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Register a new userZn-impurity effect and interplay of $s_{pm}$- and $s_{++}$-pairings in Fe-based superconductors
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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.
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Effects of disorder on electron-doped iron pnictides are investigated systematically based on self-consistent Bogoliubov-de Gennes equations. Multiply impurities with same scattering potential (SP) are randomly distributed in a square lattice. Probability distribution functions of normalized order parameters for different impurity concentrations $delta_{imp}$, different electron doping concentrations $delta$ are investigated for given SPs. Samples are found to be very robust against weak SP, in which order parameters do not have qualitative change even at very large $delta_{imp}$. While strong SP is able to easily break down the order parameters. For moderate SP, variations of order parameters on and around impurities strongly depend on $delta$, however the distribution functions of normalized order parameters have similar behavior as $delta_{imp}$ increases. Compared with superconducting (SC) order, the magnetic order is more sensitive to multi-impurity effect. The spatial spin density wave pattern has already been destroyed before the system loses its superconductivity. Dependence of SC order on temperature is similar to that of impurity-free case, with the critical temperature being remarkably suppressed for high $delta_{imp}$.
A two-band model with repulsive interband coupling and interband {it transport} (potential) scattering is considered to elucidate their effects on material properties. In agreement with previous work, we find that the bands order parameters $Delta_{1,2}$ differ and the large is at the band with a smaller normal density of states (DOS), $N_{n2}<N_{n1}$. However, the bands energy gaps, as determined by the energy dependence of the DOS, are equal due to scattering. For each temperature, the gaps turn zero at a certain critical interband scattering rate, i.e. for strong enough scattering the model material becomes gappless. In the gapless state, the DOS at the band 2 is close to the normal state value, whereas at the band 1 it has a V-shape with non-zero minimum. When the normal bands DOS are mismatched, $N_{n1} e N_{n2}$, the critical temperature $T_c$ is suppressed even in the absence of interband scattering, $T_c(N_{n1})$ has a dome-like shape. With increasing interband scattering, the London penetration depth at low temperatures evolves from being exponentially flat to the power-law and even to near linear behavior in the gapless state, the latter being easily misinterpreted as caused by order parameter nodes.
The $s_pm$ and $s_{++}$ models for the superconducting state are subject of intense studies regarding Fe-based superconductors. Depending on the parameters, disorder may leave intact or suppress $T_c$ in these models. Here we study the special case of disorder with equal values of intra- and interband impurity potentials in the two-band $s_pm$ and $s_{++}$ models. We show that this case can be considered as an isolated point and $T_c$ there has maximal damping for a wide range of parameters.
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.
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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