Bardeen-Cooper-Schrieffer (BCS) superfluidity and Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems. We report crossover behavior from the BCS limit to the BEC limit realized by varying carrier density in a two-dimensional superconductor, electron-doped zirconium nitride chloride. The phase diagram, established by simultaneous measurements of resistivity and tunneling spectra under ionic gating, demonstrates a pseudogap phase in the low-doping regime. The ratio of the superconducting transition temperature and Fermi temperature in the low–carrier density limit is consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. These results indicate that the gate-doped semiconductor provides an ideal platform for the two-dimensional BCS-BEC crossover without added complexities present in other solid-state systems.

Bardeen-Cooper-Schrieffer（BCS）超流动性和Bose-Einstein凝聚（BEC）是成对的费米子系统基态的两个极端限制。我们报告了从BCS极限到BEC极限的交叉行为，该极限行为是通过改变二维超导体电子掺杂氮化锆中的载流子密度实现的。通过在离子门控下同时测量电阻率和隧穿光谱而建立的相图，表明了低掺杂状态下的伪间隙相。在低载流子密度极限内，超导转变温度与费米温度之比与BCS-BEC跨界理论中预期的理论上限一致。