Superconductivity at NanoscaleSuperconductivity is a state of metal in which the electrical current flows without any detectable dissipation. It is a fascinating phenomenon that was used to measure magnetic fields of a living brain as well as to test the limits of the Quantum Mechanics.Superconductivity is a fascinating phenomenon that led to the development of the one of the most beautiful and precise physical theories, namely the microscopic theory of superconductivity developed by John Bardeen, Leon Cooper, and Robert Schrieffer (BCS) . Nanoscale superconductors provide a "laboratory" for testing fundamentals of quantum mechanics. For example, superconducting "Schrödinger cats" have been recently developed and tested. Very sensitive devices, such as superconducting quantum interference devices (SQUID), have been made with superconductors and have been applied, among other things, in the measurements of the magnetic fields generated by a living brain activity. Thus even the process of thinking can be investigated using SQUID technology. |
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Little-Parks oscillation provides direct proof of the quantum behavior of electrons in a superconductor. In their experiments (1962), William A. Little and Roland D. Parks demonstrated that electrons forming a quantum-coherent condensate in superconducting metals exhibit sensitivity to the vector-potential, and not only to the magnetic field, as non-quantum charged particles do. Sensitivity to the vector-potential is a general signature of charged quantum particles related to the gauge invariance principle. Little and Parks showed that a large fraction of electrons in a superconductor behave essentially as a single quantum particle.
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On the picture: Equilibrium patterns in superconducting lead: left, Prozorov’s “soap-foam” pattern; and right, the Landau laminar pattern. Both images are obtained at the same temperature and magnetic field. The only difference is how the magnetic field was increased or decreased to reach equilibrium. The lighter regions correspond to strong magnetic field and the dark regions are superconducting boundaries in which magnetic field does not enter.
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Magnetic fields have long been known to suppress superconductivity through two main effects: first, by aligning the electron spins (Zeeman effect) and second, by raising the kinetic energy of condensed electrons via Meissner screening currents. Remarkably, in contrast with these expectations that have stood for some fifty years, it was recently discovered that magnetic fields can enhance the critical supercurrent in Nb and MoGe wires with very small diameters (typically less than 10 nm). The anomalous enhancement reaches the maximum at the field of 2-4 Tesla and is followed by usual decrease at higher fields.
The graph shows curves representing how the critical supercurrent of nanowires changes with applied magnetic field. The increase of the critical current is clearly seen. It is followed by the usual decline at higher fields.
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Nanowire superconducting: strands of DNA have been used as tiny scaffolds to create superconducting nanodevices that demonstrate a new type of quantum interference phenomenon.
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Los Alamos physicists study MEG superconducting helmets which are composed of 155 SQUIDs, to provide "whole head" brain-current images. The MEG helmet offers improved capabilities that could help make magnetoencephalography, or MEG more common in hospitals.
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The left picture shows a superconducting MoGe nanowire (gray line) produced by sputter-coating a single-wall carbon nanotube placed across a deep trench etched into the substrate.
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Phase boundary (dashed line) in Rn/L versus Rn representation. Click to enlarge.
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Superconducting vortices (white spots) are imaged by the magnetic decoration technique.
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