19 research outputs found
Effective numbers of charge carriers in doped graphene: The generalized Fermi liquid approach
The single-band current-dipole Kubo formula for the dynamical conductivity of
heavily doped graphene from Kup\v{c}i\'{c} [Phys. Rev. B 91, 205428 (2015)] is
extended to a two-band model for conduction electrons in lightly doped
graphene. Using a posteriori relaxation-time approximation in the two-band
quantum transport equations, with two different relaxation rates and one
quasi-particle lifetime, we explain a seemingly inconsistent dependence of the
dc conductivity of ultraclean and dirty
lightly doped graphene samples on electron doping, in a way consistent with the
charge continuity equation. It is also shown that the intraband contribution to
the effective number of conduction electrons in vanishes at K in the ultraclean regime, but it remains finite in
the dirty regime. The present model is shown to be consistent with a picture in
which the intraband and interband contributions to are characterized by two different mobilities of conduction electrons,
the values of which are well below the widely accepted value of mobility in
ultraclean graphene. The dispersions of Dirac and plasmon resonances are
reexamined to show that the present, relatively simple expression for the
dynamical conductivity tensor can be used to study simultaneously
single-particle excitations in the dc and optical conductivity and collective
excitations in energy loss spectroscopy experiments.Comment: 13 pages, 11 figure
Two-dimensional conical dispersion in ZrTe5 evidenced by optical spectroscopy
Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal,
with a single conical band, located at the center of the Brillouin zone. The
cone's lack of protection by the lattice symmetry immediately sparked vast
discussions about the size and topological/trivial nature of a possible gap
opening. Here we report on a combined optical and transport study of ZrTe5,
which reveals an alternative view of electronic bands in this material. We
conclude that the dispersion is approximately linear only in the a-c plane,
while remaining relatively flat and parabolic in the third direction (along the
b axis). Therefore, the electronic states in ZrTe5 cannot be described using
the model of 3D Dirac massless electrons, even when staying at energies well
above the band gap 6 meV found in our experiments at low temperatures.Comment: Physical Review Letters 122, 217402 (2019). Corrected acknowledgment
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
Distinguishing the gapped and Weyl semimetal scenario in : Insights from an effective two-band model
Here we study the static and dynamic transport properties of a low-energy two-band model proposed previously in Martino et al. [PRL 122, 217402 (2019)], with an anisotropic in-plane linear momentum dependence and a parabolic out-of-plane dispersion. The model is extended to include a negative band gap, which leads to the emergence of a Weyl semimetal (WSM) state, as opposed to the gapped semimetal (GSM) state when the band gap is positive. We calculate and compare the zero- and finite-frequency transport properties of the GSM and WSM cases. The DC properties that are calculated for the GSM and WSM cases are Drude spectral weight, mobility, and resistivity. We determine their dependence on the Fermi energy and crystal direction. The in- and out-of-plane optical conductivities are calculated in the limit of the vanishing interband relaxation rate for both semimetals. The main common features are an ω1/2 in-plane and ω3/2 out-of-plane frequency dependence of the optical conductivity. We seek particular features related to the charge transport that could unambiguously point to one ground state over the other, based on the comparison with the experiment. Differences between the WSM and GSM are in principle possible only at extremely low carrier concentrations and at low temperatures
Neutron Capture on the s-Process Branching Point 171Tm via Time-of-Flight and Activation
Here we study the static and dynamic transport properties of a low-energy two-band model proposed previously in Martino et al. [PRL 122, 217402 (2019)], with an anisotropic in-plane linear momentum dependence and a parabolic out-of-plane dispersion. The model is extended to include a negative band gap, which leads to the emergence of a Weyl semimetal (WSM) state, as opposed to the gapped semimetal (GSM) state when the band gap is positive. We calculate and compare the zero- and finite-frequency transport properties of the GSM and WSM cases. The DC properties that are calculated for the GSM and WSM cases are Drude spectral weight, mobility, and resistivity. We determine their dependence on the Fermi energy and crystal direction. The in- and out-of-plane optical conductivities are calculated in the limit of the vanishing interband relaxation rate for both semimetals. The main common features are an ω^1/2 in-plane and ω^3/2 out-of-plane frequency dependence of the optical conductivity. We seek particular features related to the charge transport that could unambiguously point to one ground state over the other, based on the comparison with the experiment. Differences between the WSM and GSM are in principle possible only at extremely low carrier concentrations and at low temperatures
Manifestations of the electron-phonon interaction range in angle-resolved photoemission spectra
Numerous angle-resolved photoemission spectroscopy (ARPES) studies of a wide class of low-density metallic systems, ranging from doped transition metal oxides to quasi-two-dimensional interfaces between insulators, exhibit phonon sidebands below the quasiparticle peak as a unique hallmark of polaronic correlations. Here, we single out properties of ARPES spectra that can provide a robust estimate of the effective range (screening length) of the electron-phonon interaction, regardless of the limited experimental resolution, dimensionality, and particular features of the electronic structure, facilitating a general methodology for an analysis of a whole class of materials
Low-energy excitations in type-II Weyl semimetal Td -MoTe2 evidenced through optical conductivity
Molybdenum ditelluride, MoTe2, is a versatile material where the topological phase can be readily tuned by manipulating the associated structural phase transition. The fine details of the band structure of MoTe2, key to understanding its topological properties, have proven difficult to disentangle experientially due to the multiband character of the material. Through experimental optical conductivity spectra, we detect two strong low-energy interband transitions. Both are linked to excitations between spin-orbit split bands. The lowest interband transition shows a strong thermal shift, pointing to a chemical potential that dramatically decreases with temperature. With the help of ab initio calculations and a simple two-band model, we give qualitative and quantitative explanations of the main features in the temperature-dependent optical spectra up to 400 meV
Addressing shape and extent of Weyl cones in TaAs by Landau level spectroscopy
International audienceTaAs is a prime example of a topological semimetal with two types of Weyl nodes, W 1 and W 2 , whose bulk signatures have proven elusive. We apply Landau level spectroscopy to crystals with multiple facets and identify-among other low-energy excitations between parabolic bands-the response of a cone extending over a wide energy range. Comparison with density functional theory studies allows us to associate this conical band with nearly isotropic W 2 nodes. In contrast, W 1 cones, which are more anisotropic and less extended in energy, appear to be buried too deep beneath the Fermi level. They cannot be accessed directly. Instead, the excitations in their vicinity give rise to an optical response typical of a narrow-gap semiconductor rather than a Weyl semimetal
Optical conductivity of the type-II Weyl semimetal TaIrTe<sup>4</sup>
TaIrTe4 is an example of a candidate Weyl type-II semimetal with a minimal possible number of Weyl nodes. Four nodes are reported to exist in a single plane in k space. The existence of a conical dispersion linked toWeyl nodes has yet to be shown experimentally. Here, we use optical spectroscopy as a probe of the band structure on a low-energy scale. Studying optical conductivity allows us to probe intraband and interband transitions with zero momentum. In TaIrTe4, we observe a narrow Drude contribution and an interband conductivity that may be consistent with a tilted linear band dispersion up to 40 meV. The interband conductivity allows us to establish the effective parameters of the conical dispersion; effective velocity v = 1.1 × 104 m/s and tilt γ = 0.37. The transport data, Seebeck and Hall coefficients, are qualitatively consistent with conical features in the band structure. Quantitative disagreement may be linked to the multiband nature of TaIrTe4