Electron-hole liquid in a van der Waals heterostructure photocell at room temperature

Condensation - the familiar process underlying the formation of clouds and the distillation of ethyl alcohol into whiskey - is the phase transition of gas into liquid droplets. In semiconductors, at sufficiently high electron-hole (e-h) densities or low temperatures, the gas of non-equilibrium electrons and holes may undergo condensation into one of several potential liquid-like phases. In this talk, I present recent results on the gas-to-liquid phase transition of electrons and holes in ultrathin van der Waals heterostructure photocells revealed through multi-parameter dynamic photoresponse microscopy (MPDPM). By combining rich visualization with comprehensive analysis of very large data sets acquired through MPDPM, we find that ultrafast laser excitation at a graphene-molybdenum ditelluride-graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power. These patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional e-h liquid. While our imaging experiments mark the first observation (in over 50 years of study) of an e-h liquid at room temperature, I will discuss our results within the greater context of strongly correlated electronic condensates.