Wednesday, June 25, 2014

Superconductors and Friction

I spent most of yesterday travelling between Austin and College Station, but I did manage to get a little bit of research in.  I'm looking for other experiments related to Hirsch's theories we can do in conjunction to the search for H-rays.  One of the measurable predictions of Hirsch's theory is a change in the coefficient of sliding friction when a material enters it's superconducting state.

New to the lab book?  Scroll to the bottom for background and a summary of the experiment.

Hirsch mentions[1] that a finding of reduced friction in superconductors might be evidence in support of his hole theory of superconductivity.

Furthermore, the electronic layer outside the surface
is likely to affect the friction properties of the superconductor,
by providing a ‘lubricating layer’ on top
of which another material would slide. As a matter of
fact, an abrupt drop in sliding friction between a lead
surface and solid nitrogen has been observed when Pb
enters the superconducting state [15]. We infer that
this effect is due to the physics of the electron layer
outside the surface discussed here, and hence that a
friction drop should be observed for all superconductors.

He cites, ([15] above), an experiment reported by Dayo, Alnasrallah, and Krim[2] as having actually shown a drop in the sliding friction between Pb and solid nitrogen in a liquid helium reservoir.  The experimental setup for the DAK experiment is shown in picture 2.

A quartz resonator was covered with thin film of Pb and then nitrogen was adsorbed into the surface of the Pb.  By measuring the quality factor of the underlying quartz resontaor, the researchers claimed they were able to monitor the friction between the Pb and the layer of nitrogen.  If the nitrogen could easily slide across the Pb, then the underlying resonator could vibrate more easily than if it could not.  This apparatus described has been used freqently in friction research and is know as a quartz crystal microblalnce.  DAK reported that when the Pb entered it's usperconducting state, there was a large, almost discontinuous, reduction in the coefficient of sliding friction between the Pb and the nitrogen.

There are issues however...  Renner, Rutledge, and Taborek, repeated the DAK experiment and got a null result.  Krim replied stating that the RRT experient had used a contaminated sample.  The important takeaway for the work here is that if Krim is correct, then additional friction experiments would require more resources than we currently have at hand.

A similar test has been done with YBCO, by Krim and Altfeder[5].  The results showed that there was a roughly linear reduction between YBCO and a Fe surface as the temperature of the YBCO approached the critical temperature, but that friction held steady once the YBCO entered the superconducting state.


1.  Hirsch on superconductors and friction [open access]

2.  A. Dayo, W. Alnasrallah, J. Krim, Phys. Rev. Lett. 80 (1998)

3.  More theoretical work on the DAK experiment

4.  R. L. Renner, J. E. Rutledge, and P. Taborek, preceding
Comment, Phys. Rev. Lett. 83, 1261 (1999).

5.  YBCO and friction

6.  Friction on Wikipedia

Hirsch's theory of hole superconductivity proposes a new BCS-compatible model of Cooper pair formation when superconducting materials phase transition from their normal to their superconducting state[1].  One of the experimentally verifiable predictions of his theory is that when a superconductor rapidly transitions, (quenches), back to its normal state, it will emit x-rays, (colloquially referred to here as H-rays because it's Hirsch's theory).

A superconductor can be rapidly transitioned back to its normal state by placing it in a strong magnetic field.  My experiment will look for H-rays emitted by both a Pb and a YBCO superconductor when it is quenched by a strong magnetic field.
This series of articles chronicles both the experimental lab work and the theory work that’s going into completing the experiment.

The lab book entries in this series detail the preparation and execution of this experiment… mostly.  I also have a few theory projects involving special relativity and quantum field theory.  Occasionally, they appear in these pages.

Call for Input
If you have any ideas, questions, or comments, they're very welcome!


1.  Hirsch, J. E., “Pair production and ionizing radiation from superconductors”, 

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