Anomalous quantum criticality in an itinerant ferromagnet.

C L Huang,D Fuchs,M S Scheurer,R Schneider,J Schmalian,H V Löhneysen,M Wissinger,M C Ling

doi:10.1038/ncomms9188

Abstract

The dynamics of continuous phase transitions is governed by the dynamic scaling exponent relating the correlation length and correlation time. For transitions at finite temperature, thermodynamic critical properties are independent of the dynamic scaling exponent. In contrast, at quantum phase transitions where the transition temperature becomes zero, static and dynamic properties are inherently entangled by virtue of the uncertainty principle. Consequently, thermodynamic scaling equations explicitly contain the dynamic exponent. Here we report on thermodynamic measurements (as a function of temperature and magnetic field) for the itinerant ferromagnet Sr1-xCaxRuO3 where the transition temperature becomes zero for x=0.7. We find dynamic scaling of the magnetization and specific heat with highly unusual quantum critical dynamics. We observe a small dynamic scaling exponent of 1.76 strongly deviating from current models of ferromagnetic quantum criticality and likely being governed by strong disorder in conjunction with strong electron-electron coupling.

Highlights

The dynamics of continuous phase transitions is governed by the dynamic scaling exponent relating the correlation length and correlation time
While specific heat[3] and NMR4 measurements both concluded that the self-consistent renormalization theory[9], equivalent to the Hertz–Millis model[10,11], could describe the underlying physics of an FM quantum critical point (QCP) in Sr1 À xCaxRuO3 (SCRO), recent Kerr effect measurements on a composition-spread epitaxial film showed that a possible quantum phase transition (QPT) around a critical concentration xc were smeared by disorder originating from the difference of ionic radii between Sr and Ca ions[8]
We report on a consistent dynamical scaling analysis of the magnetization and specific heat for an x 1⁄4 0.7 sample, bearing all features of a QCP with, very unusual critical exponents

Summary

Introduction

The dynamics of continuous phase transitions is governed by the dynamic scaling exponent relating the correlation length and correlation time. We observe a small dynamic scaling exponent of 1.76 strongly deviating from current models of ferromagnetic quantum criticality and likely being governed by strong disorder in conjunction with strong electron–electron coupling. While specific heat[3] and NMR4 measurements both concluded that the self-consistent renormalization theory[9], equivalent to the Hertz–Millis model[10,11], could describe the underlying physics of an FM QCP in SCRO, recent Kerr effect measurements on a composition-spread epitaxial film showed that a possible quantum phase transition (QPT) around a (reduced) critical concentration xc were smeared by disorder originating from the difference of ionic radii between Sr and Ca ions[8]. We observe a small dynamic critical exponent z 1⁄4 1.76, which strongly deviates from current models, and may arise from an inhomogeneous electron liquid due to the strong disorder induced by Ca substitution

Methods

Results

Conclusion