Search for time reversal symmetry breaking in multiband superconductors

• Applicant: Dr. Vadim Grinenko, Ph.D.
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 273420190

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.

• DFG Programme: Research Grants