Unconventional magnetic excitations and spin dynamics of exotic quantum spin systems BaCo $$_2$$ 2 V $$_2$$ 2 O $$
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Applied Magnetic Resonance
REVIEW
Unconventional magnetic excitations and spin dynamics of exotic quantum spin systems BaCo2V2O8 and Ba3CuSb2O9 Yibo Han1 · Shojiro Kimura2 · Kouichi Okunishi3 · Masayuki Hagiwara4 Received: 3 September 2020 / Revised: 21 October 2020 / Accepted: 26 October 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract We review terahertz (THz) electron spin resonance studies of two types of exotic quantum spin systems, namely, the spin(S)-1/2 one-dimensional (1D) Ising-like antiferromagnet BaCo2V2O8 and the S=1/2 two-dimensional (2D) honeycomb-like antiferromagnet Ba3CuSb2O9 in magnetic fields of up to 50 T. For the former subject, unconventional magnetic excitations were identified below a critical magnetic field Hc(∼ 4 T), where the exotic field-induced order-to-disorder transition occurs, and magnetic excitations in a Tomonaga-Luttinger liquid state were observed above Hc . The novel magnetic excitations were analyzed with an S=1/2 1D XXZ model by considering the peculiar structure of this compound. For the latter subject, the orbital quantum dynamics of the spin liquid candidate Ba3CuSb2O9 was revealed using multifrequency electron spin resonance ranging from 9.3 GHz to 0.73 THz. The g-factor of the hexagonal Ba3CuSb2O9 single crystal possesses a weak six-fold symmetry at low frequencies, while two-fold symmetry is manifested at high frequencies. From the critical point between the two frequency regions, the frequency of the dynamic Jahn-Teller distortion is determined to be approximately 10 GHz. This dynamic distortion, accompanied by orbital quantum tunneling, proves the spin-orbital liquid state in Ba3CuSb2O9.
* Masayuki Hagiwara [email protected]‑u.ac.jp 1
Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China
2
Institute for Materials Science, Tohoku University, Sendai, Miyagi 980‑8577, Japan
3
Department of Physics, Niigata University, Niigata 950‑2181, Japan
4
Center for Advanced High Magnetic Field Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560‑0043, Japan
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Y. Han et al.
1 Introduction Terahertz (THz) electron spin resonance (ESR) spectroscopy in high magnetic fields [1] is one of the most powerful and useful means to precisely investigate the microscopic characteristics of the electronic structure, magnetic excitations, and spin dynamics in quantum spin systems that are low-dimensional (one- or two-dimensional, 1D or 2D) magnetic systems with small spin values (S). First, we present the reasons why THz ESR spectroscopy is a powerful, modern probe in magnetic materials research [1, 2]. Specifically, THz or high-frequency ESR provides advantages over low-frequency ESR, such as the following: (1) An increased resolution of g-values, which is important in systems with similar g-values where spectral lines may not be resolved at low frequencies, (2) increased absorption intensity due to the large Zeeman splitting corresponding to the
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