Magnetic Excitation in the S = 1/2 Ising-like Antiferromagnetic Chain CsCoCl3 in Longitudinal Magnetic Fields Studied by High-field ESR Measurements

High-field electron spin resonance (ESR) measurements were performed on the S = 1/2 Ising-like antiferromagnetic chain CsCoCl3 in the frequency range up to 4.4 THz and magnetic fields for H || c up to 53 T. We observed softening of the spinon excitation, which drives the spin chain into the quantum critical state at the critical field Hc = 33.0 T. Above Hc, we observed the string excitation at the wave number q = π, which is characteristic of the S = 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 field Hs = 43.9 T 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.