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

Abstract

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 (2D) superconductor, electron-doped layered material ZrNCl. 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 2D BCS-BEC crossover without added complexities present in other solid-state systems.

Unconventional superconductivity due to interband polarization

“…This strikingly contrast our approach from other mechanisms for superconductivity (such as the KL mechanism) that rely on the presence of a Fermi surface. It also lead to distinctive predictions, such as a BEC-BCS crossover upon increasing the doping concentration, which has recently been observed in ZrNCl [7,36].…”

“…Superconductivity in low carrier density materials, such as WTe 2 [1][2][3], SrTiO 3 [4,5], ZrNCl [6,7], twisted bilayer [8][9][10] and trilayer graphene [11][12][13], is a subject of intense interest. In conventional electron-phonon superconductors, the vast difference between Fermi and Debye energies is responsible for the retarded nature of the phonon-mediated attraction.…”

Cooper quartet correlations in infinite symmetric nuclear matter

“…Such a phenomenon is called BCS-BEC crossover, which has been realized in ultracold Fermi atomic gases [12][13][14] and in superconductors [15,16], and its connections to nuclear systems have also been studied extensively [17,18]. In asymmetric nuclear matter, both the isovector and isoscalar pairing states can appear and compete with each other [19][20][21].…”

Many-body selection rule for quasiparticle pair creations in centrosymmetric superconductors

“…[4,5] is formulated within the meanfield theory, it is not clear whether it applies to ultra-strongcoupling superconductors close to the Bose-Einstein condensate (BEC) regime (k F ξ < 1). Addressing this issue is relevant for the optical study of the BCS-BEC crossover [6,7] recently observed in superconductors, including iron-based superconductors [8][9][10], twisted bilayer graphene [11], twisted trilayer graphene [12,13], and Li x ZrNCl [14].…”