Nodeless multiband superconductivity in stoichiometric single-crystalline CaKFe4As4

Signature of multigap nodeless superconductivity in CaKFe4As4

“…The superconducting gap values obtained from the two-gap model fits are in excellent agreement with the reported values obtained from the STM measurements for this material [13] and also consistent with other layered Fe-based superconductors, especially from the µSR and ARPES measurements of optimally doped (Ba,K,Rb)Fe 2 As 2 system [33,34]. These are, however, more than double the values reported from the TDR measurements of CaKFe 4 As 4 [13]. Our finding of two superconducting gaps in CaKFe 4 As 4 is not unsurprising and is very much consistent with other members of Fe-based superconductors.…”

“…Along with high T c values, the A e AFe 4 As 4 family also show a very high upper critical field (≈ 92 T for CaKFe 4 As 4 ) [11], similar to the optimally doped (Ba,K)Fe 2 As 2 [12] which is well above the Pauli paramagnetic limit H p (0) = 1.86 * T c for weak-coupling BCS superconductors. Measurements of the London penetration depth λ(T ) using a Tunneldiode resonator (TDR) and tunneling conductance from a Scanning tunneling microscope (STM) spectroscopy show two nodeless gaps in CaKFe 4 As 4 with a larger gap of between 6-9 meV and a smaller gap of between 1-4 meV [13]. High-resolution angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations support multi-band superconductivity in CaKFe 4 As 4 , in which Cooper pairs form in the electron and the hole bands interacting via a dominant interband repulsive interaction enhanced by Fermi surface (FS) nesting [14].…”

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

“…22,23) This multiband nature implies multi-gap superconductivity in CaKFe 4 As 4 . Two superconducting gaps which are nodeless and isotropic were, indeed, observed by scanning tunneling microscopy, 20) optical conductivity, 24) penetration depth, 21) muon spin rota- * k iida@cross.or.jp tion, 18) and 75 As nuclear magnetic resonance (NMR) 25) measurements. The largest superconducting gaps (∆ hole = 13 meV and ∆ electron = 12 meV) for quasi-two-dimensional hole and electron pockets that have similar diameters at the Γ and M points were reported by ARPES studies, 22) suggesting that the ideal nesting condition with the (π, π) wave vector referred to the tetragonal reciprocal lattice.…”

“…One can, therefore, expect the same spin-fluctuationmediated s ± -wave pairing mechanism 27,28) for this material, which was proposed in previous studies. [20][21][22][23]25) However, the sign of the superconducting gaps in CaKFe 4 As 4 has never been investigated via a phase-sensitive technique. As is well established, the appearance of neutron spin resonance depends sensitively on the relative signs of the superconducting gaps on different portions of the Fermi surfaces separated by momentum Q AF , and thus it serves as a direct probe of the symmetry of the superconducting order parameter.…”

“…16,18) Various measurements 15,16,[18][19][20][21] indicated that CaKFe 4 As 4 shows behaviors similar to optimally or slightly overdoped (Ba 1−x K x )Fe 2 As 2 . Angle-resolved photoemission spectroscopy (ARPES) measurements supported by density functional theory calculations indicated that the Fermi surface consists of three-hole pockets at the Γ point and two electron pockets at the M point.…”