Possible experimental realization of a basic Z2 topological semimetal in GaGeTe

Abstract

We report experimental and theoretical evidence that GaGeTe is a basic Z2 topological semimetal with three types of charge carriers: bulk-originated electrons and holes as well as surface state electrons. This electronic situation is qualitatively similar to the primer 3D topological insulator Bi 2 Se 3 , but important differences account for an unprecedented transport scenario in GaGeTe. High-resolution angle-resolved photoemission spectroscopy combined with advanced band structure calculations show a small indirect energy gap caused by a peculiar band inversion in the T -point of the Brillouin zone in GaGeTe. An energy overlap of the valence and conduction bands brings both electron-and hole-like carriers to the Fermi level, while the momentum gap between the corresponding dispersions remains finite. We argue that peculiarities of the electronic spectrum of GaGeTe have a fundamental importance for the physics of topological matter and may boost the material's application potential.A variety of materials where topology of the electronic structure plays a special role in transport properties is rapidly growing [1][2][3]. The envisioned applications of topological materials in novel devices and quantum information technology will be strongly influenced by the fine balance between their charge carriers of various types. In classic 3D topological insulators, like Bi 2 Se 3 , the transport properties are dictated by the non-degenerate massless topological surface states as well as the electron-like carriers from the bulk conduction band [4]. In 3D Dirac or Weyl semimetals, the Fermi surface is formed by single points of band degeneration (type I), or hole and electronic pockets touching in discrete points (type II) [3]. A bulk material with a gapped electronic spectrum combining a comparable number of both types of the bulk charge carriers with the spin-momentum locked topological surface states has not yet been accounted for. Here we put forward GaGeTe as the first example of such conceptually different electronic situation, i. e. a basic Z 2 topological semimetal.The layered compound GaGeTe was first synthesized and structurally characterized quite long ago [5,6], and its phonon structure [7], thermoelectric [8] and transport [9] properties were consequently studied. However, the first theoretical study of its bulk electronic structure has appeared very recently [10] and has sparked strong interest in the surface electronic structure of this material [11]. In [10] structural and electronic resemblance between a structure fragment of the layered GaGeTe bulk and a heavy analogue of graphene, germanene [12], has been highlighted. 2D materials with buckled honeycomb atomic arrangements, e.g. silicene, germanene, stanene, are promising for the realization of new devices. For instance, silicene demonstrates such advantages as high carrier mobility, excellent mechanical flexibility and compatibility with existing Si-based electronics. Most recent studies establish theoretically [11] and experimentally [13] that monolayers...

Anomalous Raman modes in tellurides

“…We want to stress that while this hypothesis could give account for some second-order Raman modes in tellurides, like MoTe2 with a bandgap around 1.1 eV, 48 it is rather unlikely that it can show similar resonances for GaTe, a semiconductor with a bandgap around 1.65 eV, 18 and GaGeTe, a semimetal with very small direct and indirect bandgaps. 50 Moreover, it is very unlikely that the resonances could occur by excitation with different laser wavelenghts in so different tellurides as those already commented. The theoretical electronic band structure of monoclinic GaTe and rhombohedral GaGeTe can be viewed in the Materials Project Database, 51,52 and their completely different nature can be checked, what makes improbable the observation of double resonance in both compounds with similar features as those shown in Figure 1.…”

Theoretical investigations of the electronic and optical properties of a GaGeTe monolayer

“…Moreover, the calculated optical gap value rather matches the experimental optical gap of 1.12 eV. 53 The Kramers-Kronig relation provides a close connection between the real and imaginary components of the dielectric function, resulting in the simultaneous appearance of the pair of prominent peaks in both the real and imaginary parts. It is well known that the polarizability of a material can be estimated by the real part of the dielectric function.…”

Intrinsic Modulation Doping Enhances the Thermoelectric Performance of Monolayer GaGeTe

“…Upon interposing semimetallic germanene in GaTe, the band gap of bulk GaTe significantly decreases from a measured value of ≈1.68 eV 24 in pristine GaTe to lesser values in bulk GaGeTe. ,, However, conflicting reports on the band gap reduction in bulk GaGeTe have made it difficult to determine a single number. Initially, Kucek et al reported a band gap of 1.3 eV for GaGeTe, which was debunked by Haubold et al, who used angle-resolved photoemission spectroscopy (ARPES) and found that GaGeTe is either a gapless semimetal or a narrow-gap semiconductor. A recent optical measurement by Zhou et al reported a band gap of 0.95 eV for GaGeTe, straddling between the two previous reports.…”