Fluctuation-Induced Heat Release from Temperature-Quenched Nuclear Spins near a Quantum Critical Point

The quasi-two-dimensional quantum antiferromagnet Cr(diethylenetriamine)(O$_{2})_{2}\cdot $H$_{2}$O [1] has a magnetic-field-tuned quantum critical point (QCP) at 12.3 T, where a highly polarized antiferromagnetic phase turns into a field-induced ferromagnetic phase. We report a novel relaxation phenomenon near this QCP: quantum-fluctuation-driven annealing of hydrogen nuclear spins frozen in a non-equilibrium high-energy state by temperature quenching. This relaxation phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, reveals the extent of a quantum critical region around the QCP and provides a unique avenue to investigate the dynamics of the divergent quantum fluctuations that underlie quantum criticality. [1] C. M. Ramsey \textit{et al}., Chem. Mater. \textbf{15}, 92 (2003).