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The one-dimensional antiferromagnet with spin S = 1/2 has been studied since 1931 when H. Bethe proposed a method to obtain the exact eigenvalues and eigenstates of a system with isotropic Heisenberg-type exchange interactions. Research has revealed that the simple S = 1/2 one-dimensional antiferromagnet exhibits rich and exotic properties originating from the many-body quantum effects. In the case of the most-investigated system with isotropic Heisenberg-type interactions, the quantum fluctuations destroy the long-range order, leading to a quantum critical state with algebraic decay of the spin correlation function. The magnetic excitation from this fluctuating ground state exhibits a marked contrast from that of the classical spin wave. The lowest-energy excitation in the S = 1/2 one-dimensional antiferromagnet is a pair excitation of quasi-particles called spinons, which are mobile domain walls created by flipping the spins from its ground state. Under external magnetic fields, the magnetic excitation becomes gapless at incommensurate wave numbers in the reciprocal lattice space. In addition, recent theory has indicated that the string states, which are characterized by complex rapidities in the Bethe-anzats calculations, change asymptotically to multimagnon bound states upon increasing the field, significantly contributing to the excitation spectrum.

We investigated the magnetic excitation in the S = 1/2 one-dimensional antiferromagnet with Ising-like anisotropy CsCoCl₃ through high-field electron spin resonance (ESR) spectroscopy in a wide frequency region from 130 GHz to 4.4 THz under pulsed magnetic fields up to 53 T. In contrast to the Heisenberg-type system, the antiferromagnetic order is maintained in the ground state of the S = 1/2 Ising-like one-dimensional antiferromagnet at zero magnetic field, owing to magnetic anisotropy. However, by applying the magnetic field along the magnetic easy axis, the system transitions into the quantum critical state. Our ESR results demonstrate that the softening of the spinon by the external magnetic field drives this field-induced transition. This result suggests that the antiferromagnetic order is destroyed by the propagating domain walls induced in the ground state by the longitudinal magnetic field. Thereby, the spin chain is brought into the quantum-critical state with no long-range order. In the quantum critical state, we observe the appearance of a two-string state in the terahertz region above 2.9 THz. By increasing the magnetic fields further, to the saturation field, we can successfully observe the change in the magnetic excitation from the two-string to the two-magnon bound state, for the first time.

(Written by Shojiro Kimura on behalf of all the authors)