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Register a new userManifestation of impurity induced s_{+-} -> s_{++} transition: multiband model for dynamical response functions
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We investigate effects of disorder on the density of states, the single particle response function and optical conductivity in multiband superconductors with s_{+-} symmetry of the order parameter, where s_{+-} -> s_{++} transition may take place. In the vicinity of the transition the superconductive gapless regime is realized. It manifests itself in anomalies in the above mentioned properties. As a result, intrinsically phase-insensitive experimental methods like ARPES, tunneling and terahertz spectroscopy may be used for revealing of information about the underlying order parameter symmetry.
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We report theoretical and experimental studies of the effect of Zn-impurity in Fe-based superconductors. Zn-impurity is expected to severely suppress sign reversed s$_pm$ wave pairing. The experimentally observed suppression of T$_c$ under Zn-doping strongly depends on the materials and the charge carrier contents, which suggests competition of $s_{++}$ and $s_{pm}$ pairings in Fe-base superconductors. We study a model incorporating both $s_{++}$ and $s_{pm}$ pairing couplings by using Bogoliubov de-Gennes equation, and show that the Zn-impurity strongly suppresses $s_{pm}$ pairing and may induce a transition from $s_{pm}$ to $s_{++}$-wave. Our theory is consistent with various experiments on the impurity effect. We present new experimental data on the Zn-doping SmFe$_{1-x}$Zn$_x$AsO$_{0.9}$F$_{0.1}$ of T$_c=$ 50K, in further support of our proposal.
We study the dependence of the superconducting gaps on both the disorder and the temperature within the two-band model for iron-based materials. In the clean limit, the system is in the $s_pm$ state with the sign-changing gaps. Scattering by nonmagnetic impurities leads to the change of sign of the smaller gap thus resulting in a transition from the $s_{pm}$ to the $s_{++}$ state with the sign-preserving gaps. We show here that the transition is temperature-dependent, thus, there is a line of $s_pm to s_{++}$ transition in the temperature-disorder phase diagram. There exists a narrow range of impurity scattering rates, where the disorder-induced $s_pm to s_{++}$ transition occurs at low temperatures, but then the low-temperature $s_{++}$ state transforms back to the $s_pm$ state at higher temperatures. With increasing impurity scattering rate, temperature of such $s_{++} to s_{pm}$ transition shifts to the critical temperature $T_c$ and only the $s_{++}$ state is left for higher amount of disorder.
We consider a two-band superconductor with relative phase $pi $ between the two order parameters as a model for the superconducting state in ferropnictides. Within this model we calculate the microwave response and the NMR relaxation rate. The influence of intra- and interband impurity scattering beyond the Born and unitary limits is taken into account. We show that, depending on the scattering rate, various types of power law temperature dependencies of the magnetic field penetration depth and the NMR relaxation rate at low temperatures may take place.
Irradiation of superconductors with different particles is one of many ways to investigate effects of disorder. Here we study the disorder-induced transition between $s_pm$ and $s_{++}$ states in two-band model for Fe-based superconductors with nonmagnetic impurities. Specifically, the important question of whether the superconducting gaps during the transition change smoothly or steeply? We show that the behavior can be of either type and is controlled by the ratio of intra- and interband impurity scattering and a parameter $sigma$ that represents a scattering strength and changes from zero (Born approximation) to one (unitary limit). For the pure interband scattering potential and $sigma lesssim 0.11$, the $s_pm to s_{++}$ transition is accompanied by the steep behavior of gaps, while for larger values of $sigma$, gaps change smoothly. The steep behavior of the gaps occurs at low temperatures, $T < 0.1 T_{c0}$, otherwise it is smooth. The critical temperature $T_c$ is always a smooth function of the scattering rate in spite of the steep changes in the behavior of the gaps.
In high-superconducting transition temperature ($T_{rm c}$) iron-based superconductors, interband sign reversal ($s_{rm pm}$) and sign preserving ($s_{rm ++}$) $s$-wave superconducting states have been primarily discussed as the plausible superconducting mechanism. We study Co impurity scattering effects on the superconductivity in order to achieve an important clue on the pairing mechanism using single crystal Fe$_{1-x}$Co$_x$Se and depict a phase diagram of a FeSe system. Both superconductivity and structural transition / orbital order are suppressed by the Co replacement on the Fe sites and disappear above $x$ = 0.036. These correlated suppressions represent a common background physics behind these physical phenomena in the multiband Fermi surfaces of FeSe. By comparing experimental data and theories so far proposed, the suppression of $T_{rm c}$ against the residual resistivity is shown to be much weaker than that predicted in the case of a general sign reversal and a full gap $s_{pm}$ models. The origin of the superconducting paring in FeSe is discussed in terms of its multiband electronic structure.
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