Magnetic Excitation in S = 1/2 Antiferromagnetic Chain CsCoCl_{3 }with IsingLike Exchange Interaction
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Magnetic Excitation in the S = 1/2 Isinglike Antiferromagnetic Chain CsCoCl_{3} in Longitudinal Magnetic Fields Studied by Highfield ESR Measurementss
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 92, 094701 (2023).
The results of highfield electron spin resonance measurements in the millimeterwave to terahertz region reveal unconventional magnetic excitation in S = 1/2 onedimensional antiferromagnets.
The onedimensional 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 Heisenbergtype exchange interactions. Research has revealed that the simple S = 1/2 onedimensional antiferromagnet exhibits rich and exotic properties originating from the manybody quantum effects. In the case of the mostinvestigated system with isotropic Heisenbergtype interactions, the quantum fluctuations destroy the longrange 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 lowestenergy excitation in the S = 1/2 onedimensional antiferromagnet is a pair excitation of quasiparticles 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 Betheanzats 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 onedimensional antiferromagnet with Isinglike anisotropy CsCoCl_{3} through highfield 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 Heisenbergtype system, the antiferromagnetic order is maintained in the ground state of the S = 1/2 Isinglike onedimensional 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 fieldinduced 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 quantumcritical state with no longrange order. In the quantum critical state, we observe the appearance of a twostring 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 twostring to the twomagnon bound state, for the first time.
(Written by Shojiro Kimura on behalf of all the authors)
Magnetic Excitation in the S = 1/2 Isinglike Antiferromagnetic Chain CsCoCl_{3} in Longitudinal Magnetic Fields Studied by Highfield ESR Measurementss
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 92, 094701 (2023).
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