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Islam, F., Pertsova, A. & Canali, C. M. (2019). Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization. Physical Review B, 99(15), 1-6, Article ID 155401.
Open this publication in new window or tab >>Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 15, p. 1-6, article id 155401Article in journal (Refereed) Published
##### Abstract [en]

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.

##### Place, publisher, year, edition, pages
American Physical Society, 2019
##### National Category
Other Physics Topics
##### Research subject
Physics, Electrotechnology
##### Identifiers
urn:nbn:se:lnu:diva-81988 (URN)10.1103/PhysRevB.99.155401 (DOI)000463883800004 ()2-s2.0-85064136667 (Scopus ID)
##### Funder
Swedish Research Council, 621-2014-4785Carl Tryggers foundation , CTS 14:178 Available from: 2019-04-16 Created: 2019-04-16 Last updated: 2019-08-29Bibliographically approved
Pournaghavi, N., Holmqvist, C., Pertsova, A. & Canali, C. M. (2018). Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes [Letter to the editor]. Physica Status Solidi. Rapid Research Letters, 12(11, Special Issue), Article ID 1800210.
Open this publication in new window or tab >>Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes
2018 (English)In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 12, no 11, Special Issue, article id 1800210Article in journal, Letter (Refereed) Published
##### 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 $G=\frac{2e^{2}}{h}$ . 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.

##### Place, publisher, year, edition, pages
Wiley-Blackwell, 2018
##### Keywords
graphene nanoribbons, quantum anomalous Hall effect, quantum spin Hall effect, topological insulators
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-76947 (URN)10.1002/pssr.201800210 (DOI)000450130300007 ()2-s2.0-85050622980 (Scopus ID)
##### Funder
Carl Tryggers foundation , CTS 14:178Swedish Research Council, 621‐2014‐4785 Available from: 2018-07-19 Created: 2018-07-19 Last updated: 2019-08-29Bibliographically approved
Gooth, J., Zierold, R., Sergelius, P., Hamdou, B., Garcia, J., Damm, C., . . . Nielsch, K. (2016). Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires. ACS Nano, 10(7), 7180-7188
Open this publication in new window or tab >>Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires
2016 (English)In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 10, no 7, p. 7180-7188Article in journal (Refereed) Published
##### Abstract [en]

Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.

##### Keywords
1D confinement, magnetism, nanowire, surface, topological insulator
##### National Category
Condensed Matter Physics Atom and Molecular Physics and Optics
##### Research subject
Natural Science, Physics
##### Identifiers
urn:nbn:se:lnu:diva-56093 (URN)10.1021/acsnano.6b03537 (DOI)000380576600085 ()27351276 (PubMedID)2-s2.0-84979873036 (Scopus ID)
Available from: 2016-09-16 Created: 2016-08-31 Last updated: 2017-11-21Bibliographically approved
Pertsova, A., Canali, C. M. & MacDonald, A. H. (2016). Quantum Hall edge states in topological insulator nanoribbons. Physical Review B, 94(12), Article ID 121409.
Open this publication in new window or tab >>Quantum Hall edge states in topological insulator nanoribbons
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 12, article id 121409Article in journal (Refereed) Published
##### 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.

##### National Category
Physical Sciences
##### Research subject
Natural Science, Physics
##### Identifiers
urn:nbn:se:lnu:diva-57610 (URN)10.1103/PhysRevB.94.121409 (DOI)000384070000003 ()2-s2.0-84990882880 (Scopus ID)
Available from: 2016-10-27 Created: 2016-10-25 Last updated: 2017-11-29Bibliographically approved
Pertsova, A., Canali, C. M., Pederson, M. R., Rungger, I. & Sanvito, S. (2015). Chapter Three: Electronic Transport as a Driver for Self-Interaction-Corrected Methods. In: Ennio Arimondo, Chun C. Lin and Susanne F. Yelin (Ed.), Advances In Atomic, Molecular, and Optical Physics: Volume 64 (pp. 29-86). Academic Press, 64
Open this publication in new window or tab >>Chapter Three: Electronic Transport as a Driver for Self-Interaction-Corrected Methods
2015 (English)In: 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.

##### Series
Advances In Atomic, Molecular, and Optical Physics, ISSN 1049-250X ; 64
##### Keywords
Spin dependent transport; Coulomb blockade; Averaged self-interaction correction; Molecular magnets; Quantum information; Electronic structure
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-43575 (URN)10.1016/bs.aamop.2015.06.002 (DOI)000370490900004 ()2-s2.0-84937605309 (Scopus ID)978-0-12-802127-9 (ISBN)
##### Funder
Swedish Research Council Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2016-11-01Bibliographically approved
Aikebaier, F., Pertsova, A. & Canali, C. M. (2015). Effects of short-range electron-electron interactions in doped graphene. Physical Review B. Condensed Matter and Materials Physics, 92(15), Article ID 155420.
Open this publication in new window or tab >>Effects of short-range electron-electron interactions in doped graphene
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 15, article id 155420Article in journal (Refereed) Published
##### Abstract [en]

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

##### Keywords
graphene, impurities in graphene, electron-electron interactions
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-39636 (URN)10.1103/PhysRevB.92.155420 (DOI)000362895500003 ()2-s2.0-84944810134 (Scopus ID)
Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2017-12-05Bibliographically approved
Pertsova, A., Canali, C. M. & MacDonald, A. H. (2015). Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study. Physical Review B. Condensed Matter and Materials Physics, 91, Article ID 075430.
Open this publication in new window or tab >>Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, article id 075430Article in journal (Refereed) Published
##### 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.

##### Keywords
topological insulator thin films, Landau levels
##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-38038 (URN)10.1103/PhysRevB.91.075430 (DOI)000350207800010 ()2-s2.0-84924028254 (Scopus ID)
##### Funder
Swedish Research Council, 621-2010-3761 Available from: 2014-11-06 Created: 2014-11-06 Last updated: 2017-12-05Bibliographically approved
Mahani, M. R., Islam, F., Pertsova, A. & Canali, C. M. (2014). Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface. Physical Review B. Condensed Matter and Materials Physics, 89(16), Article ID: 165408
Open this publication in new window or tab >>Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface
2014 (English)In: 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) Published
##### Abstract [en]

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.

##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-27461 (URN)10.1103/PhysRevB.89.165408 (DOI)000334118200003 ()2-s2.0-84899714415 (Scopus ID)
Available from: 2013-07-05 Created: 2013-07-05 Last updated: 2017-04-18Bibliographically approved
Mahani, M. R., Pertsova, A., Islam, F. & Canali, C. M. (2014). Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator. In: MAR14 Meeting of The American Physical Society: . Paper presented at MAR14 Meeting of The American Physical Society, March 3-7, 2014, Denver. Denver, Colorado: American Physical Society
Open this publication in new window or tab >>Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator
2014 (English)In: MAR14 Meeting of The American Physical Society, Denver, Colorado: American Physical Society , 2014Conference paper, Oral presentation with published abstract (Refereed)
##### Place, publisher, year, edition, pages
Denver, Colorado: American Physical Society, 2014
##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-33304 (URN)
##### Conference
MAR14 Meeting of The American Physical Society, March 3-7, 2014, Denver
Available from: 2014-03-26 Created: 2014-03-26 Last updated: 2017-04-18Bibliographically approved
Mahani, M. R., Pertsova, A., Islam, F. & Canali, C. M. (2014). Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator. Physical Review B. Condensed Matter and Materials Physics, 90, Article ID: 195441
Open this publication in new window or tab >>Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator
2014 (English)In: 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) Published
##### Abstract [en]

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.

##### National Category
Condensed Matter Physics
##### Research subject
Physics, Condensed Matter Physics
##### Identifiers
urn:nbn:se:lnu:diva-31785 (URN)10.1103/PhysRevB.90.195441 (DOI)000345538500008 ()2-s2.0-84918831696 (Scopus ID)
Available from: 2014-01-29 Created: 2014-01-29 Last updated: 2017-12-06Bibliographically approved
##### Identifiers
ORCID iD: orcid.org/0000-0002-7831-7214

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