Strongly correlated two-dimensional plasma explored from entropy measurements
Charged plasma and Fermi liquid are two distinct states of electronic matter intrinsic to dilute
two-dimensional electron systems at elevated and low temperatures, respectively. Probing
their thermodynamics represents challenge because of lack of an adequate technique. Here,
we report a thermodynamic method to measure the entropy per electron in gated structures. Our technique appears to be three orders of magnitude superior in sensitivity to a.c. calorimetry, allowing entropy measurements with only 108 electrons. This enables us to investigate the correlated plasma regime, previously inaccessible experimentally in twodimensional
electron systems in semiconductors. In experiments with clean two-dimensional electron system in silicon-based structures, we traced entropy evolution from the plasma to Fermi liquid regime by varying electron density. We reveal that the correlated plasma regime can be mapped onto the ordinary non-degenerate Fermi gas with an interaction-enhanced
temperature-dependent effective mass. Our method opens up new horizons in studies of low-dimensional electron systems.