There are several FeSe based superconductors, including the bulk FeSe, monolayer FeSe thin film, intercalated ${\mathrm{K}}_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ and ${\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\mathrm{OHFeSe}$, 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, ${\mathrm{CsFe}}_{4-x}{\mathrm{Se}}_4$, 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 ${\gamma }_0=C/T{{|}_T}_{\to 0}$ 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 ${\mathrm{CsFe}}_{4-x}{\mathrm{Se}}_4$.

中文翻译：

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抑制铁-硒基莫特绝缘子CsFe4-xSe4引起的非常规超导

有几种基于FeSe的超导体，包括块状FeSe，单层FeSe薄膜，插层 ${\mathrm{ķ}}_X{铁}_{2-ÿ}{硒}_2$ 和 ${里}_{\mathrm{1个}-X}{铁}_X\mathrm{硒化铁}$等等。它们的正常状态都显示出金属行为。这里的关键因素是FeSe层，它以单层薄膜的形式展现出最高的超导转变温度。最近，一种新的基于FeSe的化合物${\mathrm{硫化铁}}_{4-X}{硒}_4$，其中的空间组 $乙米米米$被找到。有趣的是，尽管该系统与其他亲属共享相同的FeSe平面，但它仍表现出很强的类似绝缘子的行为。密度泛函理论计算表明，它应该是金属，与实验观察结果形成鲜明对比。在这里，我们通过施加压力来抑制这种类似绝缘子的行为，报告了非常规超导的出现。在环境压力下，不能将类似绝缘子的行为建模为带状绝缘子，但是可以通过相关系统的变程跳跃模型来描述它。此外，已经测量了比热低至400 mK，并且显着的残留系数${\gamma }_0=C/Ť{{|}_{Ť}}_{\to 0}$观察到，这与绝缘子状状态形成对比，并暗示了自旋动力学的量子自由度。通过施加压力，逐渐抑制了类似绝缘子的行为，系统变成了金属。最终，在约5.1 K处实现了超导性。超导转变很大程度上取决于磁场和所施加的电流，表明脆弱的超流体密度。我们的研究结果表明，超导性是由稀释的Cooper对在强相关背景下建立的。${\mathrm{硫化铁}}_{4-X}{硒}_4$。