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Journal of the Physical Society of Japan

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

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

Published Jul 28, 2023• 5 citations
Publisher

Abstract

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.

DOI
10.7566/jpsj.92.094701

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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.
We report a high-resolution terahertz spectroscopic study of quantum spin dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound BaCo_{2}V_{2}O_{8} as a function of temperature and longitudinal magnetic field. Confined spinon excitations are observed in an antiferromagnetic phase below T_{N}≃5.5 K. In a field-induced gapless phase above B_{c}=3.8 T, we identify many-body string excitations as well as low-energy fractional psinon or antipsinon excitations by comparing to Bethe ansatz calculations. In the vicinity of B_{c}, the high-energy string excitations are found to have a dominant contribution to the spin dynamics as compared with the fractional excitations.
Magnetic excitations of the exchange-dipole quasi-one-dimensional XXZ antiferromagnet are studied in the ordered phase. We observe a transformation of the electron spin resonance (ESR) spectrum when crossing the N\'eel temperature near 0.2 K. The single-mode ESR of a correlated XXZ chain transforms in the multimode spectrum in the ordered phase. The multimode spectrum consists mainly of the intensive mode of a single correlated chain, which is surrounded and/or indented by numerous weak satellites. The number of securely fixed modes is eight at $\mathbf{H}\ensuremath{\parallel}b$ and twelve at $\mathbf{H}\ensuremath{\parallel}a$. In addition to the multimode resonance observed at the transverse polarization of the microwave and static magnetic fields, we reveal a wide band of absorption by (kkk,--kkk) pairs of quasiparticles at the longitudinal polarization. This kind of absorption of microwaves occurs both in the ordered and specific spin-liquid phases, revealing the presence of quasiparticles in the specific spin-liquid phase.
This article is a review of the recent achievements in experimental studies of magnetic excitations in the copper pyrimidine dinitrate [PM–Cu(NO3)2(H2O)2]n (PM = pyrimidine), an S = 1/2 antiferromagnetic chain material with an alternating g-tensor and the Dzyaloshinskii–Moriya interaction, by means of high-field electron spin resonance (ESR). Due to this alternation, in the 1D critical regime this material exhibits a field-induced gap. The excitation spectrum is formed by solitons and their bound states, breathers, and can be effectively described using the sine-Gordon model. With increasing temperature, the soliton-breather regime can be suppressed, resulting in a substantial evolution of ESR parameters. These changes can be described using a new theoretical concept recently proposed for S = 1/2 AF chains with a staggered transverse field. High magnetic field induces a transition into the spin-polarized state with the excitation spectrum formed by magnons. Nonmonotonous behavior of the field-induced gap is observed in vicinity of the saturation field. Experimental data are compared with results of existing theoretical approaches, revealing excellent agreement with predictions.
Abstract A spin-1/2 Heisenberg antiferromagnetic chain is one of the most important paradigmatic models in quantum magnetism. Its ground state is a spin singlet, while the excitation spectrum is formed by gapless fractional excitations, spinons. The presence of alternating g-tensors and/or the staggered Dzyaloshinskii-Moriya interaction results in opening the energy gap Δ∝H2/3\varDelta \propto H^{2/3}Δ∝H2/3 Δ ∝ H 2 / 3 , once the magnetic field H is applied. A fairly good understanding of this phenomenon was achieved in the framework of the sine-Gordon quantum-field theory, taking into account the effective transverse staggered field induced by the applied uniform field. The theory predicts solitons and antisolitons as elementary excitations, as well as their bound states, breathers. Here, I review recent high-field electron spin resonance spectroscopy studies of such systems, focusing on peculiarities of their spin dynamics in the sine-Gordon regime and beyond.
The magnetic properties of $\mathrm{Cs}{\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{Cl}}_{3} (x=0.073)$ are studied by electron-spin-resonance (ESR), magnetic-susceptibility, and heat-capacity measurements to elucidate the edge state in the finite chains of the S=2S=2S=2 quasi-one-dimensional Heisenberg antiferromagnet CsCrCl3{\mathrm{Cl}}_{3}Cl3​. The results of the ESR measurements in the temperature range 14-40 K are well explained by an effective S=1S=1S=1 fractional spin. A crossover between different Curie constants observed in the susceptibility measurements is also explained by assuming S=1S=1S=1 free spins at the ends of the finite chains below 40 K. The appearance of the S=1S=1S=1 fractional spin can be understood by the valence-bond-solid model.