Tuesday, March 30, 2021

Amazingly good harmonic oscillators

One way that we judge the "quality" of a harmonic oscillator by how long it takes to ring down.  A truly perfect, lossless harmonic oscillator would ring forever, so that's the limiting ideal.  If you ding a tuning fork, it will oscillate about 1000 times before its energy falls by a factor of around \(\exp(-2\pi) \approx 1/535\).  That means that its quality factor, \(Q\), is about 1000.  (An ideal, lossless harmonic oscillator would have \(Q = \infty\).   In contrast, if you ding the side of a coffee mug, the sound dies out almost immediately - it doesn't seem bell-like at all, because it has a much lower \(Q\), something like 10-50.  The quality is limited by damping, and in a mechanical system this is the lossy frictional process that, in the simplest treatment, acts on the moving parts of the oscillator with a force proportional to the speed of the motion.  That damping can be from air resistance, or in the case of the coffee mug example, it's dominated by "internal friction".

So, how good of a mechanical oscillator can we make?  This paper on the arxiv last night shows a truly remarkable (to me, anyway) example, where \(Q \sim 10^{8}\) in vacuum.  The oscillators in question are nanofabricated (drumhead-like) membranes of silicon nitride, with resonant frequencies of about 300 kHz.  To put this in perspective, if a typical 1 kHz tuning fork had the same product of \(Q\) and frequency, it would take \(3 \times 10^{10}\) seconds, or 950 years, for its energy content to ring down by that 1/535 factor.  The product of \(Q\) and frequency is so high, it should be possible to do quantum mechanics experiments with these resonators at room temperature.  
A relevant ad from a favorite book.

That's impressive, but it's even more so if you know a bit about internal friction in most solids, especially amorphous ones like silicon nitride.  If you made a similar design out of ordinary silicon dioxide glass, it would have a \(Q\) at room temperature of maybe 1000.  About 15 years ago, it was discovered that there is something special about silicon nitride, so that when it is stretched into a state of high tensile stress, its internal friction falls dramatically.  This actually shows a failure of the widely used tunneling two-level system model for glasses.  The investigators in the present work have taken this to a new extreme, and it could really pave the way for some very exciting work in mechanical devices operating in the quantum regime.  

update:  In resonators made from silicon nitride beams with specially engineered clamping geometries, you can do even better.  How about the equivalent of a guitar string that takes 30000 years to ring down?  “Listen to that sustain!

Sunday, March 28, 2021

Brief items

Catching up after the APS meeting, here are a couple of links of interest:

  • This video has been making the rounds, and it's fun to watch.  It's an updated take on one of those powers-of-ten videos, though in this case it's really powers-of-two.  Nicely done, though I think the discussion of the Planck Length is not really correct.  As far as I know, the Planck Length is a characteristic scale where quantum gravity effects cannot be neglected - that doesn't mean that the structure of the universe is discrete on that scale.
  • There have also been a lot of articles like this one implying that new (non-Standard Model) physics has been seen at the LHC.  As is usually the case, it's premature to get too excited.  At the 3\(\sigma\) level, there is an asymmetry in decay channels (electrons vs muons) seen by the LHCb experiment when none is expected.  As the always reliable Tommaso Dorigo writes here, everyone should just take a breath before getting too excited.  At least when the LHC starts back up next year, there should be a lot of new data coming in, and either this effect will grow, or it will fade away.  Anyone want to bet on the over/under for the number of theory papers about leptoquarks that are going to show up on the arxiv in the next month?
  • We were fortunate enough to have Pablo Jarillo-Herrero give our colloquium this past Wednesday, talking about some really exciting recent results (here, here) in twisted trilayer graphene.
I'll hopefully write more soon, also touching on a recent paper of ours.

Sunday, March 21, 2021

APS March Meeting wrap-up and thoughts

Well, that was certainly an interesting experience.  Some thoughts:

  • Having the talks available as recordings and live streams has a number of real positives:  It means being able to go back and catch up on talks for which I had conflicts, and it does eliminate the problem of having a hugely popular topic placed in a tiny, suffocatingly crowded room.  It would be nice if there is a way to make this work seamlessly in a hybrid mode (e.g., combining a live talk with the zoom stream, though questions would get tricky).   
  • It would also be nice if there were a way to subsidize availability and pricing for people unable to attend in person, particularly from economically disadvantaged countries.  I know that the costs of the virtual meeting are not trivial; this was made clear at the Town Hall about the meeting.  That being said, it would be nice to make the meeting contents more broadly accessible to the whole community.
  • The whole "virtual hallway" networking thing really did not seem to catch on at all, based on my limited experience.  For example, in the session where I spoke, all of the invited speakers went there after the session, and only three additional people showed up.  Given that at one point there were apparently something like 170 people watching live, that's rather surprising.
  • I did miss much of the social interaction of the meeting - to be able to see friends, meet people in the hallway and catch up, sit down for spontaneous discussions, take my group and alumni out for dinner.   
  • I did not miss overpriced food or spending a small fortune on hotel and airfare for me and my research group.  
In the post-pandemic world, we will see whether large conferences like this (or larger ones, like the MRS or ACS national meetings) revert to the traditional format or evolve into something new.

Friday, March 19, 2021

APS March Meeting, Day 5

 More work meetings so that I had to view some talks out of sequence, but here are some highlights.  I'll post a bit of a wrap-up later.

  • In a talk that I watched on delay, here is a really fun talk by Harry Atwater, photonics expert par excellence, about photonic materials considerations for light-based propulsion for an interstellar probe, as discussed here.  A phase gradient on a flat metasurface can give the same kind of dynamic stability that you could get from a curved purely reflective sail.  The fact that serious scientists and engineers are at least thinking about and discussing interstellar probes is pretty damn cool.  
  • Also on delay, it was fun to watch the Physics for Everyone session about popularization.  (Note to self:  get brilliant, truly original inspiration for popular book approach.)  All the talks that I could see were good, but I particularly enjoyed David Weitz talking about his famous science and cooking course (edx version here), since cooking is a hobby of mine.  Jim Kakalios spoke engagingly about using superheroes as a tool for science outreach.  I was very disappointed that the recording then stopped, and somehow did not capture the last two talks of the session - it would have been nice to hear about Ainissa Ramirez's recent book.
  • This morning there was an invited session all about various approaches that check very critically for superconductor/semiconductor device effects that can look like but often are not Majorana fermions.  Javad Shabani showed a neat result, where it looks convincingly like they can use gate tuning of spin-orbit coupling to go from topologically trivial (s-wave) to topologically nontrivial (p-wave-like) superconductivity in Al/InAs/Al structures.  Again, with this session, the last talk was not recorded for some reason.  Weird.
  • Alex Hamilton from UNSW (no connection to the Ten Dollar Founding Father, as far as I am aware) gave a really nice talk about hydrodynamic flow of electrons in 2D systems, where he addressed an issue that's bugged me for a long time:  What controls the boundary condition on the fluid at the edges of the channel?  That is, what determines whether there is perfect slip, no slip, or something in between?
  • Finally, I enjoyed Mark Miodownik's excellent talk based on his book Stuff Matters, which is just a great read.  If you haven't read it, do.

APS March Meeting, Day 4

 Yesterday was also very chaotic, and so I have had to make a reminder to watch some talks later.  Very briefly:

  • Burkard Hillebrands gave a talk about creating room temperature Bose Einstein condensates out of magnons.  When first hearing about this a few years ago I wondered how this worked, since magnons are not strictly conserved.  They do have a minimum energy to be created, however, and if losses (to phonons) are sufficiently weak, then with the right population manipulation (either by rapid cooling or parametric pumping) you can create a BEC, in the same way that one can get a BEC from ultracold atoms in a somewhat leaky trap.  He showed evidence of a magnon condensate Josephson junction with the ac Josephson effect.  Neat stuff.  A magnonics roadmap has also just come out, for those interested in applications.
  • There was a nice contributed talk by Ruofan Li from the Ralph group at Cornell, looking at magnon transport in films of the magnetic insulator MgAl2O4.  The found an anisotropy in the magnon diffusion length that correlates with the magnetic anisotropy of the material along crystallographic directions.
  • Andrea Cavalleri spoke about his work on light-induced superconducting-like response in various materials, particularly K3C60.  His group has a recent result showing that they can trigger an apparently superconducting state (based on the conductivity) that is metastable for tens of nanoseconds at temperatures far above the equilibrium superconducting transition.  
  • A large part of my afternoon was spent at this session about pairing in the high-Tc normal state.  My fellow speakers gave uniformly excellent talks, and according to the session chair the turnout was actually pretty good.  As a proponent of noise measurements as interesting probes, I was very impressed by the recent results from Milan Allan, whose group has combined noise measurements with STM, and revealed clear evidence of pairing well above the bulk Tc in TiN.
There are multiple other talks that I want to watch later on as well, if I can find the time.

Wednesday, March 17, 2021

APS March Meeting, Day 3

As I warned yesterday, my work commitments (plus attending talks by three of my students)  mean that this set of highlights is attenuated.  Still some excellent talks, though, and if you are registered I encourage pulling up the recordings for some of these.

  • There was a talk this morning by Lukas Prochaska from TU Vienna, pertaining to this paper, where the charge fluctuations in the quantum critical heavy fermion compound YbRh2Si2 really blow up, as seen via THz optical conductivity measurements.  (Full disclosure, I'm working with these folks as well, and two of my colleagues are on that paper.)  A key advance is the ability to grow this comparatively exotic compound via molecular beam epitaxy (MBE).
  • Speaking of MBE, I strongly recommend the talks by this year's McGroddy Prize winners, Ivan Božović, Darrell Schlom, and Jim Eckstein.  These folks are pioneers of the growth of complex oxides by MBE, and it is really amazing how much good science has come out of the development of this technique and the resulting materials.  (Again, full disclosure, I've had the opportunity to collaborate with the first two.)
  • Speaking of pioneers, I also strongly endorse the Buckley Prize talk by Moty Heiblum.  It was simply a great explanation of how shot noise can be an incredibly useful tool to examine comparatively exotic physics (e.g., fractionally charged quasiparticles in the fractional quantum Hall regime; the breakup of neutral excitations in the fractional quantum Hall regime).   (Unfortunately I was not able to watch the other Buckley Prize talk today, but since Pablo Jarillo-Herrero is giving our colloquium next week, I get to see similar material soon.)
  • The talk by Prof. Xiaoxing Xi, very similar to his remarkable Harvard colloquium, should be required viewing.  Here is a link to the JASON report (pdf) about a much better way to handle scientific and security concerns re China.
  • Finally, you should watch this whole session if you want to see a great cross-section of the state-of-the-art on different quantum computing approaches (superconducting qubits, trapped ions, Si spin qubits (that I'd mentioned here), the ongoing Majorana business, and photonic quantum computing).  Very interesting.

Tuesday, March 16, 2021

APS March Meeting, Day 2

Another complicated day meant another selection of talks.  It's great that the talks are recorded so that (at least for now) I can go back and watch others that I missed, but somehow watching talks on screen is just as tiring if not moreso than watching them at a convention center.  At least I'm not crammed into a tiny room with 100 other people carrying backpacks, jackets, etc. and struggling to see the screen.

Some highlights:

  • Silke Bühler-Paschen from TU Wien gave a nice talk about the Weyl-Kondo semimetal Ce3Bi4Pd3.  This is an example of a topologically interesting material that has strong electronic correlations.  Rather analogous to the situation in heavy fermions, where the correlations flatten the bands and renormalize the electron effective mass to be very large, in this case the Weyl nodes and dispersion of the topological states remain but the dispersion is strongly renormalized.  Another signature of the correlations is the large (renormalized) size of the spontaneous Hall effect in this system.
  • Richard Silver from NIST gave a clear presentation about advances in creating atomically precise devices based on individual phosphorus dopants in silicon.  Recent reviews are here and here.  They are making strong progress toward being able to implement quantum simulations of things like the Hubbard model in arrays of sites, though disorder is a major challenge.  A related talk was presented yesterday by Shashank Misra from Sandia.
  • There was an interesting session about signatures of the strange metal in both iron pnictide and cuprate superconductors.  This ties in with ideas about the demise of quasiparticles and the possibility of "incoherent" charge-carrying excitations in these systems.  Aharon Kapitulnik ended the session looking at thermal transport in the high temperature limit of these materials, as strange metallicity (linear-in-T resistivity) crosses into bad metallicity (resistivity above the Mott-Ioffe-Regel limit), and concluding that neither electrons nor phonons are well-defined quasiparticles in that limit.  
  • Harold Hwang gave an overview and update on the growth of the cuprate-analog infinite layer nickelate material Nd0.8Sr0.2NiO2.  This included stabilization of films by encapsulating them in SrTiO3 during growth, understanding the electronic structure further, and expanding this family of materials.
  • The Phys Rev session was also very good, though I only caught pieces - the talks by Sachdev and Marcus were both fun.  The latter did a good job emphasizing the key role of materials in pursuing the goal of engineering topologically nontrivial superconductivity in superconductor/semiconductor hybrid structures.

Tomorrow my work schedule will be more of a constraint, so my writeup will likely be late and a bit sparse.

Monday, March 15, 2021

APS March Meeting, Day 1

As in past years, I'm going to try to give a few highlights of talks that I saw "at" the APS March Meeting.  Historically these are a blend of talks that usually have some connection to research topics that interest me, and subjects that I think are likely to be important or presented by particularly good speakers.  The meeting being virtual this year presents challenges.  On the one hand, because a very large fraction of the talks are being recorded, in principle I should be able to go back and watch anything that I otherwise would miss due to scheduling collisions or other commitments.  On the other hand, not traveling means that it's very hard to truly concentrate on the meeting without local work demanding some attention.  

(To simulate the true March Meeting experience, I was tempted to spend $4.50 on some terrible coffee this morning, and $11 on a slice of turkey, a slice of cheese, a sad slice of tomato, and a wilted lettuce leaf on white bread for lunch.)

  • Tim Hugo Taminiau from Delft presented a neat talk about using (the electron spins of) NV centers in diamond to examine and control 13C nuclear spins.  Through very impressive pulse sequences based on NMR techniques plus machine learning, his group has been able to determine the locations and couplings of tens of nuclear spins, and controllably create and manipulate entanglement among them.
  • Markus Raschke from Colorado gave a very nice presentation showcasing the impressive work that his group has done using the plasmonic resonance of a gold tip to do cavity quantum electrodynamics with individual emitters.   Even though the plasmonic cavity is leaky (low \(Q\)), the mode volume is tiny compared with the wavelength (\(V_{m} \sim 10^{-6} \lambda^{3}\)).  This lets them get into the strong coupling regime, with big splittings of the excitonic emission peaks in quantum dots and clear detection of the plexitonic (or polaritonic, depending on your terminology) states.
  • There was a nice session about strange metals, but I had to pop in and out of it.  One particularly interesting talk was given by Philip Phillips, who spoke about Noether's theorem(s) and the demise of charge quantization in the strange metal - see here.  (This relates to an experiment I'm very interested in trying.)  This talk also featured an unscheduled interruption for the first APS/Marvel's WandaVision crossover (see image).
  • Late in the day I was able to catch most of Bart van Wees's talk about spin transport in magnetic insulators, including the spin Seebeck effect.  The basic measurement approach is this one, using the inverse spin Hall effect to detect an incoming current of magnons driven either by spin injection or by a temperature gradient.  They have applied this approach to examine a number of material systems, including van der Waals antiferromagnets and the van der Waals Ising magnet CrBr3.  In the latter case, because the material is so chemically reactive, they had to do some clever sample fabrication to encapsulate it in hBN while countersinking their Pt spin Hall electrodes.
  • I also managed to see Bob Willett's talk about showing actual interferometric demonstration of non-Abelian statistics at the \(\nu = 5/2\) and \(7/2\) fractional quantum Hall states.  These devices are amazing in that they preserve the material quality despite challenging fabrication, and the experiments are about the clearest evidence you can have for exotic fractional charge and statistics in these systems.
There are some other talks from today that I want to see, but they will have to wait.  The virtual meeting format is ok, but there really is no substitute for talking to people face to face.  

Wednesday, March 10, 2021

Items leading into the APS March Meeting

This will be the first virtual APS March Meeting.  It's also taking place at a time when many universities in the US have eliminated spring recess, and no one is getting out of town for the conference.  This means that faculty and students are going to try to balance attending virtual talks and some level of networking/social interaction along with the usual business of the university.  Between that and the reluctance to sit immobile in front of a screen for many hours at a time, it will be interesting to see how this goes.  Here is the information available so far on how the meeting is actually going to work in terms of zoom/web access/discussions.  More information is reportedly on the way.

In the meantime, the biggest condensed matter news item of the week is the retraction of the Majorana fermion paper discussed here.  

  • The official investigative panel report on this matter is available here.  The panelists detail multiple issues with the paper, and conclude that "the most plausible explanation [is] that the authors were caught up in the excitement of the moment, and were themselves blind to the data that did not fit the goal they were striving for. They have "fooled themselves" in the way forewarned by Feynman in the speech we quoted at the beginning of section 3."
  • Another analysis is here.  An inescapable conclusion is that making the data sets available greatly helped in figuring out what went on here.  
  • Here is a youtube video that goes over this from the technical perspective.
In other news:
  • Here is an updated version of a paper by my postdoctoral mentor showing interferometric evidence for the braiding of other exotic quasiparticles.   This is an implementation of ideas related to these proposals (1, 2).  One point of commonality with the Majorana ideas:  exquisitely clean material is needed to see the interesting physics, and preserving that lack of disorder when fabricating devices is really hard.

Wednesday, March 03, 2021

Undergraduate labs - quick survey

 I've already posed this survey on a mailing list of US physics + P&A department chairs, but more information would certainly be helpful.  At major US universities,  I'm trying to do a bit of a survey about how departments staff their undergraduate introductory labs (both the physics-for-engineers/majors sequence and the physics-for-biosciences/premeds sequence).  If you have this information and can provide it and identify the university, I would be appreciative.

1) Do you have traditional-style intro labs, or a more active learning/discovery-based/modern pedagogy approach?

2) How many undergrads per lab section, how do they work (e.g. groups of 2) and how many lab TAs (or equivalent) per lab section?

3) Who is doing the supervision - what combo of graduate lab TAs, undergrad lab TAs, NTT instructors?

I've heard back from about 8 programs so far, but more would be helpful.  If you would prefer emailing me rather than using the comments, that's fine as well.