The monopole plasma resonance: a smoking gun of 3D $U(1)$ quantum spin liquids

Abstract

Certain 3D $U(1)$ spin liquids, such as those arising in dipolar quantum spin ice, have an emergent monopole which is the source of an emergent magnetic field that transforms under symmetries like an electric polarization. As a consequence, these monopoles carry a physical electric charge in their cores and form a plasma at low temperatures. Due to the monopole coupling to emergent gauge fields, the full system behaves as an electrical insulator for DC transport, but can display a sharp plasma resonance analogous to a metal at very low frequencies. This can serve as a clear fingerprint to detect these states in materials. We discuss the optimal conditions to observe this phenomenon in the 3D $U(1)$ spin-liquid candidate Ce$_2$Zr$_2$O$_7$.