Phase separation and suppression of critical dynamics at quantum transitions of itinerant magnets: MnSi and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$

TL;DR

This study uses muon spin relaxation to reveal phase separation and suppression of critical dynamics at quantum phase transitions in itinerant magnets MnSi and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$, challenging traditional views of continuous quantum criticality.

Contribution

It provides the first direct evidence of spontaneous phase separation and dynamic suppression at QPTs in these materials using $bc$SR measurements.

Findings

Evidence of phase separation at QPTs in MnSi and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$

Suppression of critical spin dynamics near quantum critical points

Observation of slow, diffuse spin correlations above critical pressure in MnSi

Abstract

Quantum phase transitions (QPTs) have been studied extensively in correlated electron systems. Characterization of magnetism at QPTs has, however, been limited by the volume-integrated feature of neutron and magnetization measurements and by pressure uncertainties in NMR studies using powderized specimens. Overcoming these limitations, we performed muon spin relaxation ($\mu$SR) measurements which have a unique sensitivity to volume fractions of magnetically ordered and paramagnetic regions, and studied QPTs from itinerant heli/ferro magnet to paramagnet in MnSi (single-crystal; varying pressure) and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$ (ceramic specimens; varying $x$). Our results provide the first clear evidence that both cases are associated with spontaneous phase separation and suppression of dynamic critical behavior, revealed a slow but dynamic character of the ``partial order'' diffuse spin correlations in MnSi above the critical pressure, and, combined with other known results in heavy-fermion and cuprate systems, suggest a possibility that a majority of QPTs involve first-order transitions and/or phase separation.