Giant dielectric nonlinearities at a magnetic Bose-Einstein condensation

TL;DR

This study reveals giant, highly anharmonic dielectric nonlinearities in a quantum magnet near a magnetic Bose-Einstein condensation quantum critical point, highlighting a strong magnetocapacitive effect linked to an improper ferroelectric transition.

Contribution

It provides the first detailed experimental investigation of dielectric nonlinearities associated with magnon BEC in a low-dimensional quantum magnet.

Findings

Giant dielectric nonlinearities observed near the quantum critical point.

Unusual anharmonicities in the magnetocapacitive response.

The transition exhibits improper ferroelectric behavior.

Abstract

We experimentally investigate the dielectric response of the low-dimensional gapped quantum magnet Cu$_2$Cl$_{4}\cdot$H$_8$C$_4$SO$_2$ near a magnetic field-induced quantum critical point, which separates the quantum-disordered and helimagnetic ground states. The observed magnetocapacitive effect originates from an improper ferroelectric nature of the transition, which itself is perhaps one of the best known realizations of Bose--Einstein condensation of magnons. Despite that, we find that the magnetocapacitive effect associated with the transition exhibits huge and very unusual anharmonicities.