Evidence of the Berezinskii-Kosterlitz-Thouless phase in a frustrated magnet

Ze Hu,Yang Qi,Yuan-Da Liao,Wei Li,Zi Yang Meng,Han Li,Zhen Ma,Yi Cui,Chunsheng Ma,Jinsheng Wen,Yanyan Shangguan,Zhentao Huang,Weiqiang Yu

doi:10.1038/s41467-020-19380-x

Abstract

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism, building upon proliferation of topological defects in 2D systems, is the first example of phase transition beyond the Landau-Ginzburg paradigm of symmetry breaking. Such a topological phase transition has long been sought yet undiscovered directly in magnetic materials. Here, we pin down two transitions that bound a BKT phase in an ideal 2D frustrated magnet TmMgGaO4, via nuclear magnetic resonance under in-plane magnetic fields, which do not disturb the low-energy electronic states and allow BKT fluctuations to be detected sensitively. Moreover, by applying out-of-plane fields, we find a critical scaling behavior of the magnetic susceptibility expected for the BKT transition. The experimental findings can be explained by quantum Monte Carlo simulations applied on an accurate triangular-lattice Ising model of the compound which hosts a BKT phase. These results provide a concrete example for the BKT phase and offer an ideal platform for future investigations on the BKT physics in magnetic materials.

Highlights

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism, building upon proliferation of topological defects in 2D systems, is the first example of phase transition beyond the LandauGinzburg paradigm of symmetry breaking
The BKT transition cannot be characterized by conventional order parameters and constitutes the earliest example of phase transition beyond the Landau–Ginzburg paradigm of spontaneous symmetry breaking
The ordering temperature is determined to be TL ≃ 0.9 K, consistent with neutron scattering experiments[16,18]. Both the second moments and the third moments of the spectra change dramatically below ~ 2 K, suggesting the onset of local hyperfine fields enhanced by the static or quasistatic magnetic ordering (Supplementary Fig. 4). These two characteristic temperatures signal the two-step melting of magnetic order through two BKT transitions, suggesting an intermediate floating BKT phase in the system

Summary

Introduction

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism, building upon proliferation of topological defects in 2D systems, is the first example of phase transition beyond the LandauGinzburg paradigm of symmetry breaking. Such a topological phase transition has long been sought yet undiscovered directly in magnetic materials. The experimental findings can be explained by quantum Monte Carlo simulations applied on an accurate triangular-lattice Ising model of the compound which hosts a BKT phase These results provide a concrete example for the BKT phase and offer an ideal platform for future investigations on the BKT physics in magnetic materials. It is of fundamental interest to find and identify BKT materials that could overcome the obstacle and study the topology-related low-energy dynamics

Methods

Results

Conclusion