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Unconventional Superconductivity Induced by Suppressing an Iron-Selenium-Based Mott Insulator CsFe4xSe4

Jin Si, Guan-Yu Chen, Qing Li, Xiyu Zhu, Huan Yang, and Hai-Hu Wen
Phys. Rev. X 10, 041008 – Published 12 October 2020
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Abstract

There are several FeSe based superconductors, including the bulk FeSe, monolayer FeSe thin film, intercalated KxFe2ySe2 and Li1xFexOHFeSe, etc. Their normal states all show metallic behavior. The key player here is the FeSe layer, which exhibits the highest superconducting transition temperature in the form of monolayer thin film. Recently, a new FeSe based compound, CsFe4xSe4, with the space group of Bmmm was found. Interestingly, the system shows a strong insulatorlike behavior, although it shares the same FeSe planes as other relatives. Density functional theory calculations indicate that it should be a metal, in sharp contrast with the experimental observations. Here, we report the emergence of unconventional superconductivity by applying pressure to suppress this insulatorlike behavior. At ambient pressure, the insulatorlike behavior cannot be modeled as a band insulator, but it can be described by the variable-range-hopping model for correlated systems. Furthermore, the specific heat down to 400 mK has been measured, and a significant residual coefficient γ0=C/T|T0 is observed, which contrasts the insulatorlike state and suggests some quantum freedom of spin dynamics. By applying pressure, the insulatorlike behavior is gradually suppressed, and the system becomes a metal; finally, superconductivity is achieved at about 5.1 K. The superconducting transition strongly depends on magnetic field and applied current, indicating a fragile superfluid density. Our results suggest that the superconductivity is established by diluted Cooper pairs on top of a strong correlation background in CsFe4xSe4.

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  • Received 7 May 2020
  • Revised 31 July 2020
  • Accepted 11 August 2020

DOI:https://doi.org/10.1103/PhysRevX.10.041008

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

  1. Research Areas
  1. Physical Systems
Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jin Si, Guan-Yu Chen, Qing Li, Xiyu Zhu, Huan Yang, and Hai-Hu Wen*

  • National Laboratory of Solid State Microstructures and Department of Physics, Center for Superconducting Physics and Materials, Collaborative Innovation Center for Advanced Microstructures, Nanjing University, Nanjing 210093, China

  • *hhwen@nju.edu.cn

Popular Summary

For more than 60 years, the field of superconductivity has been dominated by the so-called Bardeen-Cooper-Schrieffer (BCS) theory, which describes superconductivity as an emergent behavior from electrons pairing up in a special way. And yet, many superconductors, such as those based on copper oxides or iron, deviate from the predictions of BCS theory. Exploring these deviations could provide researchers with a more complete understanding of superconductivity, though such superconductors are generally difficult to realize. Here, we report on just such an unconventional superconductivity arising when pressure is applied to an iron-selenide-based compound, CsFe4xSe4.

This recently discovered material exhibits strong insulatorlike behavior, even though theory predicts it should be a metal. In our experiments, we successfully synthesize a high-quality sample, which allows us to explore some of its unusual properties. We confirm its insulating behavior at ambient pressure. By applying pressure, we find that the material first transitions from an insulator to a metal. Then, at a temperature of about 5.1 K, it transitions further into a superconductor. The exact temperature depends on the strength of an external magnetic field and applied current, suggesting a fragile superfluid density.

Our results indicate that the high-pressure superconductivity in CsFe4xSe4 is established by inducing diluted electron pairs on top of the FeSe planes, which exhibit a background of strong correlation among its electrons. We believe that this system can provide a powerful experimental platform for exploring similar approaches to unconventional superconductivity.

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Vol. 10, Iss. 4 — October - December 2020

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