The electrochemical gating technique is a powerful tool to tune the surface electronic conduction properties of various materials by means of pure charge doping, but its efficiency is thought to be hampered in materials with a good electronic screening. We show that, if applied to a metallic superconductor (NbN thin films), this approach allows observing reversible enhancements or suppressions of the bulk superconducting transition temperature, which vary with the thickness of the films. These results are interpreted in terms of proximity effect, and indicate that the effective screening length depends on the induced charge density, becoming much larger than that predicted by standard screening theory at very high electric fields.

Well characterized photon number resolving detectors are a requirement for many applications ranging from quantum information and quantum metrology to the foundations of quantum mechanics. This prompts the necessity for reliable calibration techniques at the single photon level. In this paper we propose an innovative absolute calibration technique for photon number resolving detectors, using a pulsed heralded photon source based on parametric down conversion. The technique, being absolute, does not require reference standards and is independent upon the performances of the heralding detector. The method provides the results of quantum efficiency for the heralded detector as a function of detected photon numbers. Furthermore, we prove its validity by performing the calibration of a Transition Edge Sensor based detector, a real photon number resolving detector that has recently demonstrated its effectiveness in various quantum information protocols.

Superconductivity (SC) in the Ba-122 family of iron-based compounds can be controlled by aliovalent or isovalent substitutions, applied external pressure, and strain, the combined effects of which are sometimes studied within the same sample. Most often, the result is limited to a shift of the SC dome to different doping values. In a few cases, the maximum SC transition at optimal doping can also be enhanced. In this work, we study the combination of charge doping together with isovalent P substitution and strain, by performing ionic gating experiments on BaFe2(As0.8P0.2)2 ultrathin films. We show that the polarization of the ionic gate induces modulations to the normal-state transport properties that can be mainly ascribed to surface charge doping. We demonstrate that ionic gating can only shift the system away from the optimal conditions, as the SC transition temperature is suppressed both by electron and hole doping. We also observe a broadening of the resistive transition, which suggests that the SC order parameter is modulated non-homogeneously across the film thickness, in contrast with earlier reports on charge-doped standard BCS superconductors and cuprates.

The microwave properties of polycrystalline MgB 2 thin films prepared by the so-called in-situ method are investigated. The characterization of the films at microwave frequencies was obtained by a coplanar resonator technique. The analysis of the experimental data results in the determination of penetration depth, surface impedance and complex conductivity. The aim of this work is to set the experimental results in a consistent framework, involving the two-band model in the presence of impurity scattering. The energy gaps are calculated and the contribution of intra-and inter-band scattering is considered. From the comparison between the calculated gap values and the experimental data it turns out that the temperature dependence of the penetration depth 1 can be accounted for by an effective mean energy gap, in agreement with the predictions of Kogan et al. [Phys. Rev. B 69, 132506 (2004)]. On the other hand, the temperature dependence of the real part of the microwave conductivity and of the surface resistance is accounted for by the single smaller gap, in agreement with the work of Jin et al. [Phys. Rev. Lett. 91, 127006 (2003)]. Since these findings rely on the same calculated gap structure, the required consistency is fulfilled.74.70. Ad, 74.25.Nf IntroductionSince the discovery of its superconducting properties, magnesium diboride (MgB 2 ) has generated a great deal of interest because of its simple structure, relatively high critical temperature and two-gap nature. The Fermi surface of MgB 2 consists of two threedimensional sheets, from the π bonding and antibonding bands, and two nearly cilindrical sheets from the two-dimensional σ bands. 1 Many physical properties of MgB 2 are reasonably described within a model with two separated energy gaps, ∆ π and ∆ σ . 2,3 Nevertheless, the role of interband and intraband scattering has to be considered: 2,3 it is still not completely clear, also due to the wide spread quality of samples used in different experiments. In fact, a significant scattering between the different Fermi sheets may reduce the effective gap structure to a single isotropic gap. Recently, the expected 2 observation of single-gap superconductivity at high impurity level has been observed in C-substituted MgB 2 single crystals by point-contact spectroscopy. 4 When the level of impurities is high enough, the two gaps merge into a single gap with a ratio 2∆/k B T c close to the standard BCS value.The role of the two bands in determining the microwave conductivity in MgB 2 thin films is controversial as well. The temperature dependence of the microwave conductivity in caxis oriented films was interpreted by Jin et al. 5 in terms of a dominant contribution of the π band. They deduced this argument from the observation of a single anomalous peak around t=T/T c =0.6. The presence of such a peak can be explained in the framework of the BCS theory. When the superconducting state is entered and the gap opens, a singularity appears in the density of states at the gap edges, increasing the microwave ...

High efficiency and negligible dark-count rates of transition-edge sensor\ud (TES) microcalorimeters as single photon detector at telecommunications and optical\ud visible wavelengths make them powerful tools for quantum information and quantum computation. In this work we report details on the fabrication of Au/Ti for\ud photon counting and analyse the effects on the critical temperature and the transition\ud steepness of the structuring process and wiring material. Au/Ti films deposited by electron-beam at substrate temperature lower than 435 K show sharp transition and reproducible Tc. Moreover, we observe that TES with Al wiring are more stable\ud and have better characteristics of TES with Nb wiring. Using 20 micron×20 micron Ti/Au TES single photon detection has been obtained in the UV-visible range