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• 1.
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.

• 2.
Lund University, Solid State Physics.
Lund University, Solid State Physics. Lund University, Solid State Physics. 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. Lund University, Solid State Physics.
Nano-scaled Ferromagnetic Single-electron transistors2007In: 2007 7TH IEEE CONFERENCE ON NANOTECHNOLOGY, 2007, p. 420-423Conference paper (Refereed)
• 3.
KNT University of Technology.
Influence of in-Plane Magnetic Field on Spin Polarization in the Presence of the Oft-neglected k3-Dresselhaus Spin-Orbit Coupling2008In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 372, no 38, p. 6022-6025Article in journal (Refereed)

The influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k3k3-Dresselhaus spin–orbit coupling was investigated. The k3k3-Dresselhaus term can produce a limited spin polarization. The in-plane magnetic field plays a great role in the tunneling process. It can generate the perfect spin polarization of the electrons and the ideal transmission coefficient for spin up and down simultaneously. In energy scale, complete separation between spin up and down resonance was obtained by a relatively higher in-plane magnetic field while a comparatively lower in-plane magnetic field vanishes the spin separation. On the other hand, the spin relaxation can be suppressed by compensating the oft-neglected k3k3-Dresselhaus spin orbit coupling using a relatively lower in-plane magnetic field.

• 4.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Magnetic solotronics near the surface of a semiconductor and a topological insulator2015Doctoral thesis, comprehensive summary (Other academic)

Technology where a solitary dopant acts as the active component of an opto-electronic device is an emerging  field known as solotronics, and bears the promise to revolutionize the way in which information is stored, processed and transmitted. Magnetic doped semiconductors and in particular (Ga, Mn)As, the archetype of dilute magnetic semiconductors, and topological insulators (TIs), a new phase of quantum matter with unconventional characteristics, are two classes of quantum materials that have the potential to advance spin-electronics technology. The quest to understand and control, at the atomic level, how a few magnetic atoms precisely positioned in a complex environment respond to external stimuli, is the red thread that connects these two quantum materials in the research presented here.

The goal of the thesis is in part to elucidate the properties of transition metal (TM) impurities near the surface of GaAs semiconductors with focus on their response to local magnetic and electric fields, as well as to investigate the real-time dynamics of their localized spins. Our theoretical analysis, based on density functional theory (DFT) and using tight-binding (TB) models, addresses the mid-gap electronic structure, the local density of states (LDOS) and the magnetic anisotropy energy of individual Mn and Fe impurities near the (110) surface of GaAs. We investigate the effect of a magnetic field on the Mn acceptor LDOS measured in cross-sectional scanning tunneling microscopy, and provide an explanation of why the experimental LDOS images depend weakly on the field direction despite the strongly anisotropic nature of the Mn acceptor wavefunction. We also investigate the effects of a local electrostatic field generated by nearby charged As vacancies, on individual and pairs of ferromagnetically coupled magnetic dopants near the surface of GaAs, providing a means to control electrically the exchange interaction of Mn pairs. Finally, using the mixed quantum-classical scheme for spin dynamics, we calculate explicitly the time evolution of the Mn spin and its bound acceptor, and analyze the dynamic interaction between pairs of ferromagnetically coupled magnetic impurities in a nanoscaled semiconductor.

The second part of the thesis deals with the theoretical investigation of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi2Se3 TI, using an approach that combines DFT and TB calculations. Our analysis clarifies the crucial role played by the spatial overlap and the quasi-resonant coupling between the Mn-acceptor and the topological surface states inside the Bi2Se3 band gap, in the opening of a gap at the Dirac point. Strong electronic correlations are also found to contribute significantly to the mechanism leading to the gap, since they control the hybridization between the p orbitals of nearest-neighbor Se atoms and the acceptor spin-polarization. Our results explain the effects of inversion-symmetry and time-reversal symmetry breaking on the electronic states in the vicinity of the Dirac point, and contribute to clarifying the origin of surface-ferromagnetism in TIs. The promising potential of magnetic-doped TIs accentuates the importance of our contribution to the understanding of the interplay between magnetic order and topological protected surface states.

• 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.
Anisotropy energy and local density of states of Mn acceptor states near the (110) surface of GaAs in the presence of an external magnetic field2011Conference paper (Refereed)
• 6.
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.
Effect of As vacancies on the binding energy and exchange splitting of Mn impurities on a GaAs surface2012In: Bulletin of the American Physical Society, APS March Meeting 2012,  Volume 57, Number 1, 2012, p. L14.00002-Conference paper (Other academic)

State-of-the-art STM spectroscopy is nowadays able to manipulate and probe the magnetic properties of individual magnetic impurities located near the surface of a semiconductor. A recent advance of these technique employs the electric field generated by a As vacancy in GaAs to affect the environment surrounding substitutional Mn impurities in the host material [1]. Here we calculate the binding energy of a single Mn dopant in the presence of nearby As vacancies, by using a recently-introduced tight-binding method [2] that is able to capture the salient features of Mn impurities near the (110) GaAs surface. The As vacancies, modeled by the repulsive potential they produce, are expected to decrease the acceptor binding energy in agreement with experiment [1]. Within this theoretical model, we investigate the possible enhancement of the exchange splitting for a pair of ferromagnetically ordered Mn impurities, observed experimentally when As vacancies are present [3]. We also calculate the response of the Mn-impurity---As-vacancy complex to an external magnetic field. \\[4pt] [1] H. Lee and J. A. Gupta, Science, 1807-1810, (2010). \\[0pt] [2] T. O. Strandberg, C. M. Canali, A. H. MacDonald, Phys. Rev. B 80, 024425, (2009). \\[0pt] [3] J.A. Gupta, private communication.

• 7.
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.
Effect of magnetic field on the local density of states of Mn acceptor magnets in GaAs2011In: Bulletin of the American Physical Society, Volume 56, Number 1: APS March Meeting 2011, 2011, p. W15.00002-Conference paper (Other academic)

Advances in atomic manipulation, real-space imagining and spectroscopic power of STM techniques have recently made it possible to investigate the local electronic properties of a few substitutional Mn impurities inserted in the GaAs surfaces [1]. Theoretical work [2] predicts that the local density of states in the vicinity of the Mn impurities should depend strongly on the direction of the Mn magnetic moment. In contrast, recent STM experiments [3] from several groups find a negligible dependence of the tunneling LDOS on the magnetic field direction for applied fields up to 7 T. Based on tight- binding calculations we interpret these findings by arguing that large LDOS signals require large angle moment rotations, and that the strength of the magnetic field used in present experiments is not strong enough to substantially modify the magnetic anisotropy landscape of Mn impurities near the GaAs surface.\\[4pt] [1] D. Kitchen et al., Nature, 442, 436 (2006); J. K. Garleff et al., Phys. Rev. B 82, 035303 (2010).\\[0pt] [2] T. O. Strandberg, C. M. Canali, and A. H. MacDonald, Phys. Rev. B 80, 024425 (2009). [3] P. M. Koenraad, Private Communication.

• 8.
Linnaeus University, Faculty of Science and Engineering, School of Engineering.
Linnaeus University, Faculty of Science and Engineering, School of Engineering.
Local manipulation of the magnetic properties of Mn impurities on a GaAs surface by As vacancies2012Conference paper (Refereed)
• 9.
KNT University of Technology.
Efficient Spin Filtering in a Disordered Semiconductor Superlattice in the Presence of Dresselhaus Spin-Orbit coupling2008In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 372, no 11, p. 1926-1929Article in journal (Refereed)

The influence of the Dresselhaus spin–orbit coupling on spin polarization by tunneling through a disordered semiconductor superlattice was investigated. The Dresselhaus spin–orbit coupling causes the spin polarization of the electron due to transmission possibilities difference between spin up and spin down electrons. The electron tunneling through a zinc-blende semiconductor superlattice with InAs and GaAs layers and two variable distance InxGa(1−x)As impurity layers was studied. One hundred percent spin polarization was obtained by optimizing the distance between two impurity layers and impurity percent in disordered layers in the presence of Dresselhaus spin–orbit coupling. In addition, the electron transmission probability through the mentioned superlattice is too much near to one and an efficient spin filtering was recommended.

• 10.
KNT University of Technology.
Particular Nanowire Superlattice as a Spin Filter2009In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 373, no 43, p. 3994-3996Article in journal (Refereed)

A nanowire superlattice of InAs and GaAs layers with In0.47Ga0.53As as the impure layers is proposed. The oft-neglected k3k3 Dresselhaus spin–orbit coupling causes the spin polarization of the electron but often can produce a limited spin polarization. In this nanowire superlattice, Dresselhaus term produce complete spin filtering by optimizing the distance between the In0.47Ga0.53As layers and the Indium (In) in the impure layers. The proposed structure is an optimized nanowire superlattice that can efficiently filter any component of electron spins according to its energy. In fact, this nanowire superlattice is an energy dependent spin filter structure.

• 11.
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)
• 12.
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.

• 13.
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.

• 14.
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.

• 15.
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.
Time-Dependent Spin Dynamics of Few Transition Metal Impurities in a Semiconductor Host2014In: 2014 MRS Spring Meeting and Exhibit, April 21-25, San Francisco, 2014Conference paper (Refereed)

Recently, remarkable progress has been achieved in describing electronic and magnetic properties of individual dopants in semiconductors, both experimentally [1] and theoretically [2, 3], offering exciting prospects for applications in future electronic devices. In view of potential novel applications, which involve communication between individual magnetic dopants, mediated by the electronic carriers of the host, the focus of this research field has been shifting towards fundamental understanding and control of spin dynamics of these atomic-scale magnetic centers. Importantly, the development of time-resolved spectroscopic techniques has opened up the possibility to probe the dynamics of single spin impurities experimentally [4]. These advances pose new challenges for theory, calling for a fully microscopic time-dependent description of spin dynamics of individual impurities in the solid states environment.We present results of theoretical investigations of real-time spin dynamics of individual and pairs of transition metal (Mn) impurities in GaAs. Our approach combines the microscopic tight-binding description of substitutional dopants in semicondutors [3] with the time-dependent scheme for simulations of spin dynamics [5], based on the numerical integration of equations of motion for the coupled system of the itinerant electronic degrees of freedom of the host and the localized impurity spins. We study the spin dynamics of impurities in finite clusters containing up to hundred atoms, over time scales of a few hundred femtoseconds. In particular, we calculate explicitly the time-evolution of the impurity spins and electrons of the host upon weak external perturbations. From the Fourier spectra of the time-dependent spin trajectories, we identify the energy scales associated with intrinsic interactions of the system, namely the spin-orbit interaction and the exchange interaction between the impurity spins and the spins of the nearest-neighbor atoms of the host. Furthermore, we investigate the effective dynamical coupling between the spins of two spatially separated Mn impurities, mediated by the host carriers. We find signatures of ferromagnetic coupling between the impurities in the time-evolution of their spins. Finally, we propose a scheme for investigating the spin relaxation of Mn dopants in GaAs, by extending the time-dependent approach for spin dynamics in an isolated conservative system to the case of an open system, with dephasing mechanisms included as an effective interaction between the system and an external bath [5].

• 16.
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.
Trend of the magnetic anisotropy for individual Mn dopants near the (110) GaAs surface2014In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 39, p. Article ID: 394006-Article in journal (Refereed)

Using a microscopic finite-cluster tight-binding model, we investigate the trend of the magnetic anisotropy energy as a function of the cluster size for an individual Mn impurity positioned in the vicinity of the (1 1 0) GaAs surface. We present results of calculations for large cluster sizes containing approximately 104 atoms, which have not been investigated so far. Our calculations demonstrate that the anisotropy energy of a Mn dopant in bulk GaAs, found to be non-zero in previous tight-binding calculations, is purely a finite size effect that vanishes with inverse cluster size. In contrast to this, we find that the splitting of the three in-gap Mn acceptor energy levels converges to a finite value in the limit of the infinite cluster size. For a Mn in bulk GaAs this feature is related to the nature of the mean-field treatment of the coupling between the impurity and its nearest neighbor atoms. We also calculate the trend of the anisotropy energy in the sublayers as the Mn dopant is moved away from the surface towards the center of the cluster. Here the use of large cluster sizes allows us to position the impurity in deeper sublayers below the surface, compared to previous calculations. In particular, we show that the anisotropy energy increases up to the fifth sublayer and then decreases as the impurity is moved further away from the surface, approaching its bulk value. The present study provides important insights for experimental control and manipulation of the electronic and magnetic properties of individual Mn dopants at the semiconductor surface by means of advanced scanning tunneling microscopy techniques.

• 17.
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.

• 18.
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)
• 19.
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.

• 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. 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.

• 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. 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).

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