Hear the Sound of Weyl Fermions

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.Comment: 9+16 pages, 2+0 figures, a new appendix added, accepted in PR

Light Induced Hall effect in semiconductors with spin-orbit coupling

We show that optically excited electrons by a circularly polarized light in a semiconductor with spin-orbit coupling subject to a weak electric field will carry a Hall current transverse to the electric field. This light induced Hall effect is a result of quantum interference of the light and the electric field, and can be viewed as a physical consequence of the spin current induced by the electric field. The light induced Hall conductance is calculated for the p-type GaAs bulk material, and the n-type and p-type quantum well structures.Comment: 5 pages, 3 figure

Metal-insulator transition in three-band Hubbard model with strong spin-orbit interaction

Recent investigations suggest that both spin-orbit coupling and electron correlation play very crucial roles in the $5d$ transition metal oxides. By using the generalized Gutzwiller variational method and dynamical mean-field theory with the hybridization expansion continuous time quantum Monte Carlo as impurity solver, the three-band Hubbard model with full Hund's rule coupling and spin-orbit interaction terms, which contains the essential physics of partially filled $t_{2g}$ sub-shell of $5d$ materials, is studied systematically. The calculated phase diagram of this model exhibits three distinct phase regions, including metal, band insulator and Mott insulator respectively. We find that the spin-orbit coupling term intends to greatly enhance the tendency of the Mott insulator phase. Furthermore, the influence of the electron-electron interaction on the effective strength of spin-orbit coupling in the metallic phase is studied in detail. We conclude that the electron correlation effect on the effective spin-orbit coupling is far beyond the mean-field treatment even in the intermediate coupling region.Comment: 8 pages, 8 figure