fangohr, Hans
    Univ Southampton, Dept Engn & Environm, Southampton SO17 1BJ, Hants, England.;European XFEL GmbH, Holzkoppel 4, D-22869 Schenefeld, Germany..
    Franchin, M.
    Univ Southampton, Dept Engn & Environm, Southampton SO17 1BJ, Hants, England..
    Delin, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Sch Engn Sci, Dept Appl Phys, Electrum 229, SE-16440 Kista, Sweden.;KTH Royal Inst Technol, Swedish E Sci Res Ctr SeRC, SE-10044 Stockholm, Sweden..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Micromagnetic simulations of spin-torque driven magnetization dynamics with spatially resolved spin transport and magnetization texture2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 9, article id 094428Article in journal (Refereed)
    Abstract [en]

    We present a simple and fast method to simulate spin-torque driven magnetization dynamics in nanopillar spin-valve structures. The approach is based on the coupling between a spin transport code based on random matrix theory and a micromagnetics finite-elements software. In this way the spatial dependence of both spin transport and magnetization dynamics is properly taken into account. Our results are compared with experiments. The excitation of the spin-wave modes, including the threshold current for steady-state magnetization precession and the nonlinear frequency shift of the modes are reproduced correctly. The giant magneto resistance effect and the magnetization switching also agree with experiment. The similarities with recently described spin-caloritronics devices are also discussed.

  • 15.
    Cardias, Ramon
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bezerra-Neto, M. M.
    Instituto de Engenharia e Geociências, Universidade Federal do Oeste do Pará, Santarém, PA, Brazil.
    Ribeiro, M. S.
    Instituto Federal do Pará, Campus Belém, PA, Brazil.
    Szilva, Attila
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kvashnin, Yaroslav
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    Klautau, A. B.
    Faculdade de Física, Universidade Federal do Pará, Belém, Pará, Brazil.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
    First-principles Dzyaloshinskii-Moryia interaction in a non-collinear frameworkManuscript (preprint) (Other academic)
    Abstract [en]

    Including dierent terms in the spin-Hamiltonian, we have derived an expression for theDzyaloshinskii-Moryia vector interaction (DMI) where all the three components of the vector canbe calculated independently of the magnetic conguration. Here, we have chosen the Cr triangulartrimer on Au(111) and Mn triangular trimes on Ag(111) and Au(111) surfaces to study the implementationof the derived DMI into the RS-LMTO-ASA method. Our results have shown thatthe DMI value (module and direction) is drastically dierent for collinear and non-collinear states.Based on relation between the spin and charge currents owing in the system and the non-collinearmagnetic conguration of the triangular trimer, we argued that the drastic change between the DMIcalculated considering a collinear and a non-collinear magnetic conguration can be explained bythe mechanism behind the spin and charge currents owing through the atoms when the spins arealigned in a non-collinear fashion.

  • 16.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplap
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Defect-controlled electronic transport in single, bilayer, and N-doped graphene: Theory2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 24, p. 245405-Article in journal (Refereed)
    Abstract [en]

    We report on a theoretical study of the electronic-structure and transport properties of single and bilayer graphene with vacancy defects, as well as N-doped graphene. The theory is based on first-principles calculations as well as model investigations in terms of real-space Green's functions. We show that increasing the defect concentration increases drastically the conductivity in the limit of zero applied gate voltage, by establishing carriers in originally carrier-free graphene, a fact which is in agreement with recent observations. We calculate the amount of defects needed for a transition from a nonconducting to a conducting regime (i.e., a metal-insulator transition) and establish the threshold of the defect concentration where the increase in impurity scattering dominates over the increase in carrier-induced conductivity.

  • 17.
    Chico, Jonathan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Etz, Corina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bergqvist, Lars
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Delin, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Thermally driven domain-wall motion in Fe on W(110)2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 1, p. 014434-Article in journal (Refereed)
    Abstract [en]

    It has recently been shown that domain walls (DWs) in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(110) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting this system to thermal gradients we observe a temperature-dependent movement of the domain wall. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. Our material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy, and the exchange interaction and shows that the larger DW velocities are found for materials with broader domain-wall width. The relatively slow DW motion of the Fe/W(110) system is hence primarily caused by its narrow domain-wall width, which results from its large magnetic anisotropy and soft exchange.

  • 18.
    Cornils, L.
    et al.
    Univ Hamburg, Dept Phys, D-20355 Hamburg, Germany..
    Kamlapure, A.
    Univ Hamburg, Dept Phys, D-20355 Hamburg, Germany..
    Zhou, L.
    Univ Hamburg, Dept Phys, D-20355 Hamburg, Germany.;Max Planck Inst Solid State Res, D-70569 Stuttgart, Germany..
    Pradhan, Saurabh
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Khajetoorians, A. A.
    Univ Hamburg, Dept Phys, D-20355 Hamburg, Germany.;Radboud Univ Nijmegen, Inst Mol & Mat, NL-6525 AJ Nijmegen, Netherlands..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Wiebe, J.
    Univ Hamburg, Dept Phys, D-20355 Hamburg, Germany..
    Wiesendanger, R.
    Univ Hamburg, Dept Phys, D-20355 Hamburg, Germany..
    Spin-Resolved Spectroscopy of the Yu-Shiba-Rusinov States of Individual Atoms2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 19, article id 197002Article in journal (Refereed)
    Abstract [en]

    A magnetic atom in a superconducting host induces so-called Yu-Shiba-Rusinov (YSR) bound states inside the superconducting energy gap. By combining spin-resolved scanning tunneling spectroscopy with simulations we demonstrate that the pair of peaks associated with the YSR states of an individual Fe atom coupled to an oxygen-reconstructed Ta surface gets spin polarized in an external magnetic field. As theoretically predicted, the electron and hole parts of the YSR states have opposite signs of spin polarizations which keep their spin character when crossing the Fermi level through the quantum phase transition. The simulation of a YSR state right at the Fermi level reveals zero spin polarization which can be used to distinguish such states from Majorana zero modes in chains of YSR atoms.

  • 19.
    Daukiya, Lakshya
    et al.
    Univ Haute Alsace, Inst Sci Mat Mulhouse, CNRS, UMR 7361, 3Bis,Rue Alfred Werner, F-68093 Mulhouse, France..
    Mattioli, Cristina
    CNRS, UPR 8011, CEMES, Nanosci Grp, 29 Rue Jeanne Marvig,BP 94347, F-31055 Toulouse, France..
    Aubel, Dominique
    Univ Haute Alsace, Inst Sci Mat Mulhouse, CNRS, UMR 7361, 3Bis,Rue Alfred Werner, F-68093 Mulhouse, France..
    Hajjar-Garreau, Samar
    Univ Haute Alsace, Inst Sci Mat Mulhouse, CNRS, UMR 7361, 3Bis,Rue Alfred Werner, F-68093 Mulhouse, France..
    Vonau, Francois
    Univ Haute Alsace, Inst Sci Mat Mulhouse, CNRS, UMR 7361, 3Bis,Rue Alfred Werner, F-68093 Mulhouse, France..
    Denys, Emmanuel
    Univ Haute Alsace, Inst Sci Mat Mulhouse, CNRS, UMR 7361, 3Bis,Rue Alfred Werner, F-68093 Mulhouse, France..
    Reiter, Guenter
    Univ Freiburg, Inst Phys, Hermann Herder Str 3, D-79104 Freiburg, Germany..
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Perrin, Elsa
    UPMC Univ Paris 06, CNRS, Dept Chim, Ecole Normale Super,PSL Res Univ, 24 Rue Lhomond, F-75005 Paris, France..
    Bocquet, Marie-Laure
    UPMC Univ Paris 06, CNRS, Dept Chim, Ecole Normale Super,PSL Res Univ, 24 Rue Lhomond, F-75005 Paris, France..
    Bena, Cristina
    CEA Saclay, Inst Phys Theor, Orme Merisiers, F-91190 Gif Sur Yvette, France.;Paris Sud, UMR 8502, CNRS, Lab Phys Solides, F-91405 Orsay, France..
    Gourdon, Andre
    CNRS, UPR 8011, CEMES, Nanosci Grp, 29 Rue Jeanne Marvig,BP 94347, F-31055 Toulouse, France..
    Simon, Laurent
    Univ Haute Alsace, Inst Sci Mat Mulhouse, CNRS, UMR 7361, 3Bis,Rue Alfred Werner, F-68093 Mulhouse, France..
    Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene2017In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 11, no 1, p. 627-634Article in journal (Refereed)
    Abstract [en]

    Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp(2) conjugation of carbon atoms was broken, and local sp(3) bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

  • 20.
    Fransson et al., Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    eriksson, olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    A perfect spin-filter quantum dot system2004In: J phys cond matt, Vol. 16, p. L249-Article in journal (Refereed)
  • 21.
    Fransson et al., Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    eriksson, olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Transport through quasi-degenerate states in coupled quantum dots2004In: pjotonics and nanostructres, Vol. 2, p. 11-Article in journal (Refereed)
  • 22.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism.
    Angular conductance resonances of quantum dots non-collinearly coupled to ferromagnetic leads2005In: Europhysics Letters, Vol. 70, no 6, p. 796-802Article in journal (Refereed)
    Abstract [en]

    The zero-bias conductance of quantum dots coupled to ferromagnetic leads is investigated. In the strong-coupling regime, it is found that the conductance is a non-monotonic function of the angle between the magnetisation directions in the two contacts. This behaviour is an effect of the presence of the leads which induces an angle-dependent spin-split of the quantum dot states, and spin-flip transitions between the quantum dot states whenever the magnetisation directions of the leads are non-collinear, which enhances the current density at the chemical potential. In the weak-coupling regime, the system reverts to normal spin valve character.

  • 23.
    Fransson, Jonas
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Theoretical Magnetism.
    Angular conductance resonances of quantum dots strongly coupled to noncollinearly oriented ferromagnetic leads2005In: Physical Reivew B, Vol. 72, no 4, p. 045415-Article in journal (Refereed)
    Abstract [en]

    The transport properties of quantum dots coupled to noncollinear magnetic leads is investigated. It is found that the conductance, and current, of the system in the strongly coupled regime is a nonmonotonic function of the angle between the magnetization directions in the two leads. Because of many-body interactions between electrons in the localized states of the quantum dot, induced by the presence of the conduction electrons in the leads, the positions of the quantum dot states are shifted in a spin-dependent way. Thus, the physics of the quantum dot is dynamically dependent on the angle between the magnetization directions of the two leads, which in combination with spin-flip transitions explains the nonmonotonic behavior of the magnetoresistance. The linear response conductance shows a rich complexity ranging from negative to positive magnetoresistance, depending on the positions of the localized states. The nonmonotonic transport characteristics persists for finite bias voltages.

  • 24.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chirality-Induced Spin Selectivity: The Role of Electron Correlations2019In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 22, p. 7126-7132Article in journal (Refereed)
    Abstract [en]

    Chirality-induced spin selectivity, discovered about two decades ago in helical molecules, is a nonequilibrium effect that emerges from the interplay between geometrical helicity and spin-orbit interactions. Several model Hamiltonians building on this interplay have been proposed, and while these can yield spin-polarized transport properties that agree with experimental observations, they simultaneously depend on unrealistic values of the spin-orbit interaction parameters. It is likely, however, that a common deficit originates from the fact that all these models are uncorrelated or single-electron theories. Therefore, chirality-induced spin selectivity is here addressed using a many-body approach, which allows for nonequilibrium conditions and a systematic treatment of the correlated state. The intrinsic molecular spin polarization increases by 2 orders of magnitude, or more, compared to the corresponding result in the uncorrelated model. In addition, the electronic structure responds to varying external magnetic conditions which, therefore, enables comparisons of the currents provided for different spin polarizations in one or both of the leads between which the molecule is mounted. Using experimentally feasible parameters and room temperature, the obtained normalized difference between such currents may be as large as 5-10% for short molecular chains, clearly suggesting the vital importance of including electron correlations when searching for explanations of the phenomenon.

  • 25.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Detection of spin reversal and nutations through current measurements2008In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 19, no 28, p. 285714-Article in journal (Refereed)
    Abstract [en]

    The dynamics of a single spin embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Moment reversal of the local spin induced by the bias voltage across the junction is shown to have a measurable effect on the tunneling current. Furthermore, the frequency of a harmonic bias voltage is picked up by the local spin dynamics and transferred back to the current, generating a double frequency component.

  • 26.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Dynamical exchange interaction between localized spins out of equilibrium2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 18, p. 180411-Article in journal (Refereed)
    Abstract [en]

    The electron mediated exchange interaction between local spins adsorbed on two- dimensional surface is studied under nonequilibrium conditions. The effective spin-spin interaction is found to depend both on the spin polarization of the substrate and the excitation spectrum of the local spins. For spatially anisotropic spin polarization of the substrate, the spatial dependence of the interaction comprise components decaying as sin (2k(F)R)/(2k(F)R) and sin (2k(F)R)/(2k(F)R)(2).

  • 27.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Formation of pure two-electron triplet states in weakly coupled quantum dots attached to ferromagnetic leads2006In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 17, no 21, p. 5344-5348Article in journal (Refereed)
    Abstract [en]

    Weakly coupled quantum dots in the Pauli spin blockade regime are considered with respect to spin-dependent transport. By attaching one half-metallic and one non-magnetic lead, the Pauli spin blockade is formed by a pure triplet state with spin moment S-z = 1 or -1. Furthermore, additional spin blockade regimes emerge because of full occupation in states with opposite spin to that of the half-metallic lead.

  • 28.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Inelastic-impurity-scattering-induced spin texture and topological transitions in surface electron waves2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 12, article id 125405Article in journal (Refereed)
    Abstract [en]

    Inelastic scattering off magnetic impurities in a spin-chiral two-dimensional electron gas, e.g., the Rashba system, is shown to generate topological changes in the spin texture of the electron waves emanating from the scattering center. While elastic scattering gives rise to a purely in-plane spin texture for an in-plane magnetic scattering potential, out-of-plane components emerge upon activation of inelastic scattering processes. This property leads to a possibility to make controlled transitions between trivial and nontrivial topologies of the spin texture.

  • 29.
    Fransson, Jonas
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Theoretical Magnetism.
    Nonequilibrium theory for a quantum dot with arbitrary on-site correlation strength coupled to leads2005In: Physical Reivew B, Vol. 72, no 7, p. 075314-Article in journal (Refereed)
    Abstract [en]

    An analytical expression for the current through a single level quantum dot for arbitrary strength of the on-site electron-electron interaction is derived beyond mean-field theory. By describing the localized states in terms of many-body operators, the employed diagrammatic technique for strong coupling between localized and delocalized states enables inclusion of electron correlation effects into the description of the local dynamics, which provides transport properties that are consistent with recent experimental data.

  • 30.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Non-Orthogonality and Electron Correlations in Nanotransport: Spin- and Time-Dependent Currents2002Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The concept of the transfer Hamiltonian formalism has been reconsidered and generalized to include the non-orthogonality between the electron states in an interacting region, e.g. quantum dot (QD), and the states in the conduction bands in the attached contacts. The electron correlations in the QD are described by means of a diagram technique for Hubbard operator Green functions for non-equilibrium states.

    It is shown that the non-orthogonality between the electrons states in the contacts and the QD is reflected in the anti-commutation relations for the field operators of the subsystems. The derived forumla for the current contains corrections from the overlap of the same order as the widely used conventional tunneling coefficients.

    It is also shown that kinematic interactions between the QD states and the electrons in the contacts, renormalizes the QD energies in a spin-dependent fashion. The structure of the renormalization provides an opportunity to include a spin splitting of the QD levels by polarizing the conduction bands in the contacts and/or imposing different hybridizations between the states in the contacts and the QD for the two spin channels. This leads to a substantial amplification of the spin polarization in the current, suggesting applications in magnetic sensors and spin-filters.

    List of papers
    1. On the non-orthogonality problem in the description of quantum devices
    Open this publication in new window or tab >>On the non-orthogonality problem in the description of quantum devices
    1999 (English)In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 272, no 1-4, p. 28-30Article in journal (Refereed) Published
    Abstract [en]

    An approach which allows to include the corrections from non-orthogonality of electron states in contacts and quantum dots is developed. Comparison of the energy levels and charge distributions of electrons in 1D quantum dot (QD) in equilibrium, obtained within orthogonal (OR) and non-orthogonal representations (NOR), with the exact ones shows that the NOR provides a considerable improvement, for levels below the top of barrier. The approach is extended to non-equilibrium states. A derivation of the tunneling current through a single potential barrier is performed using equations of motion for correlation functions. A formula for transient current derived by means of the diagram technique for Hubbard operators is given for the problem of QD with strongly correlated electrons interacting with electrons in contacts. The non-orthogonality renormalizes the tunneling matrix elements and spectral weights of Green functions.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90019 (URN)10.1016/S0921-4526(99)00343-9 (DOI)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2017-12-14Bibliographically approved
    2. Effects of nonorthogonality in the time-dependent current through tunnel junctions
    Open this publication in new window or tab >>Effects of nonorthogonality in the time-dependent current through tunnel junctions
    2001 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64, no 15, p. 153403-Article in journal (Refereed) Published
    Abstract [en]

    A theoretical technique which allows one to include contributions from nonorthogonality of the electron states in the leads connected to a tunneling junction is derived. The theory is applied to a single-barrier tunneling structure and a simple expression for the time-dependent tunneling current is derived showing explicit dependence of the overlap. The overlap proves to be necessary for a better quantitative description of the tunneling current, and our theory reproduces experimental results substantially better compared to standard approaches.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90020 (URN)10.1103/PhysRevB.64.153403 (DOI)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2017-12-14Bibliographically approved
    3. Effects of non-orthogonality and correlation effects on the time-dependent current through quantum dots
    Open this publication in new window or tab >>Effects of non-orthogonality and correlation effects on the time-dependent current through quantum dots
    2002 (English)In: Phys. Rev. B, Vol. 66, no 19, p. 195319-Article in journal (Refereed) Published
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90021 (URN)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2012-03-18Bibliographically approved
    4. Theory of time-dependent transport in quantum dot systems
    Open this publication in new window or tab >>Theory of time-dependent transport in quantum dot systems
    2003 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 92, no 6, p. 471-477Article in journal (Refereed) Published
    Abstract [en]

    The formulation of the time-dependent tunnel current in quantum dot (QD) systems in the (generalized) transfer Hamiltonian formalism is reconsidered, taking into account the nonorthogonality between the subsystems. Exploiting the fact that only the total charge in the system is conserved, in general, gives rise to a new formulation of the transport theory. As a result, it is shown necessary to depart from the orthodox picture, in which the current is treated as a local property of the system. A general formula for the current is derived. By expressing properties of the quantum dot in the Hubbard I approximation with the loop correction, thereby including strong correlations of the QD states, numerical results are given for various time-dependent source-drain voltages

    Keywords
    transport theory, time-dependence, non-equilibrium, tunneling, mesoscopic system, quantum dot, non-orthogonality, strong correlations
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90022 (URN)10.1002/qua.10527 (DOI)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2017-12-14Bibliographically approved
    5. Many-body approach to spin-dependent transport in quantum dot systems
    Open this publication in new window or tab >>Many-body approach to spin-dependent transport in quantum dot systems
    2002 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 88, no 22, p. 226601-Article in journal (Refereed) Published
    Abstract [en]

    By means of a diagram technique for Hubbard operators, we show the existence of a spin-dependent renormalization of the localized levels in an interacting region, e.g., quantum dot, modeled by the Anderson Hamiltonian with two conduction bands. It is shown that the renormalization of the levels with a given spin direction is due to kinematic interactions with the conduction subbands of the opposite spin. The consequence of this dressing of the localized levels is a drastically decreased tunneling current for ferromagnetically ordered leads compared to that of paramagnetically ordered leads. Furthermore, the studied system shows a spin-dependent resonant tunneling behavior for ferromagnetically ordered leads.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90023 (URN)10.1103/PhysRevLett.88.226601 (DOI)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2017-12-14Bibliographically approved
    6. Theory of spin-filtering in Fe-Pd-Fe nano-structures
    Open this publication in new window or tab >>Theory of spin-filtering in Fe-Pd-Fe nano-structures
    (English)Manuscript (Other academic)
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90024 (URN)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2012-03-18Bibliographically approved
    7. Cluster approach to transport through complex nanostructures
    Open this publication in new window or tab >>Cluster approach to transport through complex nanostructures
    (English)In: (Manuscript)Article in journal (Refereed) Submitted
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-90025 (URN)
    Available from: 2002-10-11 Created: 2002-10-11 Last updated: 2012-03-18Bibliographically approved
  • 31.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Subnanosecond switching of local spin-exchange coupled to ferromagnets2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 20, p. 205316-Article in journal (Refereed)
    Abstract [en]

    The dynamics of a single spin that is embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons, By using time-dependent equations of motion for the spin, which is under the influence of a spin-polarized tunneling current, it is shown that the magnetic field induced by bias voltage pulses allows for a subnanosecond switching of the local spin and the possibility of spin reversal is illustrated. Furthermore, it is shown that the time evolution of the Larmor frequency sharply peaks around the spin-flip event and it is argued that this feature can be used as an indicator for the spin flip.

  • 32.
    Fransson, Jonas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Theory of time-dependent transport in quantum dot systems2003In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 92, no 6, p. 471-477Article in journal (Refereed)
    Abstract [en]

    The formulation of the time-dependent tunnel current in quantum dot (QD) systems in the (generalized) transfer Hamiltonian formalism is reconsidered, taking into account the nonorthogonality between the subsystems. Exploiting the fact that only the total charge in the system is conserved, in general, gives rise to a new formulation of the transport theory. As a result, it is shown necessary to depart from the orthodox picture, in which the current is treated as a local property of the system. A general formula for the current is derived. By expressing properties of the quantum dot in the Hubbard I approximation with the loop correction, thereby including strong correlations of the QD states, numerical results are given for various time-dependent source-drain voltages

  • 33.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Balatsky, A. V.
    Imaging spin-inelastic Friedel oscillations emerging from magnetic impurities2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 16, p. 161401-Article in journal (Refereed)
    Abstract [en]

    We consider inelastic scattering of localized magnetic moments coupled with the electrons on the surface. We argue that spin-inelastic transitions of the magnetic impurities generate oscillations at a momentum k, corresponding to the inelastic mode, in the second derivative of the current with respect to voltage d(2)I/dV(2). These oscillations are similar in nature to Friedel oscillations. Inelastic Friedel oscillations, which were previously proposed for a spin-unpolarized setup, are here extended for spin-polarized systems. We propose to use scanning tunneling microscopy to measure spin-inelastic scattering generated at the impurity spin by imaging the d(2)I/dV(2) oscillations on the metal surface.

  • 34.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Balatsky, A. V.
    Zhu, Jian-Xin
    Dynamical properties of a vibrating molecular quantum dot in a Josephson junction2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 15, p. 155440-Article in journal (Refereed)
    Abstract [en]

    We investigate dynamical transport aspects of a combined nanomechanical-superconducting device in which Cooper pair tunneling interfere with the mechanical motion of a vibrating molecular quantum dot embedded in a Josephson junction. Six different regimes for the tunneling dynamics are identified with respect to the electron level and the charging energy in the quantum dot. In five of those regimes new time scales are introduced which are associated with the energies of the single-electron transitions within the quantum dot while there is one regime where the internal properties of the quantum dot are static.

  • 35.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Bengone, Olivier M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Larsson, J. Andreas
    Greer, James C.
    A physical compact model for electron transport across single molecules2006In: IEEE transactions on nanotechnology, ISSN 1536-125X, E-ISSN 1941-0085, Vol. 5, no 6, p. 745-749Article in journal (Refereed)
    Abstract [en]

    Prediction of current flow across single molecules requires ab initio electronic structure calculations along with their associated high computational demand, and a means for incorporating open system boundary conditions to describe the voltage sources driving the current. To date, first principle predictions of electron transport across single molecules have not fully achieved a predictive capability. The situation for molecular electronics may be compared to conventional technology computer-aided design (TCAD), whereby various approximations to the Boltzmann transport equation are solved to predict electronic device behavior, but in practice are too time consuming for most circuit design applications. To simplify device models for circuit design, analytical but physically motivated models are introduced to capture the behavior of active and passive devices; however, similar models do not vet exist for molecular electronics. We follow a similar approach by evaluating an analytical model achieved by combining a mesoscopic transport model with parameterizations taken from quantum chemical calculations of the electronic structure of single molecule bonded between two metal contacts. Using the model to describe electron transport across benzene- 1,4-dithiol and by comparing to experiment, we are able to extract the coupling strength of the molecule attached to two infinite metal electrodes. The resulting procedure allows for accurate and computationally efficient modeling of the static (dc) characteristics of a single molecule, with the added capability of being able to study the physical model parameter variations across a range of experiments. Such simple physical models are also an important step towards developing a design methodology for molecular electronics.

  • 36.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Black-Schaffer, Annica M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Balatsky, A. V.
    Engineered near-perfect backscattering on the surface of a topological insulator with nonmagnetic impurities2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 24, p. 241409-Article in journal (Refereed)
    Abstract [en]

    We show how to engineer enhanced skew scattering and gaplike openings in the surface state of three-dimensional topological insulators using only nonmagnetic impurities. Enhanced skewscattering off nonmagnetic impurities is obtained as a finite size effect of the scattering potential. Intimately related to the generated skew scattering is the emergence of a gaplike density of electron states locally around the impurities and surrounded by sharp resonances, with an extended energy gap appearing in engineered impurity structures.

  • 37.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    eriksson, olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Asymetric negative differential conductance in double quantum-dots2004In: J phys cond matt, Vol. 16, p. L85-Article in journal (Refereed)
  • 38.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    eriksson, olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV.
    Current-voltage asymmetries and negative differential conductance due to strong electron correlations in double quantum dots2004In: phys rev b, Vol. 70, p. 85301-Article in journal (Refereed)
  • 39.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Balatsky, A. V.
    Theory of spin-polarized scanning tunneling microscopy applied to local spins2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 11, p. 115454-Article in journal (Refereed)
    Abstract [en]

    We provide a theory for scanning tunneling microscopy and spectroscopy using a spin-polarized tip. It it shown that the tunneling conductance can be partitioned into three separate contributions, a background conductance which is independent of the local spin, a dynamical conductance which is proportional to the local-spin moment, and a conductance which is proportional to the noise spectrum of the local-spin interactions. The presented theory is applicable to setups with magnetic tip and substrate in noncollinear arrangement, as well as for nonmagnetic situations. The partitioning of the tunneling current suggests a possibility to extract the total spin moment of the local spin from the dynamical conductance. The dynamical conductance suggests a possibility to generate very high-frequency spin-dependent ac currents and/or voltages. We also propose a measurement of the dynamical conductance that can be used to determine the character of the effective exchange interaction between individual spins in clusters. The third contribution to the tunneling current is associated with the spin-spin correlations induced by the exchange interaction between the local-spin moment and the tunneling electrons. We demonstrate how this term can be used in the analysis of spin excitations recorded in conductance measurements. Finally, we propose to use spin-polarized scanning tunneling microscopy for detailed studies of the spin-excitation spectrum.

  • 40.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Sandalov, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    On the non-orthogonality problem in the description of quantum devices1999In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 272, no 1-4, p. 28-30Article in journal (Refereed)
    Abstract [en]

    An approach which allows to include the corrections from non-orthogonality of electron states in contacts and quantum dots is developed. Comparison of the energy levels and charge distributions of electrons in 1D quantum dot (QD) in equilibrium, obtained within orthogonal (OR) and non-orthogonal representations (NOR), with the exact ones shows that the NOR provides a considerable improvement, for levels below the top of barrier. The approach is extended to non-equilibrium states. A derivation of the tunneling current through a single potential barrier is performed using equations of motion for correlation functions. A formula for transient current derived by means of the diagram technique for Hubbard operators is given for the problem of QD with strongly correlated electrons interacting with electrons in contacts. The non-orthogonality renormalizes the tunneling matrix elements and spectral weights of Green functions.

  • 41.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Sandalov, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Effects of nonorthogonality in the time-dependent current through tunnel junctions2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 64, no 15, p. 153403-Article in journal (Refereed)
    Abstract [en]

    A theoretical technique which allows one to include contributions from nonorthogonality of the electron states in the leads connected to a tunneling junction is derived. The theory is applied to a single-barrier tunneling structure and a simple expression for the time-dependent tunneling current is derived showing explicit dependence of the overlap. The overlap proves to be necessary for a better quantitative description of the tunneling current, and our theory reproduces experimental results substantially better compared to standard approaches.

  • 42.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Sandalov, Igor
    Many-body approach to spin-dependent transport in quantum dot systems2002In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 88, no 22, p. 226601-Article in journal (Refereed)
    Abstract [en]

    By means of a diagram technique for Hubbard operators, we show the existence of a spin-dependent renormalization of the localized levels in an interacting region, e.g., quantum dot, modeled by the Anderson Hamiltonian with two conduction bands. It is shown that the renormalization of the levels with a given spin direction is due to kinematic interactions with the conduction subbands of the opposite spin. The consequence of this dressing of the localized levels is a drastically decreased tunneling current for ferromagnetically ordered leads compared to that of paramagnetically ordered leads. Furthermore, the studied system shows a spin-dependent resonant tunneling behavior for ferromagnetically ordered leads.

  • 43.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Galperin, Michael
    Inelastic scattering and heating in a molecular spin pump2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 7, p. 075311-Article in journal (Refereed)
    Abstract [en]

    We consider a model for a spin field-effect molecular transistor, where a directed pure spin current is controlled by an external electric field. Inelastic scattering effects of such molecular device are discussed within a framework of full counting statistics for a multilevel molecular system. We propose that the heating of the molecular junction can be controlled by external electric and magnetic fields. Characteristic features of the model are demonstrated by numerical calculations.

  • 44.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Galperin, Michael
    Spin seebeck coefficient of a molecular spin pump2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 32, p. 14350-14357Article in journal (Refereed)
    Abstract [en]

    Within a generalized version of previously considered model of a molecular spin pump controlled by an external electric field [J. Fransson and M. Galperin, Phys. Rev. B, 2010, 81, 075311] we discuss thermal properties of such spintronic devices. The spin Seebeck coefficient of a molecular spin pump is introduced, and several possible definitions of a nonequilibrium Seebeck coefficient are discussed. The influence of inelastic effects in spin transport on its thermospintronic characteristics are demonstrated within numerical examples.

  • 45.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Holmström, Erik
    Eriksson, Olle
    Sandalov, Igor
    Theory of spin-filtering in Fe-Pd-Fe nano-structuresManuscript (Other academic)
  • 46.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    King, M. -G
    Yoon, Y.
    Xiao, S.
    Ochiai, Y.
    Reno, J. L.
    Aoki, N.
    Bird, J. P.
    Tuning the Fano Resonance with an Intruder Continuum2014In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 14, no 2, p. 788-793Article in journal (Refereed)
    Abstract [en]

    Through a combination of experiment and theory we establish the possibility of achieving strong tuning of Fano resonances (FRs), by allowing their usual two-path geometry to interfere with an additional, "intruder", continuum. As the coupling strength to this intruder is varied, we predict strong modulations of the resonance line shape that, in principle at least, may exceed the amplitude of the original FR itself. For a proof-of-concept demonstration of this phenomenon, we construct a nanoscale interferometer from nonlocally coupled quantum point contacts and utilize the unique features of their density of states to realize the intruder. External control of the intruder coupling is enabled by means of an applied magnetic field, in the presence of which we demonstrate the predicted distortions of the FR. This general scheme for resonant control should be broadly applicable to a Variety of wave-based systems, opening up the possibility of new applications in areas such as chemical and biological sensing and secure communications.

  • 47.
    Fransson, Jonas
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics. Department of Physics and Materials Science, Theoretical Magnetism.
    Lin, J. -F.
    Rotkina, L.
    Bird, J. P.
    Bennett, P. A.
    Signatures of bandlike tunneling in granular nanowires2005In: Physical Reivew B: Breif Reports, Vol. 72, no 11, p. 113411-Article in journal (Refereed)
    Abstract [en]

    We explore the problem of tunneling through disorderd nanowires, comprised of a random distribution of metallic grains, by means of a many-body model that captures the essential physics of the system. The random configuration of grains gives rise to a smooth bandlike set of states that mediates current flow through the nanowires. Analytical and numerical calculations show the characteristic signature of this unusual bandlike transport to be a quadratic variation of the current as a function of the applied voltage (i.e., I~V^2), a variation that is clearly observed in experimental studies of Pt/C composite nanowires.

  • 48.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Manoharan, H. C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Balatsky, A. V.
    Detection and Cloaking of Molecular Objects in Coherent Nanostructures Using Inelastic Electron Tunneling Spectroscopy2010In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 10, no 5, p. 1600-1604Article in journal (Refereed)
    Abstract [en]

    We address quantum invisibility in the context of electronics in nanoscale quantum structures. We make use of the freedom of design that quantum corrals provide and show that quantum mechanical objects can be hidden inside the corral, with respect to inelastic electron scattering spectroscopy in combination with scanning tunneling microscopy, and we propose a design strategy. A simple illustration of the invisibility is given in terms of an elliptic quantum corral containing a molecule, with a local vibrational mode, at one of the foci. Our work has implications to quantum information technology and presents new tools for nonlocal quantum detection and distinguishing between different molecules.

  • 49.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ren, Jie
    Zhu, Jian-Xin
    Electrical and Thermal Control of Magnetic Exchange Interactions2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, no 25, p. 257201-Article in journal (Refereed)
    Abstract [en]

    We investigate the far-from-equilibrium nature of magnetic anisotropy and exchange interactions between molecular magnets embedded in a tunnel junction. By mapping to an effective spin model, these magnetic interactions can be divided into three types: isotropic Heisenberg, anisotropic Ising, and anisotropic Dzyaloshinski-Moriya contributions, which are attributed to the background nonequilibrium electronic structures. We further demonstrate that both the magnetic self- and exchange interactions can be controlled either electrically by gating and tuning the voltage bias, or thermally by adjusting the temperature bias. We show that the Heisenberg and Ising interactions scale linearly, while the Dzyaloshinski-Moriya interaction scales quadratically, with the molecule-lead coupling strength. The interactions scale linearly with the effective spin polarizations of the leads and the molecular coherence. Our results pave a way for smart control of magnetic exchange interactions at atomic and molecular levels.

  • 50.
    Fransson, Jonas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    She, J. -H
    Pietronero, L.
    Balatsky, A. V.
    Inelastic electron tunneling spectroscopy at local defects in graphene2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 24, p. 245404-Article in journal (Refereed)
    Abstract [en]

    We address local inelastic scattering from the vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from a weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks or dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy d(2)I/dV(2) using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative U center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative U formation and its evolution from weak to strong coupling. The negative U site effectively acts as a local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac-cone-like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining.

12 1 - 50 of 89
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