Real-space BCS-BEC crossover in FeSe monolayers

Atomic-Site-Dependent Pairing Gap in Monolayer FeSe/SrTiO3(001)–(√13 × √13)

“…FeSe resides in the crossover regime between weak coupling Bardeen–Cooper–Schrieffer (BCS) and strong coupling Bose–Einstein condensation (BEC), − for the large pairing gap magnitude and the relatively small Fermi energy. − Given the increasing experimental supports for the presence of fluctuated Cooper pairs in the monolayer FeSe, − the lattice-scale varied pairing gap with the particle–hole asymmetry observed here signals the preformed pairing, which interpreted the wide resistivity transition. − In contrast to the missing pseudogap feature in bulk FeSe, which has been attributed to the multiorbital screening effect, , the pseudogap-like state in the monolayer FeSe emerges upon exclusive M-centered electron pockets crossing the Fermi surface (the Γ-centered hole bands shift downward to approximately −80 meV below E F ) . − Momentum resolution is requisite to disclose the BCS-BEC crossover mechanism in multiband systems, e.g., quasi-particle interference spectrum and ARPES.…”

Lateral Quantum Confinement Effect on High-TC Superconducting FeSe Monolayer

“…18 Different substrates have also been used to tune the band edge of FeSe monolayers; a FeSe monolayer grown on trilayer graphene (TLG) features Δ/E F = 0.3. 40 Bulk FeSe exhibits uniquely tiny hole (E F ∼ 10 meV) and electron pockets (E F ∼ 3 meV) at the Fermi surface, 14 and Δ = 2.3 or 1.5 meV for the hole or electron band, respectively. 41 Thus, Δ/E F ∼ 0.23 or 0.5 for the hole or electron band of bulk FeSe.…”

“…Specifically, the upward triangles in Figure h refer to YBa 2 Cu 3 O x , leftward triangles to La 2‑x Sr x CuO 4 , and rightward triangles to Tl 2 Ba 2 Ca 2 Cu 3 O 10 , Bi 2 Sr 2 CaCu 2 O 8 , Bi 2 Sr 2 Ca 2 Cu 3 O 10 , Tl 2 Ba 2 CaCu 2 O 8 . − In iron-based superconductors, the effects of chemical doping have been explored, and Δ/ E F of bulk Fe 1+x Te 0.6 Se 0.4 can be raised from 0.16 to 0.50 by reducing x . Different substrates have also been used to tune the band edge of FeSe monolayers; a FeSe monolayer grown on trilayer graphene (TLG) features Δ/ E F = 0.3 . Bulk FeSe exhibits uniquely tiny hole ( E F ∼ 10 meV) and electron pockets ( E F ∼ 3 meV) at the Fermi surface, and Δ = 2.3 or 1.5 meV for the hole or electron band, respectively .…”

Electronic inhomogeneity and phase fluctuation in one-unit-cell FeSe films

“…Between these two limits, namely, the BCS-BEC crossover regime, incoherent fermion pairing occurs due to the Fermi surface instability at a pairing temperature T p , forming the so-called pseudogap phase, while superfluidity of coherent pairs occurs at a lower temperature T c . Given the characteristic superconducting gap magnitude Δ ~15–20 meV and the relatively small Fermi energy E F ~56 meV, one-unit-cell (1 UC) FeSe on SrTiO 3 (001) resides in the BCS-BEC crossover regime 5 – 7 . In coincidence with the crossover scenario, the hitherto observed zero-resistance temperature of 20–30 K is much lower than the pairing temperature T p of 50–83 K, as disclosed by angle-resolved photoemission spectroscopy (ARPES) investigations 8 – 15 .…”