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 Cp 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.
@article{arxiv.2203.07607,
title = {Magnetically hidden state on the ground floor of the magnetic Devil's staircase},
author = {Shusaku Imajo and Naofumi Matsuyama and Toshihiro Nomura and Takumi Kihara and Shintao Nakamura and Christophe Marcenat and Thierry Klein and Gabriel Seyfarth and Chengchao Zhong and Hiroshi Kageyama and Koichi Kindo and Tsutomu Momoi and Yoshimitsu Kohama},
journal= {arXiv preprint arXiv:2203.07607},
year = {2022}
}