BCS-like critical fluctuations with limited overlap of Cooper pairs in FeSe

Abstract

In conventional superconductors, very narrow superconducting fluctuation regions are observed above $T_c$ because of the strong overlapping of Cooper pairs in a coherence volume. In the bulk form of iron chalcogenide superconductor FeSe, it is argued that the system may locate in the crossover region of BCS to BEC, indicating a strong superconducting fluctuation. In this respect, we have carried out measurements of magnetization, specific heat and Nernst effect on FeSe single crystals in order to investigate the superconducting fluctuation effect near $T_c$. The region of diamagnetization induced by superconducting fluctuation seems very narrow above $T_c$. The crossing point of temperature dependent magnetization curves measured at different magnetic fields, which appears in many systems of cuprate superconductors and is regarded as indication of strong critical fluctuation, is however absent. The magnetization data can be scaled based on the Ginzburg-Landau fluctuation theory for a quasi-two-dimensional system, but the scaling result cannot be described by the theoretical function of the fluctuation theory because of the limited fluctuation regions. The specific heat jump near $T_c$ is rather sharp without the trace of strong superconducting fluctuation. This is also supported by the Nernst effect measurements which indicate a very limited region for vortex motion above $T_c$. Associated with very small value of Ginzburg number and further analyses, we conclude that the superconducting fluctuation is very weak above $T_c$ in this material. Our results are strongly against the picture of significant phase fluctuation in FeSe single crystals, although the system has a very limited overlapping of Cooper pairs in the coherence volume. This dichotomy provides new insights into the superconducting mechanism when the system is with a dilute superfluid density.

11 pages, 6 figures

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• Exotic Superconducting States in FeSe-based Materials
• On the Remarkable Superconductivity of FeSe and its Close Cousins
• Quantum vortex core and missing pseudogap in the multi-band BCS-BEC-crossover superconductor FeSe
• Fe-based superconducting thin films on metallic substrates: growth, characteristics and relevant properties
• Hydrostatic and uniaxial pressure tuning of iron-based superconductors: Insights into superconductivity, magnetism, nematicity and collapsed tetragonal transitions
• Thermodynamics of transition to BCS-BEC crossover superconductivity in FeSe${}_{1-x}$S${}_x$
• Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by ${}^{77}$Se-NMR
• Bulk superconductivity and role of fluctuations in the iron-based superconductor FeSe at high pressures
• Vortex-lattice melting and paramagnetic depairing in the nematic superconductor FeSe
• Superconducting fluctuation in FeSe investigated by precise torque magnetometry
• Superconductivity at 44.4 K achieved by intercalating EMIM+ into FeSe
• Precursor of Superfluidity in a Strongly Interacting Fermi Gas with Negative Effective Range
• Evolution of vortex pinning in the FeSe${}_{1-x}$S${}_x$ system
• Generalized Crossover in Interacting Fermions within the Low-Energy Expansion
• Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa${}_2$Fe${}_4$As${}_4$F${}_2$
• Apparent nonreciprocal transport in FeSe bulk crystals
• Local pairing versus bulk superconductivity intertwined by the charge density wave order in Cs(V${}_{1-x}$Ta${}_x$)${}_3$Sb${}_5$
• Unconventional superconductivity in Sc${}_2$Ir${}_{4-x}$Si${}_x$ by spin-orbit coupling driven flat band
• Fluctuating Cooper pairs in FeSe at temperatures exceeding double Tc
• Study on fluctuations of interface-enhanced superconductivity in ultrathin FeSe/SrTiO3 by the Nernst effect
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