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Register a new userEmergence of Superconductivity from Fully Incoherent Normal State in an Iron-Based Superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$
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In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs (Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state. High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at T$_c$. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system.
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We report the specific heat (SH) measurements on single crystals of hole doped $FeAs$-based superconductor $Ba_{0.6}K_{0.4}Fe_2As_2$. It is found that the electronic SH coefficient $gamma_e(T)$ is not temperature dependent and increases almost linearly with the magnetic field in low temperature region. These point to a fully gapped superconducting state. Surprisingly the sharp SH anomaly $Delta C/T|_{T_c}$ reaches a value of 98 $mJ/mol K^2$ suggesting a very high normal state quasiparticle density of states ($gamma_n approx 63 mJ/mol K^2$). A detailed analysis reveals that the $gamma_e(T)$ cannot be fitted with a single gap of s-wave symmetry due to the presence of a hump in the middle temperature region. However, our data indicate that the dominant part of the superconducting condensate is induced by an s-wave gap with the magnitude of about 6 meV.
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.
Recently, An electron-doped 12442-type iron-based superconductor BaTh$_2$Fe$_4$As$_4$(N$_{0.7}$O$_{0.3}$)$_2$ has been successfully synthesized with high-temperature solid-state reactions on basis of a structural design. The inter-block-layer charge transfer between the constituent units of BaFe$_2$As$_2$ and ThFeAsN$_{0.7}$O$_{0.3}$ was found to be essential to stabilize the target compound. Dominant electron-type conduction and bulk superconducting transition at ~22 K were demonstrated.
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.
We have successfully grown high quality single crystals of SrFe$_2$As$_2$ and A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr, Ba) using flux method. The resistivity, specific heat and Hall coefficient have been measured. For parent compound SrFe$_2$As$_2$, an anisotropic resistivity with $rho_c$ / $rho_{ab}$ as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivity due to the SDW instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows 2-fold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K doped A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr. Ba), the SDW instability is suppressed and the superconductivity appears with T$_c$ above 35 K. The rather low anisotropy in upper critical field between H$parallel$ab and H$parallel$c indicates inter-plane coupling play an important role in hole doped Fe-based superconductors.
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