Department of Applied Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan.
Quantum-Phase Electronics Center, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan.
Science. 2021 Apr 9;372(6538):190-195. doi: 10.1126/science.abb9860. Epub 2021 Mar 18.
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.
巴丁-库珀-施里弗（BCS）超流性和玻色-爱因斯坦凝聚（BEC）是配对费米子系统基态的两个极端极限。我们通过在二维超导体（电子掺杂氮化锆氯）中改变载流子密度，报告了从 BCS 极限到 BEC 极限的交叉行为。通过在离子门控下同时测量电阻率和隧道谱，建立了相图，在低掺杂区域显示出赝能隙相。在低载流子密度极限下，超导转变温度与费米温度的比值与 BCS-BEC 交叉区域中预期的理论上限一致。这些结果表明，栅极掺杂半导体为二维 BCS-BEC 交叉提供了一个理想的平台，而没有其他固态系统中存在的复杂情况。
