Superconductivity is reduced

Superconductivity is reduced as a system becomes more one-dimensional. Moving through very thin passages, Cooper pairs of electrons, which constitute the supercurrent, are sensitive to quantum effects not noticeable in larger wires. For example, quantum phase slips—fluctuations in which the superconducting wavefunction spontaneously tunnels from one state to another—occur well below the critical temperature. The tunneling produces a momentary voltage, and therefore a nonzero electrical resistance, even if the temperature could somehow be reduced to absolute zero. Armed with progressively thinner wires—down to 10 nm across—of molybdenum-germanium deposited onto carbon nanotubes, Michael Tinkham and his colleagues at Harvard University have definitively shown that resistance goes up as the wire diameter goes down. The quantum resistance effect only becomes noticeable for wires less than about 30 nm across. By going to lower temperatures, says Tinkham, one can eliminate resistivity arising from thermal fluctuations, but not from quantum fluctuations. (C. N. Lau et al., Phys. Rev. Lett., 87, 217003, 2001 http://dx.doi.org/10.1103/PhysRevLett.87.217003 .)