Approaching the quantum critical point in a highly-correlated all-in-all-out antiferromagnet

TL;DR

This study investigates the suppression of all-in-all-out magnetic order in Sm2Ir2O7 under high pressure, revealing a quantum critical point and its implications for exotic electronic phases like magnetic Weyl semimetals.

Contribution

It provides the first experimental observation of a quantum critical point in a 5d pyrochlore iridate using resonant x-ray diffraction under high pressure.

Findings

Magnetic order suppressed at Pc=6.30 GPa

Lattice symmetry remains cubic across the transition

Pressure and chemical tuning affect spin fluctuations differently

Abstract

Continuous quantum phase transitions involving all-in-all-out (AIAO) antiferromagnetic order in strongly spin-orbit-coupled 5d compounds could give rise to various exotic electronic phases and strongly-coupled quantum critical phenomena. Here we experimentally trace the AIAO spin order in Sm2Ir2O7 using direct resonant x-ray magnetic diffraction techniques under high pressure. The magnetic order is suppressed at a critical pressure Pc=6.30 GPa, while the lattice symmetry remains in the cubic Fd-3m space group across the quantum critical point. Comparing pressure tuning and the chemical series R2Ir2O7 reveals that the suppression of the AIAO order and the approach to the spin-disordered state is characterized by contrasting evolutions of both the pyrochlore lattice constant a and the trigonal distortion x. The former affects the 5d bandwidth, the latter the Ising anisotropy, and as such we posit that the opposite effects of pressure and chemical tuning lead to spin fluctuations with different Ising and Heisenberg character in the quantum critical region. Finally, the observed low-pressure scale of the AIAO quantum phase transition in Sm2Ir2O7 identifies a circumscribed region of P-T space for investigating the putative magnetic Weyl-semimetal state.