Mechanically Detected Terahertz Electron Spin Resonance
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Applied Magnetic Resonance
REVIEW
Mechanically Detected Terahertz Electron Spin Resonance Eiji Ohmichi1 · Tsubasa Okamoto1 · Hideyuki Takahashi2 · Hitoshi Ohta2 Received: 31 August 2020 / Revised: 25 September 2020 / Accepted: 27 September 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract In this review, mechanically detected electron spin resonance (MDESR) in the terahertz (THz) region is described to demonstrate its usability as a novel methodology for obtaining microscopic insights into local electronic structures. Using micromechanical devices, the sensitivity to a small-volume sample is greatly enhanced even for a cavityless setup. Moreover, there are plenty of options in the setup of MDESR, including detection modes, mechanical devices, and detection techniques. ESR spectroscopy of single-crystalline, powder, and frozen solution samples in the THz region are demonstrated.
1 Introduction Electron spin resonance (ESR) is widely used in many research areas such as physics, chemistry, and biology [1]. By utilizing unpaired electrons as sensitive probes, microscopic insights can be obtained from a detailed analysis of ESR spectra. In the standard setup, ESR measurements are carried out in the microwave region ( ∼ 9.4 GHz) using a high-Q microwave cavity. On the other hand, terahertz electron spin resonance (THzESR), known as a kind of ESR technique, operates at much higher frequency using terahertz waves. The THz-ESR technique can cover a broad frequency region typically from 0.1 to 1 THz, in contrast to the standard ESR ones operating at a fixed frequency. Thus, for paramagnetic species, the resonance fields are shifted to the high field region, and individual absorptions can be clearly resolved. Besides, THz-ESR is useful for the ESR measurement of samples having energy gaps such as zero-field splitting (ZFS) and spin gaps, which often hinder ESR detection in the microwave region. In this context, the THz-ESR technique has been used to investigate transition metal complexes, since they often exhibit broad ESR spectra and a rich variety of magnetic phases originating from strong magnetic interactions. * Eiji Ohmichi [email protected]‑u.ac.jp 1
Graduate School of Science, Kobe University, 1‑1 Rokkodai‑cho, Nada, Kobe 657‑8501, Japan
2
Molecular Photoscience Research Center, Kobe University, 1‑1 Rokkodai‑cho, Nada, Kobe 657‑8501, Japan
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THz-ESR spectroscopy is usually carried out using the transmission-type setup [2, 3], where a sample is mounted between a light source and a detector, and the transmitted intensity through the sample is monitored. This cavityless setup allows spectroscopic (in other words, multi-frequency) analysis from the frequency-field diagram. Meanwhile, THz-ESR often suffers from low sensitivity, and hence, a large sample volume is often needed to attain a sufficient signal-to-noise ratio. For this reason, it has been difficult to apply THz-ESR to a tiny single-crystalline sample or a small-volume sample. To over
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