Abstract
Quasiparticles and collective modes are two fundamental aspects that characterize a quantum matter in addition to its ground state features. For example, the low energy physics for Fermi liquid phase in He-III was featured not only by Fermionic quasiparticles near the chemical potential but also by fruitful collective modes in the long-wave limit, including several different sound waves that can propagate through it under different circumstances. On the other hand, it is very difficult for sound waves to be carried by the electron liquid in the ordinary metals, due to the fact that long-range Coulomb interaction among electrons will generate plasmon gap for ordinary electron density fluctuation and thus prohibits the propagation of sound waves through it. In the present paper, we propose a unique type of acoustic collective modes in Weyl semimetals under the magnetic field called chiral zero sound. The chiral zero sound can be stabilized under so-called "chiral limit", where the intra-valley scattering time is much shorter than the inter-valley one, and only propagates along an external magnetic field for Weyl semimetals with multiple-pairs of Weyl points. The sound velocity of the chiral zero sound is proportional to the field strength in the weak field limit, whereas it oscillates dramatically in the strong field limit, generating an entirely new mechanism for quantum oscillations through the dynamics of neutral bosonic excitation, which may manifest itself in the thermal conductivity measurements under magnetic field.
Highlights
Topological semimetals are unique metallic systems with a vanishing density of states at the Fermi level [1,2,3,4,5,6,7,8,9,10,11]
Weyl semimetals in the chiral limit, since the chiral zero sound” (CZS) can propagate only along the magnetic field, the thermal conductivity along the field will be contributed by both the CZS and free electrons leading to the dramatic violation of the Wiedemann-Frantz law, which is absent for thermal conductivity along the perpendicular direction
The above-mentioned quantum oscillations in specific heat and thermal conductivity can be viewed as strong evidence for the existence of the CZS but still indirect
Summary
Topological semimetals are unique metallic systems with a vanishing density of states at the Fermi level [1,2,3,4,5,6,7,8,9,10,11]. The most common collective modes in a charged Fermi-liquid system are plasmons, and for a Weyl semimetal under a magnetic field, they are such collective modes where the oscillations of the valley particle numbers cannot cancel each other and generate net-charge-density oscillation in real space [28,29,30,31,32,33,34,35,36]. Since these modes are coupled to the CME current, the plasmon frequencies significantly depend on the magnetic field [36]. An elaborate study of collective modes in Weyl systems with only one pair of WPs using the Boltzmann equation can be found in Ref. [31]
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