Critical exponents and intrinsic broadening of the field-induced transition in NiCl$_2$$\cdot$4SC(NH$_2$)$_2$

TL;DR

This study investigates the field-induced phase transition in NiCl$_2$·4SC(NH$_2$)$_2$, revealing that intrinsic broadening affects critical exponents and challenges the Bose-Einstein condensation universality class at very low temperatures.

Contribution

The paper demonstrates that accounting for transition field distribution alters the interpretation of critical exponents, providing new insights into the quantum phase transition in this material.

Findings

Critical exponent inconsistent with BEC universality class below 100 mK

Finite transition field distribution influences data interpretation

Comparison with disordered Ni(Cl$_{1-x}$Br$_x$)$_2$·4SC(NH$_2$)$_2$ studies

Abstract

The field-induced ordering transition in the quantum spin system NiCl$_2$$\cdot$4SC(NH$_2$)$_2$ is studied by means of neutron diffraction, AC magnetometry and relaxation calorimetry. The interpretation of the data is strongly influenced by a finite distribution of transition fields in the samples, which was present but disregarded in previous studies. Taking this effect into account, we find that the order-parameter critical exponent is inconsistent with the BEC universality class even at temperatures below 100 mK. All results are discussed in comparison with previous measurements and with recent similar studies of disordered Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$.