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Register a new userThe Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ superconducting four-gap temperature evolution: a multi-band Chebyshev-BdG approach
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We generalize the Chebyshev-Bogoliubov-deGennes method to treat multi-band systems to address the temperature dependence of the superconducting (SC) gaps of iron based superconductors. Four SC gaps associated with different electron and hole pockets of optimally doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ were clearly identified by angle resolved photo-emission spectroscopy. The few approaches that reproduces with success this gap structure are based on strong-coupling theories and required many adjustable parameters. We show that an approach with a redistribution of electron population between the hole and electron pockets $ u$ with evolving temperature reproduces the different coupling ratios $2Delta^{ u}(0)/k_{rm B} T_c$ in these materials. We define the values that fit the four zero temperature gaps $Delta^{ u}(0)$ and after that all $Delta^{ u}(T)$ is obtained without any additional parameter.
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Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ($|Delta_k|$). Here, we report the measurement of a strain-induced gap modulation ($partial|Delta_k|$) in uniaxially strained Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ utilizing angle-resolved photoemission spectroscopy and $in$-$situ$ strain-tuning. We found that the uniaxial strain drives Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ into a nematic superconducting state which breaks the four-fold rotational symmetry of the superconducting pairing. The superconducting gap increases on the $d_{yz}$ electron and hole pockets while it decreases on the $d_{xz}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The $d_{xz}$ and $d_{yz}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intra-orbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $partial|Delta_k|$ as an effective way to characterize the superconducting pairing from a perturbation perspective.
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.
Superfluid density ($n_s$) in the mixed state of an iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is determined by muon spin rotation for a sample with optimal doping ($x=0.4$). The temperature dependence of $n_s$ is perfectly reproduced by the conventional BCS model for s-wave paring, where the order parameter can be either a single-gap with $Delta=8.35(6)$ meV [$2Delta/k_BT_c=5.09(4)$], or double-gap structure with $Delta_1=12$ meV (fixed) [$2Delta_1/k_BT_c=7.3$] and $Delta_2=6.8(3)$ meV [$2Delta_2/k_BT_c=4.1(2)$]. The latter is consistent with the recent result of angle-resolved photo-emssion spectroscopy. The large gap parameters ($2Delta/k_BT_c$) indicate extremely strong coupling of carriers to bosons that mediate the Cooper pairing.
We performed optical spectroscopy measurement on a superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal with T$_c$=37 K. Formation of the superconducting energy gaps in the far-infared reflectance spectra below T$_c$ is clearly observed. The gap amplitudes match well with the two distinct superconducting gaps observed in angle-resolved photoemission spectroscopy experiments on different Fermi surfaces. We determined absolute value of the penetration depth at 10 K as $lambdasimeq2000 AA$. A spectral weight analysis shows that the Ferrell-Glover-Timkham sum rule is satisfied at low energy scale, less than 6$Delta$.
We have conducted a comprehensive angle-resolved photoemission study on the normal state electronic structure of the Fe-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. We have identified four dispersive bands which cross the Fermi level and form two hole-like Fermi surfaces around $Gamma$ and two electron-like Fermi surfaces around M. There are two nearly nested Fermi surface pockets connected by an antiferromagnetic ($pi$, $pi$) wavevector. The observed Fermi surfaces show small $k_z$ dispersion and a total volume consistent with Luttinger theorem. Compared to band structure calculations, the overall bandwidth is reduced by a factor of 2. However, many low energy dispersions display stronger mass renormalization by a factor of $sim$ 4, indicating possible orbital (energy) dependent correlation effects. Using an effective tight banding model, we fitted the band structure and the Fermi surfaces to obtain band parameters reliable for theoretical modeling and calculations of the important physical quantities, such as the specific heat coefficient.
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