Gate-controlled BCS-BEC crossover in a two-dimensional superconductor*

The Bardeen-Cooper-Schrieffer (BCS) condensation and Bose-Einstein condensation (BEC) are the two limiting ground states of paired Fermion systems, and the crossover between these two limits has been a source of excitement for both fields of high temperature superconductivity and cold atom superfluidity. Here we report the two-dimensional (2D) BCS-BEC crossover realized in a gate-controlled superconductor, electron doped layered material ZrNCl, and the associated transport properties. To observe this phenomenon, we utilized an ionic gating method, which is well known as a powerful tool to control the carrier density in a large scale and induced 2D superconductivity.

We have succeeded in controlling the carrier density by nearly two-orders of magnitude, and establishing an electronic phase diagram through the simultaneous experiments of resistivity and tunneling spectra on the ionic gating devices. We found T_c exhibits dome-like behavior, and more importantly, a wide pseudogap phase in the low doping regime. In the low carrier density limit, T_c scales as T_c/T_F = 0.12, where T_F is the Fermi temperature, which shows fair agreement with the theoretical prediction for the 2D BEC-BEC crossover. Furthermore, through the systematic Hall effect measurements, we have clarified the evolution of vortex dynamics along the crossover combined with the time-dependent Ginzburg-Landau theory. These results demonstrate that the Li intercalated ZrNCl and its gate-controlled superconductivity are ideal platforms towards investigations of unexplored properties in BEC superconductors.