Gate-controlled ground state crossover in a two-dimensional superconductor

In the paired fermion systems, the Bardeen-Cooper-Schrieffer (BCS) superfluidity and Bose-Einstein condensation (BEC) are two extreme limits of the ground state. In a new report in Science, Yuji Nakagawa and a team of scientists in applied physics, quantum electronics, emergent matter science and materials research in Japan, reported crossover behavior from the BCS limit to the BEC limit by varying the carrier density in a 2D superconductor electron-doped, layered material ZrNCl containing intercalated layered nitride. The team showed how the ratio of the superconducting transition temperature and Fermi temperature in the low carrier density limit was consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. The results indicated how the gate-doped semiconductor provided an ideal platform for the 2D BCS-BEC crossover without additional complexities as those noted in other solid-state systems.