论文标题
ba $ _ {\ rm 1-x} $ k $ _ {\ rm x} $ _ {2} $ as $ _ {2} $带Fermion配对和Quadrupling States
Calorimetric evidence for two phase transitions in Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_{2}$As$_{2}$ with fermion pairing and quadrupling states
论文作者
论文摘要
从理论上讲，打破多个对称性的材料在某些条件下允许在超导临界温度上方的四个弗里米式冷凝物形成。这些状态可以通过相波的稳定。最近，在ba $ _ {\ rm 1-x} $ _ {\ rm x} $ _ {\ rm x} $ fe $ _ {2} $ as $ _ {2} $ AS $ _ {2} $ [V. Grinenko等，Nat。物理。 17，1254（2021）]。新物质状态的证据来自Muon-Spin旋转，运输，热电和超声实验。观察特定的热异常是过渡到新物质状态的非常重要的标志。但是，从其他贡献的背景中解决的波动引起的特定热奇点通常非常具有挑战性。在这里，我们报告了在ba $ _ {\ rm 1-x}的特定热量中检测两个异常，$ _ {\ rm x} $ fe $ _ {2} $作为$ _ {2} $在零磁场上。较高温度下的异常伴随着自发的Nernst效果的出现，表明时间反转（$ Z_2 $）对称性损坏。较低温度的第二个异常与向零电阻态的过渡一致，表明超导性打破了$ u（1）$量规对称性。我们的数据为超导相变高于$ z_2 $相位的$ z_2 $相位形成提供了量热证据。
Theoretically, materials that break multiple symmetries allow, under certain conditions, the formation of four-fermion condensates above the superconducting critical temperature. Such states can be stabilized by phase fluctuations. Recently a fermionic quadrupling condensate that breaks the $Z_2$ time-reversal symmetry was reported in Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_{2}$As$_{2}$ [V. Grinenko et al., Nat. Phys. 17, 1254 (2021)]. Evidence for the new state of matter comes from muon-spin rotation, transport, thermoelectric, and ultrasound experiments. Observing a specific heat anomaly is a very important signature of a transition to a new state of matter. However, a fluctuation-induced specific heat singularity is usually very challenging to resolve from a background of other contributions. Here, we report on detecting two anomalies in the specific heat of Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_{2}$As$_{2}$ at zero magnetic field. The anomaly at the higher temperature is accompanied by the appearance of a spontaneous Nernst effect, indicating broken time-reversal ($Z_2$) symmetry. The second anomaly at the lower temperature coincides with the transition to a zero resistance state, indicating superconductivity breaking the $U(1)$ gauge symmetry. Our data provide calorimetric evidence for the $Z_2$ phase formation above the superconducting phase transition.
