Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling

We report on a study of the superconducting order parameter in Fe(Te$_{1-x}$Se$_{x}$) thin films (with different Se contents: x=0.3, 0.4, 0.5) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude $\Delta_E \simeq 2.75 k_B T_c$ and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude $\Delta_H \simeq 1.75 k_B T_c$. The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te$_{1-x}$Se$_{x}$) single crystals. The values of the two gaps $\Delta_E$ and $\Delta_H$, once plotted as a function of the local critical temperature $T_c^A$, turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.Comment: 8 pages, 6 figures. This is an author-created, un-copyedited version of an article published in Supercond. Sci. Technol. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at 10.1088/0953-2048/27/12/12401

Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and Ba(Fe 1-xCox)2As2/Fe thin film

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained

Point-Contact Spectroscopy in MgB_2: from Fundamental Physics to Thin-Film Characterization

In this paper we highlight the advantages of using point-contact spectroscopy (PCS) in multigap superconductors like MgB_2, both as a fundamental research tool and as a non-destructive diagnostic technique for the optimization of thin-film characteristics. We first present some results of crucial fundamental interest obtained by directional PCS in MgB_2 single crystals, for example the temperature dependence of the gaps and of the critical fields and the effect of a magnetic field on the gap amplitudes. Then, we show how PCS can provide useful information about the surface properties of MgB_2 thin films (e.g. Tc, gap amplitude(s), clean or dirty-limit conditions) in view of their optimization for the fabrication of tunnel and Josephson junctions for applications in superconducting electronics.Comment: 11 pages, 7 figures; Proceedings of 6th EUCAS Conference (14-18 September 2003, Sorrento - Italy

Control of bulk superconductivity via surface-bound electric fields in ion-gated niobium nitride thin films

Ionic gating is a very popular tool to investigate and control the electric transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, often reaching values comparable to those occurring in metallic systems. Despite finding large success in tuning the phase diagram of low-carrier density systems, including cuprates and iron-based superconductors, its applicability to conventional metallic superconductors has received significantly less attention. In my talk, I will present the work which has been carried out in my research group over several years to investigate how ionic gating can tune the properties of metallic superconductor, using niobium nitride (NbN) as an emblematic case. By fabricating EDL-FETs on NbN thin films with thickness ranging between 10 and 40 nm, we observed that small positive and negative shifts in the critical temperature Tc could be induced by changing the gate-voltage polarity, and that the magnitude of these shifts increased upon decreasing the film thickness. These findings indicated that, despite the gate-induced electric field being confined in a thin layer at the surface by electrostatic screening, the perturbation to the superconducting state extends in a region much larger than a single unit cell. Indeed, the dependence of Tc on the gate voltage and thickness could be reconciled with the Eliashberg theory of superconductivity only if this thin surface layer is coupled to the underlying, unperturbed bulk via proximity effect. We also determined that the thickness of this surface layer (i.e. the screening length of the electric field) strongly increases for large gate electric fields, reaching values of the order of 3 nm at the highest doping. Ab-initio DFT calculations reproduced these results and linked this anomalous increase of the screening length to a distortion of the pristine charge density in the material upon the application of sufficiently large electric fields. This proximity-effect-induced transformation of the quasi-2D perturbation to the electron density into a 3D bulk modification of the superconducting properties seems to be a general behavior in gated superconductors that could hinder the possibility to obtain large Tc shifts in films thicker than the screening length. Consequently, we are currently focusing on exploring the tunability of ultrathin (< 5nm-thick) NbN films in order to maximize the gate-induced Tc shift, where we developed a novel technique of self-encapsulation in ultrathin niobium oxide to ensure the full reversibility of the gate modulation in these extremely sensitive devices

Preface Special Issue on novel superconducting and magnetic materials

Superconductivity and magnetism -- and their entanglement in a single material -- are among the most studied phenomena in condensed matter physics and continue to pose new challenges for fundamental research and exciting opportunities for technological applications. The last decade has witnessed ground-breaking discoveries in both fields: high-temperature superconductivity in compressed hydrides, unconventional superconductivity in iron-based materials and new types of magnetic states in spin-orbit coupled materials with topological and nematic characteristics. The prediction of material-specific properties and the interpretation of superconducting and magnetic phase transitions have been crucially aided by advances in ab-initio electronic structure methods within the density functional theory and its extensions. This special issue gathers together selected theoretical and experimental contributions on novel aspects of superconductivity and magnetism, %that have been collected in memory of Prof. Sandro Massidda. The collection aims to provide an updated view on timing issues and challenges in this active research field that have been at the hearth of Sandro's scientific interests. As commemorated in the obituary by Continenza and Colombo, Sandro has dedicated his scientific work to the development and application of \textit{ab-initio} computational and theoretical methods, yet never losing focus to the ultimate goal of theoretical and computational physics, that is to support, complement and understand the experimental observations

Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5and Ba(Fe1?xCox)2As2/Fe thin film

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe 0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained