Unconventional magnetic excitations and spin dynamics of exotic quantum spin systems BaCo ₂ V ₂ O ₈ and Ba ₃ CuSb ₂ O ₉

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 H_c(∼ 4 T), where the exotic field-induced order-to-disorder transition occurs, and magnetic excitations in a Tomonaga-Luttinger liquid state were observed above H_c. 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.