Bose-Einstein condensation superconductivity induced by disappearance of the nematic state

Abstract

The crossover from the superconductivity of the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime holds a key to understanding the nature of pairing and condensation of fermions. It has been mainly studied in ultracold atoms, but in solid systems, fundamentally previously unknown insights may be obtained because multiple energy bands and coexisting electronic orders strongly affect spin and orbital degrees of freedom. Here, we provide evidence for the BCS-BEC crossover in iron-based superconductors FeSe1 − xSx from laser-excited angle-resolved photoemission spectroscopy. The system enters the BEC regime with x = 0.21, where the nematic state that breaks the orbital degeneracy is fully suppressed. The substitution dependence is opposite to the expectation for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in this system.

Quadrupolar charge dynamics in the nonmagnetic FeSe 1− x S x superconductors

“…5) An additional intensity which develops below TS (x ) at the lowest frequencies and is related to coupling between QEP response and acoustic lattice modes in the presence of a quasiperiodic array of the structural domain walls that appear in twinned crystals (55). 6) A weak feature at about 190 cm −1 that is related to interband transition between occupied β and unoccupied α bands (31).…”

Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$

“…Theoretical calculations [29] have shown that in the 2D case, when E F is small (BCS-BEC crossover), the system displays pseudogap behavior due to preformed pairs above T c . Given that K12442 is quasi-2D, and has shallow and incipient bands (very small E F ), the pseudogap probably originates from preformed Cooper pairs related to the BCS-BEC crossover [27,[29][30][31][32][33][34]. This scenario seems compatible with the doping dependence of the optical response as well.…”

“…Here, ∆ and E F correspond to the superconducting (SC) gap and the Fermi energy, respectively. A large ∆/E F ratio (∆/E F ∼ 1) may drive the system into the crossover regime between the weak-coupling Bardeen-Cooper-Schrieffer (BCS) and the strong-coupling Bose-Einstein condensation (BEC) limits [27][28][29][30][31][32][33][34]. One of the most intriguing phenomena in the BCS-BEC crossover regime is the opening of a pseudogap due to preformed Cooper pairs between the pairing temperature T * and the critical temperature T c at which the pairs condense into a phase-coherent quantum state [27][28][29][30].…”

Hidden Pseudogap and Excitation Spectra in a Strongly Coupled Two-Band Superfluid/Superconductor

“…Recently, the Bardeen-Cooper-Schrieffer to Bose-Einstein-condensation (BCS-BEC) crossover, where a weakly-interacting BCS state continuously changes to BEC of tightly bound molecules with increasing the attractive interaction [1][2][3][4], has gathered much attention due to its realization in ultracold Fermi gases [5,6]. Moreover, such a crossover phenomenon has been confirmed experimentally in multiband iron-based superconductors, such as the iron-chalcogenides family FeSe [7][8][9][10][11].…”