Properties of dirty two-bands superconductors with repulsive interband interaction: normal modes, length scales, vortices and magnetic response

Disorder in two-band superconductors with repulsive interband interaction induces a frustrated competition between the phase-locking preferences of the various potential and kinetic terms. This frustrated interaction can result in the formation of an $s+is$ superconducting state, that breaks the time-reversal symmetry. In this paper we study the normal modes and their associated coherence lengths in such materials. We especially focus on the consequences of the soft modes stemming from the frustration and time-reversal-symmetry breakdown. We find that two-bands superconductors with such impurity-induced frustrated interactions display a rich spectrum of physical properties that are absent in their clean counterparts. It features a mixing of Leggett's and Anderson-Higgs modes, and a soft mode with diverging coherence length at the impurity-induced second order phase transition from $s_{\pm}/s_{++}$ states to the $s+is$ state. Such a soft mode generically results in long-range attractive intervortex forces that can trigger the formation of vortex clusters. We find that, if such clusters are formed, their size and internal flux density have a characteristic temperature dependence that could be probed in muon-spin-rotation experiments. We also comment on the appearance of spontaneous magnetic fields due to spatially varying impurities.Comment: Added discussion of spontaneous magnetic fields due to spatially varying impurities; Replaced with a version in print in Phys. Rev. B; 17 pages, 8 figure

Two-electronic component behavior in the multiband FeSe0.42_{0.42}Te0.58_{0.58} superconductor

We report X-band EPR and $^{125}$Te and $^{77}$Se NMR measurements on single-crystalline superconducting FeSe$_{0.42}$Te$_{0.58}$ ($T_c$ = 11.5(1) K). The data provide evidence for the coexistence of intrinsic localized and itinerant electronic states. In the normal state, localized moments couple to itinerant electrons in the Fe(Se,Te) layers and affect the local spin susceptibility and spin fluctuations. Below $T_c$, spin fluctuations become rapidly suppressed and an unconventional superconducting state emerges in which $1/T_1$ is reduced at a much faster rate than expected for conventional $s$- or $s_\pm$-wave symmetry. We suggest that the localized states arise from the strong electronic correlations within one of the Fe-derived bands. The multiband electronic structure together with the electronic correlations thus determine the normal and superconducting states of the FeSe$_{1-x}$Te$_x$ family, which appears much closer to other high-$T_c$ superconductors than previously anticipated.Comment: 5 pages, 4 figure

Eliashberg Theory of a Multiband Non-Phononic Spin Glass Superconductor

I solved the Eliashberg equations for a multiband non-phononic s wave spin-glass superconductor, calculating the temperature dependence of the gaps and of superfluid density. Their behaviors were revealed to be unusual: showing non-monotonic temperature dependence and reentrant superconductivity. By considering particular input parameters values that could describe the iron pnictide EuFe2(As1-xPx)2, a rich and complex phase diagram arises, with two different ranges of temperature in which superconductivity appears

Lessons from Oxypnictide Thin Films

First experiments on the growth of oxypnictide F-doped LaFeAsO thin films indicated an incomplete normal-to-superconducting transition and offered a work programme challenging to overcome possible difficulties in their fabrication. In this regard the possibility of an all in-situ epitaxial growth appeared to be a matter of time and growth parameters. The following review clarifies that F-doped oxypnictide thin films are extremely difficult to grow by in-situ PLD due to the formation of very stable impurity phases such as oxyfluorides (LaOF) and oxides (La2O3) and the loss of stoichiometry possibly due to incongruent evaporation of the target or re-evaporation of volatile elements at the substrate surface. However, the review also demonstrates that the employed two-step fabrication process for oxypnictide thin films has been successfully applied in the preparation of clean polycrystalline as well as of epitaxial thin films. Fundamental investigations on the upper critical field, its temperature dependence and its anisotropy contributed to an understanding of multiband superconductivity in oxypnictides.Comment: accepted, pre-print versio

Using Gap Symmetry and Structure to Reveal the Pairing Mechanism in Fe-based Superconductors

I review theoretical ideas and implications of experiments for the gap structure and symmetry of the Fe-based superconductors. Unlike any other class of unconventional superconductors, one has in these systems the possibility to tune the interactions by small changes in pressure, doping or disorder. Thus, measurements of order parameter evolution with these parameters should enable a deeper understanding of the underlying interactions. I briefly review the "standard paradigm" for $s$-wave pairing in these systems, and then focus on developments in the past several years which have challenged this picture. I discuss the reasons for the apparent close competition between pairing in s- and d-wave channels, particularly in those systems where one type of Fermi surface pocket -- hole or electron -- is missing. Observation of a transition between $s$- and $d$-wave symmetry, possibly via a time reversal symmetry breaking "$s+id$" state, would provide an importantconfirmation of these ideas. Several proposals for detecting these novel phases are discussed, including the appearance of order parameter collective modes in Raman and optical conductivities. Transitions between two different types of $s$-wave states, involving various combinations of signs on Fermi surface pockets, can also proceed through a ${\cal T}$-breaking "$s+is$" state. I discuss recent work that suggests pairing may take place away from the Fermi level over a surprisingly large energy range, as well as the effect of glide plane symmetry of the Fe-based systems on the superconductivity, including various exotic, time and translational invariance breaking pair states that have been proposed. Finally, I address disorder issues, and the various ways systematic introduction of disorder can (and cannot) be used to extract information on gap symmetry and structure.Comment: 41 pp., Published in special focus issue of Comptes Rendus Physique (Paris) on recent progress in Fe-based Superconductivity. Full issue with 10 review articles available at http://www.sciencedirect.com/science/journal/16310705/17/1-