APS March Meeting 2022, Day 4 and wrap-up

I gave my contributed talk this (Fri) morning, and I will head to the airport shortly, so this is the end of my March Meeting blogging.  A few highlights from yesterday:

• Konrad Lehnert gave a very nice, pedagogical talk about the possibility of detecting axionic dark matter using quantum sensing.  The super short version:  it is thought that axions if they exist can, in the presence of a large magnetic field, convert at some rate into photons with energy \(\hbar \omega = m_{\mathrm{a}}c^2\).  In a microwave cavity, it is possible to detect such excess photons, and by doing clever things with "squeezing", it is possible to beat the standard quantum limit and to examine parameter space more rapidly than otherwise.  There is still a lot of room for improvement if one wants to be able to look across the whole range of potential axion masses and not have it take years and cost a gazillion dollars.  One approach using entanglement can eliminate a number of confounding factors.
• I saw two very clear talks, one by Kevin Nuckolls and one by Stevan Nadj-Perge about using STM and tunneling (and point contact) spectroscopy to examine superconductivity in magic-angle twisted bilayer and trilayer graphene, respectively.  In the former, one challenge is to decide how much of the observed gap features in tunneling are due to superconductivity, and then using the functional form of that superconducting part to consider pairing mechanisms.  It is also possible to see how band flattening increases the density of states even at angles away from the magic angle.
• In a different session, Inti Sodemann spoke about whether and how it is possible to get current rectification in semiconductors when they are illuminated by light with energy below the band gap, so that there is no absorption.  There are thermodynamic restrictions that come in - you can't get energy from nowhere, and you can't break the second law.  Thanks to Berry curvature effects, it is actually possible to have this kind of rectification under some circumstances.
• There was another extremely clear talk by N. Peter Armitage about Co-containing compounds as Kitaev spin liquid candidates.  There was some really great THz absorption data as a fn of temperature and magnetic field for CoNb2O6 that had amazing agreement with theory, and newer results looking at a more 2D system, BaCo2(AsO4)2.
• Unfortunately I was unable to attend the Kavli Symposium.  I hope to be able to watch the talks later, as these are typically of very high quality and general interest.

Closing thoughts:

• It was nice and kind of weird to finally see a good number of people in person.  Really great to catch up with old friends, though I think my conference stamina has waned since the 2019 meeting.
• When the participants skew younger, as seemed to be the case this year, the crowd definitely looks more diverse.  It would be interesting to know the demographics of the attendees.
• I don't think pre-recorded short talks work well.  The inability to ask/answer questions is a problem.  
• I wonder if we will have hybrid meetings in general from now on.  There are definitely environmental impact reasons to go that way, and it would help solve the APS's problem that prior to covid the meeting had grown so large that it was difficult to plan or host.

APS March Meeting 2022, Day 3

Highlights are brief today, because I spent more of my time seeing talks from my group and chatting with people:

• Started the day with the Keithley Prize session, and Dan Rugar talking about the history of magnetic resonance force microscopy.   Very interesting and educational.  It is inspiring to see the evolution of a technique, from the genesis of the idea (an early paper here) to initial testing to advanced developments.
• Later I saw Marcel Franz give a very clear talk about how to try to build a topological superconductor (fully gapped with topologically protected chiral edge modes) by stacking individual cuprate layers rotated by 45 degrees with respect to each other.  
• There was a neat talk by Naomi Ginsberg on her group's pump-probe interferometric technique ("stroboSCAT") that allows them to visualize and separate the diffusion of heat and the diffusion of charge in various materials.  For a review, see here.
• Later in the day I bopped back and forth a bit between the Buckley/Isakson/Onsager Prize session and a session about the BCS/BEC crossover in condensed matter systems.  It was pretty neat hearing Emmanuel Rashba speak.  

Now to figure out what to see tomorrow....

APS March Meeting 2022, Day 2

It was a busy day today, and I saw a lot of talks, some live and in person, some live via zoom, and some prerecorded.  Some highlights:

• Started off with Nai Phuan Ong's invited talk about their recent results on thermal transport measurements in the proximate Kitaev spin liquid material \(\alpha\)-RuCl3.  His group performs measurements of the longitudinal thermal conductivity \(\kappa_{xx}\) and the thermal Hall conductivity \(\kappa_{xy}\) in this system by gluing tiny thermometers to the very delicate crystals.  They see some remarkable results, including evidence that there are heat-carrying bosonic (not fermionic) edge modes.
• In a different session, I heard Dan Ralph talk about a variety of issues involved in really properly understanding all the pitfalls that can come into interpreting the different experimental attempts to measure SO torque efficiency.  
• This was followed by a nice talk by Alex McLeod about nanophotonic near-field probes of correlated materials.  The talk included a great history of the field, including this paper that I'd somehow never seen before. 
• Anand Bhattacharya gave a nice presentation about his group's work on 2D superconductivity at the interface of KTaO3 with other oxides, especially the dependence on crystallographic orientation.  They have another recent paper that explains features of the gate dependence of the superconducting transition, and there is a theoretical proposal for the underlying mechanism.
• Garnet Chan spoke on a very interesting topic:  Is there an exponential quantum advantage (relative to classical computing) to be had in using quantum computers to try to solve theoretical chemistry problems such as finding the ground state of a large molecule or material?  Such an advantage requires, among other things, that classical methods exponentially poorly with the problem size, and that initial state preparation of the quantum system is not exponentially difficult.  The short answer:  it's not clear that this is the case.  (Quantum computers could still be very useful for quantum chemistry.  Here is a relevant review article.)
• Mathieu Taupin from TU Wien spoke about superconductivity at very low temperatures in the quantum critical strange metal YbRh2Si2, and also about whether these kinds of heavy fermion strange metals are "Planckian".
• I also heard Linda Ye present data showing that Ni3In, a "kagome flat band" material, is a strange metal.  In this kind of system, because of the lattice structure and its symmetry, there is particular destructive quantum interference that happens to disfavor electronic hopping between certain lattice sites - see here.  As a result, the electrons in that band tend to localize, leading to an energy band that is flat.  In this system, that band sits at the Fermi level, and strange metallicity seems to result.

Besides the talks, I also got to see and catch up with a number of friends and colleagues for the first time since the pandemic started.  The exhibition show part of the meeting has changed quite a bit. It's really amazing how big a difference three years makes in terms of the exhibitors. Now it seems like 75% of the vendors there are "quantum"-related.  

APS March Meeting 2022, Day 1

My first impressions of this year's March Meeting are a bit limited, since I flew today and didn't make it to the convention center until around 4pm.  Still, a few thoughts:

• Population density at the meeting does seem lower than 2019, though that could partly be because the convention center is enormous.  Attendance also seems to skew younger this year.
• It was interesting attending a contributed session, with a mix of in-person speakers and the session chair playing pre-recorded presentations from those who could not or chose not to be present.  It seems to work ok, though the lack of Q&A for the recorded talks takes some getting used to.  In asking around, I get the impression that the full live streaming interactive Q&A approach from last year's virtual meeting was very expensive to implement, and that's one reason why the method this year is different (with live streaming only for invited talks).
• While I was getting my bearings, I popped into a session about spin transport by electrons and magnons.  I saw a talk where an interesting "spin diode" effect takes place in a thin film multilayer structure consisting of (from one side to the other) permalloy/gold/platinum/cobalt.  Driving the permalloy layer into ferromagnetic resonance can effectively pump spin into the gold and so forth, and in that direction spin current flows and is absorbed in the cobalt.  However, if one instead drives the cobalt layer to try to push a spin current the other way, the permalloy does not act like a "sink".  This can be modeled and the directionality makes sense.
• I saw a second talk that somewhat similar in spirit, in which magnons are launched into an insulating magnet using the spin Hall effect in a Pt wire, and then detected by another Pt wire via the inverse spin Hall effect.  By placing a NiFe pad between the Pt wires, it is possible to make it so that magnons transport more easily from one Pt wire to the other than vice versa.  This work is described here.
• Then I went to the APS special session about Ukraine.  I expected this to be largely about how to help displaced scholars and scientists, but it was more complex, and most of the time was a listening session for the APS president and the CEO.  The APS statements about the Russian invasion of Ukraine are here.  There are many issues.  For example, 200+ rectors of top Russian universities and institutes signed an open letter supporting the war. Should the APS still allow those places to have journal access? Faculty members there to have society membership and privileges? What is mandated as a result of sanctions?  There are many Ukrainian and Russian and Belarusian physicists in the APS, and feelings are intense.  On an historical note, the question was raised about what if anything the APS did in regard to German physicists and institutions (like the German Physical Society and the Kaiser Wilhelm Institutes) when Germany invaded Poland in 1939 - can any readers point to a record of what the APS actually did?  I spent a while googling and could find nothing.

It is good to see folks face to face, even if attendance is down.  It's been a long time.

2022 March APS Meeting - coming soon.

I will actually be attending the 2022 March APS Meeting in Chicago next week, so look for posting to pick up as I try to be good about my annual routine of reporting some highlights.  I have been so busy in recent weeks that I really have not had time to go through the meeting program in any depth; if there are particularly exciting sessions that people want to recommend, please point them out in the comments.

Three papers to distract from the awfulness

Here are three papers that may briefly divert you from doomscrolling about the horrific situation in Ukraine.

• 
  This paper from the Feb 11 issue of Science shows some amazing images of the calcite structures that make up starfish skeletons, with order on multiple length scales that leads to remarkable mechanical properties.   Biomineralization is amazing, especially when you think about how it works.  Cells generate protein structures that can have charge patterns that allow templating of inorganic crystal growth in specific phases and orientations.  See here for examples of experiments that get at how this works.  The cover image from the Science paper is really eye-popping.
  
• Then there's this paper from the Young group at UCSB, which shows that ordinary Bernal-stacked bilayer graphene can also superconduct, albeit at 30 mK.  The really interesting bit here is that to get superconductivity requires both a large c-directed electric field (obtained by having a voltage difference between top and bottom graphite gate electrodes above and below the bilayer) and an in-plane magnetic field.  That latter requirement suggests that this might be an exotic superconductivity where the electron pairs are spin triplets rather than the conventional singlets of ordinary superconductors.
• Finally, enjoy this paper, which was published as a commentary in ACS Photonics, and is absolutely worth reading for the tone alone.  

Brief items - fun and games, news, and lots of transistors

My busiest time of the year continues.  A few interesting links:

• I'm sure you've heard of wordle.  There are some other free games that are similar in design (and not co-opted by the New York Times) that are also good to keep your brain tuned up.  Mathler is cute (and knows order of operation), and worldle, while difficult to pronounce, is fun if you want to keep up with your geography.
• The US presidential science advisor resigned this week, apparently because he is just an awful person to work with.  That prompted this article from Stat, which advances a thesis that I don't buy, that this resignation shows that we are leaving the era of "big ego science".  I hope that we are finally entering an era where bullying and pushiness are not automatically tolerated in high profile positions, but drawing some sweeping conclusion from Lander's departure is not reasonable to me.  I do know from interactions with folks like my colleague Neal Lane that it is possible for top-flight scientific leaders to be both highly accomplished and genuinely nice people.  I hope someone in that mold ends up taking the reins.
• The US House passed the 2022 version of the America COMPETES act.  The US Senate had passed the related US Innovation and Competition Act (USICA) last summer.  Now it's up to the conference committee to try to work out a compromise bill that can pass both houses.  If this passed and the House actually appropriated the funds, it would be big news for NSF and the DOE Office of Science.  I'm a bit cynical about the prospect of this happening, and both bills have issues, but it's better to have this at least in front of Congress than languishing off-stage.
• There are rumors that Nvidia's next big chip will be built on the TSMC "5 nm node" process (where the numbering really doesn't mean that a critical lengthscale is 5 nm) and hold 140 billion transistors (!!).  If anyone asks you whether nanotechnology is meaningful, point to things like that as examples of nanoelectronics.  

Graduate stipends and tuition - a bold move by Princeton

I will write more about actual physics soon, but it has been a very busy period with other commitments.  In the meantime....

Princeton did something remarkable this week.  They raised their graduate stipends across the board to $40K/10 months, roughly a 25% increase.  That's already quite impressive, but the really wild change is less readily apparent.

It's important to understand how graduate students are paid on research grants in the US.  Grants pay for the stipend + indirect costs ("overhead") on the stipend + tuition remission.  "Tuition remission" is some effective graduate tuition rate.  Indirect costs ("overhead") go to the university and are meant to pay for things like keeping the lights on and the buildings air conditioned and the cost of running the office that does the financial reporting, etc.  Indirect cost rates are set by negotiations between the university and the US government.  Rice's indirect cost rate right now for on-campus research is 56.5%.   

Tuition is trickier.  These are funds meant to cover the university's cost of graduate education.  Different universities do different things with that money they take in on grants for tuition remission - usually it covers things like support for first-year grad students, part of the TA salary pool, etc.  In STEM doctoral programs in the US, students do not pay tuition out of pocket.  It is either waived by the university (for incoming students supported on fellowship or TA, for example) or paid through research grants for students supported by external funding.  (Note that this is money that doctoral students never actually see - that's why it's dumb that every few years (here is the 2017 example) someone in Congress tries to argue it should be taxed.)  It's unclear what a true fair value is for doctoral tuition; grad students are much more independent than undergrads, and when they are doing purely thesis research it's not clear how to think about their educational costs.  Rice has a "tuition remission rate" of 38.5%, rather than a fixed dollar amount, with the idea that this strikes a balance between beginning students taking a lot of courses and later students working only on their thesis.   That means that if our graduate stipend is $S, then on a federal grant the total cost of a doctoral student at Rice is \( (1.565 + 0.385)\times\) $S.   

Anyway, along with raising stipends drastically, Princeton also cut their graduate tuition rate to zero (!).  That means that a graduate student at Princeton will cost less on a grant now than before, even though they have jumped up stipends by 25%.  

This is pretty radical.  The university is going to take in many millions of dollars less on grants to do this, but given their roughly $38B endowment, they can afford it.  Even if they took a $40K hit per grad student per year, the $100M of "lost" income would only be 5% of their operating budget.  I assume that this also plays well against the criticism that elite institutions don't spend enough of their resources.  No idea what the implication is for, e.g., their professional masters degrees in engineering, where they surely charge students (or their employers) substantial graduate tuition.

The long-term effect of this will be interesting and complicated.  I would think that STEM faculty at other comparably wealthy universities will turn to their administrations and ask why students are so much cheaper on grants for Princeton faculty.  This zero doctoral tuition approach would require wholesale restructuring of financial models at most US universities.

Update:  President Eisgruber has publicly announced the tuition change here in his state-of-the-university address.

Brief items - papers, packings, books

 It's a very busy time, so no lengthy content, but here are a few neat things I came across this week.

•  A new PRL came out this week that seems to have a possible* analytic solution to Hilbert's 18th problem, about the density of random close-packed spheres in 2D and 3D.  This is a physics problem because it's closely related to the idea of jamming and the onset of mechanical rigidity of a collection of solid objects.  (*I say possible only because I don't know any details about any subtle constraints in the statement of the problem.)
• The Kasevich group at Stanford has an atom interferometric experiment that they claim is a gravitational analog of the Aharonov-Bohm effect.  This is a cool experiment, where there is a shift in the quantum phase of propagating atomic clouds due to the local gravitational potential caused by a nearby massive object.  (Phase goes like the argument of \(\exp(-i S(x(t))/\hbar)\), where the action can include a term related to the gravitational potential, \(m \times \Phi_{G}(x(t)\).)  At a quick read, though, I don't see how this is really analogous to the AB effect.  In the AB case, there is a relative phase due to magnetic flux enclosed by the interfering paths even when the magnetic field is arbitrarily small at the actual location of the path.  I need to read this more closely, or perhaps someone can explain in the comments.
• A colleague pointed out to me this great review article all about charge shot noise in mesoscopic electronic systems.  
• Speaking of gravity, there has been interest in recent years about "warp drives", geometries of space-time allowed by general relativity that seem to permit superluminal travel for an observer in some particular region.  One main objection to these has been that past proposed incarnations violate various energy conditions in GR - requiring enormous quantities "negative matter", for example, which does not seem to exist.  Interestingly, people have been working on normal-matter-only ideas for these, and making some progress as in this preprint.  Exercises like this can be really important for illuminating subtle issues with GR, just like worrying about "fast light" experiments can make us refine arguments about causality and signaling.  
• Thomas Wong from Creighton University has a free textbook (link on that page) to teach about quantum computing, where the assumed starting math knowledge is trig.  It looks very accessible!
• People recommended two other books to me recently that I have not yet had time to read.  The Alchemy of Us is a materials-and-people focused book from Ainissa Ramirez, and Sticky: The Secret Science of Surfaces is all about surfaces and friction, by Laurie Winkless.  Gotta make time for these once the semester craziness is better in hand....

Condensed matter and a sense of wonder

I had an interesting conversation with a colleague last week about the challenges of writing a broadly appealing, popular book about condensed matter.  This is a topic I've been mulling for (too many) years - see this post from the heady days of 2010.

He made a case that condensed matter is inherently less wondrous to the typical science-interested person than, e.g., "the God Particle" (blech) or black holes.  This is basically my first point in the old post linked above.  He was arguing that people have a hard time ever seeing something that captures the imagination in items or objects that they have around them all the time.  The smartphone is an incredible piece of technology and physics, but what people care about is how to get better download speeds, not how or why any of it works.  

I'm curious:  Do readers think this is on-target?  Is "lack of wonder" the main issue, or one of many?  

A book review, and wishes for a happy new year

 I was fortunate enough to receive a copy of Andy Zangwill's recent biography of Phil Anderson, A Mind Over Matter:  Philip Anderson and the Physics of the Very Many.  It's a great book that I would recommend to any physics student or graduate interested in learning about one of the great scientists of the 20th century.  Zangwill does an excellent job with the difficult task of describing (in a way accessible to scientists, if not necessarily always the lay-public) the rise of solid-state physics in the last century and its transformation, with significant guidance from Anderson, into what we now call condensed matter.  This alone is reason to read the book - it's more accessible than the more formally historical (also excellent) Out of the Crystal Maze and a good pairing with Solid State Insurrection (which I discussed here).  

This history seamlessly provides context for the portrait of Anderson, a brilliant, intuitive theorist who prized profound, essential models over computational virtuosity, and who had a litany of achievements that is difficult to list in its entirety.  The person described in the book gibes perfectly with my limited direct interactions with him and the stories I heard from my thesis advisor and other Bell Labs folks.  Some lines ring particularly true (with all that says about the culture of our field):  "Anderson never took very long to decide if a physicist he had just met was worth his time and respect."

On a separate note:  Thanks for reading, and I wish you a very happy new year!  I hope that you and yours have a safe, healthy, and fulfilling 2022.

US News graduate program rankings - bear this in mind

The US News rankings of graduate programs have a surprisingly out-sized influence.  Prospective graduate students seem to pay a lot of attention to these, as do some administrators.  All ranking schemes have issues:  Trying to encapsulate something as complex and multi-variate as research across a whole field + course offerings + student life + etc. in a single number is inherently an oversimplification. 

The USNWR methodology is not secret - here is how they did their 2018 rankings.   As I wrote over a decade ago, it's a survey.  That's all.  No detailed metrics about publications or research impact or funding or awards or graduate rates or post-graduation employment.  It's purely a reputational survey of department chairs/heads and "deans, other administrators and/or senior faculty at schools and programs of Ph.D. Physics programs", to quote the email I received earlier this month.   (It would be nice to know who gets the emails besides chairs - greater transparency would be appreciated.)

This year for physics, they appear to have sent the survey to 188 departments (the ones in the US that granted PhDs in the last five years), and historically the response rate is about 22%.  This implies that the opinions of a distressingly small number of people are driving these rankings, which are going to have a non-perturbative effect on graduate recruiting (for example) for the next several years.   I wish people would keep that in mind when they look at these numbers as if they are holy writ.  

(You also have to be careful about rough analytics-based approaches.  If you ranked departments based purely on publications-per-faculty-member, for example, you would select for departments that are largely made up of particle physics experimentalists.  Also, as the NRC found out the last time they did their decadal rankings, the quality of data entry is incredibly important.)

My advice to students:  Don't place too much emphasis on any particular ranking scheme, and actually look closely at department and research group websites when considering programs.  

No, a tardigrade was not meaningfully entangled with a qubit

This week this paper appeared on the arxiv, claiming to have entangled a tardigrade with a superconducting transmon qubit system.  My readers know that I very rarely call out a paper in a negative way here, because that's not the point of this blog, but this seems to be getting a lot of attention, including in Physics World and New Scientist.  I also don't know how seriously the authors were about this - it could be a tongue-in-cheek piece.  That said, it's important to point out that the authors did not entangle a tardigrade with a qubit in any meaningful sense.  This is not "quantum biology".

Tardigrades are amazingly robust.  We now have a demonstration that you can cool a tardigrade in high vacuum down to millikelvin temperatures, and if you are sufficiently gentle with the temperature and pressure changes, it is possible to revive the little creature.  

What the authors did here was put a tardigrade on top of the capacitive parts of one of two coupled transmon qubits.  The tardigrade is mostly (frozen) water, and here it acts like a dielectric, shifting the resonance frequency of the one qubit that it sat on.   (It is amazing deep down that one can approximate the response of all the polarizable bits of the tardigrade as a dielectric function, but the same could be said for any material.)

This is not entanglement in any meaningful sense. If it were, you could say by the same reasoning that the qubits are entangled with the macroscopic silicon chip substrate.  The tardigrade does not act as a single quantum object with a small number of degrees of freedom.  The dynamics of the tardigrade's internal degrees of freedom do not act to effectively decohere the qubit (which is what happens when a qubit is entangled with many dynamical degrees of freedom that are then traced over).  

Atoms and molecules in our bodies are constantly entangling at a quantum level with each other and with the environment around us.  Decoherence means that trying to look at these tiny constituents and see coherent quantum processes related to entanglement generally becomes hopeless on very short timescales.  People still argue over exactly how the classical world seems to emerge from this constant churning of entanglement - it is fascinating.  Just nothing to do with the present paper. 

Real progress on machine learning for density functional theory

(Sorry about the slow pace of posting.  The end of the semester has been very intense, including a faculty retreat for our department last week.)

I've written before (here, here, and here) about density functional theory, arguably one of the most impactful intellectual physics results of 20th century physics.   DFT is one approach to trying to solve the quantum electronic structure problem for molecules or solids containing many electrons.  As explained in the links above, the idea is powerful.  It turns out that the ground state (lowest energy state) electronic density as a function of position \(n(\mathbf{r})\), contains all the information needed to calculate basically anything you could want to know about the ground state.  There is a functional \(E[n(\mathbf{r})]\), for example, that will give you the energy of the full-on, interacting many-electron ground state.  It's possible to do a non-interacting electron model that can get you arbitrarily close to the true, correct \(n(\mathbf{r})\), The tricky bit is, there is no exact analytical expression for the functional \(E[n(\mathbf{r})]\), which includes a particularly tricky contribution called the exchange-correlation part of the functional, \(E_{\mathrm{xc}}[n(\mathbf{r})]\).  Because we are talking about functionals rather than functions,  \(E_{\mathrm{xc}}[n(\mathbf{r})]\) might depend in a non-local way on \(n(\mathbf{r})\) and its derivatives at all points in space - there is no reason to think it will be simple to write down.  

From deepmind.com

I wrote six years ago about the idea that machine learning techniques might make it possible to get a working version of something close to the exact \(E_{\mathrm{xc}}[n(\mathbf{r})]\) , even if we can't readily write it down in some closed form.  Now it seems that real progress has been made in this direction.  Here is a blog post from the DeepMind team about their paper in Science this week where they demonstrate a new functional that they claim is very good and accurate vs exact calculations on test systems, computationally tractable, and satisfies fundamental properties that have to hold for the true exact functional.  They argue that their code is more than just a fancy look-up table and that it contains generalizable knowledge so that it's useful well beyond their specific training test cases.  

If this is so, then it could be a major step forward in (for some definitions of the term) first-principles calculations of molecular and material properties.  I'm curious about whether the new functional will actually let us gain some physical insight into why physics requires that particular underlying mathematical structure.  Still, even if we end up with a "black box" that allows greatly improved calculations, that would really be something.  I'd appreciate it if knowledgable DFT/electronic structure experts could comment here on how excited we should be about this.

LEDs - condensed matter/nanostructures having real impact

I'd written two years ago about the pervasiveness of light emitting diodes for holiday decorations.  While revising some notes for my class on nanoscience and nanotechnology, I recently came upon some numbers that really highlight the LED as a great example of condensed matter (and recently nanoscience) having a serious positive impact on energy consumption and environmental impacts. 

Image from here.

Back when I was growing up, incandescent light bulbs were common, and pretty lousy at generating light for a given amount of energy input.  Incandescents produce something like 20 lumens/W, while compact fluorescent bulbs are more like 60 lm/W.  In contrast, LED lighting is well over 100 lm/W and is hitting numbers like 200 lm/W in more expensive bulbs, and in theory could reach more like 325 lm/W.   (For good sources of information about this, I recommend this report by the International Energy Agency, and this 2020 report (pdf) from the US Department of Energy.   LED "white" lighting works either by having a UV LED that excites the same kind of phosphors that are in fluorescent bulbs, or by "color mixing" through having red, green, and blue LEDs all in one package.  (The "nano" comes into this both through the precision growth of the semiconductors and in some cases nanostructuring to enhance the fraction of emitted light that actually gets out of the LED.)

Six years ago, lighting accounted for about 15% of global electricity demand.  In just a few years, LEDs have gone from a few % of market share for new lighting to well above 50% of market share, and there is no sign of this slowing down.  The transition to LEDs is expected to save hundreds of billions of dollars per year in energy costs, gigatons per year in CO₂ emissions, and to stave off the need to construct over a hundred new municipal-scale power plants over the next decade.  

This is a big deal.  One way to cast "the energy problem" is that there is no clear, environmentally reasonable path toward raising the standard of living of billions of people up to the level of per capita energy consumption seen in the most developed economies.  Cutting that per capita energy use would be great, and LED lighting is a true success story in that regard.  

Hanle magnetoresistance - always more to learn....

You would think that, by now, we would have figured out basically all there is to know about comparatively simple metals conduct electricity, even in the presence of a magnetic field.  I mean, Maxwell and Faraday etc. were figuring out electric and magnetic fields a century and a half ago.  Lorentz wrote down the force on a moving charge in a magnetic field in 1895.  The Hall Effect goes back to 1879.  Sommerfeld and his intellectual progeny laid the groundwork for a quantum theory of electronic conduction starting about a hundred years ago.  We have had good techniques for measuring electrical resistances (that is, sourcing a current and measuring the voltage differences between different places on a material) for many decades, and high quality magnets for around as long.  

Surprisingly, even in very recent times we are still finding out previously unknown effects that influence the resistance of a metal in a magnetic field.  Let me give you an example.  

I'd written here about the spin Hall effect and its inverse, which were only "discovered" relatively recently.  In brief, because of strong spin-orbit coupling (SOC) effects on the electronic structure of comparatively heavy metals (Pt, Ta, W), passing a current through a thin film strip of such a material generates a spin current, leading to the accumulation of spin at the top and bottom of the strip.  If those interfaces are in contact with magnetic materials, exchange processes can take place so that there is a net transfer of angular momentum between the metal and the magnetic system.  

There is actually a correction to the resistance of the SOC metal:  The spin accumulation can lead to a diffusive spin current between the top and bottom surfaces, which (thanks to the inverse spin Hall effect, ISHE) gives an additive kick to the charge current (and effectively lowers the resistance of the metal from what it would be in the absence of the spin Hall physics).  If the top and bottom interfaces are in contact with a magnetic system and therefore affect the spin accumulation, that correction can be modified depending on the orientation of the magnetization of the magnetic material, leading to the spin Hall magnetoresistance.  

Spin Hall/inverse spin Hall
resistive correction,
adapted from here.

That's not the end of the story, however.  Even without an adjoining magnetic material, there is an additional magnetoresistive correction, \(\delta \rho(\mathbf{H})\) to the resistivity of the SOC metal.  If the magnetic field has a component transverse to the direction of the SHE accumulated spins, the spins will precess about that field, and that can affect the ISH correction to the resistivity.  This was predicted in 2007 by Dyakanov (arxiv, PRL), and it was found experimentally several years later, as reported in PRL (arxiv version here).  There are readily measurable effects in both the longitudinal resistivity \(\rho_{xx}\) (voltage measured along the direction of the current) and the transverse resistivity \(\rho_{xy}\) (voltage measured transverse to the current, as in the Hall effect, but this holds even when the external magnetic field is in the plane of the film).

Hanle magnetoresistance idea, 
adapted from here.

This correction is called the Hanle magnetoresistance.  

(Aside:  There is some interesting scientific history behind the name.  Hanle was the first to explain an atomic physics optical effect, where the precession of magnetic moments of a gas of atoms in a magnetic field affects the polarization of light passing through the gas.  In condensed matter, the name "Hanle effect" shows up in discussions of spin transport in metals.  The first time I ever encountered the term was in this paper, which foreshadows the discovery of giant magnetoresistance.  A ferromagnetic emitter contact is used to inject spin-polarized electrons into a non-magnetic metal, aluminum.  Those electrons diffuse over to a second ferromagnetic collector contact, where their ability to enter that contact (and hence the resistance of the gadget) depends on the relative alignment of the spins and the magnetization of the collector.  If there is a magnetic field perpendicular to the plane of the device, the spins precess while the electrons diffuse, and one can analyze the magnetoresistance to infer the spin relaxation time in the metal.)

One of my students and I have been scratching our heads trying to see if we really understand the Hanle magnetoresistance, which we have been measuring recently as a by-product of other work.  I think it's pretty amazing that we are still discovering new effects in something as simple as the resistance of a metal in a magnetic field.

The community of department chairs

For the vast majority, there is no formal training process that professors go through before-hand to become chair or head of a department.  That makes access to the experiences and knowledge of others an invaluable resource.  In recent years, the APS has been sponsoring conferences of physics (or physics & astronomy) department chairs, and that's great, but pre-dating that have been electronic mailing lists for department chairs and heads*.  There is a long-standing, somewhat appropriately named "Midwest Physics Department Chairs" email listserv, and similarly there is an analogous American Astronomical Society astronomy chairs listserv.  

The chairs mailing list has been a great way to learn how processes work at other places, and to get advice or sanity checks.  Sometimes it can be very helpful to be able to say to your administration, "here is how everyone else does this."  Not everything translates, as large public universities have some real structural differences in operations compared to private universities, but it's still been informative. Examples of recent discussion topics in no particular order:

• Rough startup costs for hires in different subfields (and how those costs are borne between departments, deans, provosts, etc.)
• Qualifying/candidacy exams - what they cover (undergrad v grad), their value or lack thereof
• Promotion and tenure processes 
• Diversity/equity/inclusion at all levels
• Graduate admissions in the post-standardized-test era
• International students in the era of covid + recent changes in student visa policies
• Various curricular issues (incorporating computation; lab staffing)
• Mental health at all levels (undergrads, grad students, faculty, staff)

The group also has an annual get-together.  Last weekend I attended a meeting (face to face!) of about 30 physics department chairs at the exotic O'Hare Airport Hilton in Chicago.  While not everyone was able to make it, it was helpful to talk and compare notes.  People had a lot to say about teaching methods and what will stick around post-pandemic.  It was also very informative to learn what it takes financially and in terms of personnel to support a successful bridge program.  

Being chair or head can be isolating, and it's good having a community of people who understand the weird issues that can come up.  

* The definitions are not rigid, but a chair is often elected and expected to make decisions through consensus and voting, while a head is appointed and typically has more autonomy and authority.  As one former head at a big place once told me, though, you basically need consensus as a head, too, otherwise you can't get anything done.

The noise is the signal

 I am about to attend a gathering of some physics department chairs/heads from around the US, and I'll write some about that after the meeting, but I wanted to point out a really neat paper (arxiv version here) in a recent issue of Science.  A group at Leiden has outfitted their scanning tunneling microscope with the ability to measure not just the tunneling current, but the noise in the tunneling current, specifically the "shot noise" that results out of equilibrium because charge is transported by the tunneling of discrete carriers.  See here for a pretty extensive discussion about how charge shot noise is a way to determine experimentally whether electrons are tunneling one at a time independently, or whether they are, for example, being transported two at a time because of some kind of pairing.

Adapted from Fig. 1 of this paper.

The experiment is quite pretty, looking at disordered thin films of TiN, with a macroscopic superconducting transition temperature of \(T_{c} =\) 2.95 K.  With the shot noise measurement, the experimenters see enhanced noise at low applied voltages consistent with pairing (with pairs being transported presumably by the process of Andreev reflection).  The interesting point is that this enhanced noise persists up to temperatures as high as 2.7 times \(T_{c}\), despite the fact that the tunneling conductance \(dI/dV\) shows no sign of a gap or pseudogap up there.  This implies that superconductivity in this material dies as \(T\) exceeds \(T_{c}\) not because the pairing between electrons falls apart, but instead because of the loss of the global coherence needed for the superconducting state.  That's an exciting result.  

I'm a big fan of noise measurements and applying them to a broader class of condensed matter systems.  We'd seen enhanced noise in cuprate tunnel junctions above \(T_{c}\) and at large biases, as mentioned here, but in the cuprates such persistence of pairing is less surprising than in the comparatively "simple" TiN system.  Noise measurements on demand via STM should be quite the enabling capability!

The physics of ornithopters

One thing that the new Dune film captures extremely well is the idea that the primary small-capacity air transportation mode on Arrakis is travel by ornithopter.  The choice of flapping wings as a lift/propulsion mechanism can be in-fictional-universe justified by the idea that jet turbines probably won't do well in an atmosphere with lots of suspended dust and sand, especially on take-off and landing.  Still, I think Frank Herbert decided on ornithopters because it just sounded cool.

The actual physics and engineering of flight via flapping wings is complicated.  This site is a good place to do some reading.  The basic idea is not hard to explain.  To get net lift, in the cyclical flapping motion of a wing, somehow the drag force pushing downward on the wing during the upstroke has to be more than balanced by the flux of momentum pushed downward on the wing's downstroke.  To do this, the wing's geometry can't be unchanging during the flapping.  The asymmetry between up and down strokes is achieved through the tilting (at the wing base and along the wing) and flexing of the wing during the flapping motion.  

The ornithopters in the new movie have wings on the order of 10 m long, and wing motions that look like those of a dragonfly, and the wings are able to flap up and down and an apparent frequency of a couple of hundred hertz (!).  If you try to run some numbers on the torque, power, and material strength/weight that would be required to do this, you can see pretty quickly why this has not worked too well yet as a strategy on earth.   (As batteries, motor technology, and light materials continue to improve, perhaps ornithopters will become more than a fun hobby.)  

This issue - that cool gadgets in sci-fi or superhero movies would need apparently unachievable power densities at low masses - is common (see, e.g., Tony Stark's 3 GW arc reactor that fits in your hand, weighs a few pounds, and somehow doesn't have to radiate GW of waste heat), and that's ok; the stories are not meant to be too realistic. Still, the ornithopter fulfills its most important purpose in the movie:  It looks awesome.  

Brief items - Sarachik, Feynman, NSF postdocs and more

 Here are several items of interest:

• I was saddened to learn of the passing of Myriam Sarachik, a great experimental physicist and a generally impressive person.  I was thinking about writing a longer piece about her, but this New York Times profile from last year is better than anything I could do.  This obituary retells the story to some degree. (I know that it's pay-walled, but I can't find a link to a free version.)  In the early 1960s, after fighting appalling sexism to get a doctorate and a position at Bell Labs, she did foundational experimental work looking at the effect of dilute magnetic impurities in the conduction of nonmagnetic metals.  For each impurity, the magnetic atom has an unpaired electron in a localized orbitals.  A conduction electron of opposite spin could form a singlet to fill that orbital, but the on-site Coulomb repulsion of the electron already there makes that energetically forbidden except as a virtual intermediate state for a scattering process.  The result is that scattering by magnetic impurities gets enhanced as \(T\) falls, leading to an upturn in the resistivity \(\rho(T)\) that is logarithmic in \(T\) at low temperatures.  Eventually the localized electron is entangled with the conduction electrons to form a singlet, and the resistivity saturates.  This is known as the Kondo Effect based on the theoretical explanation of the problem, but Sarachik's name could credibly have been attached.  Her family met with a personal tragedy from which it took years to recover.  Later in her career, she did great work looking at localization and the metal-insulator transition in doped semiconductors.  She also worked on the quantum tunneling of magnetization in so-called single-molecule magnets, and was a key player in the study of the 2D metal-insulator transition in silicon MOSFETs.  I was fortunate enough to meet her when she came through Rice in about 2003, and she was very generous in her time meeting with me when I was a young assistant professor.  Sarachik also had a great service career, serving as APS President around that time.  Heck of a career! 
• The audio recordings of the famous Feynman Lectures on Physics are now available for free to stream from Caltech.  You can also get to these from the individual lectures by a link on the side of each page.
• There is a new NSF postdoctoral fellowship program for math and physical sciences.  I would be happy to talk to anyone who might be interested in pursuing one of these who might want to work with me.  Please reach out via email.
• I've written before about the "tunneling time" problem - how long does quantum mechanical tunneling of a particle through a barrier take?  Here is an experimental verification of one of the most counterintuitive results in this field:  the farther "below" the barrier the particle is (in the sense of having a smaller fraction of the kinetic energy needed classically to overcome the potential barrier), the faster the tunneling.  A key experimental technique here is the use of a "Larmor clock", with the precession of the spin of a tunneling atom acting as the time-keeping mechanism.
• Did you know that it is possible, in Microsoft Word, to turn on some simple LaTeX-style symbolic coding?  The key is to enable "Math Autocorrect", and then typing \alpha will automatically be turned into \(\alpha\).  (I know act like doing scientific writing in Word is heretical, but not everyone in every discipline is facile with LaTeX/Overleaf.)