Spin Liquid Regimes at Nonzero Temperature in Quantum Spin Ice

TL;DR

This paper explores the existence of distinct quantum spin liquid phases at nonzero temperature in quantum spin ice, identifying a thermal confinement transition that separates quantum and classical regimes, with implications for experimental observations.

Contribution

It introduces a phase diagram with quantum and thermal spin liquid phases in quantum spin ice, highlighting a first order transition at finite temperature, supported by gauge mean field theory.

Findings

Quantum spin liquids can exist as distinct phases at T>0.

A first order thermal confinement transition separates quantum and classical phases.

This transition explains low-temperature phenomena in Yb2Ti2O7.

Abstract

Quantum spin liquids are highly entangled ground states of quantum systems with emergent gauge structure, fractionalized spinon excitations, and other unusual properties. While these features clearly distinguish quantum spin liquids from conventional, mean-field-like states at zero temperature (T), their status at T>0 is less clear. Strictly speaking, it is known that most quantum spin liquids lose their identity at non-zero temperature, being in that case adiabatically transformable into a trivial paramagnet. This is the case for the U(1) quantum spin liquid states recently proposed to occur in the quantum spin ice pyrochlores. Here we propose, however, that in practical terms, the latter quantum spin liquids can be regarded as distinct phases from the high temperature paramagnet. Through a combination of gauge mean field theory calculations and physical reasoning, we argue that these systems sustain both quantum spin liquid and thermal spin liquid phases, dominated by quantum fluctuations and entropy, respectively. These phases are separated by a first order "thermal confinement" transition, such that for temperatures below the transition, spinons and emergent photons are coherently propagating excitations, and above it the dynamics is classical. Even for parameters for which the ground state is magnetically ordered and not a quantum spin liquid, this strong first order transition occurs, pre-empting conventional Landau-type criticality. We argue that this picture explains the anomalously low temperature phase transition observed in the quantum spin ice material Yb2Ti2O7.