Berlin 2024 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 18: Superconductivity: Poster

TT 18.16: Poster

Montag, 18. März 2024, 15:00–18:00, Poster C

⁷⁵As NMR investigations of the quartic-metal phase in Ba_1−xK_xFe₂As₂ — •F. Bärtl^1,2, S. Luther¹, E. Babaev³, J. Wosnitza^1,2, H. Kühne¹, R. Sarkar², K. Kihou⁴, C.-H. Lee⁴, H.-H Klauss², and V. Grinenko^2,5 — ¹HLD-HZDR, Dresden — ²IFMP-TUD, Dresden — ³Department of Physics, KTH Royal Institute of Technology, Stockholm — ⁴AIST, Tsukuba — ⁵TDLI, Shanghai Jiao Tong University

Ba_1−xK_xFe₂As₂ is a hole-doped superconductor (SC), for which samples in a doping range of 0.7 ≤ x ≤ 0.8 yield a broken-time-reversal-symmetry (BTRS) SC state (s + is). Recently, an electron-quadrupling state (pairing of Cooper pairs) was reported to form above T_c in this doping range [1], which is not possible within the mean-field BCS theory. Instead, this is a fluctuations-stabilized phase that requires the existence of pre-formed pairs above T_c, which can manifest itself as a pseudogap. The indication for superconducting fluctuations well above T_c in BKFA with an s+is state was found in measurements of the spontaneous Nernst effect [1], but spectroscopic evidence is lacking so far. Here, we present ⁷⁵As NMR measurements of Ba_0.25K_0.75Fe₂As₂, where we employ spectroscopy and relaxometry to probe the low-energy density of states at temperatures ranging across the presumed quartic-metal phase. We find a weak slope change of the temperature-dependent nuclear spin-lattice relaxation rate at temperatures above T_c = 7 K and 8 T, indicating the manifestation of a pseudo-gap-like state below T^* ≈ 12 K in the bulk of the sample.

[1] Grinenko, V. et al. Nat. Phys. 17, 1254 (2021)

Keywords: superconductivity; pnictide; electron-quadrupoling; pseudogap; nmr