Magnetically hidden state on the ground floor of the magnetic Devil's staircase

TL;DR

This study reveals a hidden magnetic state in SrCu2(BO3)2 around 26 T, characterized by thermodynamic anomalies and likely involving a spin-nematic phase, advancing understanding of complex magnetic phases in quantum materials.

Contribution

It provides experimental evidence for a thermodynamic anomaly indicating a hidden spin-nematic state in SrCu2(BO3)2 near 26 T, supported by theoretical predictions.

Findings

Detection of a thermodynamic anomaly at ~26 T.

Confirmation of magnetic crystallization above 27 T.

Proposal of a spin-nematic state as a hidden phase.

Abstract

We investigated the low-temperature and high-field thermodynamic and ultrasonic properties of SrCu2(BO3)2, which exhibits various plateaux in its magnetization curve above 27~T, called a magnetic Devil's staircase. The results of the present study confirm that magnetic crystallization, the first step of the staircase, occurs above 27~T as a 1st-order transition accompanied by a sharp singularity in heat capacity $C_p$ and a kink in the elastic constant. In addition, we observe a thermodynamic anomaly at lower fields around 26~T, which has not been previously detected by any magnetic probes. At low temperatures, this magnetically hidden state has a large entropy and does not exhibit Schottky-type gapped behavior, which suggests the existence of low-energy collective excitations. Based on our observations and theoretical predictions, we propose that magnetic quadrupoles form a spin-nematic state around 26~T as a hidden state on the ground floor of the magnetic Devil's staircase.