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

Abstract

The Bardeen-Cooper-Schrieffer (BCS) condensation and the 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 condensation to the BEC realized in the two-dimensional (2D) superconductor, electron doped layered material ZrNCl. The phase diagram, established by simultaneous experiments of resistivity and tunneling spectra under the ionic gating, demonstrates the pseudogap phase at the low doping regime. In the low carrier density limit, $T_{\mathrm{BKT}}$ (Berezinskii-Kosterlitz-Thouless transition temperature for 2D superconductors) scales as $T_{\mathrm{BKT}}/T_{\mathrm{F}}=0.12$, where $T_{\mathrm{F}}$ is the Fermi temperature, which is consistent with the theoretical upper bound expected in the BCS-BEC crossover regime. The present results indicate that the gate-doped semiconductor provides an ideal platform for the 2D BCS-BEC crossover without any added complexity, such as magnetic orders and density waves.

49 pages, 12 figures

References in corpus (13)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Advanced capabilities for materials modelling with Quantum ESPRESSO
• Field-induced superconducting phase of FeSe in the BCS-BEC cross-over
• Localization of preformed Cooper-pairs in disordered superconductors
• Observation of pseudogap behavior in a strongly interacting Fermi gas
• Observation of pair condensation in the quasi-2D BEC-BCS crossover
• Fermi surface of the most dilute superconductor
• Condensation, excitation, pairing, and superfluid density in high-$T_c$ superconductors: magnetic resonance mode as a roton analogue and a possible spin-mediated pairing
• High-temperature Superconductivity in Layered Nitrides β-Li${}_x$MNCl (M = Ti, Zr, Hf): Insights from Density-functional Theory for Superconductors
• Two-Dimensional Valley Electrons and Excitons in Noncentrosymmetric 3R MoS${}_2$
• Unconventional Superconductivity in electron-doped layered metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I)
• Anisotropic superconducting gaps in YNi${}_2$B${}_2$C: A first-principles investigation
• Universal enhancement of superconductivity in two dimensional semiconductors at low doping by electron-electron interaction