Dear Colleagues,
I am in problem in measuring the transition temperature of a Superconducting sample.
I am getting absurd behaviour.
I am using four probe method with a disc type pellet.
Is it not possible to do the experiment by circular disc?
Please suggest.
Thanks and Regards
N Das
Four point should work fine! Ensure the current connections are in line with the voltage and straddle the latter. Disc measurements are often used and sometimes resistivity is measured by a switched square arrangement of contacts - look up the van der pauw method. Try a piece of metal first. it might be that your resistivity is too high or low for your measuring instrument’s input impedance (or voltage compliance). Remember you cannot measure resistance directly - you effectively have to source a current and measure a voltage drop - so you need a low output impedance current supply, with a decent voltage swing (compliance) and a very high input impedance voltage measurement circuit (voltmeter). Your material might be outside of your instrument’s measuring capability. Also is it possible that there is some kind of diode created between your material and 4point contacts? - the assumption that they are purely ohmic, might be invalid.
Your sample is likely to be too thick and wide for four probe measrument
You can use magnetic susceptibility measurement.
Dear Rajan Gnanarajan,
My sample is 1 mm thick. Disc diameter is 13 mm.
As the sample starts cooling, the voltage drop starts increasing initially.
Then it reaches a value and then comes down.
I have tried several times but the same think occurs.
I wonder about this horrible result.
I can't think of any reason.
Thanks
N Das
Dear Nityanand Das,
According to my experience, there is no problem with sample dimension, you can do a superconductivity test of disc sample. absurd behavior may be due to the silver paste or soldering of the contact, you can try once by changing the paste or soldering wire.
I hope this will help!
When the resistance increases by decreasing the temperature before starting to decrease it's because you are probably measuring something that covers the ohmic behavior of your sample in that range of temperatures. I saw this once on very poorly connected sample (once, on a few ohms thin metal with silver paste connections, and then also on superconducting devices with very bad pads). In both cases there was unwanted oxide below the contacts, air bubbles, or non-ohmic stuff. We solved making dedicated pads on the device, or putting a "large" amount of current through the sample to break the oxide. You can try the same on your pellet disc, with 1mm thick sample it should not be a problem. By the way, wire bonding is generally preferred compared to silver paste or tin soldering, because you can make more precise contacts and ensure a good electrical connection. Hope this helps!
Prof. Nityananda Das as Dr. Halima Giovanna Ahmad points out correctly, there are other physical quantities that cover the ohmic behavior (due to contacts points).
I also saw some experimental data ~ 2003 in an HTSC sample from experimentalists, where they were measuring the phonon thermal contributions (point contacts too big) and the heat was plain the major role instead of the ratio given by Wiedemann-Franz law. The "phonon & the electron thermal" contribution were mixed out.
https://en.wikipedia.org/wiki/Wiedemann%E2%80%93Franz_law
Is it possible that your contacts are showing diode behaviour (as distinct from ohmic)? Contact behaviour is complex and often evaporating or applying metallic contacts to semiconductors or ceramics will sometimes generate non-ohmic behaviour. to get good ohmic contacts you often need to diffuse your conductive (ie metallic) atomic species into the substrate. In semiconductors this can sometimes generate 2nd phases and precipitates which again affect the ohmic behaviour - so contact “science” is not as straightforward as slapping a conductive paste on to a substrate. Remember that old “crystal“ radios used a “cat’s whisker” diode - a metallic wire on a crystal of germanium working as a diode in a simple radio detector. the wire is simply laid on the crystal - and moved around until the appropriate diode behaviour is generated! i’m not sure about superconductors however but the presence of diode behaviour at the contact point could mean your current either fails to pass (if you use a DC resistance measurement) or it passes - but passes with a finite (and often highly temperature sensitive) resistance. The fwd voltage drop across a silicon diode (for example) is around 0.6v but is also very temperature sensitive to boot. This could cause all sorts of interesting effects as T changes. If you’ve followed a standard measurement methodology, where others have not seen this behaviour, my guess is that your superconductor surface is contaminated before u make your contact or the contact material and/or superconductor are in some way different from what has been tested before. You could try evaporation as a means of making contact and use a post anneal to try and diffuse some metallic species into the first few nanometres of your SC.
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