Experimental investigation of the quantum phase transitions in spin-1/2 triangular lattice antiferromagnet

The two-dimensional spin-1/2 triangular lattice antiferromagnet (TLAF) is one of the simplest geometrically frustrated quantum magnets. The recently synthesized Na₂BaCo(PO₄)₂ (NBCP) is an ideal material incarnation of the spin-1/2 TLAF. Theoretically, it was proposed that the NBCP hosts a spin supersolid ground state that breaks both the lattice translational and spin U(1) symmetries. The applied magnetic field H could drive the system through Berezinskii-Kosterlitz-Thouless (BKT) melting of the spin supersolidity and many other richer quantum phases. However, the detection of these transitions is a huge challenge because the transitions onset at extremely low temperatures T at around 60 mK, and the measurement of the magnetic susceptibility requires high sensitivity. With the help of our newly developed gradient force magnetometer in a dilution refrigerator, we clearly map out the contour diagram of the magnetic susceptibility in the H-T phase diagram in T as cold as 20 mK. The sensitivity of the measurement is comparable to the commercial magnetometer. These results offer an ideal platform for investigations on the field-tunable quantum phase transitions and BKT melting of the spin supersolidity.