A new cubic Hall viscosity in three-dimensional topological semimetals

While nondissipative hydrodynamics in two-dimensional electron systems has been extensively studied, the role of nondissipative viscosity in three-dimensional transport has remained elusive. In this work, we address this question by studying the nondissipative viscoelastic response of three dimensional crystals. We show that for systems with tetrahedral symmetries, there exist new, intrinsically three-dimensional Hall viscosity coefficients that cannot be obtained via a reduction to a quasi-two-dimensional system. To study these coefficients, we specialize to a theoretically and experimentally motivated tight binding model for a chiral magentic metal in (magnetic) space group [(M)SG] $P2_13$ (No.~198$.$9), a nonpolar group of recent experimental interest which hosts both chiral magnets and topological semimetals. Using the Kubo formula for viscosity, we compute the nondissipative Hall viscosity for the spin-1 fermion in two ways. First we use an electron-phonon coupling ansatz to derive the phonon strain coupling and associated phonon Hall viscosity. Second we use a momentum continuity equation to derive the viscosity corresponding to the conserved momentum density. We conclude by discussing the implication of our results for hydrodynamic transport in three-dimensional magnetic metals, and discuss some candidate materials in which these effects may be observed.

Spectral and optical properties of Ag$_3$Au(Se$_2$,Te$_2$) and dark matter detection

In this work we study the electronic structure of Ag${}_3$AuSe${}_2$ and Ag${}_3$AuTe${}_2$, two chiral insulators whose gap can be tuned through small changes in the lattice parameter by applying hydrostatic pressure or choosing different growth protocols. Based on first principles calculations we compute their band structure for different values of the lattice parameters and show that while Ag${}_3$AuSe${}_2$ retains its direct narrow gap at the $Gamma$ point, Ag${}_3$AuTe${}_2$ can turn into a metal. Focusing on Ag${}_3$AuSe${}_2$ we derive a low energy model around $Gamma$ using group theory, which we use to calculate the optical conductivity for different values of the lattice constant. We discuss our results in the context of detection of light dark matter particles, which have masses of the order of a $k$eV, and conclude that Ag${}_3$AuSe${}_2$ satisfies three important requirements for a suitable detector: small Fermi velocities, $m$eV band gap and low photon screening. Our work motivates the growth of high-quality and large samples of Ag${}_3$AuSe${}_2$ to be used as target materials in dark matter detectors.