Magnesium diboride, MgB₂, is an exciting superconductor. It is a conventional BCS superconductor, in which the Cooper pairs are formed through electron-phonon coupling, with a high transition temperature T_c of 39 K. It has multiple bands with weak interband impurity scattering: the 2-dimensional σ bands and the 3-dimensional π bands. They couple to the B-B stretch modes of E_2g symmetry with different strengths, resulting in different superconducting energy gaps. For electronic applications, the high T_c of MgB₂ allows operation of MgB₂ devices and circuits above 20 K, substantially reducing the cryogenic requirements compared to the Nb-based superconducting electronics, which have to operate at 4.2 K. For applications in high magnetic field, carbon-alloyed MgB₂ films have shown higher upper critical field H_c2 values than those of the Nb-based superconductors at all temperatures. MgB₂ is of particular interest for magnets in cryogen-free magnetic resonance imaging (MRI) systems. It is further recognized that the high T_c and low resistivity make MgB₂ an attractive material for RF cavity applications.

In the past, we have made planar MgB₂-TiB₂-MgB₂ SNS Josephson junctions that show excellent properties even above 30 K. We have made trilayer MgB₂-barrier-Pb SIS Josephson junctions that show both gaps and excellent MgB₂-barrier interface properties. The next step in the development of MgB₂ Josephson junctions and circuits is to fabricate trilayer MgB₂-barrier-MgB₂ junctions. For this purpose, it is important to understand the properties of the tunnel barrier. In this PRF project, we have collaborated closely with Prof. Buhrman's group at Cornell on X-Ray Photoelectron Spectroscopy (XPS) studies of MgB₂ films grown by hybrid physical-chemical vapor deposition (HPCVD) used in our MgB₂-barrier-Pb Josephson junctions. The results indicate that an MgO layer is formed on top of the MgB₂ film when the film is exposed to ultra high purity nitrogen at 400 °C after the deposition, which is a successful process to make tunnel barriers. We believe that the sample was oxidized by the impurity oxygen in the nitrogen gas.  We estimate that the MgO thickness is roughly 33 angstroms. From these studies, a process is proposed to anneal MgB₂ film in N₂ at 400°C for 30 min as the tunnel barrier. MgB₂-barrier-Pb Josephson junctions are made using this process, and very good junction characteristics are observed. This is an important step towards trilayer MgB₂-barrier-MgB₂ junctions and circuits. The student who carried out this research has successfully defended his Ph D thesis and is pursuing postdoctoral research at the Lawrence Berkeley Laboratory.

In the previous reported carbon alloyed MgB₂ films, we added carbon into the films by adding bis(methylcyclopentadienyl)magnesium, ((MeCp)₂Mg), a metalorganic magnesium precursor, to the carrier gas during the HPCVD deposition. Transmission electron microscope (TEM) results show that the carbon-alloyed MgB₂ films are not uniform and contain grain boundaries between the carbon doped MgB₂ grains. The material at the grain boundaries is amorphous and highly resistive, which reduce the cross section area of current conduction. On the other hand, these grain boundaries may contribute to the strain in the carbon-doped MgB₂ grains, leading to the high H_c2. To find out the critical factor for the high H_c2, we have tried to make carbon doped MgB₂ films with different microstructures. We have been trying to replace (MeCp)₂Mg with Trimethylboron, (TMB, B(CH₃)₃), as the carbon source. The initial results indicate that using TMB to dope the MgB₂ films with carbon is a very promising approach for getting more uniform films. While T_c is suppressed by carbon doping, the resistivity increases with carbon content in a much slower pace than in carbon-alloyed MgB₂ films using (MeCp)₂Mg. This is an indication that carbon is doped into the MgB₂ grain more efficiently and there is less high resistive grain boundaries in the films. The carbon doped MgB₂ films using TMB show similar increase of H_c2 but higher J_c(H) than the  (MeCp)₂Mg alloyed films. This work opens up a new direction for the study of the mechanism of high H_c2 in carbon doped/alloyed films. A new graduate student is being supported by this PRF project to further investigate this carbon doping process.