Frequency Extension to THz Range in High Pressure ESR System and Its Application to Shastry-Sutherland Model Compound SrCu$_{2}$(BO$_{3}$)$_{2}$

TL;DR

This study extends the frequency range of high-pressure ESR systems into the THz region, enabling detailed investigation of quantum spin systems like SrCu$_{2}$(BO$_{3}$)$_{2}$ under pressure, revealing pressure-dependent gap suppression and proximity to quantum criticality.

Contribution

The paper introduces a novel high-pressure ESR system capable of THz frequencies up to 700 GHz, facilitating advanced quantum spin system studies under high pressure.

Findings

Successfully achieved pressure up to 1.5 GPa with THz ESR up to 700 GHz.

Observed the suppression of the spin gap energy with increasing pressure.

Identified the approach to a quantum critical point in SrCu$_{2}$(BO$_{3}$)$_{2}$ under pressure.

Abstract

We have made a survey of ceramics for the inner parts of the transmission-type pressure cell to achieve the high pressure and the high transmission in THz range. By using the optimal combination of ZrO$_{2}$-based ceramic and Al$_{2}$O$_{3}$ ceramic, we have succeeded in obtaining the pressure up to 1.5 GPa and the frequency region up to 700 GHz simultaneously. We show the high-pressure ESR results of Shastry-Sutherland compound SrCu$_{2}$(BO$_{3}$)$_{2}$ as an application. We observed the direct ESR transition modes between the singlet ground state and the triplet excited states up to the pressure of 1.51 GPa successfully, and obtained the precise pressure dependence of the gap energy. The gap energy is directly proved to be suppressed by the pressure. Moreover, we found that the system approaches the quantum critical point with pressure by comparing the obtained data with the theory. This result also shows the usefulness of high-pressure ESR measurement in THz region to study quantum spin systems.