Giant Magnetic Quantum Oscillations in the Thermal Conductivity of TaAs: Indications of Chiral Zero Sound

Abstract

Charge transport of topological semimetals has been in the focus of intensive investigations because of their non-trivial band topology. Heat transport of these materials, on the other hand, is largely unexplored and remains elusive. Here we report on an observation of unprecedented, giant magnetic quantum oscillations of thermal conductivity in the prototypical Weyl semimetal TaAs. The oscillations are antiphase with the quantum oscillating electronic density of states of a Weyl pocket, and their amplitudes amount to two orders of magnitude of the estimation based on the Wiedemann-Franz law. Our analyses show that all the conventional heat-transport mechanisms through diffusions of propagating electrons, phonons and electron-hole bipolar excitations, are far inadequate to account for these phenomena. Taking further experimental facts that the parallel field configuration favors much higher magneto-thermal conductivity, we propose that the newly proposed chiral zero sound provides a reasonable explanation to these exotic phenomena. More work focusing on other topological semimetals along the same line is badly called for.Charge and heat conductions define two entangled fundamental transport properties of a conducting solid. Controllable manipulations of both are essential for various functionalities of pertinent materials. For the recently discovered topological semimetals, electrical transport has revealed distinct topologically nontrivial properties, like the chiral anomaly in the longitudinal magneto-resistance (MR) [1][2][3][4]. On the other hand, thermal transport, in particular thermal conductivity, has remained largely unexplored. This is mainly due to the technical difficulties of precise thermal management at low temperatures. In addition, such work has been so far considered 1 uninteresting because the thermal conductivity may be estimated from the electrical conductivity through the Wiedemann-Franz (WF) law.Indeed, for most electrical conductors, the WF law can provide a straightforward and reliable approach to the electronic contribution κ e of the thermal conductivity based on the electrical conductivityσ, κ e /T = σ L 0 , (1) with the Sommerfeld value of Lorenz number L 0 ≡ π 2 /3 (k B /2) 2 = 2.44 x 10 -8 W⋅Ω⋅K -2 . The WF law holds in the case of dominant elastic scattering, e.g., from static defects as realized at sufficiently low temperatures [5]. Within the last few years, it was also found that the charge and heat conductions of the relativistic Dirac/Weyl fermions may deviate from the WF law [6][7][8][9][10][11]. For example, it was shown very recently that hydrodynamic transport by relativistic Weyl fermions in a Weyl semimetal WP 2 can lead to L/L 0 < 1 in the temperature range T < 200 K [10]. On the other hand, for the electron-hole plasma at the charge-neutrality point in graphene, considered to form a strongly coupled Dirac fluid, hydrodynamic transport was found between T = 50 and 80 K to result in a much enhanced Lorenz number L ≈ 22 L 0 [11].

The “Sound” of Weyl Fermions

“…Early theoretical studies of the electronic contribution to the thermal conductivity in Weyl semimetal without considering the CZS modes obtain the B 2 dependence for the thermal conductivity under magnetic field [53], which is quite different from the contribution from the CZS introduced above. Such a field-dependent violation of the Wiedemann-Frantz law has already been seen in the thermal conductivity measurement of TaAs under a magnetic field, indicating the possible contribution from the CZS [54]. We note that for realistic systems which are not deeply in the chiral limit, the CZS will also acquire nonzero velocity along the transverse direction of the magnetic field as well, which is caused by the accompanying normal current during the oscillation.…”

Large magnetothermopower and anomalous Nernst effect in HfTe5

“…An alternative signature of the chiral anomaly was found in the thermoelectric properties in Cd 3 As 2 studied by Jia et al [5], showing that this approach bears more promise. Very recently, a new heat-transport mechanism called chiral zero sound has been predicted in Weyl semimetals [11] and could be used to explain the observed giant thermal conductivity in Weyl semimetal TaAs [12] and anomalous heat current in GdPtBi [13]. Here, we report our systematic study of the thermoelectric properties (both Seebeck and Nernst effects) of layered Hafnium pentatelluride (HfTe 5 ) single crystals and show that the observed results are consistent with a nontrivial electronic structure.…”

Chiral fermion reversal in chiral crystals

“…Numerous theoretical and experimental studies have revealed that the chiral fermions can host rich fascinating physical phenomena, such as the chiral zero sound [39,40], colossal chiral ultrafast photocurrents [41], and photoinduced anomalous Hall effects [42]. Our discovery introduces a new degree of freedom to study the chiral fermions, which would facilitate uncovering more exotic behaviour associated with the chiral fermions in materials.…”