Magneto-optical response of the Weyl semimetal NbAs: Experimental results and hyperbolic-band computations

The magneto-optical properties of (001)-oriented NbAs single crystals have been studied in the spectral range from 5 to 150 meV and in magnetic fields of up to 13 T. A rich spectrum of inter-Landau-level transitions is revealed by these measurements. The transitions follow a square-root-like dependence with magnetic field, but the simple linear-band approximation is unable to accurately reproduce the observed behavior of the transitions in applied fields. We argue that the detected magneto-optical spectra should be related to crossing hyperbolic bands, which form the W1 cones. We propose a model Hamiltonian, which describes coupled hyperbolic bands and reproduces the shape of the relevant bands in NbAs. The magneto-optical spectra computed from this Hamiltonian nicely reproduce our observations. We conclude that the hyperbolic-band approach is a minimal model to adequately describe the magneto-optical response of NbAs and that the chiral (conical) bands do not explicitly manifest themselves in the spectra.Comment: 10 pages, SM include

Singlet and triplet trions in WS2_2 monolayer encapsulated in hexagonal boron nitride

Embedding a WS$_2$ monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron-electron exchange interaction for the trion triplet in a WS$_2$ monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures.Comment: 11 pages, 4 figure

Exchange gap in GdPtBi probed by magneto-optics

We measured the magneto-reflectivity spectra (4 - 90 meV, 0 - 16 T) of the triple-point semimetal GdPtBi and found them to demonstrate two unusual broad features emerging in field. The electronic bands of GdPtBi are expected to experience large exchange-mediated shifts, which lends itself to a description via effective Zeeman splittings with a large g factor. Based on this approach, along with an ab initio band structure analysis, we propose a model Hamiltonian that describes our observations well and allows us to estimate the effective g factor, g* = 95. We conclude that we directly observe the exchange-induced $\Gamma_{8}$ band inversion in GdPtBi by means of infrared spectroscopy.Comment: 9 pages, SM include

Landau level spectroscopy of Bi2_2Te3_3

Here we report on Landau level spectroscopy in magnetic fields up to 34 T performed on a thin film of topological insulator Bi$_2$Te$_3$ epitaxially grown on a BaF$_2$ substrate. The observed response is consistent with the picture of a direct-gap semiconductor in which charge carriers closely resemble massive Dirac particles. The fundamental band gap reaches $E_g=(175\pm 5)$~meV at low temperatures and it is not located on the trigonal axis, thus displaying either six or twelvefold valley degeneracy. Notably, our magneto-optical data do not indicate any band inversion. This suggests that the fundamental band gap is relatively distant from the $\Gamma$ point where profound inversion exists andgives rise to relativistic-like surface states of Bi$_2$Te$_3$.Comment: 12 pages, 11 figures, to be published in Phys. Rev.

Optical conductivity signatures of open Dirac nodal lines

We investigate the optical conductivity and far-infrared magneto-optical response of BaNiS$_2$, a simple square-lattice semimetal characterized by Dirac nodal lines that disperse exclusively along the out-of-plane direction. With the magnetic field aligned along the nodal line the in-plane Landau level spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical-band dispersion with a small spin-orbit coupling gap. The optical conductivity exhibits an unusual temperature-independent isosbestic line, ending at a Van Hove singularity. First-principles calculations unambiguously assign the isosbestic line to transitions across Dirac nodal states. Our work suggests a universal topology of the electronic structure of Dirac nodal lines

EuCd2_2As2_2: a magnetic semiconductor

EuCd$_2$As$_2$ is now widely accepted as a topological semimetal in which a Weyl phase is induced by an external magnetic field. We challenge this view through firm experimental evidence using a combination of electronic transport, optical spectroscopy and excited-state photoemission spectroscopy. We show that the EuCd$_2$As$_2$ is in fact a semiconductor with a gap of 0.77 eV. We show that the externally applied magnetic field has a profound impact on the electronic band structure of this system. This is manifested by a huge decrease of the observed band gap, as large as 125~meV at 2~T, and consequently, by a giant redshift of the interband absorption edge. However, the semiconductor nature of the material remains preserved. EuCd$_2$As$_2$ is therefore a magnetic semiconductor rather than a Dirac or Weyl semimetal, as suggested by {\em ab initio} computations carried out within the local spin-density approximation.Comment: Accepted for publication in Physical Review Letter

Singlet and triplet trions in WS 2 monolayer encapsulated in hexagonal boron nitride

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Temperature dependence of the energy band gap in ZrTe 5 : Implications for the topological phase

International audienceUsing Landau-level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (ZrTe 5). We find that the band gap reaches E g = (5 ± 1) meV at low temperatures and increases monotonically when the temperature is raised. This implies that ZrTe 5 is a weak topological insulator, with noninverted ordering of electronic bands in the center of the Brillouin zone. Our magnetotransport experiments performed in parallel show that the resistivity anomaly in ZrTe 5 is not connected with the temperature dependence of the band gap