Topological materials II

Resume : Three-dimensional topological insulators, Bi2Te3, Bi2Se3, and Bi2Te2Se, were grown by vertical Bridgman method. The influence of the growth from non-stoichiometric melt and doping with Ca and Mn acceptors on carrier concentration and crystal morphology will be presented. The doping with acceptors reduces successfully initially high carrier concentration in Bi2Se3, originating from native crystal lattice defects. Strong doping, however, leads to phase separation, and technological compromise needs to be maintained in order to obtain desired crystal quality. Microwave spectroscopy study was performed using conventional Electron Paramagnetic Resonance spectrometer on samples with the Fermi level engineered. Paramagnetic resonance signals (conduction electron spin resonance, shallow acceptor resonance, Mn high-spin resonance), together with signals related to magnetoconductivity (cyclotron resonance, weak localization and anti-localization, Shubnikov-de Haas oscillations) allowing investigations of electron transport properties were detected. The distinction of the signals originating from bulk and surface states will be presented and discussed. Funded by NCN, grant No. 2011/03/B/ST3/03362.

Resume : HgTe is a II-VI zinc blende crystal with an inverted band ordering of the Г6 and Г8 bands. The band inversion enables the observation of topological insulator (TI) physics in this material. Both 2- and 3-dimensional TIs can be realized by growing quantum well structures or strained bulk layers, respectively. Due to the well-established MBE growth of such layers bulk carrier contributions are negligible and the carrier mobilities exceed 100000 cm²/Vs in HgTe TI structures. Here we report on the transport properties of the topological edge and surface states in HgTe quantum wells and strained bulk HgTe layers. 2-dimensional TIs are characterized by the formation of two counter-propagating oppositely spin polarized edge states, the so called quantum spin Hall effect. The transport signatures of this edge states will be discussed, especially regarding their spin polarization. 3-dimensional TIs exhibit 2 dimensional Dirac surface states. The topological surface states in strained bulk HgTe are investigated utilizing the quantum Hall effect.

Resume : We present electron transport data for Hall microbridges and submicrometer constrictions of HgCdTe/HgTe/HgCdTe quantum wells (QWs) of the width d=8 nm which is 2D topological insulator. Both types of samples are patterned from In-modulation doped QWs, grown by MBE on semi-insulating GaAs [013] substrates [1]. In the microbridges where the edge channel length is of the order of 100 microns, nonlocal resistances data prove that in the depletion regime the current is carried by the edge states only [2]. However, high and nonquantized values of channel resistances show that the topological protection length is much shorter than 100 microns. For the constrictions, we have observed large quasiperiodic reproducible conductance fluctuations as a function of the gate voltage. Surprisingly, the resistance measured as a function of the magnetic field does not show such fluctuations. We explain these findings in terms of the charge puddles in QW, to which the electrons from the edge channels are tunnel-coupled and give rise to aperiodic Coulomb blockade oscillations. The puddles are formed due to the potential fluctuations in QW. Electrons trapped by the puddles lose their spin memory and then may be backscattered which explains the high channel resistances. The research was partially supported by Regional Development Program (Poland), grant WND-RPPK. 01.03.00-18-053/12. [1] S. Dvoretsky, et al., J. Electron. Mat., 39, 918 (2010); [2] G. Grabecki et al., Phys. Rev. B88, 165309 (2013).

Resume : Band structure engineering in 3D topological insulators has been in focus of intense research activities since several years. Various ternary compounds have been characterized in search for intrinsic semiconducting characteristics. Furthermore, electric biasing has been used to alter the population of the topological surface state, and therefore control the position of the Fermi level within the fundamental gap. We will present a thorough experimental and theoretical study of the topological p-n junction created out of Sb2Te3 [1] and Bi2Te3 [2] in thin film geometry by means of the MBE. In particular we will discuss the competing effects due to the built-in electric field and the interfacial intermixing. [1] L. Plucinski, et al, J. Appl. Phys. 113, 053706 (2013), doi: 10.1063/1.4789353. [2] A. Herdt et al. Phys. Rev. B 87, 035127 (2013), doi: 10.1103/PhysRevB.87.035127.

Resume : There is an intense current search to identify new topological compounds, of which topological superconductors are an elusive category. Here, we report the observation by angle-resolved photoemission of Dirac-like surface states in the perovskite superconductor Tl5Te3, as well as its non-superconducting Sn-doped sister compound. We show evidence from ARPES that these are spin-polarised, consistent with our calculations which suggest them to be of topological origin, opening new opportunities to probe the interplay of topological order with bulk superconductivity. We will compare and contrast this first non-trigonal topological system with the much better studied Bi2Se3 class.