Unexpected magnetic phase in the weakly ordered spin-12 chain cuprate Sr2CuO3

E. G. Sergeicheva; S. S. Sosin; D. I. Gorbunov; S. Zherlitsyn; G. D. Gu; I. A. Zaliznyak

doi:10.1103/PhysRevB.101.201107

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Unexpected magnetic phase in the weakly ordered spin-12 chain cuprate Sr2CuO3

E. G. Sergeicheva, S. S. Sosin, D. I. Gorbunov, S. Zherlitsyn, G. D. Gu, I. A. Zaliznyak

DOI 10.1103/PhysRevB.101.201107 · Physical Review B

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Abstract

The magnetic phase diagram of a spin-12 chain antiferromagnet Sr2CuO3 is studied by an ultrasound phase-sensitive detection technique. The system is in the extreme proximity of the Luttinger-liquid quantum-critical point and we observe an unusually strong effect of magnetic field, which is very weak compared to the in-chain interaction, on the Néel ordering temperature. Inside the ordered phase, we detect an unexpected, field-induced continuous phase transition. The transition is accompanied by softening of magnetic excitation observed by electron-spin resonance, which in previous work [E. G. Sergeicheva et al., Phys. Rev. B 95, 020411(R) (2017)] was associated with a longitudinal (amplitude) mode of the order parameter. These results suggest a transition from a transverse collinear antiferromagnet to an amplitude-modulated spin-density-wave phase in a very weak magnetic field, which is unexpected for a system of weakly coupled Heisenberg spin-12 chains.