Linear and nonlinear optical properties of trigonal borate crystals K7MIn2-xYbx(B5O10)3 (M = Ca, Sr, Ba; x=0…2) with isolated B5O10 units

Noncentrosymmetric borates K7MIn2−xYbx(B5O10)3 (M = Ca, Sr, Ba; x = 0…2) were synthesized by the solid state reaction and the crystals were successfully grown by the top seeded solution growth method using the K2O-B2O3-MF2 flux. According to Rietveld refinement, the crystal structure belongs to the noncentrosymmetric R32 space group. Also, the octahedrally coordinated In atoms are located at wide ranges ∼8 Å which may be promising for phosphor and laser applications. Samples with ytterbium show a characteristic emission band in the range of 950–1050 nm related to the 2F5/2 → 2F7/2 transition of Yb3+ ions that is commonly used for laser generation. IR, Raman and absorption spectra were obtained for the samples as well. The short cut edge of UV absorption, SHG intensity comparable with KDP and low concentration quenching of luminescence suggest that the K7MIn2−xYbx(B5O10)3 borates are promising self-frequency doubling materials

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

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

Synthesis and growth of rare earth borates NaSrR(BO3)2 (R = Ho− Lu, Y, Sc)

NaSrR(BO3)2(R = Ho-Lu, Y, Sc) compounds were obtained for the first time. Their structures exhibit disordered positions of Sr2+and Na+ atoms while RO6polyhedra are connected through the BO3 groups. Large distances between R atoms and high transparency in the range of 250-900 nm make them promising for phosphor applications. A pathway to obtain single crystals was shown by growing NaSrY(BO3)2and NaSrYb-(BO3)2by the top seeded solution growth method with Na2O-B2O3-NaFflux

Specific features of the electronic, spin, and atomic structures of a topological insulator Bi2Te2.4Se0.6

The specific features of the electronic and spin structures of a triple topological insulator Bi2Te2.4Se0.6, which is characterized by high-efficiency thermoelectric properties, have been studied with the use of angular- and spin-resolved photoelectron spectroscopy and compared with theoretical calculations in the framework of the density functional theory. It has been shown that the Fermi level for Bi2Te2.4Se0.6 falls outside the band gap and traverses the topological surface state (the Dirac cone). Theoretical calculations of the electronic structure of the surface have demonstrated that the character of distribution of Se atoms on the Te–Se sublattice practically does not influence the dispersion of the surface topological electronic state. The spin structure of this state is characterized by helical spin polarization. Analysis of the Bi2Te2.4Se0.6 surface by scanning tunnel microscopy has revealed atomic smoothness of the surface of a sample cleaved in an ultrahigh vacuum, with a lattice constant of ~4.23 Å. Stability of the Dirac cone of the Bi2Te2.4Se0.6 compound to deposition of a Pt monolayer on the surface is shown.This study was supported by the Ministry of Education and Science of the Russian Federation, the St. Petersburg State University (project nos. 11.38.271.2014 and 15.61.202.2015), and the Russian Foundation for Basic Research (project nos. 12-02-00226, 13-02-91327, 14-08-31110, and 13-02-12110). The research was also performed at the Resource Center “Physical Methods of Surface Investigation” at St. Petersburg State University. We are also grateful to collaborators of the Helmholtz-Zentrum (Berlin) for financial and technical support.Peer reviewe

Sample-dependent Dirac-point gap in MnBi2Te4 and its response to applied surface charge: A combined photoemission and ab initio study

Recently discovered intrinsic antiferromagnetic topological insulator MnBi2Te4 presents an exciting platform for realization of the quantum anomalous Hall effect and a number of related phenomena at elevated temperatures. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early experimental measurements reported on large DP gaps, a number of recent studies claimed to observe a gapless dispersion of the MnBi2Te4 Dirac cone. Here, using micro(μ)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi2Te4 samples, grown by two different chemists groups. Based on the careful energy distribution curves analysis, the DP gaps between 15 and 65 meV are observed, as measured below the Néel temperature at about 10–16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi2Te4 samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Furthermore, based on the density functional theory, we discuss a possible factor that might contribute to the reduction of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi2Te4 DP gap size.The authors acknowledge support by the Saint Petersburg State University Grant No. ID 73028629, Russian Science Foundation Grant No. 18-12-00062 in part of the photoemission measurements and total analysis of the results, Grant No. 18-12-00169-p in part of the electronic band structure calculations and Grant No. 20-42-08002 in part of analysis of magnetic properties and Science Development Foundation under the President of the Republic of Azerbaijan Grant No. EI F-BGM-4-RFTF1/2017-21/04/1-M-02. M.M.O. acknowledges the support by Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-I00). K.K. and O.E.T. acknowledge the support from state assignment of IGM SB RAS and ISP SB RAS.Peer reviewe

Impact of Co atoms on the electronic structure of Bi2Te3 and MnBi2Te4 topological insulators

This work is devoted to an experimental investigation of the electronic structure of the surface of topological insulators of various stoichiometry during the adsorption of Co atoms. Changes in the surface electronic structure of Bi2Te3 and MnBi2Te4 systems upon deposition of Co atoms at various temperatures have been studied using the methods of angle-resolved photoemission spectroscopy, as well as X-ray photoelectron spectroscopy. It is suggested that binding of the adsorbed Co atoms to the substrate surface modifies Dirac point position. The observed changes are associated with the possible formation of magnetic Co-containing ordered surface alloys.This work was supported by the St. Petersburg State University, grant no. 73028629, the Russian Science Foundation, grant no. 18-12-00062, the Russian Foundation for Basic Research, grant nos. 20-32-70127, 21-52-12024, and 18-29-12094, the Science Development Foundation of the President of Azerbaijan, grant no. EIF-BGM-4-RFTF-1/2017-21/04/1-M-02, and in the framework of the state assignment of Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, and the Rzhanov Institute of Semiconductors Physics, Siberian Branch of the Russian Academy of Sciences.Peer reviewe

A community effort in SARS-CoV-2 drug discovery.

peer reviewedThe COVID-19 pandemic continues to pose a substantial threat to human lives and is likely to do so for years to come. Despite the availability of vaccines, searching for efficient small-molecule drugs that are widely available, including in low- and middle-income countries, is an ongoing challenge. In this work, we report the results of an open science community effort, the "Billion molecules against Covid-19 challenge", to identify small-molecule inhibitors against SARS-CoV-2 or relevant human receptors. Participating teams used a wide variety of computational methods to screen a minimum of 1 billion virtual molecules against 6 protein targets. Overall, 31 teams participated, and they suggested a total of 639,024 molecules, which were subsequently ranked to find 'consensus compounds'. The organizing team coordinated with various contract research organizations (CROs) and collaborating institutions to synthesize and test 878 compounds for biological activity against proteases (Nsp5, Nsp3, TMPRSS2), nucleocapsid N, RdRP (only the Nsp12 domain), and (alpha) spike protein S. Overall, 27 compounds with weak inhibition/binding were experimentally identified by binding-, cleavage-, and/or viral suppression assays and are presented here. Open science approaches such as the one presented here contribute to the knowledge base of future drug discovery efforts in finding better SARS-CoV-2 treatments.R-AGR-3826 - COVID19-14715687-CovScreen (01/06/2020 - 31/01/2021) - GLAAB Enric

Temperature dependence of terahertz optical properties of LBO and perspectives of applications in down-converters

Lithium triborate LiB3O5 (LBO) crystals are widely used for frequency conversion of the near-IR lasers within main transparency windows. Their optical properties at these wavelengths are well studied. However, very little work has been published on the properties in the terahertz (THz) range. There was a lack of data on the refractive indices, the absorption coefficients spectra and their temperature dispersions. There are no reports of THz applications. Present work reveals all these topics including the prospects for use LBO crystals as down-converters of the near-IR lasers radiation. Optically finished samples of flux-grown LBO crystals were studied by THz-TDS. The refractive index dispersions were recorded and then approximated in the form of Sellmeier equations for the temperatures of 300 and 81 K. The phase-matching curves for the IR-THz and THz-THz frequency conversions were calculated. It was found that the absorption coefficients of LBO decrease significantly with cooling to cryogenic temperatures, but the overall character of optical properties changes is intricated. Experimental results are discussed in detail considering potential characteristics of THz down-converters

Optical properties of β-BBO and potential for THz applications

The anisotropy of optical properties of high quality beta barium borate crystal (β-BaB2O4, β-BBO) was studied in the main transparency window by using classic spectroscopic methods and in the range of 0.2 – 2 THz by using THz time-domain spectroscopy. β-BBO crystals were grown by the top-seeded solution technique in a highly resistive furnace with a heat field of 3-fold axis symmetry. At room temperature (RT), absorption coefficient in the maximal transparency window in grown crystals did not exceed 0.05 cm-1. Strong absorption anisotropy was observed in 3 – 5 μm and the THz range. At 1 THz absorption coefficients for e and o wave were, respectively, 7 cm-1 and 21 cm-1 at RT; 2 cm-1 and 10 cm-1 at 81 K. At the most attractive for out-of-door applications range < 0.4 THz the absorption coefficient is found to be very low: below 0.2 cm-1 at RT and 1 cm-1 at 81 K. Refractive indices dispersions measured by THz-TDS were approximated in the form of Sellmeier equations. Birefringence is found quite large for phase matched difference frequency generation (DFG) or down-conversion into the THz range (THz-DFG) under near IR pump at RT and 81 K. Type II (oe-o and eo-o), and type I (ee-e) three wave interactions can be realized at RT. THz-DFG of Nd:YAG laser and KTP OPO can be realized by type II (oe-o) three-wave interaction. For selected spectral ranges of femtosecond Ti:Sapphire laser efficient phase matched and group velocity matched optical rectification can be realized by another two types of three wave interactions. Accounting other well-known attractive physical properties of β-BBO crystal, wide application in THz technique can be forecasted