Direct Observation of the Quantum Phase Transition of SrCu$_{2}$(BO$_{3}$)$_{2}$ by High-Pressure and Terahertz Electron Spin Resonance

TL;DR

This study directly observes the quantum phase transition in SrCu₂(BO₃)₂ under high pressure using terahertz ESR, revealing a first-order transition from a dimer to a plaquette singlet phase at around 1.85 GPa.

Contribution

It provides the first direct ESR evidence of the pressure-induced quantum phase transition in SrCu₂(BO₃)₂ and quantifies the exchange interactions involved.

Findings

First-order phase transition at ~1.85 GPa

Pressure dependence of exchange interactions determined

Transition from dimer to plaquette singlet phase

Abstract

High-pressure and high-field electron spin resonance (ESR) measurements have been performed on a single crystal of the orthogonal-dimer spin system SrCu$_{2}$(BO$_{3}$)$_{2}$. With frequencies below 1 THz, ESR signals associated with transitions from the singlet ground state to the one-triplet excited states and the two-triplet bound state were observed at pressures up to 2.1 GPa. We obtained directly the pressure dependence of the gap energies, finding a clear first-order phase transition at $P_{c}=1.85\pm0.05$ GPa. By comparing this pressure dependence with the calculated excitation energies obtained from an exact diagonalization, we determined the precise pressure dependence for inter- ($J'$) and intra-dimer ($J$) exchange interactions considering the Dzyaloshinski-Moriya interaction. Thus this system undergoes a first-order quantum phase transition from the dimer singlet phase to a plaquette singlet phase above the ratio $(J'/J)_{c}=0.660\pm0.003$.