Probing multiband superconductivity by point-contact spectroscopy

Point-contact spectroscopy was originally developed for the determination of the electron-phonon spectral function in normal metals. However, in the past 20 years it has become an important tool in the investigation of superconductors. As a matter of fact, point contacts between a normal metal and a superconductor can provide information on the amplitude and symmetry of the energy gap that, in the superconducting state, opens up at the Fermi level. In this paper we review the experimental and theoretical aspects of point-contact spectroscopy in superconductors, and we give an experimental survey of the most recent applications of this technique to anisotropic and multiband superconductors.Comment: 38 pages, 28 eps figures. To appear as a Topical Review in Superconductor Science and Technolog

Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions

An intrinsic gap depression at the Superconductor-Insulator interface due to the very short value of the coherence length in High-Tc Superconductors [HTSs] is considered, in the framework of a mixed (s+id)-wave pair symmetry for the order parameter ranging from pure s to pure d-wave. This gap depression acts as the main physical agent causing the relevant reduction of IcRn(T) values with respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with various experimental data is obtained with both pure s-wave and pure d-wave symmetries of the order parameter, but with amounts of gap depression depending on the pair symmetry adopted. Regardless of the pair symmetry considered, these results prove the importance of the surface order-parameter depression in the correct interpretation of the Ic(T)Rn(T) data in HTS SIS junctions. In a case of planar YBCO-based junction the use of the de Gennes condition allowed us to tentatively obtain an upper limit for the amount of d-wave present in the order parameter of YBCO.Comment: 11 pages REVTeX file, 6 PostScript figures, to be published in J. Superconductivit

Point contact spectroscopy in Fe-based superconductors: recent advancements and future challenges

Iron-based superconductors (FeSC) present an unprecedented variety of features both in the superconducting and in the normal state. Different families differ in the value of the critical temperature, in the shape of the Fermi surface, in the existence or absence of quasi-nesting conditions, in the range of doping in which the antiferromagnetic (AFM) and the superconducting phase coexist and in the structure of the order parameter in the reciprocal space, and so on. In this paper the most important results of point-contact spectroscopy (PCS) in Fe-based superconductors are reviewed, and the most recent advances are described with the aim to discuss the future perspectives and challenges of this spectroscopic technique in the characterization of the superconducting properties of these complex compounds. One of the main challenges, faced so far only by a few researchers in the PCS field, is to fully explore the phase diagram of these materials, as a function of doping or pressure, to understand the interplay between superconductivity and magnetism, the effect of intrinsic or extrinsic inhomogeneities, the role of spin fluctuations (SFs) in the pairing, the symmetry and the structure of the order parameter(s).Comment: 12 pages, 4 figure

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.

Breakdown of Migdal's theorem and intensity of electron-phonon coupling in high-Tc superconductors

In this article we quantify the possible effects of the breakdown of Migdal's theorem on the electron-phonon (e-p) coupling constant, on the critical temperature and on the superconducting gap by examining different kinds of superconducting materials either with low and high critical temperature. We use the theoretical approach developed by Grimaldi, Pietronero and Strassler [PRB 52, 10516 & 10530 (1995)] on experimental data taken both from literature and from our recent break-junction tunneling experiments in Bi_2Sr_2CaCu_2O_{8+x} (BSCCO) [Physica C 275, 162 (1997) and cond-mat/9612220]. The results show that a large violation of the Migdal's theorem (as in BSCCO) yields to a large increase of the observed e-p coupling constant and, in a first approximation and by direct solution of the Eliashberg equations, to large but different increases of Tc and gap. The same theory gives no modifications when applied to low-Tc conventional superconductors. In BSCCO, the renormalized values of the e-p coupling constant and of the ratio 2Delta/kTc (1.85 and 4.98, respectively) are compatible with a strong-coupling electron-phonon origin of superconductivity.Comment: 11 pages REVTeX file, 3 PostScript figures, to be published in Phys. Rev. B (Rapid Commun.

Anomalous screening of an electrostatic field at the surface of niobium nitride

The interaction between an electric field and the electric charges in a material is described by electrostatic screening, which in metallic systems is commonly thought to be confined within a distance of the order of the Thomas-Fermi length. The validity of this picture, which holds for surface charges up to $\sim 10^{13}\,\mathrm{cm^{-2}}$, has been recently questioned by several experimental results when dealing with larger surface charges, such as those routinely achieved via the ionic gating technique. Whether these results can be accounted for in a purely electrostatic picture is still debated. In this work, we tackle this issue by calculating the spatial dependence of the charge carrier density in thin slabs of niobium nitride via an ab initio density functional theory approach in the field-effect transistor configuration. We find that perturbations induced by surface charges $\lesssim 10^{14}\,\mathrm{cm^{-2}}$ are mainly screened within the first layer, while those induced by larger surface charges $\sim 10^{15}\,\mathrm{cm^{-2}}$ can penetrate over multiple atomic layers, in reasonable agreement with the available experimental data. Furthermore, we show that a significant contribution to the screening of large fields is associated not only to the accumulation layer of the induced charge carriers at the surface, but also to the polarization of the pre-existing charge density of the undoped system.Comment: 8 pages, 4 figure

3D-melting features of the irreversibility line in overdoped Bi2_2Sr2_2CuO6_6 at ultra-low temperature and high magnetic field

We have measured the irreversible magnetization of an overdoped Bi$_2$Sr$_2$CuO$_6$ single crystal up to B=28 T and down to T=60 mK, and extracted the irreversibility line $B_{\rm irr}(T)$: the data can be interpreted in the whole temperature range as a 3D-anisotropic vortex lattice melting line with Lindemann number $c_{\rm L}=0.13$. We also briefly discuss the applicability of alternative models such as 2D- and quantum melting, and the connection with magnetoresistance experiments.Comment: M2S-HTSC-VI Conference paper (2 pages, 1 figure), using Elsevier style espcrc2.st

Possible d+id scenario in La_{2-x}Sr_{x}CuO_4 by point-contact measurements

We analyze the results of point-contact measurements in La_{2-x}Sr_{x}CuO_{4} (LSCO) previously reported as a clear evidence of the separation between gap and pseudogap in this copper oxide. Here we show that, in addition to this, the conductance curves of our point-contact junctions -- showing clear Andreev reflection features -- can be interpreted as supporting a nodeless d_{x^2-y^2}+id_{xy}-wave symmetry of the gap in LSCO. The results of our analysis, in particular the doping dependence of the subdominant d_{xy} gap component, are discussed and compared to the predictions of different theoretical models.Comment: 6 pages, 4 eps figures, presented at SATT11 Conference (Vietri sul Mare, March 2002). To appear in Int. J. Mod. Phy

Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals

Directional point-contact spectroscopy measurements were performed for the first time in state-of-the-art MgB_2 single crystals. The selective suppression of the superconductivity in the "pi" band by means of a suitable magnetic field allowed separating the partial contribution of each band to the total point-contact conductance. By fitting the partial conductance curves sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental uncertainty. Their temperature dependence was found to agree well with the predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa, Ital