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

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.

arXiv comment5 pages, 3 figures

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