High-Pressure THz ESR
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Applied Magnetic Resonance
REVIEW
High‑Pressure THz ESR Takahiro Sakurai1 · Hitoshi Ohta2 · Shigeo Hara1 · Yu Saito1 Received: 25 August 2020 / Revised: 26 October 2020 / Accepted: 29 October 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract In solid-state physics, pressure is an essential parameter, which can continuously change the interaction between electrons. The combination of terahertz electron spin resonance (THz ESR) with pressure is a powerful approach to clarify the spin state under pressure from the microscopic point of view, especially for quantum magnets. The most important feature of our high-pressure THz ESR system is that our pressure cell ensures the high pressure and wide frequency range using ceramics in internal parts, which are well balanced in terms of toughness and transmittance of electromagnetic waves. This approach allowed to very accurately describe the spin state under pressure. In this review, high-pressure ESR systems are reviewed with a focus on those in the high-frequency region. In addition, we show differences between other systems and our system. Moreover, we show the usefulness of our system by showing application examples of several magnetic materials of interest.
1 Introduction In solid-state physics, extreme conditions are indispensable for the search of novel quantum phenomena. Among them, pressure P is the only parameter that can continuously change the interaction between electrons while maintaining lattice symmetry, provided that no structural phase transition occurs. However, in reality, the accurate evaluation of interaction parameters under pressure is difficult, and it often causes difficulty to approach the heart of the matter. In this paper, we show that our high-pressure terahertz electron spin resonance (THz ESR) is extremely effective in this respect. Pressure is not the major experimental parameter in ESR measurements compared with nuclear magnetic resonance (NMR) measurement where pressure is frequently used [1]. This is the case because NMR has a resonance frequency of tens to hundreds MHz, and it is relatively easy to apply an oscillating magnetic field to * Takahiro Sakurai tskaurai@kobe‑u.ac.jp 1
Research Facility Center for Science and Technology, Kobe University, Kobe 657‑8501, Japan
2
Molecular Photoscience Research Center, Kobe University, Kobe 657‑8051, Japan
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the sample by a coil introduced into the pressure cell. The same technique cannot be applied to ESR owing to its much higher resonance frequency [2]. Most of the previously reported high-pressure ESR techniques have focused on the combination of pressure device and resonator to provide an oscillating field to the sample under pressure [3–5]. Meanwhile, our group has performed THz ESR measurements using a pulsed high magnetic field by a simple transmission method without a resonator [6]. Specifically, a sample is placed at the center of the magnetic field, and the transmitted intensity is detected. Using this transmiss
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