Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossover.
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.
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
S
http://orcid.org/0000-0002-6007-9617
S
Kasahara
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
T
T
Yamashita
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
A
http://orcid.org/0000-0001-5831-4924
A
Shi
Research Center for Low Temperature and Materials Sciences, Kyoto University, Kyoto 606-8501, Japan.
R
R
Kobayashi
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
Y
Y
Shimoyama
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
T
T
Watashige
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
K
K
Ishida
Research Center for Low Temperature and Materials Sciences, Kyoto University, Kyoto 606-8501, Japan.
T
T
Terashima
Institute of Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe D-76021, Germany.
T
T
Wolf
Institute of Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe D-76021, Germany.
F
F
Hardy
Institute of Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe D-76021, Germany.
C
C
Meingast
Institute of Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe D-76021, Germany.
H V
HV
Löhneysen
Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
A
A
Levchenko
Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan.
T
T
Shibauchi
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
Y
Y
Matsuda
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