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We discuss a scenario for interface-induced superconductivity involving pairing by dipolar excitations proximate to a two-dimensional electron system controlled by a transverse electric field. If the interface consists of transition metal oxide materials, the repulsive on-site Coulomb interaction is typically strong and a superconducting state is formed via exchange of non-local dipolar excitations in the d-wave channel. Perspectives to enhance the superconducting transition temperature are discussed.
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We have investigated whether the electron-phonon interaction can support a d-wave gap-anisotropy. On the basis of models derived from LDA calculations, as well as LDA linear-response calculations we argue that this is the case, for materials with buckled or dimpled CuO2 planes, for the so-called buckling modes, which involve out-of-plane movements of the plane oxygens.
Unconventional superconductivity is usually associated with symmetry breaking in the system. Here we consider a simple setup consisting of a piezoelectric film and an applied surface acoustic wave (SAW), that can break time and spatial translation symmetries. We study the symmetries of the possible SAW-induced order parameters, showing that even-frequency spin-triplet odd-parity order parameter can occur. We suggest different methods of how to engineer the symmetries of the order parameters using SAWs and the applications of such setups.
Measurements of the differential conductance spectra of YBa2Cu3O7-SrRuO3 and YBa2Cu3O7-La0.67Ca_0.33MnO3 ramp-type junctions along the node and anti-node directions are reported. The results are consistent with a crossed Andreev reflection effect only in YBa2Cu3O7-SrRuO3 junctions where the domain wall width of SrRuO3 is comparable with the coherence length of YBa2Cu3O7. No such effect was observed in the YBa2Cu3O7-La0.67Ca0.33MnO3 junctions, which is in line with the much larger (x10) domain wall width of La0.67Ca0.33MnO3. We also show that crossed Andreev exists only in the anti-node direction. Furthermore, we find evidence that crossed Andreev in YBa2Cu3O7 junctions is not sensitive to nm-scale interface defects, suggesting that the length scale of the crossed Andreev effect is larger than the coherence length, but still smaller than the La0.67Ca0.33MnO3s domain wall width.
Motivated by recent proposals of correlation induced insensitivity of d-wave superconductors to impurities, we develop a simple pairing theory for these systems for up to a moderate strength of disorder. Our description implements the key ideas of Anderson, originally proposed for disordered s-wave superconductors, but in addition takes care of the inherent strong electronic repulsion in these compounds, as well as disorder induced inhomogeneities. We first obtain the self-consistent one-particle states, that capture the effects of disorder exactly, and strong correlations using Gutzwiller approximation. These `normal states, representing the interplay of strong correlations and disorder, when coupled through pairing attractions following the path of Bardeen-Cooper-Schrieffer (BCS), produce results nearly identical to those from a more sophisticated Gutzwiller augmented Bogoliubov-de Gennes analysis.
The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.
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