# Quantum phases of SrCu2(BO3)2 from high-pressure thermodynamics

## TL;DR

This study explores the pressure-induced quantum phase transitions in SrCu$_2$(BO$_3$)$_2$, revealing a first-order transition from a quantum dimer paramagnet to a plaquette-singlet state, and then to an antiferromagnetic phase, supported by experiments and simulations.

## Contribution

It provides the first detailed phase diagram of SrCu$_2$(BO$_3$)$_2$ under high pressure, combining thermodynamic measurements with quantum spin model simulations.

## Key findings

- Identification of a first-order quantum phase transition at low pressure.
- Discovery of a new antiferromagnetic phase at higher pressures.
- Validation of the Shastry-Sutherland model with inter-layer couplings.

## Abstract

We report heat capacity measurements of SrCu$_2$(BO$_3$)$_2$ under high pressure along with simulations of relevant quantum spin models and map out the $(P,T)$ phase diagram of the material. We find a first-order quantum phase transition between the low-pressure quantum dimer paramagnet and a phase with signatures of a plaquette-singlet state below T = $2$ K. At higher pressures, we observe a transition into a previously unknown antiferromagnetic state below $4$ K. Our findings can be explained within the two-dimensional Shastry-Sutherland quantum spin model supplemented by weak inter-layer couplings. The possibility to tune SrCu$_2$(BO$_3$)$_2$ between the plaquette-singlet and antiferromagnetic states opens opportunities for experimental tests of quantum field theories and lattice models involving fractionalized excitations, emergent symmetries, and gauge fluctuations.