Items of interest

The start of the semester has been very busy, but here are some items that seem interesting:

• As many know, there has been a lot of controversy in recent years about high pressure measurements of superconductivity.  Here is a first-hand take by one of the people who helped bring the Dias scandal into the light.  It's a fascinating if depressing read.

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  Adapted from [1].

  Related, a major challenge in the whole diamond anvil cell search for superconductivity is trying to perform techniques more robust and determinative than 4-point resistance measurements and optical spectroscopy.  Back in March I had pointed out a Nature paper incorporating nitrogen-vacancy centers into the diamond anvils themselves to try in situ magnetometry of the Meissner effect.  Earlier this month, I saw this Phys Rev Lett paper, in which the authors have incorporated a tunnel junction directly onto the diamond anvil facet.  In addition to the usual Au leads for conduction measurements, they also have Ta leads that are coated with a native Ta2O5 oxide layer that functions as a tunnel barrier.  They've demonstrated clean-looking tunneling spectroscopy on sulphur at 160 GPa, which is pretty impressive.  Hopefully this will eventually be applied to the higher pressures and more dramatic systems of, e.g., H2S, reported to show 203 K superconductivity.  I do wonder if they will have problems applying this to hydrides, as one could imagine that having lots of hydrogen around might not be good for the oxide tunnel barriers. 
  
• Saw a talk this week by Dr. Dev Shenoy, head of the US DoD's microelectronics effort.  It was very interesting and led me down the rabbit hole of learning more about the extreme ultraviolet lithography machines that are part of the state of the art.  The most advanced of these are made by ASML, are as big as a freight car, and cost almost $400M a piece.  Intel put up a video about taking delivery of one.  The engineering is pretty ridiculous.  Working with 13.5 nm light, you have to use mirrors rather than lenses, and the flatness/precision requirements on the optics are absurd.  It would really be transformative if someone could pull a SpaceX and come up with an approach that works as well but only costs $50M per machine, say.  (Of course, if it were easy, someone would have done it.  I'm also old enough to remember Bell Labs' effort at a competing approach, projective electron beam lithography.)
• Lastly, Dan Ralph from Cornell has again performed a real pedagogical service to the community.  A few years ago, he put on the arXiv a set of lecture notes about the modern topics of Berry curvature and electronic topology meant to slot into an Ashcroft and Mermin solid state course.  Now he has uploaded another set of notes, this time on electron-electron interactions, the underpinnings of magnetism, and superconductivity, that again are at the right level to modernize and complement that kind of a course.  Highly recommended.

Experimental techniques: bridge measurements

When we teach undergraduates about materials and measuring electrical resistance, we tend to gloss over the fact that there are specialized techniques for this - it's more than just hooking up a battery and an ammeter.  If you want to get high precision results, such as measuring the magnetoresistance \(\Delta R(B)\), where \(B\) is a magnetic field, to a part in \(10^{5}\) or better, more sophisticated tools are needed.  Bridge techniques compose a class of these, where instead of, say, measuring the voltage drop across a sample with a known current, instead you measure the difference between that voltage drop and the voltage drop across a known reference resistor.   

Why is this good?  Well, imagine that your sample resistance is something like 1 kOhm, and you want to look for changes in that resistance on the order of 10 milliOhms.  Often we need to use relatively low currents because in condensed matter physics we are doing low temperature measurements and don't want to heat up the sample.  If you used 1 microAmp of current, then the voltage drop across the sample would be about 1 mV and the changes you're looking for would be 10 nV, which is very tough to measure on top of a 1 mV background.  If you had a circuit where you were able to subtract off that 1 mV and only look at the changes, this is much more do-able.

Wheatstone bridge, from wikipedia

Sometimes in undergrad circuits, we teach the Wheatstone bridge, shown at right.  The idea is, you dial around the variable resistor \(R_{2}\) until the voltage \(V_{G} = 0\).  When the bridge is balanced like this, that means that \(R_{2}/R_{1} = R_{x}/R_{3}\), where \(R_{x}\) is the sample you care about and \(R_{1}\) and \(R_{3}\) are reference resistors that you know.  Now you can turn up the sensitivity of your voltage measurement to be very high, since you're looking at deviations away from \(V_{G} = 0\).   

You can do better in sensitivity by using an AC voltage source instead of the battery shown, and then use a lock-in amplifier for the voltage detection across the bridge.  That helps avoid some slow, drift-like confounding effects or thermoelectric voltages. 

Less well-known:  Often in condensed matter and nanoscale physics, the contact resistances where the measurement leads are attached aren't negligible.  If we are fortunate we can set up a four-terminal measurement that mitigates this concern, so that our the voltage measured on the sample is ideally not influenced by the contacts where current is injected or collected.  

A Kelvin bridge, from wikipedia

Is there a way to do a four-terminal bridge measurement?  Yes, it's called a Kelvin bridge, shown at right in its DC version.  When done properly, you can use variable resistors to null out the contact resistances.  This was originally developed back in the late 19th/early 20th century to measure resistances smaller than an Ohm or so (and so even small contact resistances can be relevant).  In many solid state systems, e.g., 2D materials, contact resistances can be considerably larger, so this comes in handy even for larger sample resistances.  

There are also capacitance bridges and inductance bridges - see here for something of an overview.  A big chunk of my PhD involved capacitance bridge measurements to look at changes in the dielectric response with \(10^{-7}\) levels of sensitivity.

One funny story to leave you:  When I was trying to understand all about the Kelvin bridge while I was a postdoc, I grabbed a book out of the Bell Labs library about AC bridge techniques that went back to the 1920s.  The author kept mentioning something cautionary about looking out for "the head effect".  I had no idea what this was; the author was English, and I wondered whether this was some British/American language issue, like how we talk about electrical "ground" in the US, but in the UK they say "earth".  Eventually I realized what this was really about.  Back before lock-ins and other high sensitivity AC voltmeters were readily available, it was common to run an AC bridge at a frequency of something like 1 kHz, and to use a pair of headphones as the detector.  The human ear is very sensitive, so you could listen to the headphones and balance the bridge until you couldn't hear the 1 kHz tone anymore (meaning the AC \(V_{G}\) signal on the bridge was very small).  The "head effect" is when you haven't designed your bridge correctly, so that the impedance of your body screws up the balance of the bridge when you put the headphones on.  The "head effect" = bridge imbalance because of the capacitance or inductance of your head.  See here.

CHIP and Science, NSF support, and hypocrisy

Note: this post is a semi-rant about US funding for science education; if this isn't your cup of tea, read no further.

Two years ago, the CHIPS and Science Act (link goes to the full text of the bill, via the excellent congress.gov service of the Library of Congress) was signed into law.  This has gotten a lot of activity going in the US related to the semiconductor industry, as briefly reviewed in this recent discussion on Marketplace.  There are enormous investments by industry in semiconductor development and manufacturing in the US (as well as funding through US agencies such as DARPA, e.g.).  It was recognized in the act that the long-term impact of all of this will be contingent in part upon "workforce development" - having ongoing training and education of cohorts of people who can actually support all of this.  The word "workforce" shows up 222 times in the actual bill.   Likewise, there is appreciation that basic research is needed to set up sustained success and competitiveness - that's one reason why the act authorizes $81B over five years for the National Science Foundation, which would have roughly doubled the NSF budget over that period.

The reality has been sharply different.  Authorizations are not the same thing as appropriations, and the actual appropriation last year fell far short of the aspirational target.  NSF's budget for FY24 was $9.085B (see here) compared with $9.899B for FY23; the STEM education piece was $1.172B in FY24 (compared to $1.371B in FY23), a 17% year-over-year reduction.  That's even worse than the House version of the budget, which had proposed to cut the STEM education by 12.8%.  In the current budget negotiations (see here), the House is now proposing an additional 14.7% cut specifically to STEM education.  Just to be clear, that is the part of NSF's budget that is supposed to oversee the workforce development parts of CHIPS and Science.  Specifically, the bill says that the NSF is supposed to support "undergraduate scholarships, including at community colleges, graduate fellowships and traineeships, postdoctoral awards, and, as appropriate, other awards, to address STEM workforce gaps, including for programs that recruit, retain, and advance students to a bachelor's degree in a STEM discipline concurrent with a secondary school diploma, such as through existing and new partnerships with State educational agencies."  This is also the part of NSF that does things like Research Experience for Undergraduates and Research Experience for Teachers programs, and postdoctoral fellowships.  

Congressional budgeting in the US is insanely complicated and fraught for many reasons.  Honest, well-motivated people can have disagreements about priorities and appropriate levels of government spending.  That said, I think it is foolish not to support the educational foundations needed for the large investments in high tech manufacturing and infrastructure.  The people who oppose this kind of STEM education support tend to be the same people who also oppose allowing foreign talent into the country in high tech sectors.  If the US is serious about this kind of investment for future tech competitiveness, half-measures and failing to follow through are decidedly not helpful.