Real-Axis Solution of Eliashberg Equations in Various Order-Parameter Symmetries and Tunneling Conductance of Optimally-Doped HTSC

In the present work we calculate the theoretical tunneling conductance curves of SIN junctions involving high-Tc superconductors, for different possible symmetries of the order parameter (s, d, s+id, s+d, anisotropic s and extended s). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that some of the peculiar characteristics of HTSC tunneling curves (dip and hump at eV > Delta, broadening of the gap peak, zero bias and so on) can be explained in the framework of the Migdal-Eliashberg theory. The theoretical dI/dV curves calculated for the different symmetries at T=4 K are then compared to various experimental tunneling data obtained in optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit the experimental data, the scattering by non-magnetic impurities has to be taken into account, thus limiting the sensitivity of this procedure in determining the exact gap symmetry of these materials. Finally, the effect of the temperature on the theoretical tunneling conductance is also discussed and the curves obtained at T=2 K are compared to those given by the analytical continuation of the imaginary-axis solution.Comment: 6 pages, 3 figures, Proceedings of SATT10 Conference, to be published in Int. J. Mod. Phys.

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1−xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d − d state with symmetry-imposed nodes. A large London penetration depth anisotropy was revealed, weakly dependent on temperature and fully compatible with the d − d model

Evidence for Single-gap Superconductivity in Mg(B1-xCx)2 Single Crystals with x=0.132 from Point-Contact Spectroscopy

We report the results of the first directional point-contact measurements in MgsB1−xCxd2 single crystals with 0.047<x<0.132. The two-gap superconductivity typical of MgB2 persists up to x=0.105. In this region, the values of the gaps Ds and Dp were determined by fitting the Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk sBTKd model, and confirmed by the single-band BTK fit of the s- and p-band conductances, separated by means of a magnetic field. At x=0.132, when Tc=19 K, we clearly observed the merging of the two gaps into one of amplitude D=3 meV

Theoretical explanation of electric field‐induced superconductive critical temperature shifts in Indium thin films

Herein, ab initio density functional theory computations and the self-consistent solution of one-band s-wave generalized Eliashberg equations with proximity effect are combined to explain 60 years old experimental data on how a static electric field can affect the superconductive critical temperature of indium thin films. Although electronic densities of states, Fermi energy shifts, and Eliashberg spectral functions can be computed ab initio, the only parameter in the theory that cannot be determined a priori is the thickness of the surface layer affected by the electric field. However, it is shown that in the weak electrostatic field limit, Thomas–Fermi approximation is valid and, therefore, no free parameters are left, as this perturbed layer is known to have a thickness of the order of the Thomas– Fermi screening length. Moreover, it is shown that the theoretical model can reproduce experimental data, even when the magnitudes of the induced charge densities are so small to be usually neglected

Pseudogap features in the d-wave strong electron-boson density of the states of high-Tc cuprate superconductors

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Determination of the electron-phonon spectral function from tunneling measurements in Bi2Sr2CaCu2O8+x single-crystal break junctions

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Possible evidence of charge-stripe ordering in the ab-plane resistivity of strongly underdoped La_(2-x)Sr_(x)CuO_4 single crystals

We measured the ab-plane resistivity of La2-xSrxCuO4 underdoped single crystals (x = 0.052 and x=0.06) between 4.2 and 300 K by using an AC version of the Van der Pauw technique. We suggest that a possible scenario of charge-stripe ordering at the lowering of the temperature can be delineated by starting from the main features of the ρab (T) curves of these samples. In particular, the deviation from the linearity of the ab-plane resistivity, occurring at a temperature Tch, could be related to the beginning of the charge localization, and the upturn of ρab to the progressive pinning of the resulting charge stripes. By starting from an analysis of our ab-plane resistivity curves, we determined the temperature of charge ordering Tch for our samples, thus extending in consistent way the Tch vs. x curve reported in literature to the very low-doping region of the phase diagram