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1.	Aikebaier, Faluke, et al. (author) Effects of short-range electron-electron interactions in doped graphene 2015 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:15 Journal article (peer-reviewed)abstract We study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of substitutional impurities. Our computational approach is based on the π orbital tight-binding model for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. The finite impurity concentration is modeled using the supercell approach. We compare explicitly noninteracting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens up at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.
2.	Balatsky, Alexander V., et al. (author) Dynamic Quantum Matter 2020 In: Annalen der Physik. - : John Wiley & Sons. - 0003-3804 .- 1521-3889. ; 532:2 Journal article (other academic/artistic)
3.	Gooth, Johannes, et al. (author) Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires 2016 In: ACS Nano. - Washington : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 10:7, s. 7180-7188 Journal article (peer-reviewed)abstract 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.
4.	Islam, Fhokrul, et al. (author) Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization 2019 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 99:15, s. 1-6 Journal article (peer-reviewed)abstract 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.
5.	Islam, Fhokrul, et al. (author) Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform 2018 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:15 Journal article (peer-reviewed)abstract 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.
6.	Kumar, Sandeep, et al. (author) Magnetic polarons and large negative magnetoresistance in GaAs nanowires implanted with Mn ions 2013 In: Nano letters (Print). - Washington, United States : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 13:11, s. 5079-5084 Journal article (peer-reviewed)abstract We report on low-temperature magnetotransport and SQUID measurements on heavily doped Mn-implanted GaAs nanowires. SQUID data recorded at low magnetic fields exhibit clear signs of the onset of a spin-glass phase with a transition temperature of about 16 K. Magnetotransport experiments reveal a corresponding peak in resistance at 16 K and a remarkably large negative magnetoresistance, reaching 40 % at 1.6 K and 8 T. The negative magnetoresistance decreases at elevated temperatures and vanishes at about 100 K. We interpret our transport data in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons forming a paramagnetic/spin-glass phase.
7.	Mahani, Mohammad Reza, et al. (author) Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface 2014 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:16, s. Article ID: 165408- Journal article (peer-reviewed)abstract 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.
8.	Mahani, Mohammad Reza, et al. (author) Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator 2014 In: MAR14 Meeting of The American Physical Society. - Denver, Colorado : American Physical Society. Conference paper (peer-reviewed)
9.	Mahani, Mohammad Reza, et al. (author) Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi2Se3 topological insulator 2014 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90, s. Article ID: 195441- Journal article (peer-reviewed)abstract 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.
10.	Mahani, Mohammad Reza, et al. (author) Spin dynamics of Mn impurities and their bound acceptors in GaAs 2014 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:24 Journal article (peer-reviewed)abstract 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.
11.	Mahani, Mohammad Reza, et al. (author) The role of d levels of substitutional magnetic impurities at the (110) GaAs surface 2013 Conference paper (other academic/artistic)abstract 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.
12.	Mahani, Mohammad Reza, et al. (author) Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators 2013 In: MRS Online Proceedings Library/Volume 1564/2013. - : Materials Research Society. Conference paper (peer-reviewed)abstract 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.
13.	Mahani, Mohammad Reza, et al. (author) Time-Dependent Spin Dynamics of Few Transition Metal Impurities in a Semiconductor Host 2014 In: 2014 MRS Spring Meeting and Exhibit, April 21-25, San Francisco. Conference paper (peer-reviewed)abstract 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].
14.	Mahani, Mohammad Reza, et al. (author) Trend of the magnetic anisotropy for individual Mn dopants near the (110) GaAs surface 2014 In: Journal of Physics. - : Institute of Physics (IOP). - 0953-8984 .- 1361-648X. ; 26:39, s. Article ID: 394006- Journal article (peer-reviewed)abstract 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.
15.	Manchon, A., et al. (author) Currents and torques due to spin-dependent diffraction in ferromagnetic/spin spiral bilayers 2008 In: Journal of Physics. - : Institute of Physics (IOP). - 0953-8984 .- 1361-648X. ; 20:50, s. 505213- Journal article (peer-reviewed)abstract Spin-dependent transport through the interface between a ferromagnet and a spin spiral is investigated using both ballistic and diffusive models. We find that spin-dependent interferences lead to a new type of diffraction called 'spin diffraction'. It is shown that this spin diffraction leads to local spin and electrical current along the interface, as well as spin transfer torque acting on the spin spiral. This study also emphasizes that in highly inhomogeneous magnetic configurations, diffracted electrons must be taken into account to properly describe the spin transport.
16.	Paschoal Jr., Waldomiro, 1977-, et al. (author) Magnetoresistance in Mn ion-implanted GaAs:Zn nanowires 2014 In: Applied Physics Letters. - New York : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 104:15 Journal article (peer-reviewed)abstract We have investigated the magnetoresistance (MR) in a series of Zn doped (p-type) GaAs nanowires implanted with different Mn concentrations. The nanowires with the lowest Mn concentration (~0.0001%) exhibit a low resistance of a few kΩ at 300K and a 4% positive MR at 1.6K, which can be well described by invoking a spin-split subband model. In contrast, nanowires with the highest Mn concentration (4%) display a large resistance of several MΩ at 300K and a large negative MR of 85% at 1.6K. The large negative MR is interpreted in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn impurity spins. Sweeping the magnetic field back and forth for the 4% sample reveals a hysteresis that indicates the presence of a weak ferromagnetic phase. We propose co-doping with Zn to be a promising way to reach the goal of realizing ferromagnetic Ga1-xMnxAs nanowires for future nanospintronics. © 2014 AIP Publishing LLC.
17.	Pertsova, Anna, 1985-, et al. (author) Chapter Three : Electronic Transport as a Driver for Self-Interaction-Corrected Methods 2015 In: Advances In Atomic, Molecular, and Optical Physics. - : Academic Press. - 9780128021279 ; , s. 29-86 Book chapter (peer-reviewed)abstract 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.
18.	Pertsova, Anna, et al. (author) Computational search for Dirac and Weyl nodes in f-electron antiperovskites 2019 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 99:20 Journal article (peer-reviewed)abstract We present the result of an ab initio search for new Dirac materials among inverse perovskites. Our investigation is focused on the less studied class of lanthanide antiperovskites containing heavy f-electron elements in the cation position. Some of the studied compounds have not yet been synthesized experimentally. Our computational approach is based on density functional theory calculations which account for spin-orbit interaction and strong correlations of the f-electron atoms. We find several promising candidates among lanthanide antiperovskites which host bulk Dirac states close to the Fermi level. Specifically, our calculations reveal massive three-dimensional Dirac states in materials of the class A(3)BO, where A=Sm, Eu, Gd, Yb, and B=Sn, Pb. In materials with finite magnetic moment, such as Eu3BO (B=Sn, Pb), the degeneracy of the Dirac nodes is lifted, leading to appearance of Weyl nodes.
19.	Pertsova, Anna, et al. (author) Dirac node engineering and flat bands in doped Dirac materials 2021 In: Physical Review Research. - 2643-1564. ; 3:3 Journal article (peer-reviewed)abstract We suggest the tried approach of impurity band engineering to produce flat bands and additional nodes in Dirac materials. We show that surface impurities give rise to nearly flat impurity bands close to the Dirac point. The hybridization of the Dirac nodal state induces the splitting of the surface Dirac nodes and the appearance of new nodes at high-symmetry points of the Brillouin zone. The results are robust and not model dependent: our tight-binding calculations are supported by a low-energy effective model of a topological insulator surface state hybridized with an impurity band. Finally, we address the effects of electron-electron interactions between localized electrons on the impurity site. We confirm that the correlation effects, while producing band hybridization and the Kondo effect, keep the hybridized band flat. Our findings open up prospects for impurity band engineering of nodal structures and flat-band correlated phases in doped Dirac materials.
20.	Pertsova, Anna, et al. (author) Dynamically Induced Excitonic Instability in Pumped Dirac Materials 2020 In: Annalen der Physik. - : Wiley. - 0003-3804 .- 1521-3889. ; 532:2 Research review (peer-reviewed)abstract Driven and non-equilibrium quantum states of matter have attracted growing interest in both theoretical and experimental studies in condensed matter physics. Recent progress in realizing transient collective states in driven or pumped Dirac materials (DMs) is reviewed herein. In particular, the focus is on optically pumped DMs which are a promising platform for transient excitonic instabilities. Optical pumping combined with the linear (Dirac) dispersion of the electronic spectrum offers a knob for tuning the effective interaction between the photoexcited electrons and holes, and thus provides a way of reducing the critical coupling for excitonic instability. As a result, a transient excitonic condensate could be achieved in a pumped DM while it is not feasible in equilibrium. A unifying theoretical framework is provided for describing transient collective states in 2D and 3D DMs. The experimental signatures are described and numerical estimates of the size of the dynamically induced excitonic gaps and the values of the critical temperatures for several specific systems, are summarized. In addition, general guidelines for identifying promising material candidates are discussed. Finally, comments are provided regarding recent experimental efforts in realizing transient excitonic condensate in pumped DMs, and outstanding issues and possible future directions are outlined.
21.	Pertsova, Anna, et al. (author) Electrical control of spin dynamics in finite one-dimensional systems 2011 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84, s. 155436- Journal article (peer-reviewed)abstract We investigate the possibility of the electrical control of spin transfer in monoatomic chains incorporating spin impurities. Our theoretical framework is the mixed quantum-classical (Ehrenfest) description of the spin dynamics, in the spirit of thes-d model, where the itinerant electrons are described by a tight-binding model while localized spins are treated classically. Our main focus is on the dynamical exchange interaction between two well-separated spins. This can be quantified by the transfer of excitations in the form of transverse spin oscillations. We systematically study the effect of an electrostatic gate bias Vg on the interconnecting channel and we map out the long-range dynamical spin transfer as a function of Vg. We identify regions of Vg giving rise to significant amplification of the spin transmission at low frequencies and relate this to the electronic structure of the channel.
22.	Pertsova, Anna, et al. (author) Excitonic instability in optically pumped three-dimensional Dirac materials 2018 In: Physical Review B. - 2469-9950 .- 2469-9969. ; 97:7 Journal article (peer-reviewed)abstract Recently it was suggested that transient excitonic instability can be realized in optically pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by nonequilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a rich phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission, and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.
23.	Pertsova, Anna, et al. (author) Probing the wavefunction of the surface states in Bi2Se3 topological insulator : a realistic tight-binding approach 2014 In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 16, s. Article ID: 063022- Journal article (peer-reviewed)abstract We report on microscopic tight-binding modeling of surfacestates in Bi$_2$Se$_3$ three-dimensional topological insulator, based on a\textit{sp}$^3$ Slater-Koster Hamiltonian, with parameters calculated fromdensity functional theory. The effect of spin-orbit interaction on theelectronic structure of the bulk and of a slab with finite thickness isinvestigated. In particular, a phenomenological criterion of band inversion isformulated for both bulk and slab, based on the calculated atomic- andorbital-projections of the wavefunctions, associated with valence and conductionband extrema at the center of the Brillouin zone. We carry out athorough analysis of the calculated bandstructures of slabs with varyingthickness, where surface states are identified using a quantitative criterionaccording to their spatial distribution. The thickness-dependent energy gap,attributed to inter-surface interaction, and the emergence of gapless surfacestates for slabs above a critical thickness are investigated. We map out thetransition to the infinite-thickness limit by calculating explicitly themodifications in the spatial distribution and spin-character of the surfacestates wavefunction with increasing the slab thickness. Our numerical analysisshows that the system must be approximately forty quintuple-layers thick toexhibit completely decoupled surface states, localized on the oppositesurfaces. These results have implications on the effect of external perturbationson the surface states near the Dirac point.
24.	Pertsova, Anna, et al. (author) Quantum Hall edge states in topological insulator nanoribbons 2016 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 94:12 Journal article (peer-reviewed)abstract 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.
25.	Pertsova, Anna, 1985-, et al. (author) Theoretical studies of surface states in Bi2Se3 : effects of finite thickness, finite-cluster boundaries, and surface doping 2013 Conference paper (peer-reviewed)abstract 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).
26.	Pertsova, Anna, 1985-, et al. (author) Theoretical studies of surface states in three-dimensional topological-insulator thin films in a strong magnetic field 2014 In: Bulletin of the American Physical Society. - Denver, Colorado : American Physical Society. Conference paper (peer-reviewed)abstract The peculiar structure of the Landau levels (LLs) in topological insulators (TIs), in particular the existence of a field-independent (zeroth) LL, is a characteristic signature of the Dirac surface states. However, recently it has been shown that the hybridization between top and bottom surfaces in a 3D TI thin film may lead to a splitting of the zeroth LL and even to its absence in the ultra-thin film limit. We report on microscopic tight-binding modelling of Bi2Se3 thin films [1] in the presence of a strong magnetic field. We find that the zeroth LL is absent for thicknesses below 4QLs, in agreement with experiments. Calculations of the LL spectrum of a 5QL-thick slab reveal a strong asymmetry with respect to the Dirac point and a clear signature of the first LL, in good agreement with Dirac-Hamiltonian model calculations. The latter feature persists in a wide range of magnetic fields and involves an extended window of energies, including bulk states away from the Dirac point. We use our results to predict an interplay between the external magnetic field and gate-voltage dependence of the anomalous Hall effect that is characteristic of topological magnetic states.\\[4pt] [1] A.Pertsova and C.M.Canali, arXiv:1311.0691.
27.	Pertsova, Anna, et al. (author) Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study 2015 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91 Journal article (peer-reviewed)abstract 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.
28.	Pertsova, Anna, et al. (author) Time-dependent electron transport through a strongly correlated quantum dot : multiple-probe open-boundary conditions approach 2013 In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:10, s. 105501- Journal article (peer-reviewed)abstract We present a time-dependent study of electron transport through a strongly correlated quantum dot, which combines adiabatic lattice density functional theory in the Bethe ansatz local-density approximation (BALDA) to the Hubbard model, with the multiple-probe battery method for open-boundary simulations in the time domain. In agreement with the recently proposed dynamical picture of Coulomb blockade, a characteristic driven regime, defined by regular current oscillations, is demonstrated for a certain range of bias voltages. We further investigate the effects of systematically improving the approximation for the electron–electron interaction at the dot site (going from non-interacting, through Hartree-only to adiabatic BALDA) on the transmission spectrum and the I–V characteristics. In particular, a negative differential conductance is obtained at large bias voltages and large Coulomb interaction strengths. This is attributed to the combined effect of the electron–electron interaction at the dot and the finite bandwidth of the electrodes.
29.	Popescu, Adrian, et al. (author) Optical Response of MoTe2 and WTe2 Weyl Semimetals : Distinguishing between Bulk and Surface Contributions 2020 In: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 3:3 Journal article (peer-reviewed)abstract A first-principles investigation of the optical response of the Weyl Semimetals MoTe2 and WTe2 is presented. The approach, based on combining two formulations, allows to both separate the intraband and interband parts of the optical conductivity and to distinguish between the bulk and surface contributions to the optical response. It is found that the response is truly anisotropic, with peaks that can be associated with interband transitions involving either bulk or surface states. The role of the relaxation time, and the relation of the calculated results with available experimental measurements, are also discussed. Furthermore, the approach reported is transferable to any system, topologically trivial or non-trivial, thus addressing the long-standing need for comprehensive characterization of the optical response.
30.	Pournaghavi, Nezhat, et al. (author) Nonlocal sidewall response and deviation from exact quantization of the topological magnetoelectric effect in axion-insulator thin films 2021 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:20 Journal article (peer-reviewed)abstract Topological insulator (TI) thin films with surface magnetism are expected to exhibit a quantized anomalous Hall effect (QAHE) when the magnetizations on the top and bottom surfaces are parallel, and a quantized topological magnetoelectric effect (QTME) when the magnetizations have opposing orientations (axion-insulator phase) and the films are sufficiently thick. We present a unified picture of both effects that associates deviations from exact quantization of the QTME caused by finite thickness with nonlocality in the sidewall current response function. Using realistic tight-binding model calculations, we show that in Bi2Se3 TI thin films, deviations from quantization in the axion-insulator phase are reduced in size when the exchange coupling of tight-binding model basis states to the local magnetization near the surface is strengthened. Stronger exchange coupling also reduces the effect of potential disorder, which is unimportant for the QAHE but detrimental for the QTME, which requires that the Fermi energy lie inside the gap at all positions.
31.	Pournaghavi, Nezhat, et al. (author) Quantum Transport by Spin‐Polarized Edge States in Graphene Nanoribbons in the Quantum Spin Hall and Quantum Anomalous Hall Regimes 2018 In: Physica Status Solidi. Rapid Research Letters. - : Wiley-Blackwell. - 1862-6254 .- 1862-6270. ; 12:11, Special Issue Journal article (peer-reviewed)abstract Using the non-equilibrium Green’s function method and the Keldysh formalism, we study the effects of spin–orbit interactions and time-reversal symmetry breaking exchange fields on non-equilibrium quantum transport in graphene armchair nanoribbons. We identify signatures of the quantum spin Hall (QSH) and the quantum anomalous Hall (QAH) phases in nonequilibrium edge transport by calculating the spin-resolved real space charge density and local currents at the nanoribbon edges. We find that the QSH phase, which is realized in a system with intrinsic spin–orbit coupling, is characterized by chiral counter-propagating local spin currents summing up to a net charge flow with opposite spin polarization at the edges. In the QAH phase, emerging in the presence of Rashba spin–orbit coupling and a ferromagnetic exchange field, two chiral edge channels with opposite spins propagate in the same direction at each edge, generating an unpolarized charge current and a quantized Hall conductance . Increasing the intrinsic spin–orbit coupling causes a transition from the QAH to the QSH phase, evinced by characteristic changes in the non-equilibrium edge transport. In contrast, an antiferromagnetic exchange field can coexist with a QSH phase, but can never induce a QAH phase due to a symmetry that combines time-reversal and sublattice translational symmetry.
32.	Ryzhanova, N., et al. (author) Quasi-two-dimensional extraordinary Hall effect 2009 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 80:2, s. 024410- Journal article (peer-reviewed)abstract Quasi-two-dimensional transport is investigated in a system consisting of a very thin 1 nm ferromagneticlayer sandwiched between two insulating layers. Using the mechanism of skew scattering to describe theextraordinary Hall effect EHE and calculating the conductivity tensor, we compare the quasi-two-dimensionalHall resistance with the Hall resistance of a massive sample. In this study, a mechanism of EHE geometricmechanism of EHE due to nonideal interfaces and volume defects is also proposed.
33.	Sumida, K., et al. (author) Prolonged duration of nonequilibrated Dirac fermions in neutral topological insulators 2017 In: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 7 Journal article (peer-reviewed)abstract Topological insulators (TIs) possess spin-polarized Dirac fermions on their surface but their unique properties are often masked by residual carriers in the bulk. Recently, (Sb1-xBix)(2)Te-3 was introduced as a non-metallic TI whose carrier type can be tuned from n to p across the charge neutrality point. By using time-and angle-resolved photoemission spectroscopy, we investigate the ultrafast carrier dynamics in the series of (Sb1-xBix)(2)Te-3. The Dirac electronic recovery of similar to 10 ps at most in the bulk-metallic regime elongated to >400 ps when the charge neutrality point was approached. The prolonged nonequilibration is attributed to the closeness of the Fermi level to the Dirac point and to the high insulation of the bulk. We also discuss the feasibility of observing excitonic instability of (Sb1-xBix)(2)Te-3.
34.	Triola, Christopher, et al. (author) Excitonic gap formation in pumped Dirac materials 2017 In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 95:20 Journal article (peer-reviewed)abstract Recent pump-probe experiments demonstrate the possibility that Dirac materials may be driven into transient excited states describable by two chemical potentials, one for the electrons and one for the holes. Given the Dirac nature of the spectrum, such an inverted population allows the optical tunability of the density of states of the electrons and holes, effectively offering control of the strength of the Coulomb interaction. Here we discuss the feasibility of realizing transient excitonic instabilities in optically pumped Dirac materials. We demonstrate, theoretically, the reduction of the critical coupling leading to the formation of a transient condensate of electron-hole pairs and identify signatures of this state. Furthermore, we provide guidelines for experiments by both identifying the regimes in which such exotic many-body states are more likely to be observed and estimating the magnitude of the excitonic gap for a few important examples of existing Dirac materials. We find a set of material parameters for which our theory predicts large gaps and high critical temperatures and which could be realized in future Dirac materials. We also comment on transient excitonic instabilities in three-dimensional Dirac and Weyl semimetals. This study provides an example of a transient collective instability in driven Dirac materials.
35.	Yilmaz, T., et al. (author) Gap-like feature observed in the non-magnetic topological insulators 2020 In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 32:14 Journal article (peer-reviewed)abstract Non-magnetic gap at the Dirac point of topological insulators remains an open question in the field. Here, we present angle-resolved photoemission spectroscopy experiments performed on Cr-doped Bi2Se3 and showed that the Dirac point is progressively buried by the bulk bands and a low spectral weight region in the vicinity of the Dirac point appears. These two mechanisms lead to spectral weight suppression region being mistakenly identified as an energy gap in earlier studies. We further calculated the band structure and found that the original Dirac point splits into two nodes due to the impurity resonant states and the energy separation between the nodes is the low density of state region which appears to be like an energy gap in potoemission experiments. We supported our arguments by presenting photoemission experiments carried out with on- and off- resonant photon energies. Our observation resolves the widely debated questions of apparent energy gap opening at the Dirac point without long range ferromagnetic order in topological insulators.

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