Giant elastocaloric effect at low temperatures above the ferroquadrupolar phase transition in TmVO₄

Ferroquadrupole order of local atomic states provides a realization of electronic nematic order. A key physical property associated with such order is the quadrupole (nematic) susceptibility, which diverges approaching the phase transition. Here, we present a new method to measure this quantity using an elastocaloric effect (ECE) technique. We choose the representative cooperative Jahn-Teller system TmVO₄, which undergoes ferroquadrupole order at 2.2 K, as a proof of principle. A simple Maxwell relation relates the ECE that we measure to the temperature derivative of the elastic modulus c₆₆, the softening of which also heralds the phase transition. A comparison of ECE data with results obtained from ultrasound measurements demonstrates that the temperature dependence of the quadrupole strain susceptibility approaching the critical temperature is indeed faithfully captured by the ECE measurement. We also compare the experimentally reconstructed entropy landscape in temperature-strain space to that from theory to further establish the extent to which the ECE measurement captures the physics of the material. Furthermore, because the specific material that we have studied orders at such a low temperature, and because the coupling to strain is necessarily large for Jahn-Teller materials, the ECE signature at low temperatures is very large. Observation of this giant ECE at low temperatures establishes the potential for using similar low-temperature ferroquadrupolar/nematic materials for elastocaloric cooling at cryogenic temperatures.