Transport Properties Of Weyl Semimetals Research Articles

Anomalous Transport Signatures in Weyl Semimetals with Point Defects.

We present the first theoretical study of transport properties of Weyl semimetals with point defects. Focusing on a class of time-reversal symmetric Weyl lattice models, we show that dilute lattice vacancies induce a finite density of quasilocalized states at and near the nodal energy, causing strong modifications to the low-energy spectrum. This generates novel transport effects, namely, (i)an oscillatory behavior of the dc conductivity with the charge carrier density in the absence of magnetic fields, and (ii) a plateau-shaped dissipative optical response for photon frequencies below the interband threshold, E_{F}≲ℏω≲2E_{F}. Our results provide a path to engineer unconventional quantum transport effects in Weyl semimetals by means of common point defects.

Nonlinear current response of Weyl semimetals to a strong dc-ac electric field in the ultraquantum regime

The linear charge transport properties of Weyl semimetals, such as negative magnetoresistance related to chiral anomaly, have been studied extensively. In this work, the nonlinear current response of Weyl semimetals to a strong dc-ac electric field in the ultraquantum regime with a strong magnetic field is explored by employing a nonperturbative treatment based on the stochastic Liouville equation. Our systematic studies of nonlinear charge transport for two types of ac fields (the cosinusoidal electric field and the periodic pulsed field) have revealed extraordinary modulation of nonlinear current response by the ac fields. For the case with the cosinusoidal electric field, we find the following: (i) in the high-frequency regime, dynamic localization (vanishing of current) and quasienergy band collapse occur under a suitable condition of ${J}_{0}(\frac{e{E}_{A}d}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}})=0$, where ${J}_{0}$ is the Bessel function with ${E}_{A}$ and ${\ensuremath{\omega}}_{0}$ being the strength and frequency of the ac field, and $d$ denoting the lattice constant of Weyl semimetals; and (ii) in the intermediate- and low-frequency regimes, the multiple-photon-assisted transport leads to extremal values of current responses whose patterns can be tuned by the magnetic field. As for the pulsed electric field, our results show that (i) the dynamic localization and quasienergy band collapse appear under a different condition of $cos(\frac{e{E}_{A}d}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}})=0$; and (ii) the influence of the ac field on the current response disappears when $\frac{e{E}_{A}d}{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}}=m\ensuremath{\pi}$, with $m$ being an integer. The experimental conditions are also discussed and the predicted nonlinear transport effects could be observed in experiments.

Hydrodynamic description of Weyl fermions in condensed state of matter

Due to the many unique transport properties of Weyl semimetals, they are promising materials for modern electronics. We investigate the electrons in the strong coupling approximation near Weyl points based on their representation as massless Weyl fermions. We have constructed a new fluid model based on the many-particle quantum hydrodynamics method to describe the behavior of electrons gas with different chirality near Weyl points in the low-energy limit in the external electromagnetic fields, based on the many-particle Weyl equation and many-particle wave function. The derived system of equations forms a closed apparatus for describing the dynamics of the electron current, spin density and spin current density. Based on the proposed model, we considered small perturbations in the Weyl fermion system in an external uniform magnetic field and predicted the new type of eigenwaves in the systems of the electrons near the Weyl points.

Thermoelectric transport in torsional strained Weyl semimetals

In a recent paper [R. Soto-Garrido and E. Mu\~noz, J. Phys. Condens. Matter 30, 195302 (2018)], we studied the electronic transport properties in Weyl semimetals (WSMs) submitted to the combined effects of torsional mechanical strain and magnetic field, showing that this configuration induces a node-polarization effect on the current that can be used to measure the torsion angle from transmission experiments. In this article, we extend our previous work to study thermoelectric transport in WSMs under torsional strain and an external magnetic field. Our analysis involves only the electronic contribution to the transport coefficients, and it is thus valid at low temperatures where the phonon contribution is negligible. We provide exact analytical expressions for the scattering cross section and the transmitted heat current, in order to calculate the thermal conductance and the Seebeck coefficient under this configuration. Our results suggest that thermoelectric transport coefficients in these materials can be engineered by appropriately tuning the magnitude of the magnetic field, torsional strain, and the applied bias or thermal gradient, leading to a potentially very high figure of merit.

Electronic transport properties of Weyl semimetals with strain-induced gauge fields

We investigate the electronic transport properties in Weyl semimetals (WSM) in the presence of unidirectional and torsional strains. We show that unidirectional and torsional strains induce an effective pseudomagnetic field which influences the tunneling transport through a barrier in a Weyl semimetal. The perfect transmission rings and arcs are shown to be highly sensitive to the application of unidirectional and torsional strains. Interestingly, the profile of the perfect transmission rings and arcs are different under tensile and compressive strains of the same magnitude. These results, together with the fact that the precise control of strain has been demonstrated in Weyl semimetals, open an exciting prospect towards all-Weyl semimetal electronics.

Magnetopolaritons in Weyl Semimetals in a Strong Magnetic Field.

Exotic topological and transport properties of Weyl semimetals have generated a lot of excitement in the condensed matter community. Here we show that Weyl semimetals in a strong magnetic field are highly unusual optical materials. The propagation of electromagnetic waves is affected by an interplay between the plasmonic response of chiral Weyl fermions and extreme anisotropy induced by a magnetic field. The resulting magnetopolaritons possess a number of peculiar properties, such as hyperbolic dispersion, photonic stop bands, coupling-induced transparency, and broadband polarization conversion. These effects can be used for optical spectroscopy of these materials including detection of the chiral anomaly or for broadband terahertz or infrared applications.

Momentum-space electromagnetic induction in Weyl semimetals

We theoretically study the effect of the Berry curvature on the transport properties of Weyl semimetals in the nonadiabatic process, which results in nonlinear optical responses. In the adiabatic process, the Berry curvature, which involves the time derivative of the Bloch states, contributes to the transport properties such as the adiabatic Thouless pump. Although this effect is very weak in usual solids, it is enhanced in Weyl semimetals, where the Berry curvature contributes to observable nonlinear optical responses due to its nodal structure. In this paper, using semiclassical Boltzmann theory, we show that the d.c. photocurrent induced by the Berry curvature robustly persists even in the limit of short scattering time. We also show that the photocurrent is well explained as a consequence of the electromagnetic induction in momentum space. The results indicate that the electromagnetic induction gives rise to a non-dissipative photocurrent that is robust even when decoherence occurs within a time scale shorter than the periodicity of the light. We also discuss the second harmonic response of the a.c. current when the electron distribution is displaced from the ground state by an external field.

Chiral wave-packet scattering in Weyl semimetals

In quantum mechanics, a particle is best described by the wave packet instead of the plane wave. Here, we study the wave-packet scattering problem in Weyl semimetals with the low-energy Weyl fermions of different chiralities. Our results show that the wave packet acquires a chirality-protected shift in the single impurity scattering process. More importantly, the chirality-protected shift can lead to an anomalous scattering probability, thus, affects the transport properties in Weyl semimetals. We find that the ratio between the transport lifetime and the quantum lifetime increases sharply when the Fermi energy approaches to the Weyl nodes, providing an explanation on the experimentally observed the ultrahigh mobility in topological (Weyl or Dirac) semimetals.

Recent developments in transport phenomena in Weyl semimetals

The last decade has witnessed great advancements in the science and engineering of systems with unconventional band structures, seeded by studies of graphene and topological insulators. While the band structure of graphene simulates massless relativistic electrons in two dimensions, topological insulators have bands that wind non-trivially over momentum space in a certain abstract sense. Over the last couple of years, enthusiasm has been burgeoning in another unconventional and topological (although, not quite in the same sense as topological insulators) phase -- the Weyl Semimetal. In this phase, electrons mimic Weyl fermions that are well-known in high-energy physics, and inherit many of their properties, including an apparent violation of charge conservation known as the Chiral Anomaly. In this review, we recap some of the unusual transport properties of Weyl semimetals discussed in the literature so far, focusing on signatures whose roots lie in the anomaly. We also mention several proposed realizations of this phase in condensed matter systems, since they were what arguably precipitated activity on Weyl semimetals in the first place.

Chiral anomaly, charge density waves, and axion strings from Weyl semimetals

We study dynamical instability and chiral symmetry breaking in three-dimensional Weyl semimetals, which turns Weyl semimetals into ``axion insulators.'' Charge density waves (CDWs) are found to be the natural consequence of chiral symmetry breaking. The phase mode of this charge density wave state is identified as the axion, which couples to an electromagnetic field in the topological $\ensuremath{\theta}\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{B}$ term. One of our main results is that ``axion strings'' can be realized as the (screw or edge) dislocations in the charge density wave, which provides a simple physical picture for the elusive axion strings. These axion strings carry gapless chiral modes, therefore they have important implications for dissipationless transport properties of Weyl semimetals with broken symmetry.