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

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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.