Strong coupling of lattice and orbital excitations in quantum magnet Ca$_{10}$Cr$_7$O$_{28}$: Anomalous temperature dependence of Raman phonons

TL;DR

This study reveals anomalous temperature-dependent Raman phonon behavior in the quantum magnet Ca$_{10}$Cr$_7$O$_{28}$, indicating a secondary Jahn-Teller transition that influences orbital and spin interactions at low temperatures.

Contribution

It uncovers a novel secondary Jahn-Teller transition in Ca$_{10}$Cr$_7$O$_{28}$, linking orbital reorganization to anomalies in phonon modes and magnetic interactions.

Findings

Anomalies in phonon frequencies and linewidths below 100 K.

Evidence of a secondary Jahn-Teller transition affecting orbital reorientation.

Orbital reorganization impacts spin exchange interactions at low temperatures.

Abstract

We report low-temperature Raman signatures of the Heisenberg quantum magnet Ca$_{10}$Cr$_7$O$_{28}$, showing clear anomalies in phonon mode frequencies and linewidths below $\sim$100 K. This crossover temperature lies in between the Jahn-Teller (JT) temperature scale ($>$ room temperature) and the temperature scale associated with the spin exchange interactions ($<$ 12 K). Our experimental observation is well captured by a novel secondary JT transition associated with a cooperative reorientation of the orbitals giving rise to anomalies in the temperature dependence of Raman frequencies and linewidths. Such orbital reorganisation, in turn, affects the spin-spin exchange interactions that decide the fate of the magnet at lower temperatures and hence provide important clues to understand the energetics of the possible lower temperature quantum paramagnetic phase.