Enhancement of the Superconducting Gap by Nesting inCaKFe4As4: A New High Temperature Superconductor

Abstract

We use high resolution angle resolved photoemission spectroscopy and density functional theory with experimentally obtained crystal structure parameters to study the electronic properties of CaKFe4As4. In contrast to related CaFe2As2 compounds, CaKFe4As4 has high Tc of 35K at stochiometric composition. This presents unique opportunity to study properties of high temperature superconductivity of iron arsenic superconductors in absence of doping or substitution. The Fermi surface consists of three hole pockets at Γ and two electron pockets at the M point. We find that the values of the superconducting gap are nearly isotropic, but significantly different for each of the FS sheets. Most importantly we find that the overall momentum dependence of the gap magnitudes plotted across the entire Brillouin zone displays a strong deviation from the simple cos(kx) cos(ky) functional form of the gap function, proposed in the scenario of the Cooper-pairing driven by a short range antiferromagnetic exchange interaction. Instead, the maximum value of the gap is observed for FS sheets that are closest to the ideal nesting condition in contrast to the previous observations in some other ferropnictides. These results provide strong support for the multiband character of superconductivity in CaKFe4As4, in which Cooper pairing forms on the electron and the hole bands interacting via dominant interband repulsive interaction, enhanced by FS nesting.

Anomalous phonon behavior in superconducting CaKFe4As4 : An optical study

“…3(b), two SC gaps and a Lorentz component reproduce the experimental data quite well. This two gaps are determined to be ∆ 1 ≃ 9 ± 0.2 meV and ∆ 2 ≃ 14 ± 0.5 meV, in good agreement with those obtained by ARPES measurements [16]. The strong reduction of the conductivity at low frequency for T << T c suggests the absence of nodes.…”

“…The onset of the absorption occurs around 120 cm −1 in the optical conductivity. Fitting the optical conductivTypeset by REVT E X ity with Marttis-Bardeen formalism yields two SC gaps, ∆ 1 ≃ 9 meV and ∆ 2 ≃ 14 meV, which agree well with recent ARPES studies [16]. The optical properties are very similar to those of Ba 0.6 K 0.4 Fe 2 As 2 [17,18].…”

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

“…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. 23,26) These results indicated that CaKFe 4 As 4 belongs to the same paradigm as other iron pnictide systems with respect to the presence of the disconnected Fermi surfaces that give rise to antiferromagnetic fluctuations in the wave vector Q AF = (π, π).…”

“…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. 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.…”

“…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.…”