Frequency Extension to the THz Range in the High Pressure ESR System and Its Application to the Shastry-Sutherland Model Compound SrCu2(BO3)2

Development and application of 2.5 GPa-25 T high-pressure high-field electron spin resonance system using a cryogen-free superconducting magnet

We have developed a high-pressure electron spin resonance probe and successfully installed into the world's highest-field cryogen-free superconducting magnet having a maximum central field of 24.6 T. The high pressure of 2.5 GPa is achieved by the specially designed piston-cylinder pressure cell using THz-wave-transparent components. In the first application of this high-pressure high-field ESR system, we observed that the orthogonal dimer spin system SrCu₂(BO₃)₂ undergoes a quantum phase transition from the dimer singlet ground to the plaquette singlet ground states.

High-field/high-pressure ESR

We present a historical review of high-pressure ESR systems with emphasis on our recent development of a high-pressure, high-field, multi-frequency ESR system. Until 2000, the X-band system was almost established using a resonator filled with dielectric materials or a combination of the anvil cell and dielectric resonators. Recent developments have shifted from that in the low-frequency region, such as X-band, to that in multi-frequency region. High-pressure, high-field, multi-frequency ESR systems are classified into two types. First are the systems that use a vector network analyzer or a quasi-optical bridge, which have high sensitivity but a limited frequency region; the second are like our system, which has a very broad frequency region covering the THz region, but lower sensitivity. We will demonstrate the usefulness of our high-pressure ESR system, in addition to its experimental limitations. We also discuss the recent progress of our system and future plans.