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Register a new userPlanar Superconductor-Normal-Superconductor Josephson Junctions in MgB2
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Authors
G. Burnell
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Since the discovery of superconductivity in MgB2 considerable progress has been made in determining the physical properties of the material, which are promising for bulk conductors. Tunneling studies show that the material is reasonably isotropic and has a well-developed s-wave energy gap (∆), implying that electronic devices based on MgB2 could operate close to 30K. Although a number of groups have reported the formation of thin films by post-reaction of precursors, heterostructure growth is likely to require considerable technological development, making single-layer device structures of most immediate interest. MgB2 is unlike the cuprate superconductors in that grain boundaries do not form good Josephson junctions, and although a SQUID based on MgB2 nanobridges has been fabricated, the nanobridges themselves do not show junction-like properties. Here we report the successful creation of planar MgB2 junctions by localised ion damage in thin films. The critical current (IC) of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very high frequency applications.
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We propose a novel type of magnetic scanning probe sensor, based on a single planar Josephson junction with a magnetic barrier. The planar geometry together with high magnetic permeability of the barrier helps to focus flux in the junction and thus enhance the sensitivity of the sensor. As a result, it may outperform equally sized SQUID both in terms of the magnetic field sensitivity and the spatial resolution in one scanning direction. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic CuNi and a ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy miniaturization to nm-scale, facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feed-back operation in a broad dynamic range.
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Self-consistent solutions of microscopic Eilenberger theory are presented for a two-dimensional model of a superconducting channel with a geometric constriction. Magnetic fields, external ones as well as those caused by the supercurrents, are included and the relevant equations are solved numerically without further assumptions. Results concerning the influence of temperature, geometric parameters, of $kappa=lambda_L/xi_0$ and of external magnetic fields on the Andreev bound states in the weak link and on the current-phase relation are presented. We find that the Andreev bound states within the junction obtain peculiar substructure when a finite supercurrent flows. As long as the London penetration depth is comparable to or bigger than the extension of the constriction, the Josephson effect is independent of $kappa$. Furthermore, the weak link is very insensitive to external magnetic fields. Features restricted to a self-consistent calculation are discussed.
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