Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS–BEC crossover

Abstract

The physics of the crossover between weak-coupling Bardeen–Cooper–Schrieffer (BCS) and strong-coupling Bose–Einstein condensate (BEC) limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature Tc=8.5 K has been found to be deep inside the BCS–BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large non-linear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below T*∼20 K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance and transport data give evidence of pseudogap formation at ∼T*. The multiband superconductivity along with electron–hole compensation in FeSe may highlight a novel aspect of the BCS–BEC crossover physics.

s+is superconductivity with incipient bands: Doping dependence and STM signatures

“…From (16) it follows that there is a sign change of the order parameter between hole and electron pockets. Inserting then the values of |∆ h |, |∆ e | obtained from (14), (15), into (16), we arrive at…”

“…At T = T c we can linearize the self-consistency equations (14) and (15) and obtain (assuming |µ e,h1,h2 | ≫ T c ):…”

“…Interestingly, a recent quasiparticle interference (QPI) study of FeSe 11 has revealed that the ratio of the superconducting gap to the Fermi energy on the shallow electron band is very large ∆/E Fe ∼ 1 with similar ratios ∆/E F ≈ 0.6 being reported for FeSe x Te 1−x 7,12,13 . This has been followed up by observation of strong superconducting fluctuations in FeSe far above T c [14][15][16] . The large values of ∆/E F observed initially in FeSe have been interpreted 11 as a signature of a crossover from conventional Cooper pairing (BCS) to Bose-Einstein condensation (BEC) regime 17 .…”

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1−xSx

“…FeSe shows no long-range magnetic order at ambient pressure, but complex magnetic fluctuations are present at high energies over a large temperature range [9]. Below T s , the spin-lattice relaxation rate from NMR experiments is enhanced as it captures the low-energy tail of the stripe spin-fluctuations [10,11]. Furthermore, recent µSR studies invoke the close proximity of FeSe to a magnetic quantum critical point as the muon relaxation rate shows unusual temperature dependence inside the nematic state [12].The changes in the electronic structure and magnetic fluctuations of FeSe can have profound implication on its transport and superconducting properties.…”

Intrinsic pinning by naturally occurring correlated defects in FeSe1−xTexsuperconductors

“…: the "11" compounds, have the simplest structure but a very complex electronic and magnetic behaviour [1].…”