Spin-Orbital Entangled Liquid State in the Copper Oxide Ba$_3$CuSb$_2$O$_9$

TL;DR

This paper reviews the discovery of a spin-orbital entangled liquid state in Ba₃CuSb₂O₉, where spins and orbitals remain fluctuating without freezing, indicating a novel quantum state stabilized by entanglement.

Contribution

It reports the identification of a spin-orbital liquid state in a copper oxide, highlighting the role of entanglement between spins and orbitals in stabilizing this novel phase.

Findings

No magnetic or Jahn-Teller transition down to low temperatures

Spins and orbitals remain fluctuating and entangled

Evidence of a spin-orbital liquid state

Abstract

Structure with orbital degeneracy is unstable toward spontaneous distortion. Such orbital correlation usually has a much higher energy scale than spins, and therefore, magnetic transition takes place at a much lower temperature, almost independently from orbital ordering. However, when the energy scales of orbitals and spins meet, there is a possibility of spin-orbital entanglement that would stabilize novel ground state such as spin-orbital liquid and random singlet state. Here we review on such a novel spin-orbital magnetism found in the hexagonal perovskite oxide Ba$_3$CuSb$_2$O$_9$, which hosts a self-organized honeycomblike short-range order of a strong Jahn-Teller ion Cu$^{2+}$. Comprehensive structural and magnetic measurements have revealed that the system has neither magnetic nor Jahn-Teller transition down to the lowest temperatures, and Cu spins and orbitals retain the hexagonal symmetry and paramagnetic state. Various macroscopic and microscopic measurements all indicate that spins and orbitals remain fluctuating down to low temperatures without freezing, forming a spin-orbital entangled liquid state.