Wenqing Dai

Impact of substitutional and interstitial carbon defects on lattice parameters in MgB2

Carbon (C)-doping has been found to increase the upper critical field HC2 in superconducting MgB2 thin-film and bulk samples. However, the C effects on both HC2 and lattice parameters are very different between thin films and bulk, suggesting C may incorporate differently in the two cases. This paper combines ab initio calculations and thin-film lattice parameter measurements to explore the connection between substitutional and interstitial C in MgB2 and experimental bulk and thin-film lattice parameters.

Momentum-dependent multiple gaps in magnesium diboride probed by electron tunnelling spectroscopy

The energy gap is the most fundamental property of a superconductor. MgB(2), a superconductor discovered in 2001, exhibits two different superconducting gaps caused by the different electron-phonon interactions in two weakly interacting bands. Theoretical calculations predict that the gap values should also vary across the Fermi surface sheets of MgB(2). However, until now, no such variation has been observed. It has been suggested that two gap values were sufficient to describe real MgB(2) samples. Here we present an electron tunnelling spectroscopy study on MgB(2)/native oxide/Pb tunnel junctions that clearly shows a distribution of gap values, confirming the importance of the anisotropic electron-phonon interaction. The gap values, and their spreads found from the tunnel junction measurements, provide valuable experimental tests for various theoretical approaches to the multi-band superconductivity in MgB(2).

Clean epitaxial MgB2 films fabricated by the ex situ annealing of chemical vapour deposition-grown B films in Mg vapour

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.

Energy gap substructures in conductance measurements of MgB2-based Josephson junctions: beyond the two-gap model

Several theoretical analyses of the two superconducting energy gaps of magnesium diboride, and , predict substructures within each energy gap, rather than two pure numbers. Recent experiments have revealed similar structures. We report tunneling conductance data providing additional experimental evidence for these features. 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 tting the data, we illustrate that some distinctions between theoretical models of energy gap substructures are experimentally accessible.

Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB2

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrow...

Growth of

We have grown MgB2 thin films using RF magnetron sputtering combined with a pocket heater. This technique relies on a low-pressure environment for sputter deposition of boron and a high-pressure environment for thermal evaporation of Mg. We have obtained superconducting MgB2 thin films using substrate temperatures of 480-540degC and Mg furnace temperatures of 730-750degC. The Tc onset of the thin films increased from 21.6 K to 35 K with increasing substrate temperature due to better crystallization. Higher boron deposition rates also increase Tc. The highest Jc of the films at 5 K and near zero magnetic field is 1.5 MA/cm2 which is comparatively lower than the films grown using a pocket heater with boron deposition by chemical vapor deposition or electron beam evaporation. The chemical composition analysis by WDS exhibits a high concentration of oxygen and carbon in the MgB2 films, which is due to a high background base pressure and an impure B sputtering target. These results suggest that the Tc, Jc, and resistivity of MgB2 films are mainly determined by an inhomogeneous microstructure and superconducting percolation paths through impurity phases such as MgO. By comparison with carbon or oxygen doped films, the high impurity content in the sputtered MgB2 films might act as method to achieve high Hc2.

{\rm MgB}_{2}

Microwave resonant activation is a powerful, straightforward technique to study classical and quantum systems, experimentally realized in Josephson junction devices cooled to very low temperatures. These devices typically consist of two single-gap superconductors separated by a weak link. We report the results of the first resonant activation experiments on hybrid thin film Josephson junctions consisting of a multi-gap superconductor (MgB2) and a single-gap superconductor (Pb or Sn). We can interpret the plasma frequency in terms of theories both for conventional and hybrid junctions. Using these models, we determine the junction parameters including critical current, resistance, and capacitance and find moderately high quality factors of Q0∼ 100 for these junctions.