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تسجيل مستخدم جديدFull Gap Superconductivity in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Probed by Muon Spin Rotation
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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.
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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 optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ have been determined in the normal state for a number of temperatures over a wide frequency range. Two Drude terms, representing two groups of carriers with different scattering rates ($1/
tau$), well describe the real part of the optical conductivity, $sigma_{1}(omega)$. A broad Drude component results in an incoherent background with a $T$-independent $1/tau_b$, while a narrow Drude component reveals a $T$-linear $1/tau_n$ resulting in a resistivity $rho_n equiv 1/sigma_{1n}(omegarightarrow 0)$ also linear in temperature. An arctan($T$) low-frequency spectral weight is also a strong evidence for a $T$-linear 1/$tau$. Comparison to other materials with similar behavior suggests that the $T$-linear $1/tau_n$ and $rho_n$ in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ originate from scattering from spin fluctuations and hence that an antiferromagnetic quantum critical point is likely to exist in the superconducting dome.
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