Kavli Affiliate: Gang Su
| First 5 Authors: Junsen Xiang, Junsen Xiang, , ,
| Summary:
Bose-Einstein condensation (BEC), a macroscopic quantum phenomenon arising
from phase coherence and bosonic statistics, has been realized in quantum
magnets. Here, we report the observation of a universal magnetocaloric effect
(MCE) near a BEC quantum critical point (QCP) in copper sulfate crystal
($CuSO_4 cdot 5H_2O$). By conducting magnetocaloric and nuclear magnetic
resonance measurements, we uncover a field-driven BEC QCP, evidenced by the
universal scaling law $T_c propto (B_c – B)^2/3$ and the perfect data
collapse of the magnetic Gr"uneisen ratio. Thermal excitation triggers a
dimensional crossover to a 1D quantum-critical regime, where the MCE scaling
strictly matches the universality class of 1D Fermi gases. Notably, the
quantum-critical MCE enables cooling down to 12.8 mK without helium-3, with
very fast thermal relaxation rate that is critical for high cooling power. This
work demonstrates the universal MCE in magnon BEC systems, using a common
copper sulfate compound as a paradigmatic example, and paves the way for
next-generation sub-Kelvin cooling.
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