Magnetic excitation in the =1/2 Ising-like antiferromagnetic chain CsCoCl in longitudinal magnetic fields studied by high-field ESR measurements

Kimura, Shojiro*; Onishi, Hiroaki  ; Okunishi, Koichi*; Akaki, Mitsuru*; Narumi, Yasuo*; Hagiwara, Masayuki*; Kindo, Koichi*; Kikuchi, Hikomitsu*

High-field electron spin resonance (ESR) measurements were performed on the =1/2 Ising-like antiferromagnetic chain CsCoCl in the frequency range up to 4.4THz and magnetic fields for up to 53T. We observed softening of the spinon excitation, which drives the spin chain into the quantum critical state at the critical field =33.0T. Above , we observed the string excitation at the wave number =, which is characteristic of the =1/2 spin chain in the quantum critical state in the magnetic fields. We also observed a crossover of the string excitation to the magnon bound state above the saturation fields =43.9T upon increasing the magnetic fields. Finally, we discuss how the string excitation and magnon bound state can be excited by the electric dipole transition due to spin quadrupolar resonance.