Critical dielectric susceptibility at a magnetic BEC quantum critical point

TL;DR

This study investigates dielectric and magnetic quantum phase transitions in a frustrated quantum magnet, revealing the nature of BEC criticality and identifying new exotic phases through dielectric measurements.

Contribution

It provides direct dielectric evidence of magnetic BEC quantum criticality and uncovers magnetic presaturation phases with potential exotic order.

Findings

Dielectric anomaly at 8 K is due to impurity, not chiral spin liquid.

Two field-induced quantum phase transitions observed.

Critical divergence of BEC susceptibility matches 3D BEC theory.

Abstract

Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet Rb$_{2}$Cu$_{2}$Mo$_3$O$_{12}$ through dielectric and calorimetric measurements on single-crystal samples. It is clarified that the previously reported dielectric anomaly at 8~K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric and paramagnetic and ferroelectric and magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point but a primary one at the saturation transition. Having clearly identified the magnetic Bose-Einstein condensation (BEC) nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three-dimensional BEC. Finally, dielectric data reveal magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries.