Observation of the orbital quantum dynamics in the spin-1/2 hexagonal antiferromagnet Ba3CuSb2O9

TL;DR

This study investigates orbital quantum fluctuations in Ba3CuSb2O9, revealing an orbital liquid state in a quantum spin liquid, using multi-frequency ESR to observe dynamic orbital behavior at low temperatures.

Contribution

It provides direct evidence of orbital quantum fluctuations and an orbital liquid state in a spin liquid candidate, using multi-frequency ESR measurements.

Findings

Orbital quantum fluctuations persist below 20 K.

No orbital ordering observed in stoichiometric crystal down to 1.5 K.

Orbital liquid state evidenced by g-factor anisotropy changes.

Abstract

We explore orbital dynamics in the spin liquid candidate Ba3CuSb2O9 using multi-frequency electron spin resonance. We prepared two high quality single crystals. The crystal with a slight copper deficiency shows a structural phase transition at around 200 K due to the cooperative Jahn-Teller effect, accompanied with orbital ordering. In contrast, the crystal with almost perfect stoichiometry shows no orbital ordering down to the lowest temperature of 1.5 K. Dramatic change in the g-factor anisotropy as a function of frequency and temperature demonstrates orbital quantum fluctuations at a nearly constant time scale of ~ 100 ps below 20 K, evidencing the emergence of an orbital liquid state in this quantum spin liquid compound.