Matthäus A. Wolak

Characterization of MgB2 Superconducting Hot Electron Bolometers

Hot-Electron Bolometer (HEB) mixers have proven to be the best tool for high-resolution spectroscopy at the Terahertz frequencies. However, the current state of the art NbN mixers suffer from a small intermediate frequency (IF) bandwidth as well as a low operating temperature. MgB2 is a promising material for HEB mixer technology in view of its high critical temperature and fast thermal relaxation allowing for a large IF bandwidth. In this work, we have fabricated and characterized thin-film (~15 nm) MgB2-based spiral antenna-coupled HEB mixers on SiC substrate. We achieved the IF bandwidth greater than 8 GHz at 25 K and the device noise temperature <; 4000 K at 9 K using a 600 GHz source. Using temperature dependencies of the radiation power dissipated in the device we have identified the optical loss in the integrated microantenna responsible as a cause of the limited sensitivity of the current mixer devices. From the analysis of the current-voltage (IV) characteristics, we have derived the effective thermal conductance of the mixer device and estimated the required local oscillator power in an optimized device to be ~4 μW.

Fabrication and Characterization of Ultrathin MgB 2 Films for Hot-Electron Bolometer Applications

Hot-electron bolometer mixers employing thin films of conventional superconducting materials have already been successfully fabricated in the past. Magnesium diboride (MgB2) is a promising alternative to conventional superconductors, and we report the fabrication and study of ultrathin MgB2 films of down to 10 nm deposited by hybrid physical-chemical vapor deposition technique. The MgB2 films showed Tc of above 36 K, while residual resistivities of up to 26 μΩ · cm were achieved. Critical currents of more than 6 × 106 A · cm-2 at 20 K have been measured for the films with thicknesses ranging from 10 to 100 nm. Fishtail structures have been observed in the magnetic field dependence of the critical current density for the thinnest of these films, indicating the presence of defects, which act as vortex pinning centers. From the magnetic field dependence, an average distance between adjacent pinning centers of 35 nm has been obtained for the thinnest films.

MgB2 ultrathin films fabricated by hybrid physical chemical vapor deposition and ion milling

In this letter, we report on the structural and transport measurements of ultrathin MgB2 films grown by hybrid physical-chemical vapor deposition followed by low incident angle Ar ion milling. The ultrathin films as thin as 1.8 nm, or 6 unit cells, exhibit excellent superconducting properties such as high critical temperature (Tc) and high critical current density (Jc). The results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB2 at the 2D limit.

Low-noise THz MgB2 Josephson mixer

The potential applications for high frequency operation of the Josephson effect in MgB2 include THz mixers, direct detectors, and digital circuits. Here we report on MgB2 weak links which exhibit the Josephson behavior up to almost 2 THz and using them for low-noise heterodyne detection of THz radiation. The devices are made from epitaxial film grown in the c-axis direction by the hybrid physical-chemical vapor deposition method. The current in the junctions travels parallel to the surface of the film, thus making possible a large contribution of the quasi-two-dimensional σ-gap in transport across the weak link. These devices are connected to a planar spiral antenna with a dielectric substrate lens to facilitate coupling to free-space radiation for use as a detector. The IcRn product of the junction is 5.25 mV, giving confirmation of a large gap parameter. The sensitivity of the mixer was measured from 0.6 THz to 1.9 THz. At a bath temperature of over 20 K, a mixer noise temperature less than 2000 K (DSB)...

Epitaxial strain and its relaxation at the LaAlO3/SrTiO3 interface

A series of LaAlO3 thin films with different thicknesses were deposited by pulsed laser deposition at temperatures from 720 °C to 800 °C. The results from grazing incidence x-ray diffraction and reciprocal space mapping indicate that a thin layer of LaAlO3 adjacent to the SrTiO3 substrate remains almost coherently strained to the substrate, while the top layer starts to relax quickly above a certain critical thickness, followed by a gradual relaxation at larger film thickness when they are grown at lower temperatures. The atomic force microscopy results show that the fast relaxation is accompanied by the formation of cracks on the film surface. This can be ascribed to the larger energy release rate when compared with the resistance of LaAlO3 to cracking, according to calculations from the Griffith fracture theory. For films grown at 720 °C, a drop in sheet resistance by two orders of magnitude is observed when the top layer starts to relax, indicating a relationship between the strain and the conductivity of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface. The strain engineered by growth temperature provides a useful tool for the manipulation of the electronic properties of oxide heterointerfaces.

Enhancement of lower critical field by reducing the thickness of epitaxial and polycrystalline MgB2 thin films

For potential applications in superconducting RF cavities, we have investigated the properties of polycrystalline MgB2films, including the thickness dependence of the lower critical fieldHc1. MgB2thin films were fabricated by hybrid physical-chemical vapor deposition on (0001) SiC substrate either directly (for epitaxialfilms) or with a MgO buffer layer (for polycrystalline films). When the film thickness decreased from 300 nm to 100 nm, Hc1 at 5 K increased from around 600 Oe to 1880 Oe in epitaxialfilms and to 1520 Oe in polycrystalline films. The result is promising for using MgB2/MgO multilayers to enhance the vortex penetration field.

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB2. MgB2 thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB2 thin layers, the presented data show that the superconducting critical temperature Tc exceeds the bulk value only when the MgB2 films are grown on atomically flat (0001) SiC single crystals and on MgB2 bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB2 thin films.

Superconducting nanowire detectors based on MgB 2

We fabricated and characterized the optical response of MgB2 nanowires with critical temperature TC = 33 K. The devices showed optical response at 4 K and sub-nanosecond relaxation time. The detectors responded to the simultaneous absorption of three photons, but not to single photons.