Search for time reversal symmetry breaking in multiband superconductors

在传统金属中，磁性和超导电性是相互对立的。然而，有几种超导体，其中超导可以与磁性共存，只有少数例子是已知的，当超导本身诱导自发磁性。在这个项目中，我们在中心对称多带超导体Ba1-xKxFe2As2(0.7x&lt；0.85)的宽广S波SC区内发现了一种新型的S+是具有时间反转对称性的新型超导(SC)相。此外，我们详细的比热研究表明，BTRS穹顶看起来非常接近费米面的拓扑变化(Lifshitz转变)。从而从实验上证明了当电子系统发生拓扑变化时，会出现一种新的量子态。类似地，在单轴应变下利用零场Muon自旋旋转/弛豫(ZF-µSR)测量，我们发现了对Sr2RuO4中两组分手征序参数的强烈支持，这是最已知的具有TRSB态的化合物。然而，尽管零外磁场中增强的Muon自旋去极化是TRSB超导电性的一个公认的经验指标，但与TRSB态相关的自发电场的微观起源尚不清楚。在更新项目中，我们解决了三个主题：1)我们的目标是阐明具有TRSB态的超导体中自发磁场的起源。通过对两个具有代表性的超导体Ba1-xKxFe2As2和Sr2RuO4的重离子和电子辐照样品的系统的µSR研究，我们将探索增强的Muon自旋去极化是由于TRSB序参数的幅度和相位的空间变化引起的边缘、磁畴壁和缺陷的静电场。2)在具有TRSB态的Ba1-xKxFe2As2体系中，我们在相图中发现了一个附加的未知相变，其位置恰好位于TRSB顶端的T_c之上。结合热力学、输运和局域探测技术，我们的目标是探索这种未知相变的本质。3)我们还将利用µSR技术继续研究单轴应变对接近Lifshitz相变的多带超导电性的影响。我们的目标是确认和研究在本项目中观察到的单轴应变下KFe2As2中的TRBS超导电性，我们将继续研究单轴应变对Sr2RuO4中TRSB相的影响。4)此外，我们还将继续在FeSe_(1-x)(S，Te)_x体系中寻找接近向列相量子临界点的TRSb超导电性。预期的结果将有助于揭示自发磁场的性质和超导体与TRSB态的配对机制。

In conventional metals magnetism and superconductivity are antagonistic to each other. However, there are several families of superconductors, in which superconductivity may coexist with magnetism, and only a few examples are known, when superconductivity itself induces spontaneous magnetism. Within this project, we have discovered a narrow dome of a novel s + is superconducting (SC) phase with broken time-reversal symmetry (TRSB) inside the broad s-wave SC region of the centrosymmetric multiband superconductor Ba1-xKxFe2As2 (0.7 < x < 0.85). Furthermore, our detailed specific heat study reveals that the BTRS dome appears very close to a change in the topology of the Fermi surface (Lifshitz transition). Thereby, the emergence of a novel quantum state at the topological change of the electronic system has been experimentally demonstrated. Similarly, using zero-field muon spin rotation/relaxation (ZF- µSR) measurements under uniaxial strain, we have found strong support for a two-component chiral order parameter in Sr2RuO4, which is the most known compound with a TRSB state. However, in spite of the fact that an enhanced muon spin depolarization in zero external magnetic field is an accepted empirical indicator of TRSB superconductivity, the microscopic origin of the spontaneous fields associated with a TRSB state is unknown. Within the renewal project we address three topics: 1) We target to elucidate the origin of the spontaneous magnetic fields in superconductors with a TRSB state. Using a systematic µSR study of heavy-ion and electron irradiated samples of two representative superconductors Ba1-xKxFe2As2 and Sr2RuO4 we will explore theoretical proposals that the enhanced muon spin depolarization is due to static fields at edges, domain walls, and defects, caused by the spatial variation in amplitude and phase of the TRSB order parameter. 2) In the Ba1-xKxFe2As2 system with TRSB state we found an additional unknown phase transition just above Tc on the top of the TRSB dome in the phase diagram. Combining thermodynamic, transport and local probe techniques we target to explore the nature of this unknown phase transition. 3) Also, we will continue to study the effect of the uniaxial strain on multiband superconductivity in the systems that are close to a Lifshitz transition using µSR technique. We target to confirm and investigate a TRBS superconductivity in KFe2As2 under uniaxial strain observed within this project and we will continue to investigate an effect of the uniaxial strain on TRSB phase in Sr2RuO4. 4) In addition, we will continue our search for a TRSB superconductivity close to nematic quantum critical point in the FeSe1-x(S,Te)x system. The anticipated results will shed light both on the nature of the spontaneous magnetic fields and on the pairing mechanism in superconductors with TRSB states.