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

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