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Fransson, J. (2019). Chirality-Induced Spin Selectivity: The Role of Electron Correlations. Journal of Physical Chemistry Letters, 10(22), 7126-7132
Open this publication in new window or tab >>Chirality-Induced Spin Selectivity: The Role of Electron Correlations
2019 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 22, p. 7126-7132Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2019
National Category
Theoretical Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-398420 (URN)10.1021/acs.jpclett.9b02929 (DOI)000497261200025 ()31657931 (PubMedID)
Funder
Swedish Research CouncilCarl Tryggers foundation
Available from: 2019-12-11 Created: 2019-12-11 Last updated: 2019-12-11Bibliographically approved
Huttmann, F., Rothenbach, N., Kraus, S., Ollefs, K., Arruda, L. M., Bernien, M., . . . Wende, H. (2019). Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator. Journal of Physical Chemistry Letters, 10(5), 911-917
Open this publication in new window or tab >>Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator
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2019 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 5, p. 911-917Article in journal (Refereed) Published
Abstract [en]

We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M-5,M-4 edge XMCD, orbital and spin magnetic moments of (-0.1 +/- 0.3)mu(B) and (+7.0 +/- 0.6)mu(B), respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M-5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-380459 (URN)10.1021/acs.jpclett.8b03711 (DOI)000461271700003 ()30717591 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation, 2013.0020Knut and Alice Wallenberg Foundation, 2012.0031Swedish Foundation for Strategic Research
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-03-28Bibliographically approved
Hammar, H., Jaramillo, J. D. & Fransson, J. (2019). Spin-dependent heat signatures of single-molecule spin dynamics. Physical Review B, 99(11), Article ID 115416.
Open this publication in new window or tab >>Spin-dependent heat signatures of single-molecule spin dynamics
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 11, article id 115416Article in journal (Refereed) Published
Abstract [en]

We investigate transient spin-dependent thermoelectric signatures in a single-molecule magnet under the effect of a time-dependent voltage pulse. We model the system using nonequilibrium Green's functions and a generalized spin equation of motion incorporating the dynamic electronic structure of the molecule. We show that the generated heat current in the system is due to both charge and spin contributions, related to the Peltier and the spin-dependent Peltier effect. There is also a clear signature in the heat current due to the spin dynamics of the single molecule and a possibility to control the spin-dependent heat currents by bias, tunneling coupling, and exchange interaction. A reversal of the net heat transfer in the molecule is found for increasing bias voltage due to the local Zeeman split and we can correlate the net heat transfer with the local anisotropies and dynamic exchange fields in the system.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-380458 (URN)10.1103/PhysRevB.99.115416 (DOI)000460722300011 ()
Funder
Swedish Research Council, SNIC 2018/8-29
Available from: 2019-03-28 Created: 2019-03-28 Last updated: 2019-03-28Bibliographically approved
Katcko, K., Urbain, E., Taudul, B., Schleicher, F., Arabski, J., Beaurepaire, E., . . . Bowen, M. (2019). Spin-driven electrical power generation at room temperature. Communications Physics, 2, Article ID 116.
Open this publication in new window or tab >>Spin-driven electrical power generation at room temperature
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2019 (English)In: Communications Physics, E-ISSN 2399-3650, Vol. 2, article id 116Article in journal (Refereed) Published
Abstract [en]

On-going research is exploring novel energy concepts ranging from classical to quantum thermodynamics. Ferromagnets carry substantial built-in energy due to ordered electron spins. Here, we propose to generate electrical power at room temperature by utilizing this magnetic energy to harvest thermal fluctuations on paramagnetic centers using spintronics. Our spin engine rectifies current fluctuations across the paramagnetic centers' spin states by utilizing so-called 'spinterfaces' with high spin polarization. Analytical and ab-initio theories suggest that experimental data at room temperature from a single MgO magnetic tunnel junction (MTJ) be linked to this spin engine. Device downscaling, other spintronic solutions to select a transport spin channel, and dual oxide/organic materials tracks to introduce paramagnetic centers into the tunnel barrier, widen opportunities for routine device reproduction. At present MgO MTJ densities in next-generation memories, this spin engine could lead to 'always-on' areal power densities that are highly competitive relative to other energy harvesting strategies.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2019
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-395839 (URN)10.1038/s42005-019-0207-8 (DOI)000488110600002 ()
Funder
Swedish Research Council
Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2019-10-25Bibliographically approved
Aydin, A., Fransson, J. & Sisman, A. (2019). Thermosize voltage induced in a ballistic graphene nanoribbon junction. Journal of Applied Physics, 126(10), Article ID 104302.
Open this publication in new window or tab >>Thermosize voltage induced in a ballistic graphene nanoribbon junction
2019 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 126, no 10, article id 104302Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2019
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-393081 (URN)10.1063/1.5111504 (DOI)000486028500025 ()
Available from: 2019-09-16 Created: 2019-09-16 Last updated: 2019-10-31Bibliographically approved
Pradhan, S. & Fransson, J. (2019). Time-dependent potential impurity in a topological insulator. Physical Review B, 100(12), Article ID 125163.
Open this publication in new window or tab >>Time-dependent potential impurity in a topological insulator
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 12, article id 125163Article in journal (Refereed) Published
Abstract [en]

We consider periodically driven potential impurities coupled to the surface states of a two-dimensional topological insulator. The problem is addressed by means of two models, of which the first model is an effective continuum Hamiltonian for the surface states, whereas the Kane-Mele lattice model is our second approach. While both models result in drastic changes in the local density of electron states with increasing amplitude and frequency of the driving field, the linearly low-energy local density of electron states remains in the continuum model, however, with an increased Fermi velocity. The spectrum of the continuum model remains gapless under the emergence of new impurity resonances near the Fermi energy. The Kane-Mele lattice model represents a finite size system, with edge states appearing at the boundary of the system. We, thus, consider the impurity at two different positions, one at the boundary and one at the center of the lattice. In the former case, a reduction and broadening of the low-energy local density of electron states result with increasing amplitude of the driving field. On the other hand, there are no new resonances emerging in the spectrum. In the latter case, the spectrum is gapped both in the absence of the impurity as well as for weak amplitudes of the driving field, while the gap tends to fill up with impurity states with increasing amplitude.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-395833 (URN)10.1103/PhysRevB.100.125163 (DOI)000488258600005 ()
Funder
Swedish Research CouncilStiftelsen Olle Engkvist Byggmästare
Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2019-10-25Bibliographically approved
Hammar, H. & Fransson, J. (2018). Dynamical exchange and phase induced switching of a localized molecular spin. Physical Review B, 98, Article ID 174438.
Open this publication in new window or tab >>Dynamical exchange and phase induced switching of a localized molecular spin
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, article id 174438Article in journal (Refereed) Published
Abstract [en]

We address the dynamics of a localized molecular spin under the influence of external voltage pulses using a generalized spin equation of motion which incorporates anisotropic fields, nonequilibrium conditions, and non-adiabatic dynamics. We predict a recurring 4π-periodic switching of the localized spin by application of a voltage pulse of temporal length τ. The switching phenomena can be explained by dynamical exchange interactions, internal transient fields, and self-interactions acting on the localized spin moment. 

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-368540 (URN)10.1103/PhysRevB.98.174438 (DOI)000451602400004 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC), 2018/8-29
Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2019-01-25Bibliographically approved
Jaramillo, J. D., Hammar, H. & Fransson, J. (2018). Electronically Mediated Magnetic Anisotropy in Vibrating Magnetic Molecules. ACS Omega, 3(6), 6546-6553
Open this publication in new window or tab >>Electronically Mediated Magnetic Anisotropy in Vibrating Magnetic Molecules
2018 (English)In: ACS Omega, E-ISSN 2470-1343, Vol. 3, no 6, p. 6546-6553Article in journal (Refereed) Published
Abstract [en]

We address the electronically induced anisotropy field acting on a spin moment comprised in a vibratingmagnetic molecule located in the junction between ferromagnetic metals. Under weak coupling between theelectrons and molecular vibrations, the nature of the anisotropy can be changed from favoring a high spin (easyaxis) magnetic moment to a low spin (easy plane) by applying a temperature difference or a voltage bias acrossthe junction. For unequal spin-polarizations in the ferromagnetic metals it is shown that the character of theanisotropy is essentially determined by the properties of the weaker ferromagnet. By increasing the temperaturein this metal, or introducing a voltage bias, its influence can be suppressed such that the dominant contributionto the anisotropy is interchanged to the stronger ferromagnet. With increasing coupling strength between themolecular vibrations and the electrons, the nature of the anisotropy is locked into favoring easy plane magnetis

National Category
Atom and Molecular Physics and Optics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-348657 (URN)10.1021/acsomega.8b00449 (DOI)000436340500071 ()
Funder
Swedish Research Council
Available from: 2018-04-16 Created: 2018-04-16 Last updated: 2018-12-05Bibliographically approved
Shiranzaei, M., Fransson, J., Cheraghchi, H. & Parhizgar, F. (2018). Nonlinear spin susceptibility in topological insulators. Physical Review B, 97(18), Article ID 180402.
Open this publication in new window or tab >>Nonlinear spin susceptibility in topological insulators
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 18, article id 180402Article in journal (Refereed) Published
Abstract [en]

We revise the theory of the indirect exchange interaction between magnetic impurities beyond the linear response theory to establish the effect of impurity resonances in the surface states of a three-dimensional topological insulator. The interaction is composed of isotropic Heisenberg, anisotropic Ising, and Dzyaloshinskii-Moriya types of couplings. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya-type contribution. We show that the spin-independent component of the impurity scattering can generate large values of the Dzyaloshinskii-Moriya-type coupling in comparison with the Heisenberg and Ising types of couplings, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and noncollinear configurations are allowed magnetic configurations of the impurities.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-356499 (URN)10.1103/PhysRevB.97.180402 (DOI)000432024200001 ()
Funder
Swedish Research Council
Available from: 2018-07-30 Created: 2018-07-30 Last updated: 2018-07-30Bibliographically approved
Pradhan, S. & Fransson, J. (2018). Shot noise as a probe of spin-correlated transport through single atoms. Physical Review B, 97(11), Article ID 115409.
Open this publication in new window or tab >>Shot noise as a probe of spin-correlated transport through single atoms
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 11, article id 115409Article in journal (Refereed) Published
Abstract [en]

We address the shot noise in the tunneling current through a local spin, pertaining to recent experiments on magnetic adatoms and single molecular magnets. We show that both uncorrelated and spin-correlated scattering processes contribute vitally to the noise spectrum. The spin-correlated scattering processes provide an additional contribution to the Landauer-Buttiker shot noise expression, accounting for correlations between the tunneling electrons and the localized spin moment. By calculating the Fano factor, we show that both super-and sub-Poissonian shot noise can be described within our approach. Our theory provides transparent insights into noise spectroscopy, consistent with recent experiments using local probing techniques on magnetic atoms.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350620 (URN)10.1103/PhysRevB.97.115409 (DOI)000427010900004 ()
Funder
Swedish Research CouncilStiftelsen Olle Engkvist Byggmästare
Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2018-05-22Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-9217-2218

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