ESR and magnetic phase diagram of the weakly ordered spin-1/2 chain antiferromagnet Sr2CuO3

Aalto Quantum Physics Seminar (Nanotalo). Speaker: MSc. Elena Sergeicheva (Kapitza Institute, Moscow, Russia).

We report on the results of magnetic resonance and ultrasonic experiments in a weakly ordered antiferromagnet Sr2CuO3 known as one of the best realizations of a spin-1/2 chain Heisenberg system with extremely small inter- to intra-chain exchange constants ratio J'/J~10^{-4} (J=2800 K). The proximity to a Luttinger-liquid state reveals itself in a very low ordering temperature T_N=5.4K<<J and unusually small value of an order parameter 0.06µ_B [1].
Our electron-spin resonance studies [2] uncovered two different types of magnetic excitations in an ordered phase of Sr2CuO3. Apart from conventional Goldstone modes with small gaps induced by weak biaxial anisotropy, we observed a novel resonance branch arising below T_N which is supposedly related to a specific gapped longitudinal mode predicted to exist in the proximity to a critical point [3]. This gap decreases linearly with a magnetic field and softens at a field ~9T indicating quantum phase transition.
In addition, a magnetic phase diagram is established using high sensitive phase-locked loop ultrasonic measurements. The presence of the field-induced phase transition is confirmed, with the nature of a high field phase still remaining unclear. A considerable increase of T_N in magnetic field up to 20% at ~10T is also observed. Both findings challenge further theoretical studies of the system.

[1] K.M. Kojima et al., PRL 78, 1787 (1997).
[2] E.G. Sergeicheva et al., PRB 95, 020411(R) (2017).
[3] H.J. Schulz, PRL 77, 2790 (1996).