Study of $\hbox{MgB}_{2}$ Josephson Junction Arrays and Sub- $\mu\hbox{m}$ Junctions

Abstract

Sandwich-type MgB 2 /MgO/MgB 2 Josephson junctions with sizes ranging from 0.5 μm × 0.5 μm to 10 μm × 10 μm were fabricated using electron beam lithography. The junctions show critical current densities as high as 130 kA/cm 2 . The smallest of the junctions operated free of hysteresis below 4.2 K. They may be ideal for use in rapid single flux quantum circuits with no requirement for external shunt resistors. The 10-μm junctions show good agreement with the expected Fraunhofer-like modulation of critical current with magnetic field; however, the smaller junctions show a sinusoidal modulation, indicating an anomalous current distribution that needs to be further investigated. Series arrays of Josephson junctions were made to quantitatively study the uniformity of junction parameters, which shows a 54% spread in critical current of a 100 junction array of 4-μm junctions.

High‐Quality Epitaxial MgB2 Josephson Junctions Grown by Molecular Beam Epitaxy

“…The inset of Figure a shows the geometry of our device with three junctions on it. Electrical transport mearements were perpformed with four‐terminal method at 4.2 K. At zero magnetic field, the I – V characteristics of junctions with different sizes are reproducible and consistent with previous reports . We next focus our discussion on the junction with area 8 × 16 µm 2 and look at its field performance.…”

“…Being a standard BCS (Bardeen‐Cooper‐Schrieffer) superconductor, MgB 2 has the highest critical temperature at ambient pressure among the BCS class and a unique dual gap nature with meV and meV, originating from its layered structure and the resultant in‐plane and out‐of‐plane lattice vibrations, respectively. Its high T c and large superconducting energy gap allow MgB 2 based superconducting electronics, such as Josephson junctions and superconducting quantum interference devices (SQUID), to operate at relatively higher temperatures (≥20 K, more suitable for field operations) and higher speeds . MgB 2 films also have very high critical current density on the order of 10 7 A cm −2 , and critical field up to 29 T .…”

Thermal Performances of an Improved Package for Cryocooled Josephson Standards

“…Refrigeration of Josephson voltage standards is generally achieved with liquid helium: this limits the spread of these quantum devices outside National Metrology Institutes, due to the increasing cost of liquid helium, driven by shortages rumors, and to the specific skills required for handling the potentially hazardous cryogen. Moreover, the difficulty of fabricating large and uniform arrays with high critical temperature superconductors, as YBCO and the more recent MgB 2 [4], still prevents the development of reliable Josephson standards working at higher temperatures.…”