Teng Tan

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.

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.

Study of Components for

In order to realize superconducting circuits using MgB2, an in-depth study was conducted to generate a baseline of parameters necessary for circuit design. With a transition temperature (Tc) of 39 K, devices and circuits can be made to operate at low temperatures to achieve higher operating frequencies, and at high temperatures to lower the cooling requirements. The Josephson junction critical current density (Jc) and the product of critical current (Ic) and normal resistance (Rn) are necessary design parameters along with the sheet inductance (Lsq) of a superconducting microstrip. The need for a shunt resistor is also required for low-temperature operation. This paper describes the current status of these parameters for the multilayer process of all-MgB2 Josephson junctions. The study includes the fabrication of Josephson junctions and DC SQUIDs to perform microstrip inductance measurements as well as research on materials suitable for an on chip resistor. The results have been used to design a Rapid Single Flux Quantum Toggle-Flip-Flop circuit operating at around 20 K.

\hbox{MgB}_{2}

A scaled-up hybrid physical-chemical vapor deposition (HPCVD) method has been developed to fabricate large-area high-quality magnesium diboride (MgB₂) thin films. The 2-in-diameter MgB₂ thin films on sapphire (0001) show uniform superconducting properties, including <i>Tc</i> ~ 39 K, residual-resistivity ratio ~ 14, and <i>Jc</i> (4.2 K) ~ 10⁷ A/cm². X-ray diffraction showed that the films are epitaxial with <i>c</i>-axis normal to the film surface. The film properties are uniform across the 2-in substrate.

RSFQ Digital Circuits

Bulk niobium Superconducting Radio-Frequency cavities are a leading accelerator technology. Their performance is limited by the cavity loss and maximum acceleration gradient, which are negatively affected by vortex penetration into the superconductor when the peak magnetic field at the cavity wall surface exceeds the vortex penetration field (Hvp). It has been proposed that coating the inner wall of an SRF cavity with superconducting thin films increases Hvp. In this work, we utilized Nb ellipsoid to simulate an inverse SRF cavity and investigate the effect of coating it with magnesium diboride layer on the vortex penetration field. A significant enhancement of Hvp was observed. At 2.8 K, Hvp increased from 2100 Oe for an uncoated Nb ellipsoid to 2700 Oe for a Nb ellipsoid coated with ~200 nm thick MgB2 thin film. This finding creates a new route towards achieving higher acceleration gradient in SRF cavity accelerator beyond the theoretical limit of bulk Nb.

Large-Area

The magnetic vortex penetration field (Hvp) is an important property of superconducting radio frequency (SRF) cavities. However, measuring Hvp of an SRF cavity directly is usually a difficult task. As an alternative, a superconducting ellipsoid in an axial magnetic field would have a similar but inversed field geometry of an SRF cavity and would allow for the characterization of Hvp. In this work, we deposited a uniform MgB2 layer on Nb ellipsoids and used those ellipsoids to mimic the behavior of MgB2 coated Nb SRF cavities. The Hvp of such a structure was measured via zero-field-cool (ZFC) magnetization method. At 1.8 K, the Hvp for a coated Nb ellipsoid is 100 Oe higher than Hvp for a bare Nb ellipsoid.