16 research outputs found
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 WS monolayer encapsulated in hexagonal boron nitride
Embedding a WS 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
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
band inversion in GdPtBi by means of infrared spectroscopy.Comment: 9 pages, SM include
Landau level spectroscopy of BiTe
Here we report on Landau level spectroscopy in magnetic fields up to 34 T
performed on a thin film of topological insulator BiTe epitaxially
grown on a BaF 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 ~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 point where profound inversion exists
andgives rise to relativistic-like surface states of BiTe.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, 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 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
EuCdAs: a magnetic semiconductor
EuCdAs 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 EuCdAs 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. EuCdAs 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
International audienc
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