Qiuyang Li
U-M Physics

Excitons and Light-Matter Interaction in Two-Dimensions

Two-dimensional (2D) van der Waals (vdW) semiconductors possess unique properties: atomically thin thickness, well-defined exciton state, and strong transition oscillator strength, providing an interesting platform for many-body interactions and optical and electronic applications. In this talk, I will discuss exciton and exciton-polariton properties in two different 2D vdW semiconductors. In the first part, I will report strongly coupled exciton-polaritons of an anisotropic 2D magnet, CrSBr, integrated with an anisotropic photonic crystal (PhC) cavity. In this system, the spin, atomic lattice, and photonic lattices anisotropies are strongly correlated, giving rise to exceptionally strong coupling between engineered anisotropic optical modes and anisotropic excitons in CrSBr and a highly tunable polariton polarization by tens of degrees by many means. In the second part, I will discuss how the twist between two CrSBr monolayers leads to tunable exciton states and anisotropies. Cavity- and twist-tuning on 2D materials provide new opportunities for both fundamental understanding of quantum materials and photonic technology.


Dechen Zhang
U-M Physics

Field Tunable BKT and Quantum Phase Transitions in Spin-1/2 Triangular Lattice Antiferromagnet

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, detecting these transitions is a huge challenge 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 temperatures 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.