MgB 2 , a two-gap superconductor for practical applications
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Introduction In 2001, Nagamatsu et al.1 discovered that MgB2 was a superconductor with a surprisingly high transition temperature for a binary compound, Tc = 40 K. Despite its conventional electronphonon coupling mechanism, MgB2 was shown to possess two superconducting energy gaps originating in two different electron bands, σ and π, both of which cross the Fermi level, EF. Since the beginning, a huge effort was expended to understand multiband effects in MgB2 and to explore its potential for affordable, dry applications of superconductivity. Because grain boundaries do not create serious weak links to reduce the critical current density (Jc) in polycrystalline forms of the material,2 long multifilamentary MgB2 wires with a relatively high Jc of 105–106 A/cm2 were quickly manufactured by the scalable powder-in-tube (PIT) technique.3 These favorable characteristics yielded, less than 10 years after its discovery, commercial MRI systems based on MgB2 cooled by a cryo-generator, thus operating in a cryogen-free environment. Further developments are now necessary to push the material toward high magnetic field applications, where superconductors have historically played a central role. Epitaxial MgB2 thin films exhibit much higher Jc than polycrystals (by two orders of magnitude),4 very close to the depairing current density. Moreover, their upper critical magnetic field Hc2 is near 60 T,5 which far exceeds the performance of industrial Nb-based superconductors currently used in virtually all commercial
superconducting magnets. Unfortunately, these outstanding characteristics are far from being reached in polycrystalline MgB2 material forms, which suggests the presence of limiting factors for supercurrent transport that still need to be fully explained. The article will review (1) the basic properties of MgB2, including its multiband nature, its two-gap superconductivity and critical parameters, and the ways in which these features affect the superconducting properties; (2) the superconducting properties, especially progress in understanding and improving the important upper critical field Hc2, critical current density Jc, vortex pinning, and connectivity; (3) development and fabrication of wires by ex situ and in situ manufacturing processes; and (4) existing and future applications, from medical devices to compact and efficient rotating machines.
Basic properties of MgB2 Multi-gap superconductivity was predicted since the 1950s,40 but MgB2 was the first clear example of a two-gap superconductor. The two-gap nature was pointed out immediately after the discovery of superconductivity in this compound, because it significantly affects the superconducting properties (for a review, see Reference 6). It was observed that the Tc value of 40 K was far too high to be explained within a conventional Bardeen-Cooper-Schrieffer (BCS) framework, and also Hc2 was unusually high when compared to single-band superconductors. While the electronic specific heat showed a significant
Marina Putti, CNR-SPIN and Dipartimento di Fisica, Università
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