Conductance oscillations and zero-bias anomaly in a single superconducting junction to a three-dimensional Bi2Te3Bi_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

Ferromagnetic HfO2/Si/GaAs interface for spin-polarimetry applications

In this letter, we present electrical and magnetic characteristics of HfO2-based metal-oxide-semiconductor capacitors (MOSCAPs), along with the effect of pseudomorphic Si as a passivating interlayer on GaAs(001) grown by molecular beam epitaxy. Ultrathin HfO2 high-k gate dielectric films (3–15 nm) have been grown on Si/GaAs(001) structures through evaporation of a Hf/HfO2 target in NO2 gas. The lowest interface states density Dit at Au/HfO2/Si/GaAs(001) MOS-structures were obtained in the range of (6−13)×101

Giant magnetic band gap in the rashba-split surface state of vanadium-doped BiTeI: A combined photoemission and Ab initio study

One of the most promising platforms for spintronics and topological quantum computation is the two-dimensional electron gas (2DEG) with strong spin-orbit interaction and out-of-plane ferromagnetism. In proximity to an s-wave superconductor, such 2DEG may be driven into a topologically non-Trivial superconducting phase, predicted to support zero-energy Majorana fermion modes. Using angle-resolved photoemission spectroscopy and ab initio calculations, we study the 2DEG at the surface of the vanadium-doped polar semiconductor with a giant Rashba-Type splitting, BiTeI. We show that the vanadium-induced magnetization in the 2DEG breaks time-reversal symmetry, lifting Kramers degeneracy of the Rashba-split surface state at the Brillouin zone center via formation of a huge gap of about 90 meV. As a result, the constant energy contour inside the gap consists of only one circle with spin-momentum locking. These findings reveal a great potential of the magnetically-doped semiconductors with a giant Rashba-Type splitting for realization of novel states of matter.The work was partially supported by grant of Saint Petersburg State University for scientific investigations (N. 15.61.202.2015). This study was supported by the Russian Science Foundation (project N. 17-12-01047, in part of crystal growth, structural characterization and ARPES measurements (Figs 1, 2)). The funding by the University of the Basque Country (Grant Nos GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P) and Tomsk State University Academic D.I. Mendeleev Fund Program in 2015 (research grant N 8.1.05.2015) are also gratefully acknowledged. The authors also acknowledge support from the Russian-German laboratory at BESSY II, the “German-Russian Interdisciplinary Science Center”(G-RISC) program and the Impuls- und Vernetzungsfonds der Helmholtz-Gemeinschaft (Grant No. HRJRG-408).Peer Reviewe

Magnetic Dirac semimetal state of (Mn,Ge)Bi2_2Te4_4

For quantum electronics, the possibility to finely tune the properties of magnetic topological insulators (TIs) is a key issue. We studied solid solutions between two isostructural Z$_2$ TIs, magnetic MnBi$_2$Te$_4$ and nonmagnetic GeBi$_2$Te$_4$, with Z$_2$ invariants of 1;000 and 1;001, respectively. For high-quality, large mixed crystals of Ge$_x$Mn$_{1-x}$Bi$_2$Te$_4$, we observed linear x-dependent magnetic properties, composition-independent pairwise exchange interactions along with an easy magnetization axis. The bulk band gap gradually decreases to zero for $x$ from 0 to 0.4, before reopening for $x>0.6$, evidencing topological phase transitions (TPTs) between topologically nontrivial phases and the semimetal state. The TPTs are driven purely by the variation of orbital contributions. By tracing the x-dependent $6p$ contribution to the states near the fundamental gap, the effective spin-orbit coupling variation is extracted. As $x$ varies, the maximum of this contribution switches from the valence to the conduction band, thereby driving two TPTs. The gapless state observed at $x=0.42$ closely resembles a Dirac semimetal above the Neel temperature and shows a magnetic gap below, which is clearly visible in raw photoemission data. The observed behavior of the Ge$_x$Mn$_{1-x}$Bi$_2$Te$_4$ system thereby demonstrates an ability to precisely control topological and magnetic properties of TIs

Semiconducting Electronic Structure of the Ferromagnetic Spinel HgCr2Se4\mathbf{Hg}\mathbf{Cr}_2\mathbf{Se}_4 Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy

We study the electronic structure of the ferromagnetic spinel $\mathrm{Hg}\mathrm{Cr}_2\mathrm{Se}_4$ by soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and first-principles calculations. While a theoretical study has predicted that this material is a magnetic Weyl semimetal, SX-ARPES measurements give direct evidence for a semiconducting state in the ferromagnetic phase. Band calculations based on the density functional theory with hybrid functionals reproduce the experimentally determined band gap value, and the calculated band dispersion matches well with ARPES experiments. We conclude that the theoretical prediction of a Weyl semimetal state in $\mathrm{Hg}\mathrm{Cr}_2\mathrm{Se}_4$ underestimates the band gap, and this material is a ferromagnetic semiconductor.Comment: 6+13 pages, 4+13 figure

Structural, Optical and Electronic Properties of the Wide Bandgap Topological Insulator Bi1.1Sb0.9Te2S

Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm−1) and the frequencies of two infrared phonons (177.7 cm−1 and 77.4 cm−1). © 2021 Elsevier B.V.The reported study was funded by RFBR, project number 19-29-12061 . The part of optical research was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (theme "Spin" No AAAA-A18-118020290104-2 and No AAAA-A19-119081990020-8 and theme "Electron" No AAAAA18-118020190098-5 ). The study was also supported by the Russian Science Foundation (Project No. 17-12-01047 ) in the part of the crystal growth and state assignment of ISP SB RAS ( 0306–2019-0007 ) and IGM SB RAS. The Raman measurements were partially supported by the grant of the Russian Foundation for Basic Research (Project No. 19-52-18008 ). This work is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through project-ID 258499086 – SFB 1170 (A01), the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter–ct.qmat Project-ID 390858490 – EXC 2147