该研究团队报道了对FeSe1-xSx超导体进行的缪子自旋弛豫(μSR)测量结果,涵盖了0 ≤ x ≤ 0.22范围内的正交(向列)晶相和四方晶相。研究发现,在所有组分中,超导转变温度Tc以下,零磁场缪子弛豫速率增加,表明超导态在向列相和四方相均破坏了时间反演对称性。此外,横向磁场μSR测量结果显示,在四方相(x > 0.17)中,超流体密度意外而显著地降低。
这意味着在零温极限下仍有相当比例的电子未成对,无法用已知具有点状或线状节点的非常规超导态来解释。四方相中的时间反演对称性破缺、抑制的超流体密度以及已报道的增强零能激发与具有Bogoliubov费米面的超节点配对态一致。这些结果揭示了FeSe1-xSx中两种不同的破坏时间反演对称性的超导态,在向列临界点分隔开,需要微观起源理论来解释向列性和超导性之间的关系。
据介绍,Iron-chalcogenide超导体FeSe1-xSx具有独特的电子性质,例如非磁性向列序和其量子临界点,这些性质对于理解非常规超导机制至关重要。最近的理论提出了在该体系中可能存在一种全新的超导态,即所谓的Bogoliubov费米面(Bogoliubov Fermi surfaces, BFSs)。然而,这种超节点对称的配对态需要在超导态中破缺时间反演对称性 (TRS),而这一点尚未在实验中观察到。
附:英文原文
Title: Two superconducting states with broken time-reversal symmetry in FeSe1-xSx
Author: Matsuura, Kohei, Roppongi, Masaki, Qiu, Mingwei, Sheng, Qi, Cai, Yipeng, Yamakawa, Kohtaro, Guguchia, Zurab, Day, Ryan P., Kojima, Kenji M., Damascelli, Andrea, Sugimura, Yuichi, Saito, Mikihiko, Takenaka, Takaaki, Ishihara, Kota, Mizukami, Yuta, Hashimoto, Kenichiro, Gu, Yilun, Guo, Shengli, Fu, Licheng, Zhang, Zheneng, Ning, Fanlong, Zhao, Guoqiang, Dai, Guangyang, Jin, Changqing, Beare, James W., Luke, Graeme M., Uemura, Yasutomo J., Shibauchi, Takasada
Issue&Volume: 2023-5-15
Abstract: Iron-chalcogenide superconductors FeSe1-xSx possess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe1-xSx superconductors for 0 ≤ x ≤ 0.22 covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperature Tc for all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-field μSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x > 0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe1-xSx, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity.
DOI: 10.1073/pnas.2208276120
Source: https://www.pnas.org/doi/10.1073/pnas.2208276120