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Free from the Science Reviews archives - Superconductivity

Posted on 9. March, 2015.

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Superconductivity is one of the most explicit and dramatic forms of electronic order. We are baffled by its consequences, for example the complete breakdown of electrical resistance, the expulsion of small magnetic fields, the Josephson effect and magnetic levitation. But at a more conceptual level, the processes underlying these phenomena are equally captivating: why does a superconductor superconduct?

What are the mechanisms that can bind electrons together to Cooper pairs? What types of superconductivity can there be? These general questions are difficult to answer. There are parallels, however, with more familiar types of phase transitions: why, for example, do the
same intermolecular interactions lead to liquid water at high temperatures, and ice at low temperatures? The analogue to supercurrents in this case is the transmission of stresses under static deformation from one part of a solid to another, a phenomenon which is absent in liquids. The first part of this review will attempt to exploit our intuitive knowledge of crystalline order to arrive at a better understanding of superconductivity.

Read the full article. for free, in  Science Progress, Volume 87, Number 1, February 2004, pp. 51-78.

Author: F.M.Grosche

Keywords: Superconductivity, superfluidity, broken symmetry

Image: Example of a circuit set up for demonstrating perfect conductivity. A measurement current is produced in the current source I. When switch S1 is closed and switch S2 is open, this current passes through the reference resistor Rref and the superconductor. The voltage drop across the reference resistor can be measured (Vref) to determine the magnitude of the current. Picking up the voltage drop Vs across the superconductor, using a second pair of leads, allows the determination of the sample resistance. When the sample is indeed superconducting, Vs = 0 for a finite measurement current. It is possible to wire up part of the circuit using superconducting elements, here denoted by thick lines. Once a current is set up as before, switch S2 can be closed and switch S1 opened. The current is now permanently circulating without dissipation in a purely superconducting loop, inducing a magnetic field. Strong superconducting magnets use this principle to produce a persistent magnetic field without the need for a large drive current, once energised.