Superconductivity of the FeSe/SrTiO₃ Interface in View of BCS–BEC Crossover^*

Abstract

In paired Fermi systems, strong many-body effects exhibit in the crossover regime between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensation (BEC) limits. The concept of the BCS-BEC crossover, which is studied intensively in the research field of cold atoms, has been extended to condensed matters. Here, by analyzing the typical superconductors within the BCS-BEC phase diagram, we find that FeSe-based superconductors are prone to shift their positions in the BCS-BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of a single-layer FeSe on SrTiO 3 substrate, the superconductivity is relocated closer to the crossover unitary than other doped FeSe-based materials, indicating that the pairing interaction is effectively modulated. We further show that hole-doping can drive the interfacial system into the phase with possible prepaired electrons, demonstrating its flexible tunability within the BCS-BEC crossover regime.Based on the assumption of Fermi systems with attractive interaction, Bardeen-Cooper-Schrieffer (BCS) theory [1,2] has been well applied not only to electronic systems, but also applicable to other Fermi systems, such as the superfluid state of paired Fermionic atoms [3]. Accordingly the BCS picture can be linked to the concept of Bose-Einstein condensation (BEC), as demonstrated in experiments by directly tuning the attractive interaction or scattering length of ultra-cold atoms [4][5][6][7]. In this context, the

NMR Evidence for Universal Pseudogap Behavior in Quasi-Two-Dimensional FeSe-Based Superconductors

“…Except for bulk FeSe, in most heavily electron-doped FeSe-based superconductors including K0.8Fe2Se2 and (Li,Fe)OHFeSe, the 𝛥/𝐸F is closer to the BCS limit rather than the BCS-BEC crossover regime. [59,61] In our case, since the organic-ion-intercalated FeSe-based superconductors own a comparable pairing gap and similar electron doping level with (Li,Fe)OHFeSe, the BCS-BEC crossover is also less possible for the emergence of preformed Cooper pairing.…”

Heterostructural one-unit-cell FeSe/SrTiO3: from high-temperature superconductivity to topological states

“…The spin-fluctuation channel is the electronic pairing intrinsic to FeSe with heavy electron doping, and the EPC channel is the phononic pairing extrinsic to FeSe by interface engineering [94]. The cooperative picture is indirectly supported by the summarized BCS-Bose-Einstein condensation (BEC) phase diagram (T c /T F −Δ/E F ; T F : Fermi temperature) for FeSe-derived superconductors [115]. To be specific, while FeSe and FeSe derivatives with purely electronic pairing are either relatively close to the BCS limit, or relatively close to the BEC limit, 1-UC FeSe is located in the middle and closer to the BCS-BEC crossover regime (unitary point) in contrast.…”

Superconductivity enhancement in FeSe/SrTiO3: a review from the perspective of electron–phonon coupling

“…The BCS-BEC crossover in FeSe-based bulk superconductors have been studied by various techniques including ARPES [136][137][138], STS [139] and magnetic torque measurements [140]. Recently, by analyzing the typical superconductors within the BCS-BEC phase diagram, Zhang et al [141] nd that FeSe-based superconductors are prone to shift their positions in the BCS-BEC crossover regime by charge doping or substrate substitution, since their Fermi energies and the superconducting gap sizes are comparable. Especially at the interface of 1 uc-FeSe/STO, the superconductivity is shifted closer to the crossover unitary than other doped FeSebased materials, as shown in gure 11, indicating that the pairing interaction has been effectively modulated.…”