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Inherent Origins of Higher Tc Superconductivity

Gerwin Hassink
Malcolm Beasley

Nowadays, superconducting materials are typically grouped into two types: the ′low-temperature superconductors′ which have a maximum critical temperature (Tc) of around 30 K, but have excellent current-carrying properties and the ′high-temperature superconductors′, many of which have a Tc above 77 K (the boiling point of liquid nitrogen), but have typically a low critical current (Jc) and magnetic field (Hc). This limits the practical application of the latter type, while the costs in terms of cooling power limit the former type.

These critical properties (Tc, Jc and Hc) seem to be interlinked with the structural properties such as the (crystal) anisotropy, the nature of the Cooper pair formation and the influence of the electron density. For example, the low-temperature superconductors are generally isotropic, but have low Tc′s. The reverse is true for high-temperature superconductors which are highly anisotropic and have high Tc′s. A similar change is seen in the Cooper pair formation mechanism; where for low-temperature superconductors the Cooper pair electrons are widely separated, for high-temperature superconductors they often seem to be located within a single unit cell of the material.

My research focuses on how properties as the anisotropy and binding mechanisms influence the superconducting properties. Currently there are two oxide systems under investigation. With the pulsed laser deposition (PLD) and e-beam evaporator set-ups in the lab thin-film structures are fabricated which are then characterized structurally and their transport properties investigated. Often further investigations in collaboration with other groups, national and international, are carried out.

The first material system is one of the few isotropic superconducting oxides, Ba1-xKxBiO3-d (dx.doi.org/10.1038/332814a0). Aside from that, the traditional low-temperature superconductor pairing mechanism of electron-phonon coupling does not seem to fully explain the superconductivity ( dx.doi.org/10.1103/PhysRevB.44.12521). A ′negative-U′ model has been proposed based on the valence-skipping property of the bismuth ion ( dx.doi.org/10.1103/PhysRevB.52.1368). Basically, this means that the Bi4+ valence state does not exist, but is split up into equal amounts of Bi3+ and Bi5+ ( dx.doi.org/10.1088/1742-6596/108/1/012011). This could give Cooper pair sizes comparable to high-temperature superconductors. As such, the BKBO system provides an interesting mix of properties that could help to figure out just what′s going on in those high-temperature superconductors.

The second material system is the doped tungsten bronze, AxWO3-d. As a function of doping and strain, this material undergoes a variety of structural transitions from cubic to tetragonal and even hexagonal. In addition, for certain doping ranges, it becomes metallic upon doping with alkali metals such as Na+, K+,Rb+ and Cs+. Typically, however, these compounds have Tc′s below 5 K. More recently, however, Tc′s above 100 K have been observed in Na+ and H+ doped WO3 ( dx.doi.org/10.1007/s10948-009-0443-3, dx.doi.org/10.1023/A:1007867710512). In both cases, it is thought that the superconductivity is due to a doped surface layer as opposed to the bulk doping from earlier experiments. By fabricating thin films of tungsten bronze and doping them from the surface, we hope to investigate this behavior.

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