Limits to magnetic quantum criticality from nuclear spins

TL;DR

This paper explores how nuclear spins influence magnetic quantum criticality, revealing that hyperfine interactions can modify or suppress electronic quantum phase transitions at ultra-low temperatures.

Contribution

It introduces simple spin models to analyze the impact of nuclear spins on quantum critical phenomena, providing estimates of crossover scales where electronic criticality is affected.

Findings

Nuclear spins can significantly alter phase diagrams and excitation spectra.

Hyperfine interactions set a temperature scale below which electronic quantum criticality is suppressed.

Emergence of novel low-temperature physics due to nuclear spin coupling.

Abstract

The phenomenology of quantum phase transitions concerns physics at low temperatures and energies, and corresponding solid-state experiments often reach millikelvin temperatures. However, this is a scale where in many solids the influence of nuclear spins and their hyperfine interaction is no longer negligible. This may limit the observability of electronic quantum critical phenomena. Here we discuss how continuous magnetic quantum phase transitions get influenced, modified, or destroyed by the coupling to nuclear spins. We use simple yet paradigmatic spin models for magnetic quantum criticality and determine modifications to the phase diagram, the excitation spectrum, and thermodynamics due to the presence of nuclear spins. We estimate crossover scales below which purely electronic quantum criticality is no longer observable, and discuss the novel physics emerging at low temperatures. Our results are relevant for a variety of compounds displaying magnetic quantum phase transitions and, more generally, highlight the sensitivity of quantum critical systems to small perturbations.