Apologies for slow posting. Real life has been very intense, and I also was rather concerned when one of my readers mentioned last weekend that these days my blog was like concentrated doom-scrolling. I will have more to say about the present university research crisis later, but first I wanted to give a hopefully diverting example of the kind of problem-solving and following-your-nose that crops up in research.
Recently in my lab we have had a need to measure very small changes in electrical resistance of some devices, at the level of a few milliOhms out of kiloOhms - parts in \(10^6\). One of my students put together a special kind of resistance bridge to do this, and it works very well. Note to interested readers: if you want to do this, make sure that you use components with very low temperature coefficients of their properties (e.g., resistors with a very small \(dR/dT\)), because otherwise your bridge becomes an extremely effective thermometer for your lab. It’s kind of cool to be able to see the lab temperature drift around by milliKelvins, but it's not great for measuring your sample of interest.
There are a few ways to measure resistance. The simplest is the two-terminal approach, where you drive currents through and measure voltages across your device with the same two wires. This is easy, but it means that the voltage you measure includes contributions from the contacts those wires make with the device. A better alternative is the four-terminal method, where you use separate wires to supply/collect the current.
Anyway, in the course of doing some measurements of a particular device's resistance as a function of magnetic field at low temperatures, we saw something weird. Below some rather low temperatures, when we measured in a 2-terminal arrangement, we saw a jump up in resistance by around 20 milliOhms (out of a couple of kOhms) as magnetic field was swept up from zero, and a small amount of resistance hysteresis with magnetic field sweep that vanished above maybe 0.25 T. This vanished completely in a 4-terminal arrangement, and also disappeared above about 3.4 K. What was this? Turns out that I think we accidentally rediscovered the superconducting transition in indium. While our contact pads on our sample mount looked clean to the unaided eye, they had previously had indium on there. The magic temperature is very close to the bulk \(T_{c}\) for indium.
For one post, rather than dwelling on the terrible news about the US science ecosystem, does anyone out there have other, similar fun experimental anecdotes? Glitches that turned out to be something surprising? Please share in the comments.
2 comments:
Yes! A friend wanted me to quickly image what was going on in an electrical device of his. We could image the current density with our technique, and so he wanted to see if current was contained to his doped nanowire region. Of course, shining a laser (required for our measurement) at some silicon will produce a photocurrent, which we knew would modulate the signal, but we were surprised that the local flow was not as simple as we naively expected (there are many 'circuits' entirely *on* the device, not just the external circuit that leaves the chip via your wire bonds... obviously).
Ended up getting a paper out of it: https://doi.org/10.1103/PhysRevApplied.18.014041
The other day, I was doing some routine checks before cleaning the sample by Argon ion sputtering inside our UHV chamber. And, I noticed something peculiar in the background scans of the residual gas analyser after leaking Argon gas into the chamber. Usually, there are only two distinguishable m/z peaks at 40 and 20. For some reason, I was noticing a sizeable peak at 36. I mistook this to be a possible contamination of HCl. However, the m/z peak ratios at 38, 36 and 35 didn't seem to corroborate the existence of Cl. After scratching my head for a while, I had feeling that it could be one of Argon's stable isotopes. And, when I checked for the ratio of intensities of the 36 and 40 m/z peaks, it roughly came out to nearly to be the same as the relative abundance of these isotopes (~0.33% and 99.6% respectively). It just that I had leaked in a slight excess of argon gas than usual and the partial pressure of the stable isotope came out to be within the detection limit of the RGA and we started seeing the 36 m/z peak. A funny little incident that teaches you there are many little things to be fascinated about every day things we do in our labs.
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