There are several FeSe based superconductors, including the bulk FeSe, monolayer FeSe thin film, intercalated K${}_x$Fe${}_{2-y}$Se${}_2$ and Li${}_{1-x}$Fe${}_x$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, CsFe${}_{4-x}$Se${}_4$ with the space group of $Bmmm$ was found. Interestingly the system shows a strong insulator-like 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 insulator-like behavior. At ambient pressure, the insulator-like behavior cannot be modeled as a band insulator, but 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 insulator-like state and suggests some quantum freedom of spin dynamics. By applying pressure the insulator-like 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 CsFe${}_{4-x}$Se${}_4$.

中文翻译：

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通过抑制铁硒基莫特绝缘子CsFe4Se4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K${}_X$铁${}_{2-ÿ}$硒${}_2$ 和李${}_{\mathrm{1个}-X}$铁${}_X$OHFeSe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CsFe${}_{4-X}$硒${}_4$ 与空间群 $乙米米米$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，但它仍表现出强大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，类似绝缘子的行为不能建模为带状绝缘子，但是可以通过相关系统的变程跳跃模型来描述。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$观察到，这与类似绝缘子的状态形成对比，并暗示了自旋动力学的量子自由度。通过施加压力，逐渐抑制了类似绝缘子的行为，并且该系统变成了金属，最终在约5.1 K处实现了超导性。超导转变很大程度上取决于磁场和施加的电流，表明脆弱的超流体密度。我们的结果表明，超导电性是由稀释的Cooper对在CsFe中的强相关背景之上建立的${}_{4-X}$硒${}_4$。