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Determining Tc value of various superconductors

  1. Nov 18, 2006 #1
    I am plotting resistance vs. thermocouple voltage (which is to be used to determine the temperature from a conversion chart) for two superconductors. I need to determine where to take the Tc value. This is difficult to determine since the "knee" is not well defined. Also, how should I guess my uncertainty in my Tc value? Are there typical criteria that physicists go by when doing these measurements?

    Thanks.
     
  2. jcsd
  3. Nov 21, 2006 #2

    ZapperZ

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    It depends on how "accurate" you want it. What most people do is to show such a curve and then indicate where they would consider to be the value of Tc. Usually such a thing varies by, at most a +/- 5K.

    If you want to be conservative, then pick the location where the resistivity flattens out at zero at the bottom of the inflection. Or you can pick the middle of the inflection and write it down with an uncertainty equal to the spread of the transition.

    As long as you make it clear how you picked the value of Tc, it shouldn't be that big of an issue where you chose it. Of course, if you want to have a more accurate determination, you should accompany the resistivity measurement with the susceptibility measurement. A value consistent with both measurements is looked upon with a greater degree of certainty than simply from one.

    Zz.
     
  4. Nov 23, 2006 #3
    Do other methods to measure critical temperature exist? If yes, does the thermal dependence of resistence plot remain the most accurate one? However, is the measurement accuracy (whatever method used) sensitive to superconductor's kind (soft or hard), or what really matter is only the specific material?
     
  5. Nov 23, 2006 #4

    ZapperZ

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    Er... say what?

    What is "thermal dependence of resistance plot"?

    For most cases, the susceptibility measurement is considered to be the "slam-bam" evidence of superconductivity, with resistivity measurement a close second. Other supporting evidence would come from tunneling measurement that indicates for formation of an energy gap to the spectral-weight pile-up in optical conductivity measurement.

    Zz.
     
  6. Nov 24, 2006 #5
    With the thermal dependance of resistance plot I just mean the function R(Tc).... ;-) Thank you.
    But in superconductors of II kind, experimentally speaking, does Hc1 give such a relevant corresponding point on the magnetization chart?
     
  7. Nov 24, 2006 #6

    ZapperZ

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    That's temperature dependence of the Resistivity, i.e. R(T), not R(Tc). That is what I said earlier about the resistivity measurement.

    Yes it does. The drop in susceptibility is present no matter if it is Type I or Type II. The Meissner effect kicks in even if there are flux penetration.

    Zz.
     
  8. Nov 25, 2006 #7
    You misunderstand me about "thermal".... What about your "kicks in"? I thought the size effects may affect superconductors of Type II. How about the susceptibility measurement technique?
    Thanks
     
  9. Nov 25, 2006 #8

    ZapperZ

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    I have no idea what you just asked here. Are you asking what is a susceptibility measurement? Or are you not understanding how the drop in susceptibility occurs?

    Zz.
     
  10. Nov 28, 2006 #9
    The diamagnetic behaviour of the sample below Tc makes the internal field vanishing. How CAN this effect be measured? So, if you prefeer, what is a susceptibility measurement? Thank you
     
  11. Nov 28, 2006 #10

    ZapperZ

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    You attach a SQUID to the sample you're measuring, and stick it into a weak external field. Then you measure the susceptibility from the SQUID as you lower the temp. That's the standard technique of measuring any magnetic susceptibility.

    Zz.
     
  12. Dec 1, 2006 #11
    What is the most appropriate, dc-SQUID or rf-SQUID?
     
  13. Dec 1, 2006 #12

    ZapperZ

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    Depends on what you mean by "appropriate".

    A DC SQUID (or any type of pickup coil, really), is easier to set up and measure. Often, this is the type of "good enough" results that most people want. And it is quick.

    An AC SQUID requires more work, more setup, and a more accurate measurement procedure for there to be any benefit to the technique.

    Zz.
     
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