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Superconductivity found at 175 degrees Kelvin?

  1. Oct 19, 2007 #1

    Hans de Vries

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  2. jcsd
  3. Oct 19, 2007 #2

    f95toli

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    I am not convinced. There have been so many reports of higher Tc accompanied by noisy curves in the pase, most of them have turned out to be false.
    Neither the susceptibilty data nor the resisitiviy is convincing. A transtion to a phase with magnetic ordering could -at least in principle- give the same results.
     
  4. Oct 19, 2007 #3

    Gokul43201

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    I don't understand either graph!!! Why are [itex]d\rho/dT[/itex] and [itex]d\chi/dT[/itex] the same in the normal (insulating) state and in the SC state?
     
  5. Oct 20, 2007 #4

    f95toli

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    I guess they are just trying to convince us that a part of the sample goes superconducting (i.e. it is very inhomogenous) which means you would get a curve similar to what they show. The resisitive transition in high-Tc can look quite strange if the sample is very inhomogenous with some parts being normal, some parts superconducting but with varying degrees of under/overdoping. Susceptibilty data is usually "cleaner" since it is less local but in this case it is too noisy to draw any conclusions.
    It would be interesting to see the result of a scan with a SQUID magnetometer.
     
  6. Oct 20, 2007 #5

    Gokul43201

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    But isn't this true of any type 2 SC between Tc1 and Tc2?
     
  7. Oct 20, 2007 #6

    f95toli

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    Well, yes and no. It is possible to see the "signature" of the mixed state in a measurement of the resistance; if the vortices are moving around you get a finite flux-flow resistance even after going below Tc. However, in reality this effect is tiny compared to the drop in resistance (and in order to see it clearly you need to cool the sample in a relatively large field) meaning the transition should still be very clear (the width of the transition is often used to measure the quality of a superconducting film, for e.g. a thin film of YBCO it should be about 1K).
    In some cases several transitions can be seen; this happens e.g. if part of the sample has been damaged or oxygen depleted.

    Morever, a susceptibility measurement is usually done at low fields with just a small AC coil meaning there shouldn't be many vortices moving around, one should therefore see a very clear vertical drop if the sample is reasonably homogenous.
     
    Last edited: Oct 20, 2007
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