Observation of a Highly Spin Polarized Topological Surface State in GeBi2_{2}Te4_{4}

Spin polarization of a topological surface state for GeBi$_2$Te$_4$, the newly discovered three-dimensional topological insulator, has been studied by means of the state of the art spin- and angle-resolved photoemission spectroscopy. It has been revealed that the disorder in the crystal has a minor effect on the surface state spin polarization and it exceeds 75% near the Dirac point in the bulk energy gap region ($\sim$180 meV). This new finding for GeBi$_{2}$Te$_{4}$ promises not only to realize a highly spin polarized surface isolated transport but to add new functionality to its thermoelectric and thermomagnetic properties.Comment: 5 pages, 4 figure

Interplay of surface and Dirac plasmons in topological insulators: the case of Bi2Se3

We have investigated plasmonic excitations at the surface of Bi2Se3(0001) via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer q∥, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with q∥. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta (q∥∼0.04  Å−1), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of Bi2Se3. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators

Geometric and electronic structure of the Cs-doped Bi2Se3(0001) surface

Using surface x-ray diffraction and scanning tunneling microscopy in combination with first-principles calculations, we have studied the geometric and electronic structure of Cs-deposited Bi2Se3(0001) surface kept at room temperature. Two samples were investigated: a single Bi2Se3 crystal, whose surface was Ar sputtered and then annealed at ∼500∘C for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial Bi2Se3 film that was not subject to sputtering and was annealed only at ∼350∘C. In the first case, a considerable fraction of Cs atoms occupy top layer Se atoms sites both on the terraces and along the upper step edges where they form one-dimensional-like structures parallel to the step. In the second case, Cs atoms occupy the fcc hollow site positions. First-principles calculations reveal that Cs atoms prefer to occupy Se positions on the Bi2Se3(0001) surface only if vacancies are present, which might be created during the crystal growth or during the surface preparation process. Otherwise, Cs atoms prefer to be located in fcc hollow sites in agreement with the experimental finding for the MBE-grown sample

Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBi2_2Te4_4

Modification of the gap at the Dirac point (DP) in antiferromagnetic (AFM) axion topological insulator MnBi$_2$Te$_4$ and its electronic and spin structure has been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation with variation of temperature (9-35~K), light polarization and photon energy. We have distinguished both a large (62-67~meV) and a reduced (15-18~meV) gap at the DP in the ARPES dispersions, which remains open above the N\'eel temperature ($T_\mathrm{N}=24.5$~K). We propose that the gap above $T_\mathrm{N}$ remains open due to short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for large-gap sample and significantly reduced effective magnetic moment for the reduced-gap sample. These effects can be associated with a shift of the topological DC state towards the second Mn layer due to structural defects and mechanical disturbance, where it is influenced by a compensated effect of opposite magnetic moments

Nature of the Dirac gap modulation and surface magnetic interaction in axion antiferromagnetic topological insulator MnBi2Te4A

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced (< 20 meV) gaps at the DP in the ARPES dispersions, which remain open above the Neel temperature (T-N = 24.5 K). We propose that the gap above T-N remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the "large gap" sample and apparently significantly reduced effective magnetic moment for the "reduced gap" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.The authors acknowledge support by the Saint Petersburg State University (Grant No. 51126254), Russian Science Foundation (Grant No. 18-12-00062 in part of the photoemission measurements and Grant No. 18-12-00169 in part of the electronic band structure calculations) and by Russian Foundation of Basic Researches (Grants Nos. 18-52-06009 and 20-32-70179) 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). A. Kimura was financially supported by KAKENHI (Grants No. 17H06138, No. 17H06152, and No. 18H03683). S.V.E. and E.V.C. acknowledge support by the Fundamental Research Program of the State Academies of Sciences (line of research III.23.2.9). The authors kindly acknowledge the HiSOR staff and A. Harasawa at ISSP for technical support and help with the experiment. The ARPES measurements at HiSOR were performed with the approval of the Proposal Assessing Committee (Proposal Numbers: 18BG027 and 19AG048). XAS and XMCD measurements were performed at BL23SU of SPring-8 (Proposal Nos. 2018A3842 and 2018B3842) under the Shared Use Program of JAEA Facilities (Proposal Nos. 2018A-E25 and 2018B-E24) with the approval of Nanotechnology Platform project supported by MEXT, Japan (Proposal Nos. A-18-AE-0020 and A-18-AE-0042). M. M. Otrokov acknowledges the support by Spanish Ministerio de Ciencia e Innovacion (Grant no. PID2019-103910GB-I00). K. Yaji was financially supported by KAKENHI (Grants No. 18K03484)

The charge transport mechanism in a new magnetic topological insulator MnBi0.5Sb1.5Te4

A new layered magnetic topological insulator with the composition MnBi0.5Sb1.5Te4 is obtained. The electrical conductivity in the plane of the layers and in the direction normal to the layers is studied in the range of temperatures of 1.4–300 K. It is found that a “metallic” character of the temperature dependence of the resistivity ρ(T) is observed in the range of temperatures of 50–300 K in both directions. Below T = 50 K, the value of ρ increases and demonstrates an uncommon temperature dependence with a characteristic feature in the region of the critical temperature Tc = 23 K. The increase in the resistance in the temperature range of 50–23 K is determined by the spin fluctuations and magnetic phase transition. Below Tc and down to 1.4 K, ρ(T) demonstrates a behavior characteristic for the weak localization effect, which is confirmed by the analysis of the data obtained when studying magnetoresistance.This work was financially supported by the Science Development Foundation under the President of the Republic of Azerbaijan (grants nos. EİF-BGM-4-RFTF-1/2017-21/04/1-M-02, EİF/MQM/Elm-Tehsil-1-2016-1(26)-71/16/1), Russian Foundation for Basic Research (grant no. 18-52-06009), St. Petersburg State University (grant no. 73028629) as well as the Spanish Ministerio de Ciencia e Innovación Foundation (grant no. PID2019-103910GB-I00).Peer reviewe