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Publications (10 of 31) Show all publications
Grånäs, O., Timneanu, N., Eliah Dawod, I., Ragazzon, D., Trygg, S., Souvatzis, P., . . . Caleman, C. (2019). Femtosecond bond breaking and charge dynamics in ultracharged amino acids. Journal of Chemical Physics, 151(14), Article ID 144307.
Open this publication in new window or tab >>Femtosecond bond breaking and charge dynamics in ultracharged amino acids
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2019 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 14, article id 144307Article in journal (Refereed) Published
Abstract [en]

Historically, structure determination of nanocrystals, proteins, and macromolecules required the growth of high-quality crystals sufficiently large to diffract X-rays efficiently while withstanding radiation damage. The development of the X-ray free-electron laser has opened the path toward high resolution single particle imaging, and the extreme intensity of the X-rays ensures that enough diffraction statistics are collected before the sample is destroyed by radiation damage. Still, recovery of the structure is a challenge, in part due to the partial fragmentation of the sample during the diffraction event. In this study, we use first-principles based methods to study the impact of radiation induced ionization of six amino acids on the reconstruction process. In particular, we study the fragmentation and charge rearrangement to elucidate the time scales involved and the characteristic fragments occurring.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-395440 (URN)10.1063/1.5116814 (DOI)000500356200030 ()31615216 (PubMedID)
Funder
Swedish National Infrastructure for Computing (SNIC), SNIC 2019/8-30Swedish National Infrastructure for Computing (SNIC), SNIC 2018/3-221Swedish Research Council, 637-2013-7303Swedish Research Council, 2013-3940Swedish Foundation for Strategic Research , ICA16-0037
Available from: 2019-10-18 Created: 2019-10-18 Last updated: 2020-01-13Bibliographically approved
Grånäs, O., Kolesov, G. & Kaxiras, E. (2017). Impact of Vibrations and Electronic Coherence on Electron Transfer in Flat Molecular Wires. MRS Advances, 2(14), 811-816
Open this publication in new window or tab >>Impact of Vibrations and Electronic Coherence on Electron Transfer in Flat Molecular Wires
2017 (English)In: MRS Advances, ISSN 2316-7858, E-ISSN 1610-191X, Vol. 2, no 14, p. 811-816Article in journal (Refereed) Published
Abstract [en]

Electron transfer in molecular wires are of fundamental importance for a range of optoelectronic applications. The impact of electronic coherence and ionic vibrations on transmittance are of great importance to determine the mechanisms, and subsequently the type of wires that are most promising for applications. In this work, we use the real-time formulation of time-dependent density functional theory to study electron transfer through oligo-pphenylenevinylene (OPV) and the recently synthesized carbon bridged counterpart (COPV). A system prototypical of organic photovoltaics is setup by bridging a porphyrin-fullerene dyad, allowing a photo-excited electron to flow between the Zn-porphyrin (ZnP) chromophore and the C60 electron acceptor through the molecular wire. The excited state is described using the fully self-consistent.-SCF method. The state is then propagated in time using the real-time TD-DFT scheme, while describing ionic vibrations with classical nuclei. The charge transferred between porphyrin and C60 is calculated and correlated with the velocity autocorrelation functions of the ions. This provides a microscopic insight to vibrational and tunneling contributions to electron transport in linked porphyrin-fullerene dyads. We elaborate on important details in describing the excited state and trajectory sampling.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-337770 (URN)10.1557/adv.2017.157 (DOI)000412730300008 ()
Funder
Swedish Research Council, 637-2013-7303
Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2018-01-12Bibliographically approved
Schött, J., Locht, I. L. M., Lundin, E., Grånäs, O., Eriksson, O. & Di Marco, I. (2016). Analytic continuation by averaging Pade approximants. PHYSICAL REVIEW B, 93(7), Article ID 075104.
Open this publication in new window or tab >>Analytic continuation by averaging Pade approximants
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 7, article id 075104Article in journal (Refereed) Published
Abstract [en]

The ill-posed analytic continuation problem for Green's functions and self-energies is investigated by revisiting the Pade approximants technique. We propose to remedy the well-known problems of the Pade approximants by performing an average of several continuations, obtained by varying the number of fitted input points and Pade coefficients independently. The suggested approach is then applied to several test cases, including Sm and Pr atomic self-energies, the Green's functions of the Hubbard model for a Bethe lattice and of the Haldane model for a nanoribbon, as well as two special test functions. The sensitivity to numerical noise and the dependence on the precision of the numerical libraries are analyzed in detail. The present approach is compared to a number of other techniques, i.e., the nonnegative least-squares method, the nonnegative Tikhonov method, and the maximum entropy method, and is shown to perform well for the chosen test cases. This conclusion holds even when the noise on the input data is increased to reach values typical for quantum Monte Carlo simulations. The ability of the algorithm to resolve fine structures is finally illustrated for two relevant test functions.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-279570 (URN)10.1103/PhysRevB.93.075104 (DOI)000369399500001 ()
Funder
Swedish Research CouncileSSENCE - An eScience CollaborationKnut and Alice Wallenberg Foundation, KAW-2013.0020
Available from: 2016-03-02 Created: 2016-03-02 Last updated: 2018-10-10
Peters, L., Di Marco, I., Grånäs, O., Sasioglu, E., Altun, A., Rossen, S., . . . Eriksson, O. (2016). Correlation effects and orbital magnetism of Co clusters. PHYSICAL REVIEW B, 93(22), Article ID 224428.
Open this publication in new window or tab >>Correlation effects and orbital magnetism of Co clusters
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 22, article id 224428Article in journal (Refereed) Published
Abstract [en]

Recent experiments on isolated Co clusters have shown huge orbital magnetic moments in comparison with their bulk and surface counterparts. These clusters hence provide the unique possibility to study the evolution of the orbital magnetic moment with respect to the cluster size and how competing interactions contribute to the quenching of orbital magnetism. We investigate here different theoretical methods to calculate the spin and orbital moments of Co clusters, and assess the performances of the methods in comparison with experiments. It is shown that density-functional theory in conventional local density or generalized gradient approximations, or even with a hybrid functional, severely underestimates the orbital moment. As natural extensions/corrections, we considered the orbital polarization correction, the LDA+U approximation as well as the LDA+DMFT method. Our theory shows that of the considered methods, only the LDA+DMFT method provides orbital moments in agreement with experiment, thus emphasizing the importance of dynamic correlations effects for determining fundamental magnetic properties of magnets in the nanosize regime.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-299872 (URN)10.1103/PhysRevB.93.224428 (DOI)000378811500006 ()
Funder
Swedish Research CouncileSSENCE - An eScience CollaborationStandUpSwedish National Infrastructure for Computing (SNIC)Knut and Alice Wallenberg Foundation, 2013.0020; 2012.0031EU, European Research Council, 338957
Available from: 2016-07-28 Created: 2016-07-28 Last updated: 2016-07-28Bibliographically approved
Panda, S. K., Di Marco, I., Grånäs, O., Eriksson, O. & Fransson, J. (2016). Electronic and magnetic properties of single Fe atoms on a CuN surface: Effects of electron correlations. PHYSICAL REVIEW B, 93(14), Article ID 140101.
Open this publication in new window or tab >>Electronic and magnetic properties of single Fe atoms on a CuN surface: Effects of electron correlations
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 14, article id 140101Article in journal (Refereed) Published
Abstract [en]

The electronic structure and magnetic properties of a single Fe adatom on a CuN surface have been studied using density functional theory in the local spin density approximation (LSDA), the LSDA+U approach, and the local density approximation plus dynamical mean-field theory (LDA+DMFT). The impurity problem in LDA+DMFT is solved through exact diagonalization and in the Hubbard-I approximation. The comparison of the one-particle spectral functions obtained from LSDA, LSDA+U, and LDA+DMFT show the importance of dynamical correlations for the electronic structure of this system. Most importantly, we focused on the magnetic anisotropy and found that neither LSDA nor LSDA+U can explain the measured high values of the axial and transverse anisotropy parameters. Instead, the spin excitation energies obtained from our LDA+DMFT approach with exact diagonalization agree significantly better with experimental data. This affirms the importance of treating fluctuating magnetic moments through a realistic many-body treatment when describing this class of nanomagnetic systems. Moreover, it facilitates insight to the role of the hybridization with surrounding orbitals.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-283304 (URN)10.1103/PhysRevB.93.140101 (DOI)000373311500001 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationeSSENCE - An eScience Collaboration
Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2016-05-18Bibliographically approved
Kolesov, G., Grånäs, O., Hoyt, R., Vinichenko, D. & Kaxiras, E. (2016). Real-Time TD-DFT with Classical Ion Dynamics: Methodology and Applications. Journal of Chemical Theory and Computation, 12(2), 466-476
Open this publication in new window or tab >>Real-Time TD-DFT with Classical Ion Dynamics: Methodology and Applications
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2016 (English)In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 12, no 2, p. 466-476Article in journal (Refereed) Published
Abstract [en]

We present a method for real-time propagation of electronic wave functions, within time-dependent density functional theory (RT-TDDFT), coupled to ionic motion through mean-field classical dynamics. The goal of our method is to treat large systems and complex processes, in particular photocatalytic reactions and electron transfer events on surfaces and thin films. Due to the complexity of these processes, computational approaches are needed to provide insight into the underlying physical mechanisms and are therefore crucial for the rational design of new materials. Because of the short time step required for electron propagation (of order similar to 10 attoseconds), these simulations are computationally very demanding. Our methodology is based on numerical atomic-orbital-basis sets for computational efficiency. In the computational package, to which we refer as TDAP-2.0 (Time-evolving Deterministic Atom Propagator), we have implemented a number of important features and analysis tools for more accurate and efficient treatment of large, complex systems and time scales that reach into a fraction of a picosecond. We showcase the capabilities of our method using four different examples: (i) photodissociation into radicals of opposite spin, (ii) hydrogen adsorption on aluminum surfaces, (iii) optical absorption of spin-polarized organic molecule containing a metal ion, and (iv) electron transfer in a prototypical dye sensitized solar cell.

National Category
Other Physics Topics Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-281964 (URN)10.1021/acs.jctc.5b00969 (DOI)000370112900002 ()26680129 (PubMedID)
Funder
Swedish Research Council, 637-2013-7303
Available from: 2016-04-01 Created: 2016-04-01 Last updated: 2017-11-30Bibliographically approved
Kvashnin, Y. O., Grånäs, O., Di Marco, I., Katsnelson, M. I., Lichtenstein, A. I. & Eriksson, O. (2015). Exchange parameters of strongly correlated materials: Extraction from spin-polarized density functional theory plus dynamical mean-field theory. Physical Review B. Condensed Matter and Materials Physics, 91(12), Article ID 125133.
Open this publication in new window or tab >>Exchange parameters of strongly correlated materials: Extraction from spin-polarized density functional theory plus dynamical mean-field theory
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 12, article id 125133Article in journal (Refereed) Published
Abstract [en]

In this paper we present an accurate numerical scheme for extracting interatomic exchange parameters (J(ij)) of strongly correlated systems, based on first-principles full-potential electronic structure theory. The electronic structure is modeled with the help of a full-potential linear muffin-tin orbital method. The effects of strong electron correlations are considered within the charge self-consistent density functional theory plus dynamical mean-field theory. The exchange parameters are then extracted using the magnetic force theorem; hence all the calculations are performed within a single computational framework. The method allows us to investigate how the Jij parameters are affected by dynamical electron correlations. In addition to describing the formalism and details of the implementation, we also present magnetic properties of a few commonly discussed systems, characterized by different degrees of electron localization. In bcc Fe, treated as a moderately correlated metal, we found a minor renormalization of the Jij interactions once the dynamical correlations are introduced. However, generally, if the magnetic coupling has several competing contributions from different orbitals, the redistribution of the spectral weight and changes in the exchange splitting of these states can lead to a dramatic modification of the total interaction parameter. In NiO we found that both static and dynamical mean-field results provide an adequate description of the exchange interactions, which is somewhat surprising given the fact that these two methods result in quite different electronic structures. By employing the Hubbard-I approximation for the treatment of the 4f states in hcp Gd we reproduce the experimentally observed multiplet structure. The calculated exchange parameters result in being rather close to the ones obtained by treating the 4f electrons as noninteracting core states.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-252005 (URN)10.1103/PhysRevB.91.125133 (DOI)000352077800003 ()
Available from: 2015-05-07 Created: 2015-04-28 Last updated: 2017-12-04Bibliographically approved
Ganguly, S., Granas, O. & Nordstrom, L. (2015). Nontrivial order parameter in Sr2IrO4. Physical Review B. Condensed Matter and Materials Physics, 91(2), 020404
Open this publication in new window or tab >>Nontrivial order parameter in Sr2IrO4
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 2, p. 020404-Article in journal (Refereed) Published
Abstract [en]

A thorough analysis of the ground state of the relativistic magnetic insulator Sr2IrO4 is performed. The results are in accordance with the small antiferromagnetic moment and gapped state found in experiment. The solution, obtained using the DFT+SO+U methodology, is thoroughly analyzed in terms of Landau theory. We find that the ordered magnetic moment only forms a secondary order parameter while the primary order parameter is a higher order magnetic multipole of rank five. It is further observed that the electronic structure in the presence of this order parameter is related to the earlier proposed j(eff) = 1/2 model, but in contrast to that model, the present picture can exactly explain the small magnitude of the ordered magnetic moments.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:uu:diva-246367 (URN)10.1103/PhysRevB.91.020404 (DOI)000348473700001 ()
Available from: 2015-03-06 Created: 2015-03-05 Last updated: 2017-12-04Bibliographically approved
Grånäs, O., Di Marco, I., Eriksson, O., Nordström, L. & Etz, C. (2014). Electronic structure, cohesive properties, and magnetism of SrRuO3. Physical Review B. Condensed Matter and Materials Physics, 90(16), 165130
Open this publication in new window or tab >>Electronic structure, cohesive properties, and magnetism of SrRuO3
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 16, p. 165130-Article in journal (Refereed) Published
Abstract [en]

We have performed an extensive test of the ability of density functional theory within several approximations for the exchange-correlation functional, local density approximation + Hubbard U, and local density approximation + dynamic mean field theory to describe magnetic and electronic properties of SrRuO3. We focus on the ferromagnetic phase, illustrating differences between the orthorhombic low-temperature structure versus the cubic high-temperature structure. We assess how magnetism, spectral function, and cohesive properties are affected by methodology, onsite Hubbard U, and double-counting corrections. Further, we compare the impact of the impurity solver on the quasiparticle weight Z, which is in turn compared to experimental results. The spectral functions resulting from the different treatments are also compared to experimental data. Finally, the impact of spin-orbit coupling is studied, allowing us to determine the orbital moments. In the orthorhombic phase, the orbital moments are found to be tilted with respect to the spin moments, emphasizing the importance of taking into account the distortion of the oxygen octahedra.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-238564 (URN)10.1103/PhysRevB.90.165130 (DOI)000343944400003 ()
Available from: 2014-12-17 Created: 2014-12-14 Last updated: 2017-12-05Bibliographically approved
Grånäs, O., Dutta, B., Ghosh, S. & Sanyal, B. (2012). A new first principles approach to calculate phonon spectra of disordered alloys. Journal of Physics: Condensed Matter, 24(1), 015402
Open this publication in new window or tab >>A new first principles approach to calculate phonon spectra of disordered alloys
2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 1, p. 015402-Article in journal (Refereed) Published
Abstract [en]

The lattice dynamics in substitutional disordered alloys with constituents having large size differences is driven by strong disorder in masses, inter-atomic force constants and local environments. In this paper, a new first principles approach based on special quasirandom structures and an itinerant coherent potential approximation to compute the phonon spectra of such alloys is proposed and applied to Ni(0.5)Pt(0.5) alloy. The agreement between our results and experiments is found to be much better than for previous models of disorder due to an accurate treatment of the interplay of inter-atomic forces among various pairs of chemical species. This new formalism serves as a potential solution to the longstanding problem of a proper microscopic understanding of lattice dynamical behavior of disordered alloys.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-167160 (URN)10.1088/0953-8984/24/1/015402 (DOI)000298542500013 ()
Available from: 2012-01-31 Created: 2012-01-23 Last updated: 2017-12-08Bibliographically approved
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