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  • 51.
    Michalak, Lukasz
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Pederson, M.R.
    Navy Research Laboratory, Washington (U.S.A.).
    Giant-spin Hamiltonians versus first-principles approaches in the tunneling transport in single-molecule magnetsManuscript (preprint) (Other academic)
  • 52.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lund University.
    Canali, Carlo M.
    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)
    Abstract [en]

    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.

     

  • 53.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Cotunneling signatures of spin-electric coupling in frustrated triangularmolecular magnets2013In: APS MARCH MEETING 2013, 2013Conference paper (Refereed)
  • 54.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Effects of spin-orbit interaction in spin-polarized single-electron transistors.2010In: APS MARCH MEETING 2010, 2010Conference paper (Refereed)
    Abstract [en]

    We consider a model of an artificial atom with interacting electrons having both spin degrees of freedom and orbital degeneracies. The interaction includes both spin and orbital exchange couplings, which favour a spin polarized ground state with nonzero orbital moment. For the two-electron problem with l=1 orbital degeneracy we enumerate all the eigenstates of the system with and without spin-orbit interaction. We then study quantum transport for the case in which the atom is weakly connected to metallic leads, focusing in particular on the effect of the spin-orbit interaction on the tunnelling conductance. We also discuss how spin-orbit interaction and an external magnetic field influence the conductance when the leads are spin-polarized and tunnelling magneto-resistance is expected.

  • 55.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lunds universitet.
    Islam, Fhokrul
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pederson, M. R.
    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)
    Abstract [en]

    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.

  • 56.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Islam, Fhokrul
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Pederson, Mark
    Naval Research Laboratory, Washington DC.
    DFT calculations of the charged states of N@C60 and {\{}Fe4{\}} single molecule magnets investigated in tunneling spectroscopy2012Conference paper (Other academic)
    Abstract [en]

    For device applications of single molecule magnets (SMMs) in high-density information storage and quantum-state control it is essential that the magnetic properties of the molecules remain stable under the influence of metallic contacts or surface environment. Recent tunneling experiments [1, 2] on N@C60 and {\{}Fe4{\}} SMM have shown that these molecules preserve their magnetic characteristics when they are used as the central island of single-electron transistors. Although quantum spin models have been used extensively to study theoretically tunneling spectroscopy of SMMs, it has been shown recently that the orbital degrees of freedom, which is absent in spin models, can significantly affect the tunneling conductance [3]. In this work we present first-principles calculations of the neutral and charged states of N@C60 and {\{}Fe4{\}} SMMs, and discuss a strategy to include their properties into a theory of quantum transport. We also present results of the magnetic anisotropy for the different charge states of Fe4 and discuss their relevance for experiments [2] in the sequential tunneling and cotunnelling regimes.

  • 57.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Islam, Fhokrul
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Pederson, Mark
    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)
    Abstract [en]

    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.

  • 58.
    Nossa, Javier
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Islam, Fhokrul
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Pederson, Mark
    Spin-orbit interaction and magnetic field in antiferromagnetic triangular molecular magnets2011In: APS MARCH MEETING 2011, 2011Conference paper (Refereed)
  • 59.
    Nossa, J.F.
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Effects of spin-orbit interaction in molecular-magnet single electron transistorsManuscript (preprint) (Other academic)
  • 60.
    Paschoal Jr, W.
    et al.
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden;Dept. of Mathematics, Physics and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden .
    Kumar, Sandeep
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden;Dept. of Mathematics, Physics and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden .
    Jacobsson, D.
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden.
    Johannes, A.
    Institute for Solid State Physics, Friedrich-Schiller-University Jena, Max-Wien-Platz 1, D-07743 Jena, Germany.
    Jain, V.
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden;Dept. of Mathematics, Physics and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden .
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Ronning, C.
    Institute for Solid State Physics, Friedrich-Schiller-University Jena, Max-Wien-Platz 1, D-07743 Jena, Germany.
    Dick, C. A.
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden;Center for Analysis and Synthesis, Lund University, Box 124, S-221 00 Lund, Sweden .
    Samuelson, L.
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden.
    Pettersson, H.
    Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden;Dept. of Mathematics, Physics and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden .
    Magnetoresistance in Mn ion-implanted GaAs:Zn nanowires2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 104, p. Article ID: 153112-Article in journal (Refereed)
    Abstract [en]

    We have investigated the magnetoresistance (MR) in a series of Zn doped (p-type) GaAs nanowires implanted with different Mn concentrations. The nanowires with the lowest Mn concentration (~0.0001%) exhibit a low resistance of a few kΩ at 300K and a 4% positive MR at 1.6K, which can be well described by invoking a spin-split subband model. In contrast, nanowires with the highest Mn concentration (4%) display a large resistance of several MΩ at 300K and a large negative MR of 85% at 1.6K. The large negative MR is interpreted in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn impurity spins. Sweeping the magnetic field back and forth for the 4% sample reveals a hysteresis that indicates the presence of a weak ferromagnetic phase. We propose co-doping with Zn to be a promising way to reach the goal of realizing ferromagnetic Ga1-xMnxAs nanowires for future nanospintronics.

  • 61.
    Paschoal, Waldomiro, Jr.
    et al.
    Lund University.
    Kumar, Sandeep
    Lund University.
    Borschel, Christian
    Jena University.
    Wu, Phillip
    Lund University.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Ronning, Carsten
    Jena University.
    Samuelson, Lars
    Lund University.
    Pettersson, Håkan
    Lund University.
    Hopping Conduction in Mn Ion-Implanted GaAs Nanowires2012In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 12, no 9, p. 4838-4842Article in journal (Refereed)
    Abstract [en]

    We report on temperature-dependent charge transport in heavily doped Mn+-implanted GaAs nanowires. The results clearly demonstrate that the transport is governed by temperature-dependent hopping processes, with a crossover between nearest neighbor hopping and Mott variable range hopping at about 180 K. From detailed analysis, we have extracted characteristic hopping energies and corresponding hopping lengths. At low temperatures, a strongly nonlinear conductivity is observed which reflects a modified hopping process driven by the high electric field at large bias.

  • 62.
    Pertsova, Anna
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Probing the wavefunction of the surface states in Bi2Se3 topological insulator: a realistic tight-binding approach2014In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 16, p. Article ID: 063022-Article in journal (Refereed)
    Abstract [en]

    We report on microscopic tight-binding modeling of surfacestates in Bi$_2$Se$_3$ three-dimensional topological insulator, based on a\textit{sp}$^3$ Slater-Koster Hamiltonian, with parameters calculated fromdensity functional theory. The effect of spin-orbit interaction on theelectronic structure of the bulk and of a slab with finite thickness isinvestigated. In particular, a phenomenological criterion of band inversion isformulated for both bulk and slab, based on the calculated atomic- andorbital-projections of the wavefunctions, associated with valence and conductionband extrema at the center of the Brillouin zone. We carry out athorough analysis of the calculated bandstructures of slabs with varyingthickness, where surface states are identified using a quantitative criterionaccording to their spatial distribution. The thickness-dependent energy gap,attributed to inter-surface interaction, and the emergence of gapless surfacestates for slabs above a critical thickness are investigated. We map out thetransition to the infinite-thickness limit by calculating explicitly themodifications in the spatial distribution and spin-character of the surfacestates wavefunction with increasing the slab thickness. Our numerical analysisshows that the system must be approximately forty quintuple-layers thick toexhibit completely decoupled surface states, localized on the oppositesurfaces. These results have implications on the effect of external perturbationson the surface states near the Dirac point.

  • 63.
    Pertsova, Anna
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. KTH Royal Inst Technol ; Stockholm University.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    MacDonald, A. H.
    Univ Texas Austin, USA.
    Quantum Hall edge states in topological insulator nanoribbons2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 12, article id 121409Article in journal (Refereed)
    Abstract [en]

    We present a microscopic theory of the chiral one-dimensional electron gas system localized on the sidewalls of magnetically doped Bi2Se3-family topological insulator nanoribbons in the quantum anomalous Hall effect (QAHE) regime. Our theory is based on a simple continuum model of sidewall states whose parameters are extracted from detailed ribbon and film geometry tight-binding model calculations. In contrast to the familiar case of the quantum Hall effect in semiconductor quantum wells, the number of microscopic chiral channels depends simply and systematically on the ribbon thickness and on the position of the Fermi level within the surface state gap. We use our theory to interpret recent transport experiments that exhibit nonzero longitudinal resistance in samples with accurately quantized Hall conductances.

  • 64.
    Pertsova, Anna
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    MacDonald, Allan H.
    The University of Texas at Austin, Austin, Texas 78712-0264, USA.
    Theoretical studies of surface states in three-dimensional topological-insulator thin films in a strong magnetic field2014In: Bulletin of the American Physical Society, Denver, Colorado: American Physical Society , 2014Conference paper (Refereed)
    Abstract [en]

    The peculiar structure of the Landau levels (LLs) in topological insulators (TIs), in particular the existence of a field-independent (zeroth) LL, is a characteristic signature of the Dirac surface states. However, recently it has been shown that the hybridization between top and bottom surfaces in a 3D TI thin film may lead to a splitting of the zeroth LL and even to its absence in the ultra-thin film limit. We report on microscopic tight-binding modelling of Bi2Se3 thin films [1] in the presence of a strong magnetic field. We find that the zeroth LL is absent for thicknesses below 4QLs, in agreement with experiments. Calculations of the LL spectrum of a 5QL-thick slab reveal a strong asymmetry with respect to the Dirac point and a clear signature of the first LL, in good agreement with Dirac-Hamiltonian model calculations. The latter feature persists in a wide range of magnetic fields and involves an extended window of energies, including bulk states away from the Dirac point. We use our results to predict an interplay between the external magnetic field and gate-voltage dependence of the anomalous Hall effect that is characteristic of topological magnetic states.\\[4pt] [1] A.Pertsova and C.M.Canali, arXiv:1311.0691.

  • 65.
    Pertsova, Anna
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    MacDonald, Allan H.
    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)
    Abstract [en]

    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.

  • 66.
    Pertsova, Anna
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pederson, Mark R.
    Johns Hopkins University, USA.
    Rungger, Ivan
    Trinity College, Ireland.
    Sanvito, Stefano
    Trinity College, Ireland.
    Chapter Three: Electronic Transport as a Driver for Self-Interaction-Corrected Methods2015In: Advances In Atomic, Molecular, and Optical Physics: Volume 64 / [ed] Ennio Arimondo, Chun C. Lin and Susanne F. Yelin, Academic Press, 2015, Vol. 64, p. 29-86Chapter in book (Refereed)
    Abstract [en]

    While spintronics often investigates striking collective spin e ects in large systems, a very important research direction deals with spin-dependent phenomena in nanostructures, reaching the extreme of a single spin conned in a quantum dot, in a molecule, or localized on an impurity or dopant. The issue considered in this chapter involves taking this extreme to the nanoscale and the quest to use rst-principles methods to predict and control the behavior of a few \spins" (down to 1 spin) when they are placed in an interesting environment. Particular interest is on environments for which addressing these systems with external elds and/or electric or spin currents is possible. The realization of such systems, including those that consist of a core of a few transition-metal (TM) atoms carrying a spin, connected and exchanged-coupled through bridging oxo-ligands has been due to work by many experimental researchers at the interface of atomic, molecular and condensed matter physics. This chapter addresses computational problems associated with understanding the behaviors of nanoand molecular-scale spin systems and reports on how the computational complexity increases when such systems are used for elements of electron transport devices. Especially for cases where these elements are attached to substrates with electronegativities that are very di erent than the molecule, or for coulomb blockade systems, or for cases where the spin-ordering within the molecules is weakly antiferromagnetic, the delocalization error in DFT is particularly problematic and one which requires solutions, such as self-interaction corrections, to move forward. We highlight the intersecting elds of spin-ordered nanoscale molecular magnets, electron transport, and coulomb blockade and highlight cases where self-interaction corrected methodologies can improve our predictive power in this emerging field.

  • 67.
    Pertsova, Anna
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Mahani, Mohammad Reza
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Islam, Fhokrul
    Virginia Commonwealth University, USA.
    MacDonald, A.H.
    Department of Physics, University of Texas at Austin, U.S.A..
    Theoretical studies of surface states in Bi2Se3: effects of finite thickness, finite-cluster boundaries, and surface doping2013Conference paper (Refereed)
    Abstract [en]

    Recently, a family of bismuth-based materials, in particular bismuth chalcogenides (Bi2Se3, Bi2Te3), have been identified as three-dimensional (3D) topological insulators (TIs), i.e. materials characterized by a non-trivial bulk insulating gap and topologically protected surface states with linear dispersion and helical spin texture, traversing the gap [1].  A question of great fundamental and practical importance is how electronic and spin properties of topological surface states are modified in the presence of external perturbations, in particular time-reversal-breaking ones, such as magnetic dopants [2]. Experimentally, this question can be addressed by using advanced experimental probes, such as spin-sensitive angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM). On the theoretical side, there is a need for atomistic modelling of TIs that enables quantitative analysis and comparison with experiments, while keeping the computational overhead to a minimum. Microscopic tight-binding models, combined with input from ab initio calculations, provide a convenient platform to study surface states in TIs [3].

    We present results of realistic tight-binding modelling of 3D TIs, with particular focus on Bi2Se3. Our implementation is based on the sp3 tight-binding model for Bi2Se3 by Kobayashi [4], with parameters calculated from density functional theory. We start with a thorough analysis of the calculated band structure of a slab of Bi2Se3 of varying thickness, with surface states identified using quantitative criteria according to their actual spatial distribution. We investigate the thickness-dependent energy gap for thin slabs, attributed to inter-surface interaction, and the emergence of gapless surface states for slabs above a critical thickness. A quantitative description of the transition to infinite-thickness limit is provided by calculating explicitly the associated modifications in the spatial distribution and spin character of the wave function. We find that the system must be at least forty quintuple-layers thick to displace surface states that are essentially localized on either surface. In addition, we discuss the effect of an external magnetic field on the electronic structure of Bi2Se3. The peculiar structure of the Landau levels, found experimentally in this system [5], is a characteristic signature of the presence of Dirac surface states and can be used to extract the dispersion of the surface band. Furthermore, building upon previous work on GaAs [6], we develop a finite-cluster tight-binding approach, where an infinite slab is represented by a large but finite cluster with the same thickness. We find that the electronic structure for a finite cluster, with periodic boundary conditions along the length and width of the cluster, is in excellent agreement with that of an infinite slab. On the other hand, the finite-cluster approach allows us to directly probe mesoscopic effects, associated with real boundaries of a finite system, especially when the topological surface states are present. The approach enables also the investigation of individual impurities and defects. As a first application we present a case study of substitutional Bi defect at Se site near the surface of a slab of Bi2Se3 of varying thickness, focusing in particular on the interplay between defect-induced states and gapless surface states.  The analysis of spatial features of the calculated local density of states around the defect reveals similarities with STM studies [7]. Finally, we discuss strategies for incorporating single transition-metal dopants (Mn, Fe) in the current model and draw connections to recent STM experiments [8].

     

    [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010); X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).

    [2] H. Beidenkopf et al., Nature Physics 7, 939 (2011); L. A. Wray et al., Nature Physics 7, 21 (2011).

    [3] W. Zhang, R. Yu, H.-J. Zhang, X. Dai, and Z. Fang, New. J. Phys. 12, 065013 (2010); M. S. Bahramy et al., Nature Communications 3, 1159 (2012).

    [4] K. Kobayashi, Phys. Rev. B 84, 205424 (2011).

    [5] T. Hanaguri et al., Phys. Rev. B 82, 081305(R) (2010).

    [6] T. O. Strandberg et al., Phys. Rev. B 80, 024425 (2009).

    [7] S. Urazhdin et al., Phys. Rev. B 66, 161306(R) (2002); S. Urazhdin et al., Phys. Rev. B 69, 085313 (2004).

    [8] Y. S. Hor et al., Phys. Rev. B 81, 195203 (2010);  T. Schlenk et al., Phys. Rev. Lett. 110, 126804 (2013).

  • 68.
    Pournaghavi, Nezhat
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Holmqvist, Cecilia
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Pertsova, Anna
    Nordita, Stockholm.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes2018In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 12, no 11, Special Issue, article id 1800210Article in journal (Refereed)
    Abstract [en]

    Using the non-equilibrium Green’s function method and the Keldysh formalism, we study the effects of spin–orbit interactions and time-reversal symmetry breaking exchange fields on non-equilibrium quantum transport in graphene armchair nanoribbons. We identify signatures of the quantum spin Hall (QSH) and the quantum anomalous Hall (QAH) phases in nonequilibrium edge transport by calculating the spin-resolved real space charge density and local currents at the nanoribbon edges. We find that the QSH phase, which is realized in a system with intrinsic spin–orbit coupling, is characterized by chiral counter-propagating local spin currents summing up to a net charge flow with opposite spin polarization at the edges. In the QAH phase, emerging in the presence of Rashba spin–orbit coupling and a ferromagnetic exchange field, two chiral edge channels with opposite spins propagate in the same direction at each edge, generating an unpolarized charge current and a quantized Hall conductance  . Increasing the intrinsic spin–orbit coupling causes a transition from the QAH to the QSH phase, evinced by characteristic changes in the non-equilibrium edge transport. In contrast, an antiferromagnetic exchange field can coexist with a QSH phase, but can never induce a QAH phase due to a symmetry that combines time-reversal and sublattice translational symmetry.

  • 69.
    Sadowski, Janusz
    et al.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lund University.
    Kret, S.
    Polish Academy of Sciences, Poland.
    Siusys, A.
    Polish Academy of Sciences, Poland.
    Wojciechowski, T.
    Polish Academy of Sciences, Poland.
    Gas, K.
    Polish Academy of Sciences, Poland.
    Islam, Fhokrul
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Sawicki, M.
    Polish Academy of Sciences, Poland.
    Wurtzite (Ga,Mn)As nanowire shells with ferromagnetic properties2017In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 9, no 6, p. 2129-2137Article in journal (Refereed)
    Abstract [en]

    (Ga,Mn)As having a wurtzite crystal structure was coherently grown by molecular beam epitaxy on the 1100 side facets of wurtzite (Ga,In)As nanowires and further encapsulated by (Ga,Al)As and low temperature GaAs. For the first time, a truly long-range ferromagnetic magnetic order is observed in non-planar (Ga,Mn)As, which is attributed to a more effective hole confinement in the shell containing Mn by the proper selection/choice of both the core and outer shell materials. © The Royal Society of Chemistry.

  • 70.
    Sjöqvist, Erik
    et al.
    Uppsala University.
    Mousolou, Vahid Azimi
    Univ Isfahan, Iran.
    Canali, Carlo M.
    Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
    Conceptual aspects of geometric quantum computation2016In: Quantum Information Processing, ISSN 1570-0755, E-ISSN 1573-1332, Vol. 15, no 10, p. 3995-4011Article in journal (Refereed)
    Abstract [en]

    Geometric quantum computation is the idea that geometric phases can be used to implement quantum gates, i.e., the basic elements of the Boolean network that forms a quantum computer. Although originally thought to be limited to adiabatic evolution, controlled by slowly changing parameters, this form of quantum computation can as well be realized at high speed by using nonadiabatic schemes. Recent advances in quantum gate technology have allowed for experimental demonstrations of different types of geometric gates in adiabatic and nonadiabatic evolution. Here, we address some conceptual issues that arise in the realizations of geometric gates. We examine the appearance of dynamical phases in quantum evolution and point out that not all dynamical phases need to be compensated for in geometric quantum computation. We delineate the relation between Abelian and non-Abelian geometric gates and find an explicit physical example where the two types of gates coincide. We identify differences and similarities between adiabatic and nonadiabatic realizations of quantum computation based on non-Abelian geometric phases.

  • 71.
    Strandberg, Olof
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    MacDonald, A.H.
    University of Texas at Austin, USA.
    Chern number spins of Mn acceptor magnets in GaAs2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, article id 017202Article in journal (Refereed)
    Abstract [en]

    We determine the effective total spin J of local moments formed from acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern numbers. When individual Mn atoms are close to the sample surface, the total spin changes from J=1 to J=2, due to quenching of the acceptor orbital moment. For Mn pairs in bulk, the total J depends on pair orientation in the GaAs lattice and on the separation between the Mn atoms. We point out that Berry curvature variation as a function of local moment orientation can profoundly influence the quantum-spin dynamics of these magnetic entities.

  • 72.
    Strandberg, Olof
    et al.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    MacDonald, Allan
    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)
    Abstract [en]

    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.

  • 73.
    Strandberg, Olof
    et al.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    Canali, Carlo M.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    MacDonald, Allan
    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)
    Abstract [en]

    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.

  • 74.
    Strandberg, Olof
    et al.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    Canali, Carlo M.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    MacDonald, Allan H.
    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)
    Abstract [en]

    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.

  • 75.
    Strandberg, Olof
    et al.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    Canali, Carlo M.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    MacDonald, Allan H.
    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)
  • 76.
    Strandberg, Olof
    et al.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    Canali, Carlo M.
    University of Kalmar, School of Pure and Applied Natural Sciences.
    MacDonald, Allan H.
    Transition-Metal Dimers and physical limits on magnetic anisotropy2007In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 6Article in journal (Refereed)
  • 77.
    Strandberg, T.O.
    et al.
    lunds universitet.
    Canali, Carlo M.
    MacDonald, A.H.
    University of Texas at Austin.
    Chern number effective spin Hamiltonians for (Mn, Ga)AsManuscript (preprint) (Other academic)
  • 78.
    Wu, Philip
    et al.
    Lund University.
    Paschoal Jr, Waldomiro
    Lund University.
    Kumar, Sandeep
    Lund University.
    Borschel, Christian
    Jena University.
    Ronning, Carsten
    Jena University.
    Canali, Carlo M.
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Samuelson, Lars
    Lund University.
    Pettersson, Håkan
    Halmstad University.
    Linke, Heiner
    Lund University.
    Thermoelectric Characterization of Electronic Properties of GaMnAs Nanowires2012In: Journal of Nanotechnology, ISSN 1687-9503, E-ISSN 1687-9511, p. Article ID: 480813-Article in journal (Refereed)
    Abstract [en]

    Nanowires with magnetic doping centers are an exciting candidate for the study of spin physics and proof-of-principle spintronics devices. The required heavy doping can be expected to have a significant impact on the nanowires’ electron transport properties.

    Here, we use thermopower and conductance measurements for transport characterization of Ga0.95Mn0.05As nanowires over a broad temperature range. We determine the carrier type (holes) and concentration and find a sharp increase of the thermopower below temperatures of 120 K that can be qualitatively described by a hopping conduction model. However, the unusually large thermopower suggests that additional mechanisms must be considered as well.

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