German-Ukrainian Seminar on Quantum Materials

Lecture #7: March 16 2023 at 14:00 Kyiv time (13:00 Dresden time)

Speaker: Hans-Henning Klauss, TU Dresden

Title: Degenerated superconductivity and fluctuation induced phases in multiband systems: Sr2RuO4 and Ba1-xKxFe2As2



From the very beginning of quantum mechanics the concept of degeneracy of states has been an important topic throughout the quantum world. For example, the Landau-Zener dynamics of avoided level crossing describes quantum tunneling in single-molecule magnets as well as the deficiency in the flux of solar electron neutrinos at the earth. In solid state physics degeneracy can lead to multi-component order-parameters in symmetry broken phases such as superconductors and magnets. The topologically non-trivial skyrmion phase in the itinerant ferromagnet MnSi can be described as a triple-q antiferromagnet. 

In particular in multi-band superconductors the degeneracy of superconducting order parameters with the same or different symmetry can lead to a coherent superposition. Unlike in magnets these states are experimentally difficult to detect. One indication is the appearance of time-reversal symmetry breaking (TRSB) superconductivity.

In my talk I will discuss two important systems in this context. In Sr2RuO4 we used uniaxial and hydrostatic pressure muon spin relaxation (µSR) experiments [1,2] to prove the two-component nature of the superconducting state using the concept of explicit symmetry breaking under uniaxial pressure. I will present uniaxial pressure studies along (100) and (110) directions in this system and discuss the implications on the superconducting state.

 In Ba1-xKxFe2As2 we found TRSB superconductivity in µSR experiments at high doping x~0.8 due to the competition of single-component order parameters with different symmetry [3]. This competition appears close to a Lifshitz transition of the multi-band fermi surface. Supported by detailed thermodynamic and transport experiments we found a new TRSB phase also above TC. This state can be understood as a fluctuation-induced Z2 symmetry breaking with four fermion correlations [4]. 

[1] V. Grinenko, S. Ghosh,  et al.,  Nat. Phys. 17, 748 (2021), [2] V. Grinenko, et al., Nat. Comm. 12, 3920 (2021), [3] V. Grinenko, et al., Nat. Phys. 16, 789 (2020), [4] V. Grinenko, et al., Nat. Phys. 17, 1245 (2021).