The condensation of electrons and holes in 2D photocells of ultrathin MoTe₂

In semiconductors, electrons and holes above a certain critical density may condense into a liquid phase containing a macroscopic population of highly correlated electrons and holes. Strikingly, under optical excitation, ultrathin MoTe₂ photocells undergo this transition even at room temperature (295 K). Using a new technique called Multi-Parameter Dynamic Photoresponse Microscopy (MPDPM) we image the photoresponse of two-dimensional graphene-MoTe₂-graphene photocells over several dynamical variables, including laser power, pump-probe delay and applied voltage. MPDPM reveals that above a critical threshold a ring-like spatial feature abruptly forms in the photoresponse as a function of increasing optical power. This pattern, combined with time-resolved photocurrent dynamics also revealed by MPDPM, provides strong evidence for the formation of an electron-hole condensate at room temperature.