Hans Mooij "Summary of experimental results of quantum phase slips in superconducting nanowires"

Hans Mooij substituted at last minute for Bezryadin. He said he was going to give a general introduction to topic of quantum phase slips in superconducting nanowires.

Asked us to consider long thin superconducting wire of constant cross section A and what happens when A is made smaller and smaller. As current run through the wire it is suspectible to phase slip in superconducting order parameter Psi = Delta e^i phi(r,t). What are the variations of the phase phi at Ts near Tc and at T=0?

Phase slips by thermal activation well studied in 60s-70s. Ginzburg-Landau theory gives a functional form for exponential activation. For experiment, see for instance in Nebower Beasley and Tinkham 1972. Theory fits R vs. T curves well. Thermal phase slip centers gives steps in IV curves

Quantum phase slips 1st addressed experimentally by Giordano 1988. These are phase slip events which are not thermally activated over a barrier, but are quantum tunneling through the barrier. Difficulties in addressing this is mostly in that current nanofabrication technology is at the edge of being able to make nanowires thin enough. One needs few nm size wires.

Mooij emphasizes that Bezryadin has made a big impact in this field. He has overcomes the limitations of nanofabrication by using nanotube as a template lying over a slot. Evaporates amorphous MoGe on top.

There is a simple extension of Ginzburg-Landua theory to T=0 that was address in Lau et al. 2001. Fits data well, but there are many free parameters in this expression. It it is bloggers opinion that it is very hard to fit exponential definitively when there are many free parameters.


As a summary, it appears that quantum phase slips have been observed. In some wires, there are so many slips that it appears to drive the wire insulating. Whether a wire is insulating or phase slipping superconductor aka a "metal" seems to be controlled by the normal state resistance of the wire. Wires with normal state resistance greater than quantum of resistance for Cooper pairs (6.45 kiloOhms) are insulators and with less resistance are "metal." In Mooij's opinion it is is remarkable that the distinction is so sharp and it is not clear why the division should be set by the quantum of resistance of Cooper pairs, instead of just number close to it.

There is a question from this blogger about the role of filtering. Mooij says that these are high impedance objects and the experiments are incredibly difficult. Believs that experimental results are robust.

Blogged by N. Peter Armitage