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Register a new userEvidence for exclusion of the possibility of $d$-wave superconducting-gap symmetry in Ba-doped KFe$_2$As$_2$
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We have investigated the superconducting(SC)-gap anisotropy for several Ba-doped KFe$_2$As$_2$ samples using laser-based angle-resolved photoemission spectroscopy. We show that the SC-gap anisotropy and node positions drastically change with a small amount of Ba doping. Our results totally exclude a possibility of $d$-wave symmetry and strongly suggest that both spin and orbital fluctuations are important for the paring interaction in the Ba-doped K122.
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The in-plane resistivity $rho$ and thermal conductivity $kappa$ of extremely overdoped KFe$_2$As$_2$ ($T_c$ = 3.0 K) single crystal were studied. It is found that $rho sim T^{1.5}$ at low temperature, a typical non-Fermi liquid behavior of electrons scattered by antiferromagnetic spin fluctuations. In zero field, we observed a large residual linear term $kappa_0/T$, about one third of the normal-state value. In low magnetic fields, $kappa_0/T(H)$ increases very fast. Such a behavior of $kappa_0/T$ mimics the d-wave cuprate superconductors, therefore provides clear evidence for nodes in the superconducting gap of KFe$_2$As$_2$. Based on the Fermi surface topology of KFe$_2$As$_2$, it is believed that the dominant intraband pairing via antiferromagnetic spin fluctuations results in the unconventional superconducting gap with nodes.
The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional cite{3D_SC} (3D) electronic structure of these high-temperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the KZ-capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on BKFAOP samples. We found a marked KZ dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer $Delta_1$ and interlayer $Delta_2$ pairing. The anisotropy ratio $Delta_2/Delta_1$, determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling $J_c/J_{ab}$ in the parent compound cite{NeutronParent}. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.
The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor with Tc = 32K (BKFA) was determined from angle-resolved photoemission spectroscopy (ARPES) via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner Gamma-barrel, X-pocket, and blade-pocket, and equals 9 meV, while the gap on the outer Gamma-barrel is estimated to be less than 4 meV, resulting in 2Delta/kT_c=6.8 for the large gap, and 2Delta/kT_c<3 for the small gap. A large (77 pm 3%) non-superconducting component in the photoemission signal is observed below T_c. Details of gap extraction from ARPES data are discussed in Appendix.
We report a doping dependent electronic Raman scattering measurements on iron-pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals. A strongly anisotropic gap is found at optimal doping for x=0.065 with $Delta_{max}sim 5Delta_{min}$. Upon entering the coexistence region between superconducting (SC) and spin-density-wave (SDW) orders, the effective pairing energy scale is strongly reduced. Our results are interpreted in terms of a competition between SC and SDW orders for electronic state at the Fermi level. Our findings advocate for a strong connection between the SC and SDW gaps anisotropies which are both linked to interband interactions.
We report measurements of ac magnetic susceptibility $chi_{ac}$ and de Haas-van Alphen (dHvA) oscillations in KFe$_2$As$_2$ under high pressure up to 24.7 kbar. The pressure dependence of the superconducting transition temperature $T_c$ changes from negative to positive across $P_c sim 18$ kbar as previously reported. The ratio of the upper critical field to $T_c$, i.e, $B_{c2} / T_c$, is enhanced above $P_c$, and the shape of $chi_{ac}$ vs field curves qualitatively changes across $P_c$. DHvA oscillations smoothly evolve across $P_c$ and indicate no drastic change in the Fermi surface up to 24.7 kbar. Three dimensionality increases with pressure, while effective masses show decreasing trends. We suggest a crossover from a nodal to a full-gap $s$ wave as a possible explanation.
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