Unusual BCS-BEC crossover in FeSe<sub>1-<i>x</i></sub>S<sub><i>x</i></sub> superconductors
ORAL
Abstract
The BCS-BEC crossover from strongly overlapping Cooper pairs to non-overlapping
composite bosons in the strong coupling limit has been a long-standing issue of interacting many-body fermion systems. It has been extremely challenging to realize such a crossover state in electronic systems over the past decade. Recently, FeSe semimetal emerged as a high-Tc superconductor located in the BCS-BEC crossover regime, owing to its very small Fermi energies. In FeSe, however, an ordinary BCS-like heat-capacity jump is observed at Tc, posing a fundamental question on the characteristics of the BCS-BEC crossover. Here we report on high resolution heat capacity, magnetic torque, and scanning tunneling spectroscopy measurements in FeSe1-xSx. When the nematic order is suppressed at x > 0.17, an unusual BEC-like transition is found in the thermodynamic quantities with giant superconducting fluctuations extending far above Tc. However, no pseudogap formation is observed in the tunneling spectra. These results illuminate highly unusual features of the BCS-BEC crossover in superconductors with multiband electronic structure and competing electronic instabilities.
composite bosons in the strong coupling limit has been a long-standing issue of interacting many-body fermion systems. It has been extremely challenging to realize such a crossover state in electronic systems over the past decade. Recently, FeSe semimetal emerged as a high-Tc superconductor located in the BCS-BEC crossover regime, owing to its very small Fermi energies. In FeSe, however, an ordinary BCS-like heat-capacity jump is observed at Tc, posing a fundamental question on the characteristics of the BCS-BEC crossover. Here we report on high resolution heat capacity, magnetic torque, and scanning tunneling spectroscopy measurements in FeSe1-xSx. When the nematic order is suppressed at x > 0.17, an unusual BEC-like transition is found in the thermodynamic quantities with giant superconducting fluctuations extending far above Tc. However, no pseudogap formation is observed in the tunneling spectra. These results illuminate highly unusual features of the BCS-BEC crossover in superconductors with multiband electronic structure and competing electronic instabilities.
*This work has been supported by KAKENHI (Nos.19H00649,18H05227) from JSPS.
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Presenters
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Yuta Mizukami
- Department of Advanced Materials Science, The University of Tokyo
- Univ of Tokyo-Kashiwanoha