Discontinuity in the transport of strongly correlated two-dimensional hole systems in zero magnetic field

Adopting undoped ultraclean two-dimensional hole systems, we approach a strongly correlated limit by reducing the carrier density down to 1 x 10(9) cm(-2). The temperature dependence of the resistivity as a function of the carrier density reveals a characteristic energy scale displaying a benchmark critical behavior near a critical density of p(c) similar to 4 x 10(9) cm(-2). The insulating state below p(c) exhibits a sharp resistance discontinuity in response to heating across a critical temperature T-c1 similar to 30 mK, consistent with a first-order transition. The dc response also identifies a second critical temperature T-c2 where linear IV behavior is recovered. Similar effects are also demonstrated by varying an external electric field. The results support a complex quantum phase transition with intermediate phases.