Search

We have investigated the normal-state and superconducting properties of Co-doped BaFe2As2 (Ba122) and MgB2 thin films irradiated at room temperature using a 30 keV focused He+ ion beam with doses between 1012 and 1017 cm2. We show that superconductivity is suppressed and the normal-state resistivity is increased upon irradiation. The critical dose for the complete suppression of superconductivity is ∼5 × 1014 cm−2 for Ba122 and ∼8 × 1015 cm−2 for MgB2. The dependence of the normal-state and superconducting properties on irradiation dose is discussed, taking into account the spatial distribution of ion-induced damage. Hillock formation due to the substrate swelling, arising from the He+ ions stopped in the substrate, is also observed. The findings provide guidelines for exploiting focused He+ ion beam irradiation in fabricating iron pnictide and MgB2 planar Josephson junctions. [ABSTRACT FROM AUTHOR]

We report on the growth of high quality MgB2 thin films on silicon and silicon-on-insulator substrates by hybrid physical chemical vapor deposition. A boron buffer layer was deposited on all sides of the Si substrate to prevent the reaction of Mg vapor and Si. Ar ion milling at a low angle of 1° was used to reduce the roughness of the boron buffer layer before the MgB2 growth. An Ar ion milling at low angle of 1° was also applied to the MgB2 surface to reduce its roughness. The resultant MgB2 films showed excellent superconducting properties and a smooth surface. The process produces thin MgB2 films suitable for waveguide-based superconducting hot electron bolometers and other MgB2-based electronic devices. [ABSTRACT FROM AUTHOR]

Magnesium diboride (MgB2) coating is a potential candidate to replace bulk niobium (Nb) for superconducting radio frequency cavities due to the appealing superconducting properties of MgB2. MgB2 coating on copper may allow cavity operation near 20–25 K as a result of the high transition temperature (Tc) of MgB2 and excellent thermal conductivity of Cu. We have grown MgB2 films on 2 inch diameter Cu discs by hybrid physical–chemical vapor deposition for radio frequency characterization. Structural and elemental analyses showed a uniform MgB2 coating on top of a Mg–Cu alloy layer with occasional intrusion of Mg–Cu alloy regions. High Tc values of around 37 K and high critical current density (Jc) on the order of 107 A cm−2 at zero field were observed. Radio frequency measurements at 11.4 GHz confirmed a high Tc and showed a quality factor (Q0) much higher than for Cu and close to that of Nb. [ABSTRACT FROM AUTHOR]

Several theoretical analyses of the two superconducting energy gaps of magnesium diboride, and , predict substructures within each energy gap. We report additional experimental evidence for these features, in tunneling conductance data using multiple samples measured at very low temperatures. The absence of these features in c-axis tunneling, and a sharp peak in the subgap (associated with the counterelectrode material), support the conclusion that these features are intrinsic to MgB2. By demonstrating the inadequacy of a simple two-gap model in fitting the data, we illustrate that distinctions between theoretical models of energy gap substructures are experimentally accessible. [ABSTRACT FROM AUTHOR]

A comprehensive microstructural study was conducted on optimally-doped epitaxial Ba(Fe1−xCox)2As2 thin films grown by pulsed laser deposition on various substrates of a wide range of lattice constants: SrTiO3, LaAlO3, (La,Sr)(Al,Ta)O3, MgO, CaF2, and BaF2. We found that epitaxial strain directly affects the superconductivity in the film, with the transition temperature decreasing linearly with increasing in-plane lattice constant of the film. However, the strain is not determined by the lattice mismatch between the film and substrate. Instead, the mosaic spread of the grain orientation in the film and the thermal expansion coefficient of the substrate were found to correlate well with the in-plane lattice constant of the film. The result confirms the importance of structural distortions to the superconductivity in the Ba(Fe1−xCox)2As2 films. [ABSTRACT FROM AUTHOR]

We report that C-doped MgB2 thin films deposited by hybrid physical-chemical vapor deposition (HPCVD) using CH4 as the carbon source have Hc2(0 K)[?]60 T, similar to that of HPCVD films obtained using (MeCp)2Mg for the carbon. Using transmission electron microscopy, we show that in the films doped using CH4 there is a MgB2C2 layer on top of the MgB2 film, which does not degrade the MgB2 connectivity or Jc. We also find a high density of coherent MgO nanoplatelets in the MgB2 which create strain fields which may give rise to strong p-band scattering and the very high Hc2. [ABSTRACT FROM AUTHOR]

Phase pure and clean MgB2 films have been fabricated on different types of single-crystal substrate, including SiC, sapphire, MgO, LaAlO3, SrTiO3, and yttrium-stabilized ZrO2 (YSZ), via the hybrid physical-chemical vapor deposition (HPCVD) technique. Pure MgB2 films with different structural textures are obtained by selecting the substrate and its orientation, such as off-axis epitaxial MgB2 on MgO(211) or textured MgB2 on YSZ(110) substrates. Films show Tc(0) in the range 37-41 K, and residual resistivity r(42 K) between 0.3 and 5 µO cm as the substrate and film thickness are varied. The crystal structures and film morphologies are presented and discussed. Further investigations are focused on c-axis epitaxial films. Epitaxial MgB2 films with thickness about 300 nm manifest Tc(0) = 41.2 K and residual resistivity r(42 K) = 0.35 µO cm on SiC(0001) while Tc(0) = 39.5 K and r(42 K) = 0.27 µO cm on c-cut sapphire, reflecting that the films are of high quality and in the 'clean limit'. Low upper critical field H_{\mathrm {c}2}^{ \parallel ab}(0) and large anisotropy of the upper critical field g comparable to the results from MgB2 single crystals are also revealed. The smooth surface of the epitaxial films, with root-mean-square (RMS) roughness less than 5 nm, makes them well qualified for device applications. Microwave performance measurements are carried out using a coplanar resonator on a 300 nm thick film on sapphire substrate and no power dependence of unloaded quality factor in the output power range of [?]10-25 dB is found. [ABSTRACT FROM AUTHOR]

Low residual resistivity in superconducting thin films is required for their applications in radio frequency (RF) cavities. Here we report on clean epitaxial MgB2 films fabricated by the ex situ annealing of B films, grown by chemical vapour deposition, in Mg vapour. The films show sharp superconducting transitions at about 40 K and a high residual resistivity ratio of about 10. The result indicates that a clean precursor B film and a contamination-free annealing procedure are important for a viable MgB2 film fabrication process for RF cavity applications. [ABSTRACT FROM AUTHOR]