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• 1.
Virginia Commonwealth University, USA.
Stable magnetic order and charge induced rotation of magnetizationin nano-clusters2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 152409Article in journal (Refereed)

Efficient control of magnetic anisotropy and the orientation of magnetization are of centralimportance for the application of nanoparticles in spintronics. Conventionally, magnetization iscontrolled directly by an external magnetic field or by an electric field via spin-orbit coupling.Here, we demonstrate a different approach to control magnetization in small clusters. We firstshow that the low magnetic anisotropy of a Co5 cluster can be substantially enhanced by attachingbenzene molecules due to the mixing between p states of C and the d states of Co sites. We thenshow that the direction of magnetization vector of Co5 sandwiched between two benzene moleculesrotates by 90 when an electron is added or removed from the system. An experimental set up torealize such effect is also suggested.

• 2.
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.

• 3.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. NORDITA, Stockholm. NORDITA, Stockholm. CNR, Italy. CNR, Italy. CNR, Italy. Novosibirsk State Univ, Russia. Novosibirsk State Univ, Russia. European Synchrotron Radiat Facil, France. European Synchrotron Radiat Facil, France. ALBA Synchrotron Light Source, Spain. ALBA Synchrotron Light Source, Spain. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany. Univ Wurzburg, Germany.
Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 15, article id 155429Article in journal (Refereed)

The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here, we report on a detailed and systematic investigation of transition metal (TM) doped Sb2Te3. By combining density functional theory calculations with complementary experimental techniques, i.e., scanning tunneling microscopy, resonant photoemission, and x-raymagnetic circular dichroism, we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM doped topological insulators. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V-and Fe-doped Sb2Te3 display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity dependent and can vary from in plane to out of plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM doped Sb2Te3 in the ferromagnetic state.

• 4.
Virginia Commonwealth University, USA.
Virginia Commonwealth University, USA.
On the enhancement of magnetic anisotropy in cobalt clusters via non-magnetic doping2014In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 125303Article in journal (Refereed)

We show that the magnetic anisotropy energy (MAE) in cobalt clusters can be significantlyenhanced by doping them with group IV elements. Our firstprincipleselectronic structurecalculations show that Co4C2 and Co12C4 clusters have MAEs of 25 K and 61 K, respectively. The large MAE is due to controlled mixing between Co dandC pstatesand can be furthertuned by replacing C by Si. Larger assemblies of such primitive units are shown to be stablewith MAEs exceeding 100 K in units as small as 1.2 nm, in agreement with the recentobservation of large coercivity. These results may pave the way for the use of nanoclustersinhigh density magnetic memory devices for spintronics applications

• 5.
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.

• 6.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Dept. of physics, Linnéuniversitetet.
Nordita, Sweden. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 15, p. 1-6, article id 155401Article in journal (Refereed)

The quantum anomalous Hall effect (QAHE), characterized by dissipationless quantized edge transport, relies crucially on a nontrivial topology of the electronic bulk band structure and a robust ferromagnetic order that breaks time-reversal symmetry. Magnetically doped topological insulators (TIs) satisfy both these criteria, and are the most promising quantum materials for realizing the QAHE. Because the spin of the surface electrons aligns along the direction of the magnetic-impurity exchange field, only magnetic TIs with an out-of-plane magnetization are thought to open a gap at the Dirac point (DP) of the surface states, resulting in the QAHE. Using a continuum model supported by atomistic tight-binding and first-principles calculations of transition-metal doped Bi2Se3, we show that a surface-impurity potential generates an additional effective magnetic field which spin polarizes the surface electrons along the direction perpendicular to the surface. The predicted gap-opening mechanism results from the interplay of this additional field and the in-plane magnetization that shifts the position of the DP away from the Γ point. This effect is similar to the one originating from the hexagonal warping correction of the band structure but is one order of magnitude stronger. Our calculations show that in a doped TI with in-plane magnetization the impurity-potential-induced gap at the DP is comparable to the one opened by an out-of-plane magnetization.

• 7.
Cent Michigan Univ, USA.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Univ Texas El Paso, USA.
A multiferroic molecular magnetic qubit2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 17, p. 1-7, article id 174105Article in journal (Refereed)

The chiral Fe3O(NC5H5)3(O2CC6H5)6 molecular cation, with C-3 symmetry, is composed of three six-fold coordinated spin-carrying Fe3+ cations that form a perfect equilateral triangle. Experimental reports demonstrating the spin-electric effect in this system also identify the presence of a magnetic uniaxis and suggest that this molecule may be a good candidate for an externally controllable molecular qubit. Here, we demonstrate, using standard density-functional methods, that the spin-electric behavior of this molecule could be even more interesting as there are energetically competitive reference states associated with both high and low local spins (S = 5/2 vs S = 1/2) on the Fe3+ ions. Each of these structures allow for spin-electric ground states. We find that qualitative differences in the broadening of the Fe(2s) and O(1s) core levels, shifts in the core-level energies, and the magnetic signatures of the single-spin anisotropy Hamiltonian may be used to confirm whether a transition between a high-spin manifold and a low spin manifold occurs.

• 8.
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.
As vacancies in MnGaAs: tight binding and first-principles studies2012Conference paper (Refereed)
• 9.
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.

• 10.
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. Department of Physics, University of Texas at Austin, U.S.A.
Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators2013In: MRS Online Proceedings Library/Volume 1564/2013, Materials Research Society, 2013Conference paper (Refereed)

We present results of theoretical studies of transition metal dopants in GaAs, based on microscopic tight-binding model and ab-initio calculations. We focus in particular on how the vicinity of surface affects the properties of the hole-acceptor state, its magnetic anisotropy and its magnetic coupling to the magnetic dopant.  In agreement with STM experiments, Mn substitutional dopants on the (110) GaAs surface give rise to a deep acceptor state, whose wavefunction is localized around the Mn center. We discuss a refinement of the theory that introduces explicitly the d-levels for the TM dopant. The explicit inclusion of d-levels is particularly important for addressing recent STM experiments on substitutional Fe in GaAs. In the second part of the paper we discuss an analogous investigation of single dopants in Bi2Se3 three-dimensional topological insulators, focusing in particular on how substitutional impurities positioned on the surface affect the electronic structure in the gap.  We present explicit results for BiSe antisite defects and compare with STM experiments.

• 11.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Virginia Commonwealth University, USA. 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: MAR14 Meeting of The American Physical Society, Denver, Colorado: American Physical Society , 2014Conference paper (Refereed)
• 12.
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.

• 13.
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.
The role of d levels of substitutional magnetic impurities at the (110) GaAs surface2013Conference paper (Other academic)

The study of the spin of individual transition-metal dopants in a semiconductor host is an emergent field known as magnetic solotronics, bearing exciting prospects for novel spintronics devices at the atomic scale. Advances in different STM based techniques allowed experimentalists to investigate substitutional dopants at a semiconductor surface with unprecedented accuracy and degree of details [1]. Theoretical studies based both on microscopic tight-binding (TB) models and DFT techniques have contributed in elucidating the experimental findings. In particular, for the case of Mn dopants on the (110) GaAs surface, TB models [2] have provided a quantitative description of the properties of the associated acceptor states. Most of these TB calculations ignore dealing explicitly with the Mn d-levels and treat the associated magnetic moment as a classical vector. However recent STM experiments [3] involving other TM impurities, such as Fe, reveal topographic features that might be related to electronic transitions within the d-level shell of the dopant. In this work we have included explicitly the d levels in the Hamiltonian. The parameters of the model have been extracted from DFT calculations. We have investigated the role that d levels play on the properties of the acceptor states of the doped GaAs(110) surface, and analyzed their implications for STM spectroscopy.

• 14.
Virginia Commonwealth University, USA.
Virginia Commonwealth University, USA. Virginia Commonwealth University, USA. Virginia Commonwealth University, USA.
Robust Magnetic Moments on Impurities in Metallic Clusters: Localized Magnetic States in Superatoms2013In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 117, no 20, p. 4297-4303Article in journal (Refereed)

Introducing magnetic impurities into clusters of simplemetals can create localized states for higher angular momentum quantumnumbers (l = 2 or 3) that can breed magnetism analogous to that invirtual bound states in metallic hosts, offering a new recipe for magneticsuperatoms. In this work, we demonstrate that MnCan clusterscontaining 6−15 Ca atoms show a spin magnetic moment of 5.0 μBirrespective of the cluster size. Theoretical analysis reveals that the Mn dstates hybridize only partially with superatomic states and introduce extramajority and minority d states, largely localized at the Mn site, with alarge gap. Successive addition of Ca atoms introduces superatomic statesof varying angular momentum that are embedded in this gap, allowingcontrol over the stability of the motifs without altering the moment. Assemblies of such clusters can offer novel electronic features due to theformation of localized magnetic “quasibound states” in a confined nearlyfree electron gas.

• 15.
Purdue University, USA.
University of Texas, USA.
Quantum Percolation in Two Dimensions2009In: Quantum and Semi-classical Percolation and Breakdown in Disordered Solids / [ed] Bikas K. Chakrabarti, Kamal K. Bardhan, Asok K. Sen, Springer, 2009Chapter in book (Other academic)

We overview the recent work on the transport of a quantum particle through a two-dimensional, disordered network produced by the percolation algorithm. This problem, known as quantum percolation, has been controversial regarding the nature of localization and presence or absence of phase transitions in its transmission behavior.

• 16.
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.

• 17.
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, Washington DC.
DFT calculations of the charged states of N@C60 and {\{}Fe4{\}} single molecule magnets investigated in tunneling spectroscopy2012Conference paper (Other academic)

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.

• 18.
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.

• 19.
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.
Spin-orbit interaction and magnetic field in antiferromagnetic triangular molecular magnets2011In: APS MARCH MEETING 2011, 2011Conference paper (Refereed)
• 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. Virginia Commonwealth University, USA. 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)

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).

• 21.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Lund University.
Polish Academy of Sciences, Poland. Polish Academy of Sciences, Poland. Polish Academy of Sciences, Poland. Polish Academy of Sciences, Poland. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. 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)

(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. Â© The Royal Society of Chemistry.

• 22.
Virginia Commonwealth University.
Virginia Commonwealth University. Virginia Commonwealth University.
Using graphene to control magnetic anisotropy and interaction between supported clusters2015In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 17, article id 053052Article in journal (Refereed)

Stabilization of magnetic order in clusters/nanoparticles at elevated temperatures is a fundamentallychallenging problem. The magnetic anisotropy energy (MAE) that prevents the thermal fluctuationsof the magnetization direction can be around 1–10Kin free transition metal clusters of around adozen atoms. Here we demonstrate that a graphene support can lead to an order of magnitudeenhancement in the anisotropy of supported species. Our studies show that theMAEof supportedCo5 and Co13 clusters on graphene increase by factors of 2.6 and 25, respectively. The enhancement islinked to the splitting of selected electronic orbitals that leads to the different orbital contributionsalong the easy and hard axis. The conductive support enables a magnetic interaction between thedeposited species and the nature of themagnetic interaction can be controlled by the separation betweensupported clusters or by vacancies offering an unprecedented ability to tune characteristics of assemblies.

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