Sunday, May 15, 2022

Flat bands: Why you might care, and one way to get them

When physicists talk about the electronic properties of solids, we often talk about "band theory".  I've written a bit about this before here.  In classical mechanics, a free particle of mass \(m\) and momentum \(\mathbf{p}\) has a kinetic energy given by \(p^2/2m\).  In a crystalline solid, we can define a parameter, the crystal momentum, \(\hbar \mathbf{k}\), that acts a lot like momentum (accounting for the ability to transfer momentum to and from the whole lattice).  The energy near the top or bottom of a band is often described by an effective mass \(m_{*}\), so that \(E(\mathbf{k}) = E_{0} + (\hbar^2 k^2/2m_{*})\).  The whole energy band spans some range of energies called the bandwidth, \(\Delta\). If a band is "flat", that means that its energy is independent of \(\mathbf{k}\) and \(\Delta = 0\).  In the language above, that would imply an infinite effective mass; in a semiclassical picture, that implies zero velocity - the electrons are "localized", stuck around particular spatial locations.  

Why is this an interesting situation?  Well, the typical band picture basically ignores electron-electron interactions - the assumption is that the interaction energy scale is small compared to \(\Delta\).  If there is a flat band, then interactions can become the dominant physics, leading potentially to all kinds of interesting physics, like magnetism, superconductivity, etc.  There has been enormous excitement in the last few years about this because twisting adjacent layers of atomically thin materials like graphene by the right amount can lead to flat bands and does go along with a ton of cool phenomena.  

How else can you get a flat band?  Quantum interference is one way.  When worrying about quantum interference in electron motion, you have to add the complex amplitudes for different electronic trajectories.  This is what gives you the interference pattern in the two-slit experiment.   When trajectories to a certain position interfere destructively, the electron can't end up there.  

It turns out that destructive interference can come about from lattice symmetry. Shown in the figure is a panel adapted from this paper, a snapshot of part of a 2D kagome lattice.  For the labeled hexagon of atoms there, you can think of that rather like the carbon atoms in benzene, and it turns out that there are states such that the electrons tend to be localized to that hexagon.  Within a Wannier framework, the amplitudes for an electron to hop from the + and - labeled sites to the nearest (red) site are equal in magnitude but opposite in sign.  So, hopping out of the hexagon does not happen, due to destructive interference of the two trajectories (one from the + site, and one from the - site).  

Of course, if the flat band is empty, or if the flat band is buried deep down among the completely occupied electronic states, that's not likely to have readily observable consequences.  The situation is much more interesting if the flat band is near the Fermi level, the border between filled and empty electronic states.  Happily, this does seem to happen - one example is Ni3In, as discussed here showing "strange metal" response; another example is the (semiconducting?) system Nb3Cl8, described here.  These flat bands are one reason why there is a lot of interest these days in "kagome metals".

Saturday, May 14, 2022

Grad students mentoring grad students - best practices?

I'm working on a physics post about flat bands, but in the meantime I thought I would appeal to the greater community.  Our physics and astronomy graduate student association is spinning up a mentoring program, wherein senior grad students will mentor beginning grad students.  It would be interesting to get a sense of best practices in this.  Do any readers have recommendations for resources about this kind of mentoring, or examples of departments that do this particularly well?  I'm aware of the program at UCI and the one at WUSTL, for example.

Sunday, May 01, 2022

The multiverse, everywhere, all at once

The multiverse (in a cartoonish version of the many-words interpretation of quantum mechanics sense - see here for a more in-depth writeup) is having a really good year.  There's all the Marvel properties (Spider-Man: No Way Home; Loki, with its Time Variance Authority; and this week's debut of Doctor Strange in the Multiverse of Madness), and the absolutely wonderful film Everything, Everywhere, All at Once, which I wholeheartedly recommend.  

While it's fun to imagine alternate timelines, the actual many-worlds interpretation of quantum mechanics (MWI) is considerably more complicated than that, as outlined in the wiki link above.  The basic idea is that the apparent "collapse of the wavefunction" upon a measurement is a misleading way to think about quantum mechanics.  Prepare an electron so that its spin is aligned along the \(+x\) direction, and then measure \(s_{z}\).  The Copenhagen interpretation of quantum would say that prior to the measurement, the spin is in a superposition of \(s_{z} = +1/2\) and \(s_{z}=-1/2\), with equal amplitudes.  Once the measurement is completed, the system (discontinuously) ends up in a definite state of \(s_{z}\), either up or down.  If you started with an ensemble of identically prepared systems, you'd find up or down with 50/50 probability once you looked at the measurement results.    

The MWI assumes that all time evolution of quantum systems is (in the non-relativistic limit) governed by the Schrödinger equation, period.  There is no sudden discontinuity in the time evolution of a quantum system due to measurement.  Rather, at times after the measurement, the spin up and spin down results both occur, and there are observers who (measured spin up, and \(s_{z}\) is now +1/2) and observers who (measured spin down, and \(s_{z}\) is now -1/2).  Voila, we no longer have to think about any discontinuous time evolution of a quantum state; of course, we have the small issues that (1) the universe becomes truly enormously huge, since it would have to encompass this idea that all these different branches/terms in the universal superposition "exist", and (2) there is apparently no way to tell experimentally whether that is actually the case, or whether it is just a way to think about things that makes some people feel more comfortable.  (Note, too, that exactly how the Born rule for probabilities arises and what it means in the MWI is not simple.) 

I'm not overly fond of the cartoony version of MWI.  As mentioned in point (2), there doesn't seem to be an experimental way to distinguish MWI from many other interpretations anyway, so maybe I shouldn't care.  I like Zurek's ideas quite a bit, but I freely admit that I have not had time to sit down and think deeply about this (I'm not alone in that.).  That being said, lately I've been idly wondering if the objection of the "truly enormously huge" MWI multiverse is well-founded beyond an emotional level.  I mean, as a modern physicist, I already have come to accept (because of observational evidence) that the universe is huge, possibly infinite in spatial extent, appears to have erupted into an inflationary phase 13.6 billion years ago from an incredibly dense starting point, and contains incredibly rich structure that only represents 5% of the total mass of everything, etc.  I've also come to accept that quantum mechanics makes decidedly unintuitive predictions about reality that are borne out by experiment.  Maybe I should get over being squeamish about the MWI need for a zillion-dimensional hilbert space multiverse.  As xkcd once said, the Drake Equation should include a factor for "amount of bullshit you're willing to buy from Frank Drake".  Why should MWI's overhead be a bridge too far?  

It's certainly fun to speculate idly about roads not taken.  I recommend this thought-provoking short story by Larry Niven about this, which struck my physics imagination back when I was in high school.  Perhaps there's a branch of the multiverse where my readership is vast :-)



Monday, April 25, 2022

Science Communications Symposium

 I will be posting more about science very soon, but today I'm participating in a science communications symposium here in the Wiess School of Natural Sciences at Rice.  It's a lot of fun and it's great to hear from some amazing colleagues who do impressive work.   For example, Lesa Tran Lu and her work on the chemistry of cooking, Julian West and his compelling scientific story-telling, Scott Solomon and his writing about evolution, and Kirsten Siebach and her work on Mars rovers and geology.

(On a side note, I've now been blogging for almost 17 years - that makes me almost 119 blog-years old.)


Friday, April 08, 2022

Brief items

It's been a while since the APS meeting, with many things going on that have made catching up here a challenge.  Here are some recent items that I wanted to point out:

  • Igor Mazin had a very pointed letter to the editor in Nature last week, which is rather ironic since much of what he was excoriating is the scientific publishing culture promulgated by Nature.  His main point is that reaching for often-unjustified exotic explanations is rewarded by glossy journals - a kind of inverse Occam's Razor.   He also points out correctly that it's almost impossible for experimentalists to get a result published in a fancy journal without claiming some theoretical explanation.
  • We had a great physics colloquium here this week by Vincenzo Vitelli of the University of Chicago.  He spoke about a number of things, including "odd elasticity".  See, when relating stresses \(\sigma_{ij}\) to strains \(u_{kl}\), in ordinary elasticity there is a tensor that connects these things: \(\sigma_{ij} = K_{ijkl} u_{kl}\), and that tensor is symmetric:  \(K_{ijkl} = K_{klij}\).  Vitelli and collaborators consider what happens when there is are antisymmetric contributions to that tensor.  This means that a cycle of stress/strain ending back at the original material configuration could add or remove energy from the system, depending on the direction of the cycle.  (Clearly this only makes sense in active matter, like driven or living systems.)  The results are pretty wild - see the videos about halfway down this page.
  • Here's something I didn't expect to see:  a new result out of the Tevatron at FermiLab, which is interesting since the Tevatron hasn't run since 2011.  Quanta has a nice write-up.  Basically a new combined analysis of FermiLab data has a new estimate out for the mass of the W boson along with a claimed improved understanding of systematic errors and backgrounds.  The result is a statement that the W boson is heavier than expectations from the Standard Model by an amount that is estimated to be 7 standard deviations.  The exotic explanation (perhaps favored by the inverse Occam's Razor above) is that the Standard Model calculation is off because it's missing some added contributions from so-far-undiscovered particles.  The less exotic explanation is that the new analysis and small error estimates have some undiscovered flaw.  Time will tell - I gather that the LHC collaborations are working on their own measurements. 
  • This result is very impressive.  Princeton investigators have made qubits using spins of single electrons trapped in Si quantum dots, and they have achieved fidelity in 2-qubit operations greater than 99%.  If this is possible in (excellent) university-level fabrication, it does make you wonder whether great things may be possible in a scalable way with industrial-level process control.
  • This is a great interview with John Preskill.  In general the AIP oral history project is outstanding.
  • Well, this is certainly suggestive evidence that the universe really is a simulation.

Friday, March 18, 2022

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.

Wednesday, March 16, 2022

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....

Tuesday, March 15, 2022

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.  

Monday, March 14, 2022

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.

Thursday, March 10, 2022

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.

Friday, February 25, 2022

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.  


Sunday, February 13, 2022

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.  

Saturday, January 29, 2022

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.

Saturday, January 15, 2022

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....

Saturday, January 08, 2022

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?