Continuous and Discontinuous Quantum Phase Transitions in a Model Two-Dimensional Magnet

TL;DR

This study investigates quantum phase transitions in the Shastry-Sutherland model by applying pressure to a real material, revealing a continuous magnetic transition and a subsequent structural change.

Contribution

It demonstrates the pressure-induced quantum phase transition and structural distortion in SrCu2(BO3)2, linking magnetic and structural properties through high-resolution x-ray measurements.

Findings

Singlet-triplet gap decreases with pressure and vanishes at 2 GPa.

Continuous magnetic transition observed without structural change.

Structural distortion occurs at higher pressure after the magnetic transition.

Abstract

The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a 2-dimensional square lattice, is simple and soluble, but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu2(BO3)2, in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution x-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.