arXiv subjects

O. E. Tereshchenko

Publications and source records attributed to O. E. Tereshchenko.

34 records · Page 2Linked to original sources

arXiv2019

Band gap opening at the Dirac point in Co/BiSbTeSe2(0001) system

Sub-angstrom Co coverage, being deposited on BiSbTeSe2(0001) surface at 200-330 C, opens a band gap at the Dirac point, with the shift of the Dirac point position caused by RT adsorbate pre-deposition. Temperature dependent measurements in 15-150 K range have shown no band gap width change. This fact indicates the nonmagnetic nature of the gap which may be attributed to the chemical hybridization of surface states upon the introduction of Co adatoms, which decrease crystallographic symmetry and eliminate topological protection of the surface states.

cond-mat.mtrl-sci↗

arXiv2019

Variety of magnetic topological phases in the (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ family

Quantum states of matter combining non-trivial topology and magnetism attract a lot of attention nowadays; the special focus is on magnetic topological insulators (MTIs) featuring quantum anomalous Hall and axion insulator phases. Feasibility of many novel phenomena that \emph{intrinsic} magnetic TIs may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here, using angle- and spin-resolved photoemission spectroscopy along with \emph{ab initio} calculations we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ with $m=0, ..., 6$. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the $m$ value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m=0) to MnBi4Te7 (m=1), changes to ferromagnetic (FM) one in MnBi6Te10 (m=2) and disappears with further increase in m. In this way, the AFM and FM TI states are respectively realized in the $m=0,1$ and $m=2$ cases, while for $m \ge 3$ a novel and hitherto-unknown topologically-nontrivial phase arises, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the \MBT\, building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi$_2$Te$_4$)(Bi$_2$Te$_3$)$_m$ allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

cond-mat.mtrl-sci↗

arXiv2019

Electronic correlation determining correlated plasmons in Sb-doped Bi$_2$Se$_3$

Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magneto resistance and high temperature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy and Hall measurements all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas those in the bulk states increase, and that the newly-discovered correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining new correlated plasmons and opens a new path in engineering plasmonic-based topologically-insulating devices.

cond-mat.mtrl-sci↗

arXiv2018

Crystalline structure and XMCD studies of Co40Fe40B20 grown on Bi2Te3, BiTeI and Bi2Se3

Epitaxial films of Co40Fe40B20 (further - CoFeB) were grown on Bi2Te3(001) and Bi2Se3(001) substrates by laser molecular beam epitaxy (LMBE) technique at 200-400C. Bcc-type crystalline structure of CoFeB with (111) plane parallel to (001) plane of Bi2Te3 was observed, in contrast to polycrystalline CoFeB film formed on Bi2Se3(001) at RT using high-temperature seeding layer. Therefore, structurally ordered ferromagnetic thin films were obtained on the topological insulator surface for the first time. Using high energy electron diffraction (RHEED) 3D reciprocal space mapping, epitaxial relations of main crystallographic axes for the CoFeB/ Bi2Te3 heterostructure were revealed. MOKE and AFM measurements showed the isotropic azimuthal in-plane behavior of magnetization vector in CoFeB/ Bi2Te3, in contrast to 2nd order magnetic anisotropy seen in CoFeB/Bi2Se3. XPS measurements showed more stable behavior of CoFeB grown on Bi2Te3 to the oxidation, in compare to CoFeB grown on Bi2Se3. XAS and XMCD measurements of both concerned nanostructures allowed calculation of spin and orbital magnetic moments for Co and Fe. Additionally, crystalline structure and XMCD response of the CoFeB/BiTeI and Co55Fe45/BiTeI systems were studied, epitaxial relations of main crystallographic axes were found, and spin and orbital magnetic moments were calculated.

cond-mat.mtrl-sci↗

arXiv2017

Conductance oscillations and zero-bias anomaly in a single superconducting junction to a three-dimensional $Bi_2Te_3$ topological insulator

We experimentally investigate Andreev transport through a single junction between an s-wave indium superconductor and a thick film of a three-dimensional $Bi_2Te_3$ topological insulator. We study $Bi_2Te_3$ samples with different bulk and surface characteristics, where the presence of a topological surface state is confirmed by direct ARPES measurements. All the junctions demonstrate Andreev transport within the superconducting gap. For junctions with transparent $In-Bi_2Te_3$ interfaces we find a number of nearly periodic conductance oscillations, which are accompanied by zero-bias conductance anomaly. Both effects disappear above the superconducting transition or for resistive junctions. We propose a consistent interpretation of both effects as originating from proximity-induced superconducting correlations within the $Bi_2Te_3$ topological surface state.

cond-mat.mes-hall↗

arXiv2017

Synchrotron radiation induced magnetization in magnetically-doped and pristine topological insulators

Quantum mechanics postulates that any measurement influences the state of the investigated system. Here, by means of angle-, spin-, and time-resolved photoemission experiments and ab initio calculations we demonstrate how non-equal depopulation of the Dirac cone (DC) states with opposite momenta in V-doped and pristine topological insulators (TIs) created by a photoexcitation by linearly polarized synchrotron radiation (SR) is followed by the hole-generated uncompensated spin accumulation and the SR-induced magnetization via the spin-torque effect. We show that the photoexcitation of the DC is asymmetric, that it varies with the photon energy, and that it practically does not change during the relaxation. We find a relation between the photoexcitation asymmetry, the generated spin accumulation and the induced spin polarization of the DC and V 3d states. Experimentally the SR-generated in-plane and out-of-plane magnetization is confirmed by the $k_{\parallel}$-shift of the DC position and by the splitting of the states at the Dirac point even above the Curie temperature. Theoretical predictions and estimations of the measurable physical quantities substantiate the experimental results.

cond-mat.mtrl-sci↗

arXiv2016

Ultrafast Energy- and Momentum-resolved Surface Dirac Photocurrents in the Topological Insulator Sb$_2$Te$_3$

We present energy-momentum mapping of surface Dirac photocurrent in the topological insulator Sb$_2$Te$_3$ by means of time- and angle-resolved two-photon photoemission spectroscopy combined with polarization-variable mid-infrared pulse laser. It is demonstrated that the direct optical transition from the occupied to the unoccupied part of the surface Dirac-cone permits the linear and circular photogalvanic effect which thereby enables us to coherently control the surface electric-current by laser polarization. Moreover, the surface current mapping directly visualizes ultrafast current dynamics in the Dirac cone in the time domain. We unravel the ultrafast intraband relaxation dynamics of the inelastic scattering and momentum scattering separately. Our observations pave the pathway for coherent optical control over surface Dirac electrons in topological insulators.

cond-mat.mtrl-sci↗

arXiv2016

Mesoscopic electron focusing in topological insulators

The particle wave duality sets a fundamental correspondence between optics and quantum mechanics. Within this framework, the propagation of quasiparticles can give rise to superposition phenomena which, like for electromagnetic waves, can be described by the Huygens principle. However, the utilization of this principle by means of propagation and manipulation of quantum information is limited by the required coherence in time and space. Here we show that in topological insulators, which in their pristine form are characterized by opposite propagation directions for the two quasiparticles spin channels, mesoscopic focusing of coherent charge density oscillations can be obtained at large nested segments of constant energy contours by magnetic surface doping. Our findings provide evidence of strongly anisotropic Dirac fermion-mediated interactions. Even more remarkably, the validity of our findings goes beyond topological insulators but applies for systems with spin orbit lifted degeneracy in general. It demonstrates how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in this fascinating class of materials.

cond-mat.mes-hall↗

arXiv2015

Experimental realization of a topological p-n junction by intrinsic defect-grading

A junction between an n- and p-type semiconductor results in the creation of a depletion region whose properties are at the basis of nowadays electronics. If realized using topological insulators as constituent materials, p-n junctions are expected to manifest several unconventional effects with great potential for applications. Experimentally, all these fascinating properties remained unexplored so far, mainly because prototypical topological PNJs, which can be easily realized and investigated, were not readily available. Here, we report on the creation of topological PNJs which can be as narrow as few tenths of nm showing a built-in potential of 110meV. These junctions are intrinsically obtained by a thermodynamic control of the defects distribution across the crystal. Our results make Bi2Te3 a robust and reliable platform to explore the physics of topological p-n junction.

cond-mat.mes-hall↗

arXiv2015

Scattering properties of the three-dimensional topological insulator Sb2Te3: Coexistence of topologically trivial and non-trivial surface states with opposite spin-momentum helicity

The binary chalcogenides Bi2Te3 and Bi2Se3 are the most widely studied topological insulators. Although the quantum anomalous Hall effect has recently been observed in magnetically doped Sb2Te3 this compound has been studied to a much lesser extend. Here, by using energy resolved quasiparticle interference mapping, we investigate the scattering events of pristine Sb2Te3 surfaces. We find that, in addition to the Dirac fermions, another strongly spin polarized surface resonance emerges at higher energies in the unoccupied electronic states. Although the two surface states are of different origin, i.e. topologically protected and trivial, respectively, both show strongly directional scattering properties and absence of backscattering. Comparison with ab-initio calculations demonstrates that this is a direct consequence of their spin momentum locked spin texture which is found to exhibit an opposite rotational sense for the trivial state and the Dirac state.

cond-mat.mes-hall↗

arXiv2015

Spin current and magnetization induced by circularly polarized synchrotron radiation in magnetically-doped topological insulator Bi$_{1.37}$V$_{0.03}$Sb$_{0.6}$Te$_2$Se

We propose a hole-induced mechanism of spin-polarized current generation by circularly polarized synchrotron radiation and corresponding induced magnetization in magnetically-doped topological insulators Bi$_{1.37}$V$_{0.03}$Sb$_{0.6}$Te$_2$Se. Considered spin-polarized current is generated due to the spin-dependent depopulation of the Dirac cone topological surface states at the Fermi level and subsequent compensation of the generated holes. We have found experimentally and theoretically a relation between the generated spin-polarized current and the shift of the electrochemical potential. The out-of-plane magnetization induced by circularly polarized synchrotron radiation and its inversion with switching the direction of circular polarization were experimentally shown and theoretically confirmed.

cond-mat.mtrl-sci↗

arXiv2013

Unoccupied topological surface state in Bi$_{2}$Te$_{2}$Se

Bias voltage dependent scattering of the topological surface state is studied by scanning tunneling microscopy/spectroscopy for a clean surface of the topological insulator Bi$_2$Te$_2$Se. A strong warping of constant energy contours in the unoccupied part of the spectrum is found to lead to a spin-selective scattering. The topological surface state persists to higher energies in the unoccupied range far beyond the Dirac point, where it coexists with the bulk conduction band. This finding sheds light on the spin and charge dynamics over the wide energy range and opens a way to designing opto-spintronic devices.

cond-mat.mes-hall↗

arXiv2012

Unoccupied Topological States on Bismuth Chalcogenides

The unoccupied part of the band structure of topological insulators Bi$_2$Te$_{x}$Se$_{3-x}$ ($x=0,2,3$) is studied by angle-resolved two-photon photoemission and density functional theory. For all surfaces linearly-dispersing surface states are found at the center of the surface Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical analysis shows that this feature appears in a spin-orbit-interaction induced and inverted local energy gap. This inversion is insensitive to variation of electronic and structural parameters in Bi$_2$Se$_3$ and Bi$_2$Te$_2$Se. In Bi$_2$Te$_3$ small structural variations can change the character of the local energy gap depending on which an unoccupied Dirac state does or does not exist. Circular dichroism measurements confirm the expected spin texture. From these findings we assign the observed state to an unoccupied topological surface state.

cond-mat.mes-hall↗

arXiv2012

Topological Surface States with Persistent High Spin Polarization across Dirac Point in Bi$_{2}$Te$_{2}$Se and Bi$_{2}$Se$_{2}$Te

Helical spin textures with the marked spin polarizations of topological surface states have been firstly unveiled by the state-of-the-art spin- and angle-resolved photoemission spectroscopy for two promising topological insulators Bi$_2$Te$_2$Se and Bi$_2$Se$_2$Te. The highly spin-polarized natures are found to be persistent across the Dirac point in both compounds. This novel finding paves a pathway to extending their utilization of topological surface state for future spintronic applications.

cond-mat.mes-hall↗

arXiv2009

Cs-induced charge transfer on (2x4)-GaAs(001) studied by photoemission

Cesium adsorption on 2x4 GaAs (001) was studied by photoemission and low energy electron diffraction. The different Cs induced changes of the As 3d and Ga 3d core level spectra show that charge transfer is almost complete for Ga surface sites, but is negligible to surface As at a coverage smaller than 0.3 ML. The situation is opposite for a coverage larger than 0.3ML, at which transfer occurs to As but no longer to Ga. Charge transfer to As atoms leads to disordering and destabilization and induces surface conversion from the As-rich surface to the Ga-rich 4x2 one after annealing at a reduced temperature of 450 C.

cond-mat.mtrl-sci↗

arXiv2009

GaAs(111)A and B in hydrazine sulfide solutions : extreme polarity dependence of surface adsorption processes

Chemical bonds formed by hydrazine-sulfide treatment of GaAs(111) were studied by synchrotron photoemission spectroscopy. At the B surface, the top arsenic atoms are replaced by nitrogen atoms, while GaAs(111)A is covered by sulfur, also bonded to underlying gallium, despite the sulfide molar concentration being 103 times smaller than that of the hydrazine. This extreme dependence on surface polarity is explained by competitive adsorption processes of HS- and OH- anions and of hydrazine molecules, on Ga- adsorption sites, which have distinct configurations on the A and B surfaces.

cond-mat.mtrl-sci↗