Universal Magnetocaloric Effect near Quantum Critical Point of Magnon Bose-Einstein Condensation

TL;DR

This paper demonstrates a universal magnetocaloric effect near a quantum critical point in a magnon Bose-Einstein condensate, enabling efficient cooling to very low temperatures without helium-3 and revealing universal scaling laws.

Contribution

It provides the first experimental observation of universal MCE near a magnon BEC quantum critical point in copper sulfate, linking quantum criticality with practical cooling applications.

Findings

Universal scaling law $T_c \\propto (B_c - B)^{2/3}$ confirmed

Magnetic Gr"uneisen ratio data collapse demonstrates universality

Achieved cooling down to 12.8 mK without helium-3

Abstract

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.