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TT: Fachverband Tiefe Temperaturen

TT 16: Superconductivity: Properties and Electronic Structure II

TT 16.3: Vortrag

Dienstag, 18. März 2025, 10:00–10:15, H32

NMR evidence of pseudogap and against spin magnetism in the time-reversal symmetry breaking state of Ba_1−xK_xFe₂As₂ — •F. Bärtl¹, F. Caglieris^2,3,4, Y. Li⁵, Q. Hu⁵, Y. Zheng⁵, C.-H. Yim⁵, J. Wosnitza^1,6, R. Sarkar², H.-H. Klauss², E. Babaev⁷, J. Garaud⁸, H. Kühne¹, and V. Grinenko⁵ — ¹HLD-EMFL, HZDR, Dresden — ²University of Genoa — ³CNR-SPIN, Genoa — ⁴IFW, Dresden — ⁵TDLI, Shanghai Jiao Tong University — ⁶IFMP, TU Dresden — ⁷Department of Physics, KTH, Stockholm — ⁸Institut Denis Poisson, Université de Tours

Recently, we focused on the investigation of samples from the narrow doping range of 0.7 ≤ x ≤ 0.8 in the hole-doped superconductor Ba_1−xK_xFe₂As₂. Here, the proximity to a Lifshitz transition results in a multiband s + is superconducting state, which spontaneously breaks time-reversal symmetry (BTRS), manifested as spontaneous currents around non-magnetic impurities. This is theoretically predicted and was experimentally revealed by µ⁺SR measurements, where the depolarization rate below T_c is only non-zero in the mentioned narrow doping regime. Moreover, the µ⁺SR together with Nernst-effect measurements suggest the emergence of the BTRS already at a temperature T_c^Z₂ > T_c, which indicates a behavior beyond mean-field approximation, described by a four-fermion order parameter, hence termed quartic metal. Here, we present ⁷⁵As NMR spectroscopy and relaxometry measurements of a sample with x = 0.77, which contradict the presence of conventional spin magnetism and hint at pseudogap behavior in the critical regime.