Characterization of superconducting multilayers samples

Best RF bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H 200 mT close to the thermodynamic critical field Hc. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > HcNb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities [1]. Depositing good quality layers inside a whole cavity is rather difficult but we have sputtered high quality samples by applying the technique used for the preparation of superconducting electronics circuits and characterized these samples by X-ray reflectivity, dc resistivity (PPMS) and dc magnetization (SQUID). Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating clearly exhibit different behaviors. Because SQUID measurements are influenced by edge and shape effects we propose to develop a specific local magnetic measurement of HC1 based on ac third harmonic analysis in order to reveal the screening effect of multilayers

Calculation of surface quantum levels in tellurium inversion layers

The energies of surface quantum levels of inversion layers in [0001] planes at the surface of tellurium, calculated from Shubnikov de Haas measurements, are compared with theoretical values obtained with the assumption of a triangular surface potential well. In the experimental concentration range, results are explained by the existence of a single sub-band, separated in two levels due to the constitution in two sheets of the conduction band of tellurium.Les énergies des niveaux quantifiés dans les couches d'inversion sur des plans [0001] en surface du tellure, calculées à partir de mesures Shubnikov de Haas, sont comparées avec des valeurs théoriques obtenues avec l'hypothèse d'un puits de potentiel triangulaire en surface. Dans la gamme expérimentale de concentration, les résultats s'expliquent par l'existence d'une seule sousbande, séparée en deux niveaux du fait de la conformation en deux nappes de la bande de conduction du tellure

Frequency scaling with miniature Compact Microwave and Coaxial ion sources

International audienc

Characterization of superconducting nanometric multilayer samples for superconducting rf applications: First evidence of magnetic screening effect

The best rf bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H≈200  mT close to the thermodynamic critical field H_{c}. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of H_{c}>H_{c}^{Nb} for magnetic shielding of bulk niobium to increase the breakdown magnetic field of superconducting rf cavities. Depositing good quality layers inside a whole cavity is rather difficult, so as a first step, characterization of single layer coating and multilayers was conducted on high quality sputtered samples by applying the technique used for the preparation of superconducting electronics circuits. The samples were characterized by x-ray reflectivity, dc resistivity (PPMS), and dc magnetization (SQUID) measurements. Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating exhibit different behaviors and clearly show an enhancement of the magnetic penetration field. Because SQUID measurements are influenced by edge and shape effects, we propose to develop a specific local magnetic measurement of H_{C1} based on ac third harmonic analysis in order to reveal the true screening effect of multilayers

Epitaxial Growth of Sputtered Ultra-thin NbN Layers and Junctions on Sapphire

International audienceHigh crystalline quality of ultra-thin NbN layers and of NbN-MgO-NbN tri-layers, epitaxially grown by DC-magnetron sputtering in the superconducting B1-cubic phase has been achieved in a reproducible way on three different orientations of sapphire substrates i.e. R-, A- and M-planes. Significant improvements such as higher Tc, higher Jc and lower resistivity have been obtained by growing untwined (110) oriented NbN layers on M-plane orientation of sapphire. Uniform, low roughness, 3-5 nm thick films with Tc above 12 K and Jc above 5 cm at 4.2 K were obtained. Characterizations by TEM, AFM and X-Ray diffraction evidence that growth of untwined NbN on M-plane lead to a better epitaxy in comparison with twinned films observed on other sapphire orientations. We observe that the reduction of the substrate temperature from 600 to 300 during the deposition of NbN or NbN-MgO-NbN layers thicker than 20 nm prevents the nucleation of the competing HCP NbN phase. Moreover, 1.5 nm thick AlN or MgO over-layers sputtered in-situ prevent ultra-thin NbN films degradation through aging. The formation of Nb2Ny05-x ( 2.2 nm) at the unprotected NbN surface and of interfacial NbO ( 0.7 nm) native oxides has been observed by XPS. It is forecasted that such improvements in ultra-thin NbN films deposited uniformly on 3 and 4 inch sapphire wafers is a key in the future development of superconducting single photon detectors, THz HEB mixers and also in low noise quantum analogical and digital Josephson devices