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We investigate the penetration depth of high-quality Ba1-xKxFe2As2 single crystals by a planar waveguide resonator technique, in a cavity perturbation approach. The experimental λL is compared to calculations based on the three-band Eliashberg equations within the s± wave model. To this end, the anisotropy of the penetration depth is taken into account. In fact, the agreement between theory and experiment is remarkable. The low-temperature value of the in-plane penetration depth, λL,ab(5K)=220nm, and the total plasma frequency, ωp=1.0eV, are also consistent with earlier results. This overall consistency validates the model itself, thus allowing us to estimate parameters that are missing in literature, such as the plasma frequency for each band: it turns out that ωp,1=ωp,3=0.32eV and ωp,2=0.89eV, with the subscripts 1 and 2 denoting the two hole bands and 3 the equivalent electron band. [ABSTRACT FROM AUTHOR]

The dependence of the superconducting gaps in epitaxial Ba(Fe1-xCox)2As2 thin films on the nominal doping x (0.04⩽x⩽0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the two-dimensional Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps—although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps ΔS and ΔL show similar monotonic trends as a function of the local critical temperature TcA (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e., to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with Tc according to the empirical law Ω0=4.65kBTc, and with a total electron-boson coupling strength λtot=2.22 for x⩽0.10 (i.e., up to optimal doping) that slightly decreases to λtot=1.82 in the overdoped samples (x=0.15). [ABSTRACT FROM AUTHOR]

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO1− xF x in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T c=27.3–28.6 K within the two-band BTK model, are Δ1=2.75–3.8 meV for the small gap and Δ2=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Néel temperature of the parent compound, TN≈140 K. [ABSTRACT FROM AUTHOR]

Microwave measurements of the London penetration depth and critical temperature Tc were used to show evidence of a disordered-driven transition from s± to s++ order parameter symmetry in optimally doped Ba(Fe1-xRhx)2As2 single crystals, where disorder was induced by means of 3.5 MeV proton irradiation. Signatures of such a transition, as theoretically predicted [V. D. Efremov et al., Phys. Rev. B 84, 180512(R) (2011)], are found as a drop in the low-temperature values of the London penetration depth and a virtually disorder-independent superconducting Tc. We show how these experimental observations can be described by multiband Eliashberg calculations in which the effect of disorder is accounted for in a suitable way. To this aim, an effective two-band approach is adopted, allowing us to treat disorder in a range between the Born approximation and the unitary limit. [ABSTRACT FROM AUTHOR]

We study the graphite intercalated compound CaC6 by means of Eliashberg theory. We perform an analysis of the electron-phonon coupling and define a minimal six-band anisotropic structure, which leads to a Fermi-surface dependence of the superconducting gap. A comparison of the superconducting gap structure obtained using the Eliashberg and the superconducting density functional theory is performed. We further report the anisotropic properties of the electronic spectral function, the polaronic quasiparticle branches, and their interplay with Bogoljubov excitations. [ABSTRACT FROM AUTHOR]

A key aspect of field effect experiments is the possibility to induce charges on the first layers of a sample as a function of an applied gate voltage. It is therefore possible to study correlated phases of matter as a function of the induced charge density and the applied electric field. Moreover, resulting charge modulation along the direction of the applied electric field gives rise to junctions between perturbed and uneffected regions of the sample. In the framework of proximity effect Eliashberg theory, we investigate the consequence of an applied static electric field on the transition temperature of a two-band s-wave superconductor magnesium diboride. In most cases the only free parameter in the theory is the penetration depth of the applied electric field, whereas there is no freedom when the static perturbation is sufficiently weak. We come to the conclusion that the optimal way to enhance the critical temperature is to have a very thin film of magnesium diboride, otherwise the external electric field would not have substantial effect on superconductivity in this material.

Microwave measurements of the London penetration depth and critical temperature T_{c} were used to show evidence of a disordered-driven transition from s_{±} to s_{++} order parameter symmetry in optimally doped Ba(Fe_{1-x}Rh_{x})_{2}As_{2} single crystals, where disorder was induced by means of 3.5 MeV proton irradiation. Signatures of such a transition, as theoretically predicted [V. D. Efremov et al., Phys. Rev. B 84, 180512(R) (2011)PRBMDO1098-012110.1103/PhysRevB.84.180512], are found as a drop in the low-temperature values of the London penetration depth and a virtually disorder-independent superconducting T_{c}. We show how these experimental observations can be described by multiband Eliashberg calculations in which the effect of disorder is accounted for in a suitable way. To this aim, an effective two-band approach is adopted, allowing us to treat disorder in a range between the Born approximation and the unitary limit.

One of the open issues concerning iron-based superconductors is whether the s± wave model is able to account for the overall effects of impurity scattering, including the low rate of decrease of the critical temperature with the impurity concentration. Here we investigate Ba 1-x K x Fe 2 As 2 crystals where disorder is introduced by Au-ion irradiation. Critical temperature, T c , and London penetration depth, λ L , were measured by a microwave resonator technique, for different values of the irradiation fluence. We compared experimental data with calculations made on the basis of the three-band Eliashberg equations, suitably accounting for the impurity scattering. We show that this approach is able to explain in a consistent way the effects of disorder both on T c and on λ L (T), within the s± wave model. In particular, a change of curvature in the low-temperature λ L (T) curves for the most irradiated crystals is fairly well reproduced.

We show that the phenomenology of the iron chalcogenide superconductor FeTe(1-x)Se(x) can be explained within an effective three-band s±-wave Eliashberg model. In particular, various experimental data reported in literature-the critical temperature, the energy gaps, the upper critical field, the superfluid density-can be reproduced by this model in a moderate strong-coupling regime provided that both an intraband phononic term and an interband antiferromagnetic spin-fluctuations term are included in the coupling matrix. The intraband coupling is unusual in Fe-based compounds and is required to explain the somehow anomalous association between gap amplitudes and Fermi surfaces, already evidenced by ARPES.

Superconductivity is due to an attractive interaction between electrons that, below a critical temperature, drives them to form Cooper pairs and to condense into a ground state separated by an energy gap from the unpaired states. In the simplest cases, the pairing is mediated by lattice vibrations and the wavefunction of the pairs is isotropic. Less conventional pairing mechanisms can favour more exotic symmetries of the Cooper pairs. Here, we report on point-contact spectroscopy measurements in PuCoGa(5), a moderate heavy-fermion superconductor with a record high critical temperature T(c)=18.5 K. The results prove that the wavefunction of the paired electrons has a d-wave symmetry, with four lobes and nodes, and show that the pairing is likely to be mediated by spin fluctuations. Electronic structure calculations, which take into account the full structure of the f-orbital multiplets of Pu, provide a hint of the possible origin of these fluctuations.

We present the first results of directional point-contact spectroscopy in high-quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6.

We present the first results of directional point-contact measurements in Mg1-xMnxB2 single crystals, with x up to 0.015 and bulk Tc down to 13.3 K. The order parameters Deltasigma and Deltapi were obtained by fitting the conductance curves with the two-band Blonder-Tinkham-Klapwijk model. BothDeltapi and Deltasigma decrease with the critical temperature of the junctions TAC, but remain clearly distinct up to the highest Mn content. Once analyzed within the Eliashberg theory, the results indicate that spin-flip scattering is dominant in the sigma band, as also confirmed by first-principles band-structure calculations.