Rapidly fluctuating orbital occupancy above the orbital ordering transition in spin-gap compounds

TL;DR

This paper investigates the rapid orbital occupancy fluctuations above the orbital ordering transition in spin-gap compounds, revealing a phonon-glass to phonon-crystal transition driven by orbital-liquid states and strong orbital-lattice coupling.

Contribution

It provides a microscopic analysis of thermal conductivity showing orbital fluctuations and dynamic bond-length variations above the transition in spin-dimer systems.

Findings

Orbital occupancy fluctuates rapidly above the transition temperature.

Strong orbital-lattice coupling causes glass-like thermal conductivity behavior.

The phonon-glass to phonon-crystal transition is observed in multiple spin-dimer systems.

Abstract

Several spin systems with low dimensionality develop a spin-dimer phase within a molecular orbital below TS, competing with long-range antiferromagnetic order. Very often, preferential orbital occupancy and ordering are the actual driving force for dimerization, as in the so-called orbitally-driven spin-Peierls compounds (MgTi2O4, CuIr2S4, La4Ru2O10, NaTiSi2O6, etc.). Through a microscopic analysis of the thermal conductivity k (T) in La4Ru2O10, we show that the orbital occupancy fluctuates rapidly above TS, resulting in an orbital-liquid state. The strong orbital-lattice coupling introduces dynamic bond-length fluctuations that scatter the phonons to produce a k (T) proportional to T (i.e. glass-like) above TS. This phonon-glass to phonon-crystal transition is shown to occur in other spin-dimer systems, like NaTiSi2O6, pointing to a general phenomenon.