High-field ESR Spectroscopy of a Quantum Spin Liquid System CuHpCl

Yuji Inagaki, Takahiro Sakurai, Makoto Yoshida, Susumui Okubo, Hikomitsu Kikuchi, Keiichi Koyama and Hitoshi Ohta

High-filed ESR measurements are performed on a single crystal of the spin gap system
Cu2(C5H12N2)2Cl4 (CuHpCl). Observed ESR spectra at T = 1.7 K show beautiful hyperbolic anomalies
at around 200 GHz and 300 GHz when estimated g-values are plotted as a function of the resonance
frequency. The first anomaly point coincides with a critical magnetic field of Hc1 = 7.5 T (∼
10 K) where the magnetization starts to increase due to softening of a triplet excitation branch into a
singlet ground state, while the second one corresponds approximately to the higher energy excitation
mode reported by the inelastic neutron measurement studies at around 1.2 meV (14 K or 10 T). Observed
ESR modes at around Hc1 are explained by taking account of a small off-diagonal element,
typically 0.45 K, which is probably coming from DM interaction, in the energy scheme with the
singlet-triplet excitation gap of Δ/kB = 10.3 K. In addition, direct ESR transitions from the ground
state to excited triplet states are observed. On the basis of these experimental facts, we discuss the
nature of double spin excitations.

JPS Conf. Proc.

38

011141