In this article we have seen a Meissner like diamagnetism as in case of SC materials but it was electrically insulator. Besides, such a diamagnetic transition starts at quite high temperatures (near 280 K, see our articles in J. Supercond. Novel Magnetism and materials characterization). We have no idea about the theoretical explanation of this behavior. Is there anybody who has idea about this behavior?
Dear Ugur,
Did you ask about your paper: J Supercond Nov Magn (2011) 24: 957–959?
i think that referring on it, we can not claim superconductivity (or other type of transition) not only at 280 K but also at 90 K in your samples.
You need to analyze thoroughly experimental errors when you have deal with so small changes of magnetic moments (around or less than 10^-6 emu). Possible error sources:
1. How sample was mounted in the magnetometer: polyethylene tubing (even scratches and holes in it), cotton, thread, capsule could provide such effects.
2. Not good air purge in magnetometer and so presence of condensed gases in sample space or on sample surface. Well-known, that thin layers of Oxygen (melting-50 K, boiling-90 K) give different ferro-/antifferro- effects.
2. Non-homogeneity of external field.
3. Residual field in magnet.
4. Sample centering along with field direction and in the plane which is perpendicular to the direction. Check shapes of SQUID signals.
5. If we suspect a transition (near 280 K as example), we need to set more points around suspected temperature and to do centering of sample near it.
6. Automatic changing of SQUID ranges can do effects at temperature dependences.
7. Field distribution inside of multi-phase sample having phases with different magnetic properties could be bizarre, so that we will not know which field (and its direction) acts on a specific grain.
Sincerely,
Gregory.
Dear Gregory
Thanks your suggestions. Believe me that I was very careful on the experimenat errors. The most challenging property of these samples are the observation of diamagnetic response. If you look at the video on our publication in journal of "materials characterization" it is clearly seen that permanent magnet is repelled by the sample at liquid nitrogen. In time sample becomes warm and stays over the sample. It is behind the experimental errors of SQUID. This is real observation with eye. And such a temperature dependent transition can not be explanined by classical diamagnetism on which diamagnetism is not depend on temperature. So, it must be a phase transition. But, what kind of transition? This is the question requiring to be answered.
Ugur, please look at the attachment. It may you can explain it as localized superconducting regions on surface.
Dear Gregory
Thanks for the article. It really explains most of the questions we couldnt understand. zfc and fc curves are quite similar to ours. So, our samples require further investigation, We have no facility for local measurements but somebody may do it.
I may think that approximately as follows.
It may be normal state Josephson arrays.
When the superconducting coherence length is smaller than insulating size for superconductor-insulator-superconductor, the total phase is normal metallic or insulating. -20140323.Sun.PM8:29-Seoul Time-
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