MTL Annual Report

Figure 1: Electron microscope image of the weak-link region in a MgB₂ thin film lateral device. The proximity of the Fe in the bilayer region of Fe/MgB₂ causes local weakening or complete suppression of superconductivity, depending on the thickness of the MgB₂ thin film.

MgB₂ is a multi-gap superconductor with a T_C of 39K and a hexagonal structure. This simple and stable compound is very attractive for device applications. We have developed thin-film all-MgB₂ devices, both in the vertical geometry with a sputtered MgO barrier, and in the lateral geometry by creating weak-link junctions using proximity with Fe. In the first case, we have deposited and patterned micron-sized SIS tunnel junctions with highly textured MgB₂ electrodes grown by MBE co-evaporation with sputter-deposited MgO tunnel barriers, in an entirely in-situ process. This method enabled us to obtain low-resistance junctions with very good oxide coverage. The I-V and dI/dV data display Josephson pair tunneling as well as the quasi-particle tunneling signature of both the Pi- and Sigma-bands of the MgB₂, with gap values agreeing with previous reports. Although our MgB₂ films were c-axis oriented, the growth-related roughness of the bottom MgB₂ enabled a/b-axis tunneling, giving rise to Sigma-band features. The layers for lateral junctions were also deposited entirely in situ, the Fe film directly in contact with the MgB₂ film, and these films were subsequently patterned into lateral bars. This configuration creates S1/U/S1 junctions, where U is the Fe/MgB₂ bilayer region, and is either a weakened superconductor S2, or a normal metal N in the case where superconductivity is completely suppressed in the MgB₂ thin film. Figure 1 is an electron microscope image of the weak link region in one of our typical devices. Current transport in these junctions is in the plane of the film, along the a/b-axis of the superconductor. We observe signs of pair tunneling through the weakened region.