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• 1.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Effects of short-range electron-electron interactions in doped graphene2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 15, article id 155420Article in journal (Refereed)

We study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of substitutional impurities. Our computational approach is based on the π orbital tight-binding model for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. The finite impurity concentration is modeled using the supercell approach. We compare explicitly noninteracting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens up at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.

• 2. Benza, V G
University of Kalmar, School of Pure and Applied Natural Sciences.
Landau-Zener quantum tunneling in disordered nanomagnets2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 18, p. 1844261-9-184426-9Article in journal (Refereed)
• 3.
Photonics and Semiconductor Nanophysics, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, NL-5600 MB Eindhoven, The Netherlands.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. London Center for NanoTechnology, 17-19 Gordon Street, WC1H 0AH, London, U.K.. 5Department of Physics, University of New Hampshire, Durham, New Hampshire 03824-3520, USA. London Center for NanoTechnology, 17-19 Gordon Street, WC1H 0AH, London, U.K.. London Center for NanoTechnology, 17-19 Gordon Street, WC1H 0AH, London, U.K.. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242-1479,U.S.A.. Photonics and Semiconductor Nanophysics, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, NL-5600 MB Eindhoven, The Netherlands. Department of Chemistry, UCL, London, WC1H 0AJ, United Kingdom. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Photonics and Semiconductor Nanophysics, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, NL-5600 MB Eindhoven, The Netherlands.
Magnetic anisotropy of single Mn acceptors in GaAs in an external magnetic field2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, p. Article ID: 205203-Article in journal (Refereed)

We investigate the effect of an external magnetic field on the physical properties of the acceptor hole statesassociated with single Mn acceptors placed near the (110) surface of GaAs. Cross-sectional scanning tunnelingmicroscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported bytheoretical calculations based on a tight-binding model of Mn acceptors in GaAs. For Mn acceptors on the (110)surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantlythe magnetic anisotropy landscape. However, the acceptor hole wave function is strongly localized around theMn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mnacceptors placed on deeper layers below the surface, the acceptor hole wave function is more delocalized andthe corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However, themagnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 Tcan hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis.We predict that substantially larger magnetic fields are required to observe a significant field dependence of thetunneling current for impurities located several layers below the GaAs surface.

• 4. Cehovin, Alexander
University of Kalmar, School of Pure and Applied Natural Sciences.
Elementary Excitations of Ferromagnetic Metal Nanoparticles2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 1, p. 144231-1442317Article in journal (Refereed)
• 5. Cehovin, Alexander
University of Kalmar, School of Pure and Applied Natural Sciences.
Magnetization orientation dependence of the quasiparticle and hysteresis in ferromagnetic metal nanoparticles2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 9, p. 944301-9443015Article in journal (Refereed)
• 6. Cehovin, Alexander
University of Kalmar, School of Pure and Applied Natural Sciences.
Orbital and Spin Contributions to the g-Tensors in Metal Nanoparticles2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 4, p. 454111-8-45411-8Article in journal (Refereed)
• 7.
Donostia Int Phys Ctr DIPC UPV EHU, San Sebastian 20018, Spain.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Toyama Univ, Grad Sch Sci & Engn, Div Nano & New Funct Mat Sci, Toyama 930855, Japan. Toyama Univ, Grad Sch Sci & Engn, Div Nano & New Funct Mat Sci, Toyama 930855, Japan.
Theory of action spectroscopy for single-molecule reactions induced by vibrational excitations with STM2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 3, p. 035427-Article in journal (Refereed)

A theory of action spectroscopy, i.e., a reaction rate or yield as a function of bias voltage, is presented for single-molecule reactions induced by the inelastic tunneling current with a scanning tunneling microscope. A formula for the reaction yield is derived using the adsorbate resonance model, which provides a versatile tool to analyze vibrationally mediated reactions of single adsorbates on conductive surfaces. This allows us to determine the energy quantum of the excited vibrational mode, the effective broadening of the vibrational density of states (as described by Gaussian or Lorentzian functions), and a prefactor characterizing the elementary process behind the reaction. The underlying approximations are critically discussed. We point out that observation of reaction yields at both bias voltage polarities can provide additional insight into the adsorbate density of states near the Fermi level. As an example, we apply the theory to the case of flip motion of a hydroxyl dimer (OD)(2) on Cu(110) which was experimentally observed by Kumagai et al. [Phys. Rev. B 79, 035423 (2009)]. In combination with density functional theory calculations for the vibrational modes, the vibrational damping due to electron-hole pair generation, and the potential energy landscape for the flip motion, a detailed microscopic picture for the switching process is established. This picture reveals that the predominant mechanism is excitation of the OD stretch modes which couple anharmonically to the low-energy frustrated rotation mode.

• 8. Frederiksen, Thomas
University of Kalmar, School of Pure and Applied Natural Sciences.
From tunneling to contact: Inelastic signals in an atomic gold junction from first principles2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 23, article id 235441Article in journal (Refereed)
• 9. Frederiksen, Thomas
University of Kalmar, School of Pure and Applied Natural Sciences.
Inelastic transport theory from first principles: Methodology and application to nanoscale devices2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 20, article id 205413Article in journal (Refereed)
• 10.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Toyama Univ, Japan. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Theory of vibrationally assisted tunneling for hydroxyl monomer flipping on Cu(110)2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 16, p. Article ID: 165413-Article in journal (Refereed)

To describe vibrationally mediated configuration changes of adsorbates on surfaces we have developed a theory to calculate both reaction rates and pathways. The method uses the T-matrix to describe excitations of vibrational states by the electrons of the substrate, adsorbate, and tunneling electrons from a scanning tunneling probe. In addition to reaction rates, the theory also provides the reaction pathways by going beyond the harmonic approximation and using the full potential energy surface of the adsorbate which contains local minima corresponding to the adsorbates different configurations. To describe the theory, we reproduce the experimental results in [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)], where the hydrogen/deuterium atom of an adsorbed hydroxyl (OH/OD) exhibits back and forth flipping between two equivalent configurations on a Cu(110) surface at T = 6 K. We estimate the potential energy surface and the reaction barrier, similar to 160 meV, from DFT calculations. The calculated flipping processes arise from (i) at low bias, tunneling of the hydrogen through the barrier, (ii) intermediate bias, tunneling electrons excite the vibrations increasing the reaction rate although over the barrier processes are rare, and (iii) higher bias, overtone excitations increase the reaction rate further.

• 11. Hayashi, M.
University of Kalmar, School of Pure and Applied Natural Sciences.
Lateral hopping of CO molecules on Pt(111) surface by femtosecond laser pulses2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no Article number: 245409Article in journal (Refereed)

Theory of heat transfer between adsorbate vibrational degrees of freedom and ultrafast laser heated hot electrons including vibrational intermode coupling is applied to calculate two-pulse correlation, laser fluence dependence and time dependence of lateral hopping of CO molecules from a step to terrace site on a stepped Pt (111) surface. The intermode coupling is a key ingredient to describe vibrational heating of the frustrated translation mode responsible for the CO hopping. The calculated results are in good agreement with the experimental results, especially if we scale down the experimentally determined absorbed fluence. It is found that CO hopping is induced by indirect heating of the FT mode by the FR mode with a strong frictional coupling to hot electrons.

• 12.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Ab initio calculations of the magnetic properties of Mn impurities on GaAs (110) surfaces2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. B 85, p. Article ID: 155306-Article in journal (Refereed)

We present a computational study of individual and pairs of substitutional Mn impurities on the (110) surface of GaAs samples based on density functional theory. We focus on the anisotropy properties of these magnetic centers and their dependence on on-site correlations, spin-orbit interaction, and surface-induced symmetry-breaking effects. For a Mn impurity on the surface, the associated acceptor-hole wave function tends to be more localized around the Mn than for an impurity in bulk GaAs. The magnetic anisotropy energy for isolated Mn impurities is of the order of 1 meV, and can be related to the anisotropy of the orbital magnetic moment of the Mn acceptor hole. Typically Mn pairs have their spin magnetic moments parallel aligned, with an exchange energy that strongly depends on the pair orientation on the surface. The spin magnetic moment and exchange energies for these magnetic entities are not significantly modified by the spin-orbit interaction, but are more sensitive to on-site correlations. Correlations in general reduce the magnetic anisotropy for most of the ferromagnetic Mn pairs.

• 13.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics. Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics. Naval Research Laboratory, USA.
First-principles study of spin-electric coupling in a {Cu3} single molecular magnet2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 15, p. article number-155446Article in journal (Refereed)

We report on a study of the electronic and magnetic properties of the triangular antiferromagnetic {Cu3} single-molecule magnet, based on spin-density-functional theory. Our calculations show that the low-energy magnetic properties are correctly described by an effective three-site spin s = 1/2 Heisenberg model, with an antiferromagnetic exchange coupling J approximate to 5 meV. The ground-state manifold of the model is composed of two degenerate spin S = 1/2 doublets of opposite chirality. Due to lack of inversion symmetry in the molecule these two states are coupled by an external electric field, even when spin-orbit interaction is absent. The spin-electric coupling can be viewed as originating from a modified exchange constant delta J induced by the electric field. We find that the calculated transition rate between the chiral states yields an effective electric dipole moment d = 3.38 x 10(-33) C m approximate to e10(-4)a, where a is the Cu separation. For external electric fields epsilon approximate to 10(8) V/m this value corresponds to a Rabi time tau approximate to 1 ns and to a delta J on the order of a few mu eV.

• 14. Kristensen, IS
University of Kalmar, School of Pure and Applied Natural Sciences.
Inelastic scattering in metal-H-2-metal junctions2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no Article number: 235411Article in journal (Refereed)

We present first-principles calculations of the dI/dV characteristics of an H-2 molecule sandwiched between Au and Pt electrodes in the presence of electron-phonon interactions. The conductance is found to decrease by a few percentages at threshold voltages corresponding to the excitation energy of longitudinal vibrations of the H-2 molecule. In the case of Pt electrodes, the transverse vibrations can mediate transport through otherwise nontransmitting Pt d channels leading to an increase in the differential conductance even though the hydrogen junction is characterized predominately by a single almost fully open transport channel. In the case of Au, the transverse modes do not affect the dI/dV because the Au d states are too far below the Fermi level. A simple explanation of the first-principles results is given using scattering theory. Finally, we compare and discuss our results in relation to experimental data.

• 15. Liang, GC
University of Kalmar, School of Pure and Applied Natural Sciences.
Electrostatic potential profiles of molecular conductors2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 11, article id 115302Article in journal (Refereed)
• 16. Liu, R. S.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
CoFe alloy as middle layer for strong spin dependent quantum well resonant tunneling in MgO double barrier magnetic tunnel junctions2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 2, p. 024411-Article in journal (Refereed)

We report on spin dependent quantum well (QW) resonances in the CoFe alloy middle layer of CoFe/MgO/CoFe/MgO/CoFeB double barrier magnetic tunnel junctions. The dI/dV spectra reveal clear resonant peaks for the parallel magnetization configurations, which can be related to the existence of QW resonances obtained from first-principles calculations. We observe that the differential tunneling magnetoresistance (TMR) exhibits an oscillatory behavior as a function of voltage with a sign change as well as a pronounced TMR enhancement at resonant voltages at room temperature. The observation of strong QW resonances indicates that the CoFe film possesses a long majority spin mean-free path, and the substitutional disorder does not cause a significant increase of scattering. Both points are confirmed by first-principles electronic structure calculation. DOI: 10.1103/PhysRevB.87.024411

• 17.
Uppsala University, Sweden.
Uppsala University, Sweden. Uppsala University, Sweden. Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering. Fysikavdelningen. Uppsala University, Sweden.
Large magnetic circular dichroism in resonant inelastic x-ray scattering at the Mn L-edge of Mn-Zn ferrite2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 17, article id 172409Article in journal (Refereed)

We report resonant inelastic x-ray scattering (RIXS) excited by circularly polarized x rays on Mn-Zn ferrite at the Mn L2,3 resonances. We demonstrate that crystal-field excitations, as expected for localized systems, dominate the RIXS spectra and thus their dichroic asymmetry cannot be interpreted in terms of spin-resolved partial density of states, which has been the standard approach for RIXS dichroism. We observe large dichroic RIXS at the L2 resonance which we attribute to the absence of metallic core hole screening in the insulating Mn ferrite. On the other hand, reduced L3-RIXS dichroism is interpreted as an effect of longer scattering time that enables spin-lattice core hole relaxation via magnons and phonons occurring on a femtosecond time scale.

• 18.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Virginia Commonwealth Univ, Richmond, VA 23284 USA. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 16, p. Article ID: 165408-Article in journal (Refereed)

Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically theelectronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+](0) plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity d electrons are in good agreement with approaches that treat the spin ofthe impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+](0) and the ionized [Fe2+](-)states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of d orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state [Fe2+, h](0), the magnetic-anisotropy dependence on the impurity sublayer resembles the case of [Mn2+, h](0). In contrast, for [Fe3+](0) electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e. g., by an external magnetic field, with detectable consequences in the local density of states probed by STM.

• 19.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
University of Texas at Austin, USA. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Electric manipulation of the Mn-acceptor binding energy and the Mn-Mn exchange interaction on the GaAs (110) surface by nearby As vacancies2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 4, article id 045304Article in journal (Refereed)

We investigate theoretically the effect of nearby As (arsenic) vacancies on the magnetic properties of substitutional Mn (manganese) impurities on the GaAs (110) surface, using a microscopic tight-binding model which captures the salient features of the electronic structure of both types of defects in GaAs. The calculations show that the binding energy of the Mn-acceptor is essentially unaffected by the presence of a neutral As vacancy, even at the shortest possible VAs--Mn separation. On the other hand, in contrast to a simple tip-induced-band-bending theory and in agreement with experiment, for a positively charged As vacancy the Mn-acceptor binding energy is significantly reduced as the As vacancy is brought closer to the Mn impurity. For two Mn impurities aligned ferromagnetically, we find that nearby charged As vacancies enhance the energy level splitting of the associated coupled acceptor levels, leading to an increase of the effective exchange interaction. Neutral vacancies leave the exchange splitting unchanged. Since it is experimentally possible to switch reversibly between the two charge states of the vacancy, such a local electric manipulation of the magnetic dopants could result in an efficient real-time control of their exchange interaction.

• 20.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Spin dynamics of Mn impurities and their bound acceptors in GaAs2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 24, article id 245406Article in journal (Refereed)

We present results of tight-binding spin-dynamics simulations of individual and pairs of substitutionalMn impurities in GaAs. Our approach is based on the mixed quantum-classical schemefor spin dynamics, with coupled equations of motions for the quantum subsystem, representing thehost, and the localized spins of magnetic dopants, which are treated classically. In the case ofa single Mn impurity, we calculate explicitly the time evolution of the Mn spin and the spins ofnearest-neighbors As atoms, where the acceptor (hole) state introduced by the Mn dopant resides.We relate the characteristic frequencies in the dynamical spectra to the two dominant energy scalesof the system, namely the spin-orbit interaction strength and the value of the p-d exchange couplingbetween the impurity spin and the host carriers. For a pair of Mn impurities, we find signaturesof the indirect (carrier-mediated) exchange interaction in the time evolution of the impurity spins.Finally, we examine temporal correlations between the two Mn spins and their dependence on theexchange coupling and spin-orbit interaction strength, as well as on the initial spin-configuration andseparation between the impurities. Our results provide insight into the dynamic interaction betweenlocalized magnetic impurities in a nano-scaled magnetic-semiconductor sample, in the extremelydilute(solotronics) regime.

• 21.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, p. Article ID: 195441-Article in journal (Refereed)

We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi$_2$Se$_3$ topological insulator. Combining \textit{ab initio} calculations with microscopic tight-binding modeling, we identify the effects of inversion-symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in ${+2}$ valence state and introduces an acceptor in the bulk band gap. The Mn-acceptor has predominantly $p$--character, and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn $d$--levels and the $p$--levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasi-resonant coupling and the strong real-space overlap between the spin-chiral surface states and the mid-gap spin-polarized Mn-acceptor states.

• 22.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lund University.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lund Univ,.
Cotunneling signatures of Spin-Electric coupling in frustrated triangularmolecular magnets2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 23, p. Article ID: 235435-Article in journal (Refereed)

The ground state of frustrated (antiferromagnetic) triangular molecular magnets is characterized by two total-spin S = 1/2 doublets with opposite chirality. According to a group theory analysis [M. Trif et al., Phys. Rev. Lett. 101, 217201 (2008)] an external electric field can efficiently couple these two chiral spin states, even when the spin-orbit interaction (SOI) is absent. The strength of this coupling, d, is determined by an off-diagonal matrix element of the dipole operator, which can be calculated by ab-initio methods [M. F. Islam et al., Phys. Rev. B 82, 155446 (2010)]. In this work we propose that Coulomb-blockade transport experiments in the cotunneling regime can provide a direct way to determine the spin-electric coupling strength. Indeed, an electric field generates a d-dependent splitting of the ground state manifold, which can be detected in the inelastic cotunneling conductance. Our theoretical analysis is supported by master-equation calculations of quantum transport in the cotunneling regime. We employ a Hubbard-model approach to elucidate the relationship between the Hubbard parameters t and U, and the spin-electric coupling constant d. This allows us to predict the regime in which the coupling constant d can be extracted from experiment.

• 23.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lunds universitet.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. US DOE, Off Basic Energy Sci, Washington, DC 20585 USA.
Electric control of a {Fe4} single-molecule magnet in a single-electron transistor2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 22, p. Article ID: 224423-Article in journal (Refereed)

Using first-principles methods, we study theoretically the properties of an individual {Fe-4} single-molecule magnet (SMM) attached to metallic leads in a single-electron transistor geometry. We show that the conductive leads do not affect the spin ordering and magnetic anisotropy of the neutral SMM. On the other hand, the leads have a strong effect on the anisotropy of the charged states of the molecule, which are probed in Coulomb blockade transport. Furthermore, we demonstrate that an external electric potential, modeling a gate electrode, can be used to manipulate the magnetic properties of the system. For a charged molecule, by localizing the extra charge with the gate voltage closer to the magnetic core, the anisotropy magnitude and spin ordering converges to the values found for the isolated {Fe-4} SMM. We compare these findings with the results of recent quantum transport experiments in three-terminal devices.

• 24.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics. Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics. US Department of Energy SC22.1, Washington DC 20585-1290.
First-principles studies of spin-orbit and Dzyaloshinskii-Moriya interactions in the {Cu3} single-molecule magnet2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 8, p. 085427-1-085427-10Article in journal (Refereed)

Frustrated triangular molecule magnets such as {Cu3} are characterized by two degenerate S = 1/2 ground states with opposite chirality. Recently, it has been proposed theoretically [M. Trif et al., Phys. Rev. Lett. 101, 217201 (2008)] and verified by ab initio calculations [M. F. Islam et al., Phys. Rev. B 82, 155446 (2010)] that an external electric field can efficiently couple these two chiral spin states, even in the absence of spin-orbit interaction (SOI). The SOI is, nevertheless, important since it introduces a splitting in the ground-state manifold via the Dzyaloshinskii-Moriya (DM) interaction. In this paper, we present a theoretical study of the effect of the SOI on the chiral states within spin-density functional theory. We employ a recently introduced Hubbard-model approach to elucidate the connection between the SOI and the Dzyaloshinskii-Moriya interaction. This allows us to express the Dzyaloshinskii-Moriya interaction constant D in terms of the microscopic Hubbard-model parameters, which we calculate from first principles. The small splitting that we find for the {Cu3} chiral state energies (≈ 0.02 meV) is consistent with experimental results. The one-band Hubbard-model approach adopted and analyzed here also yields a better estimate of the isotropic exchange constant than the ones obtained by comparing total energies of different spin configurations. The method used here for calculating the DM interaction unmasks its simple fundamental origin, which is the off-diagonal spin-orbit interaction between the generally multireference vacuum state and single-electron excitations out of those states.

• 25.
Kyoto University.
Kyoto University. Kyoto University. Kyoto University. Kyoto University. University of Toyama. Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics. University of Toyama.
Modifying current-voltage characteristics of a single molecule junction by isotope substitution: OHOD dimer on Cu(110)2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 20, p. Article ID: 205424-Article in journal (Refereed)

Vibrationally induced configurational change and nonlinear current-voltage (I-V) characteristics are investigated within the scanning tunneling microscope (STM) junction, including hydroxyl dimers on a Cu(110) surface. H-bonded hydroxyl dimers composed of OH and/or OD have a unique inclined geometry that can be switched back and forth by vibrational excitations via the inelastic electron tunneling process of the STM. The relative occupation change between the high-and low-conductance states as a function of bias voltage critically depends on the isotopic compositions, and thus the I-V characteristics can be modified to exhibit negative differential resistance by H/D substitution. The experimental results of the occupation change and I-V curves are nicely reproduced using a recently proposed analytical model combined with comprehensive density functional calculations for the input parameters (vibrational modes and their emission rates by tunneling electrons, conductance, and relative occupation change of high-and low-conductance states), and they underlines the different roles played by the free and shared O-H(D) stretch modes of the hydroxyl dimers on a Cu(110) surface.

• 26.
Toyama University, Japan.
Donostia Int Phys Ctr, Spain. Toyama University, Japan. Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Vibrationally induced flip motion of a hydroxyl dimer on Cu(110)2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, p. Article ID 193403-Article in journal (Refereed)
• 27.
University of Kalmar, School of Pure and Applied Natural Sciences.
Transmission eigenchannels from nonequilibrium Green's functions2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 11, article id 115117Article in journal (Refereed)
• 28.
University of Kalmar, School of Pure and Applied Natural Sciences.
Thermoelectric effect in molecular electronics2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, no 24, p. 2414031-2414034Article in journal (Refereed)
• 29.
University of Kalmar, School of Pure and Applied Natural Sciences.
Modeling inelastic phonon scattering in atomic- and molecular-wire junctions2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 20, article id 201101Article in journal (Refereed)

Computationally inexpensive approximations describing electron-phonon scattering in molecular-scale conductors are derived from the nonequilibrium Green's function method. The accuracy is demonstrated with a first-principles calculation on an atomic gold wire. Quantitative agreement between the full nonequilibrium Green's function calculation and the newly derived expressions is obtained while simplifying the computational burden by several orders of magnitude. In addition, analytical models provide intuitive understanding of the conductance including nonequilibrium heating and provide a convenient way of parameterizing the physics. This is exemplified by fitting the expressions to the experimentally observed conductances through both an atomic gold wire and a hydrogen molecule.

• 30.
University of Kalmar, School of Pure and Applied Natural Sciences.
Effects of doping and interchain interactions on the metal-insulator transition in trans-polyacetylene1999In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 60, no 11, p. 7939-7943Article in journal (Refereed)
• 31.
University of Kalmar, School of Pure and Applied Natural Sciences.
Numerical investigation of electron localization in polymer chains1998In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 57, no 4, p. 2197-2202Article in journal (Refereed)
• 32.
University of Kalmar, School of Pure and Applied Natural Sciences.
Self-consistent-field study of conduction through conjugated molecules2001In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 64, no 3, p. 0354161-03541610Article in journal (Refereed)
• 33.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. University of Texas at Austin.
Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, article id 075430Article in journal (Refereed)

The response of thin films of Bi$_2$Se$_3$ to a strong perpendicular magnetic field is investigated  by performing magnetic bandstructure calculations for a realistic multi-band tight-binding model.   Several crucial features of Landau quantization in a realistic three-dimensional topological insulator are revealed.  The $n=0$ Landau level is absent in ultra-thin  films, in agreement with experiment.  In films with a crossover thickness of five quintuple layers, there is     a signature of the $n=0$ level, whose overall trend as a function of magnetic field matches the established  low-energy effective-model result.  Importantly, we find a field-dependent splitting and a strong spin-polarization of the $n=0$ level which can be measured experimentally at reasonable field strengths. Our calculations      show  mixing between the surface and bulk Landau levels      which causes the character of levels to evolve with magnetic field.

• 34.
School of Physics and CRANN, Trinity College, Dublin 2, Ireland.
School of Physics and CRANN, Trinity College, Dublin 2, Ireland. School of Physics and CRANN, Trinity College, Dublin 2, Ireland.
Electrical control of spin dynamics in finite one-dimensional systems2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, p. 155436-Article in journal (Refereed)

We investigate the possibility of the electrical control of spin transfer in monoatomic chains incorporating spin impurities. Our theoretical framework is the mixed quantum-classical (Ehrenfest) description of the spin dynamics, in the spirit of thes-d model, where the itinerant electrons are described by a tight-binding model while localized spins are treated classically. Our main focus is on the dynamical exchange interaction between two well-separated spins. This can be quantified by the transfer of excitations in the form of transverse spin oscillations. We systematically study the effect of an electrostatic gate bias Vg on the interconnecting channel and we map out the long-range dynamical spin transfer as a function of Vg. We identify regions of Vg giving rise to significant amplification of the spin transmission at low frequencies and relate this to the electronic structure of the channel.

• 35.
Institut National Polytechnique de Grenoble .
Institut National Polytechnique de Grenoble . Trinity College Dublin . Institut National Polytechnique de Grenoble .
Quasi-two-dimensional extraordinary Hall effect2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 2, p. 024410-Article in journal (Refereed)

Quasi-two-dimensional transport is investigated in a system consisting of a very thin 1 nm ferromagneticlayer sandwiched between two insulating layers. Using the mechanism of skew scattering to describe theextraordinary Hall effect EHE and calculating the conductivity tensor, we compare the quasi-two-dimensionalHall resistance with the Hall resistance of a massive sample. In this study, a mechanism of EHE geometricmechanism of EHE due to nonideal interfaces and volume defects is also proposed.

• 36.
Russian Acad Sci / Toyama Univ. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Toyama Univ. Toyama Univ.
Isotope effect in acetylene C2H2 and C2D2 rotations on Cu(001)2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 16, p. Article ID: 165418-Article in journal (Refereed)

A comprehensive analysis of the elementary processes behind the scanning tunneling microscope controlled rotation of C2H2 and C2D2, isotopologues of a single acetylene molecule adsorbed on the Cu(001) surface, is given, with a focus on the isotope effects. With the help of density-functional theory we calculate the vibrational modes of C2H2 and C2D2 on Cu(001) and estimate the anharmonic couplings between them, using a simple strings-on-rods model. The probability of the elementary processes, nonlinear and combination band, is estimated using the Keldysh diagram technique. This allows us to clarify the main peculiarities and the isotope effects of the C2H2 and C2D2 on Cu(001) rotation, discovered in the pioneering work [B. C. Stipe et al., Phys. Rev. Lett. 81, 1263 (1998)], which have not been previously understood.

• 37.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics. University of Texas at Austin (U.S.A.).
Magnetic interactions of substitutional Mn pairs in GaAs2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 5, p. Article ID: 054401-Article in journal (Refereed)

We employ a kinetic-exchange tight-binding model to calculate the magnetic interaction and anisotropy energies of a pair of substitutional Mn atoms in GaAs as a function of their separation distance and direction. We find that the most energetically stable configuration is usually one in which the spins are ferromagnetically aligned along the vector connecting the Mn atoms. The ferromagnetic configuration is characterized by a splitting of the topmost unoccupied acceptor levels, which is visible in scanning tunneling microscope studies when the pair is close to the surface and is strongly dependent on pair orientation. The largest acceptor splittings occur when the Mn pair is oriented along the < 110 > symmetry direction and the smallest when they are oriented along < 100 >. We show explicitly that the acceptor splitting is not simply related to the effective exchange interaction between the Mn local moments. The exchange interaction constant is instead more directly related to the width of the distribution of all impurity levels-occupied and unoccupied. When the Mn pair is at the < 110 > GaAs surface, both acceptor splitting and effective exchange interaction are very small except for the smallest possible Mn separation.

• 38.
University of Kalmar, School of Pure and Applied Natural Sciences.
University of Kalmar, School of Pure and Applied Natural Sciences.
Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layers2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no Article number: 024425Article in journal (Refereed)

Motivated by recent scanning tunnel microscopy (STM) experiments, we present a theoretical study of the electronic and magnetic properties of the Mn-induced acceptor level obtained by substituting a single Ga atom in the (110) surface layer of GaAs or in one of the atoms layers below the surface. We employ a kinetic-exchange tight-binding model in which the relaxation of the (110) surface is taken into account. The acceptor wave function is strongly anisotropic in space and its detailed features depend on the depth of the sublayer in which the Mn atom is located. The local-density-of-states (LDOS) on the (110) surface associated with the acceptor level is more sensitive to the direction of the Mn magnetic moment when the Mn atom is located further below the surface. We show that the total magnetic anisotropy energy of the system is due almost entirely to the dependence of the acceptor level energy on Mn spin orientation, and that this quantity is strongly dependent on the depth of the Mn atom.

• 39.
University of Kalmar, School of Pure and Applied Natural Sciences.
University of Kalmar, School of Pure and Applied Natural Sciences.
Calculation of Chern number spin Hamiltonians for magnetic nano-clusters by DFT  methods2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, article id 174416Article in journal (Refereed)

By combining field-theoretical methods and ab initio calculations, we construct an effective Hamiltonian with a single giant-spin degree of freedom, which is capable of describing the low-energy spin dynamics of ferromagnetic metal nano-clusters consisting of up to a few tens of atoms. In our procedure, the magnetic moment direction of the Kohn–Sham spin density functional wave function is constrained by means of a penalty functional, which allows us to explore the entire parameter space of directions, and to extract the magnetic anisotropy energy and Berry curvature functionals. The average of the Berry curvature over all magnetization directions is a Chern number—a topological invariant that can only take on values equal to multiples of one-half, which represents the dimension of the Hilbert space of the effective spin system. The spin Hamiltonian is obtained by quantizing the classical anisotropy energy functional, after performing a change of variables to a constant Berry curvature space. The purpose of this paper is to examine the impact of the topological effect from the Berry curvature on the low-energy total-spin-system dynamics. To this end, we study small transition-metal clusters: Con (n=2,…,5), Rh2, Ni2, Pd2, MnxNy, and Co3Fe2.

• 40.
University of Kalmar, School of Pure and Applied Natural Sciences.
University of Kalmar, School of Pure and Applied Natural Sciences.
Magnetic anisotropy of isolated cobalt nanoplatelets2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Physical review. B, Vol. 73, no 14, article id 144415Article in journal (Refereed)
• 41. Ueba, H
Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
Lateral hopping of CO on Cu(111) induced by femtosecond laser pulses2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 12, article id 121411Article in journal (Refereed)
• 42. Ueba, Hiromu
University of Kalmar, School of Pure and Applied Natural Sciences.
Adsorbate hopping via vibrational-mode coupling induced by femtosecond laser pulses2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, p. Article ID: 113408-Article in journal (Refereed)

We study the heat transfer from femtosecond laser-heated hot electrons in a metal to adsorbates in the presence of vibrational-mode coupling. The theory is successfully applied to the experimental result of atomic oxygen hopping on a vicinal Pt(111) surface. The effective friction coupling between hot electrons and the vibrational mode relevant to the hopping motion depends on the transient temperature of the partner mode excited by hot electrons. The calculated two-pulse correlation and fluence dependence of the hopping probability reproduce the experimental results, which were previously analyzed using the hot-electron temperature (Te)-dependent friction ηa(Te) in a conventional heat transfer equation. A possible elementary process behind such a hypothetic modeling using ηa(Te) is discussed in terms of an indirect heating of the vibrational mode for hopping at the surface.

• 43. Zahid, Ferdows
University of Kalmar, School of Pure and Applied Natural Sciences.
Charging-induced asymmetry in molecular conductors2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 24, article id 245317Article in journal (Refereed)
1 - 43 of 43
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