Quantum solid phase and Coulomb drag in two-dimensional electron-electron bilayers of MoS2

Coulomb drag experiments can give us information about the interaction state of double-layer systems. Here, we demonstrate anomalous Coulomb drag behaviours in a two-dimensional electron-electron bilayer system constructed by stacking atomically thin MoS2 on opposite sides of thin dielectric layers of boron nitride. In the low temperature regime, the measured drag resistance does not follow the behaviour predicted by the Coulomb drag models of exchanging momenta and energies with the particles in Fermi-liquid bilayer systems. Instead, it shows an upturn to higher and higher values. We investigate quantum solid/fluid phases and the Kosterlitz-Thouless/Wigner two-dimensional quantum melting transition in this bilayer system and describe this interesting phenomenon based on thermally activated carriers of quantum defects from the formation of the correlation-induced electron solid phases with enhanced stabilization by the potential due to the boron nitride dielectric layers.Comment: published in Advanced Electronic Material

Grain-Boundary Roughening in Colloidal Crystals

In polycrystals, faceted grains may become round and rough at high temperatures. Such a roughening phenomenon remains poorly understood, partly because of the lack of experimental observations. Here, we directly visualize the roughening dynamics of grain boundaries inside thin-film colloidal crystals at the single-particle level using video microscopy. The thermal fluctuations of grain boundaries appear to exhibit both static and dynamic critical-like behaviors, in contrast to the Kosterlitz-Thouless transition in typical free surface roughening. The roughening point shifts towards the melting point as the grain boundary’s mismatch angle θ decreases and is preempted by melting when θ<18°. Counterintuitively, the amplitude of grain-boundary fluctuations decreases above the roughening point. This could be attributed to the observed widening of the grain boundary. The roughening strongly affects the mobility of the grain boundary but not the stiffness. These results provide new guidance for the control of microstructures in polycrystals and further development of roughening theory