Field tunable BKT and quantum phase transitions in spin-1/2 triangular lattice antiferromagnet

TL;DR

This study investigates field-tunable quantum phase transitions and BKT melting in a spin-1/2 triangular lattice antiferromagnet, using advanced magnetometry to explore low-temperature magnetic susceptibility and phase diagrams.

Contribution

It introduces a new gradient force magnetometer enabling precise detection of quantum phase transitions at ultra-low temperatures in NBCP, revealing detailed BKT and supersolid phase behaviors.

Findings

Identification of field-driven BKT and quantum phase transitions.

Observation of giant magnetocaloric effects near phase boundaries.

Construction of detailed magnetic susceptibility phase diagram at 30 mK.

Abstract

Quantum magnetism is one of the most active fields for exploring exotic phases and phase transitions. The recently synthesized Na2BaCo(PO4)2 (NBCP) is an ideal material incarnation of the spin-1/2 easy axis triangular lattice antiferromagnet (TLAF). Experimental evidence shows that NBCP hosts the spin supersolid state with a giant magnetocaloric effect. It was also proposed that the applied magnetic field B can drive the system through Berezinskii-Kosterlitz-Thouless (BKT) and other richer quantum phase transitions. However, the detection of these transitions is challenging because they onset at extremely low temperature T at around 60 mK, and the measurement of the magnetic susceptibility of these transitions requires high sensitivity. With the help of our newly developed gradient force magnetometer in a dilution refrigerator, we constructed the contour diagram of the magnetic susceptibility in the B-T phase diagram in T as cold as 30 mK. These results provide a more comprehensive and accurate understanding of the several field-tunable quantum phase transitions and BKT melting of the spin supersolidity, which are especially significant when their giant magnetocaloric effects highlight potential applications for sub-Kelvin refrigeration under concerns about global helium shortages.