NMR Evidence of Charge Fluctuations in Multiferroic CuBr2

TL;DR

This study combines multiple experimental techniques to reveal charge fluctuations and competing instabilities in the multiferroic CuBr2, especially below 60 K, indicating complex interactions between magnetic, electric, and charge degrees of freedom.

Contribution

It provides new evidence of charge fluctuations in CuBr2 using NMR and other measurements, highlighting emergent competing instabilities in this multiferroic material.

Findings

Charge fluctuations detected below 60 K via NMR relaxation rates

Increase in NMR linewidth and magnetic susceptibility at low temperatures

Reduction of dielectric constant indicating competing phases

Abstract

We report combined magnetic susceptibility, dielectric constant, nuclear quadruple resonance (NQR) and zero-field nuclear magnetic resonance (NMR) measurements on single crystals of multiferroics CuBr$_2$. High quality of the sample is demonstrated by the sharp magnetic and magnetic-driven ferroelectric transition at $T_N=T_C\approx$ 74~K. The zero-field $^{79}$Br and $^{81}$Br NMR are resolved below $T_N$. The spin-lattice relaxation rates reveal charge fluctuations when cooled below 60~K. Evidences of an increase of NMR linewidth, a reduction of dielectric constant, and an increase of magnetic susceptibility are also seen at low temperatures. These data suggest an emergent instability which competes with the spiral magnetic ordering and the ferroelectricity. Candidate mechanisms are discussed based on the quasi-one-dimensional (1D) nature of the magnetic system.