抑制铁硒基莫特绝缘体CsFe4-xSe4引起的非常规超导电性
Unconventional superconductivity induced by suppressing an iron-selenium based Mott insulator CsFe4-xSe4
摘要：
FeSe基超导体有多种，包括体FeSe、单层FeSe薄膜、插层KxFe 2-ySe 2和Li 1-xFexOHFeSe等，它们的正常态都表现出金属性。这里的关键是FeSe层，其以单层薄膜的形式表现出最高的超导转变温度。最近发现了一种新的FeSe基化合物CsFe_（4-x）Se_（4-x），空间群为Bmmm，有趣的是，该体系虽然与其他同类化合物具有相同的FeSe面，但仍表现出很强的绝缘体性质。密度泛函理论计算表明它应该是一种金属，与实验观察形成鲜明对比。在这里，我们报告了非常规超导性的出现，通过施加压力来抑制这种绝缘体样的行为。在环境压力下，类似绝缘体的行为不能被建模为带状绝缘体，但可以通过相关系统的可变距离跳跃模型来描述。此外，还测量了低至400 mK的比热，并得到了很大的剩余系数γ_0 =C/T|观察到T->0，这与类绝缘体状态形成对比，并表明自旋动力学的某些量子自由。通过施加压力，绝缘体的行为逐渐被抑制，并且系统变成金属，最终在约5.1K处实现超导性。超导转变强烈依赖于磁场和外加电流，表明一个脆弱的超流密度。我们的研究结果表明，超导电性建立稀释库珀对顶部的强关联背景在CsFe 4-xSe 4。
英文摘要：
There are several FeSe based superconductors, including the bulk FeSe, monolayer FeSe thin film, intercalated KxFe2-ySe2 and Li1-xFexOHFeSe, 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, CsFe4-xSe4 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->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 CsFe4-xSe4.
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