Strong coupling of lattice and orbital excitations in the quantum magnet Ca₁₀Cr₇O₂₈: Anomalous temperature dependence of Raman phonons

We report low-temperature Raman signatures of the Heisenberg quantum magnet Ca10Cr7O28, showing clear anomalies in phonon mode frequencies and linewidths below similar to 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 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 reorganization, 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.