Tuesday, July 20, 2021

Quantum computing + hype

 Last Friday, Victor Galitski published a thought-provoking editorial on linkedin, entitled "Quantum Computing Hype is Bad for Science".  I encourage people to read it.

As a person who has spent years working in the nano world (including on topics like "molecular electronics"), I'm intimately familiar with the problem of hype.  Not every advance is a "breakthrough" or "revolutionary" or "transformative" or "disruptive", and that is fine - scientists and engineers do themselves a disservice when overpromising or unjustifiably inflating claims of significance.  Incentives often point in an unfortunate direction in the world of glossy scientific publications, and the situation is even murkier when money is involved (whether to some higher order, as in trying to excite funding agencies, or to zeroth order, as in raising money for startup companies).   Nano-related research advances overwhelmingly do not lead toward single-crystal diamond nanofab or nanobots swimming through our capillaries.  Not every genomics advance will lead to a global cure for cancer or Alzheimers.  And not every quantum widget will usher in some quantum information age that will transform the world.  It's not healthy for anyone in the long term for unsupported, inflated claims to be the norm in any of these disciplines.

I am more of an optimist than Galitski.  

I agree that we are a good number of years away from practical general-purpose quantum computers that can handle problems large enough to be really interesting (e.g. breaking 4096-bit RSA encryption).  However, I think there is a ton of fascinating and productive research to be done along the way, including in areas farther removed from quantum computing, like quantum-enhanced sensing.  Major federal investments in the relevant science and engineering research will lead to real benefits in the long run, in terms of whatever technically demanding physics/electronics/optics/materials work force needs we will have.  There is very cool science to be done.  If handled correctly, increased investment will not come at the expense of non-quantum-computing science.  It is also likely not a zero-sum game in terms of human capital - there really might be more people, total, drawn into these fields if prospects for employment look more exciting and broader than they have in the past.  

Where I think Galitski is right on is the concern about what he calls "quantum Ponzi schemes".  Some people poured billions of dollars into anything with the word "blockchain" attached to it, even without knowing what blockchain means, or how it might be implemented by some particular product.  There is a real danger that investors will be unable to tell reality from science fiction and/or outright lying when it comes to quantum technologies.  Good grief, look how much money went into Theranos when lots of knowledgable people knew that single-drop-of-blood assays have all kinds of challenges and that the company's claims seemed unrealistic. 

I also think that it is totally reasonable to be concerned about the sustainability of this - anytime there is super-rapid growth in funding for an area, it's important to think about what comes later.  The space race is a good example.  There were very cool knock-on benefits overall from the post-Sputnik space race, but there was also a decades-long hangover in the actual aerospace industry when the spending fell back to earth.  

Like I said, I'm baseline pretty optimistic about all this, but it's important to listen to cautionary voices - it's the way to stay grounded and think more broadly about context.  

APS Division of Condensed Matter Physics Invited Symposium nominations

Hopefully the 2022 APS March Meeting in Chicago will be something closer to "normal", though (i) with covid variants it's good to be cautious about predictions, and (ii) I wouldn't be surprised if there is some hybrid content.  Anyway, I encourage submissions.  Having been a DCMP member-at-large and seen the process, it's to all of our benefit if there is a large pool of interesting contributions.


The Division of Condensed Matter Physics (DCMP) program committee requests your proposals for Invited Symposium sessions for the APS March Meeting 2022. DCMP hosts approximately 30 Invited symposia during the week of the March Meeting highlighting cutting-edge research in the broad field of condensed matter physics. These symposia consist of 5 invited talks centered on a research topic proposed by the nominator(s). Please submit only Symposium nominations. DCMP does not select individual speakers for invited talks.

Please use the APS nominations website for submission of your symposium nomination.

Submit your nomination

Nominations should be submitted as early as possible, and no later than August 13. Support your nomination with a justification, a list of five confirmed invited speakers with tentative titles, and a proposed session chair. Thank you for spending the time to help organize a strong DCMP participation at next year’s March Meeting.

Jim Sauls, Secretary/Treasurer for DCMP

Friday, July 16, 2021

Slow blogging + a couple of articles

Sorry - blogging has been slow in recent days because, despite it being summer, it's been a very busy time for various reasons.

Here are a couple of articles that I've come across that seem interesting.  On the news/popular writing front:

On the science front, there have been several cool things that I haven't had time to look at in depth.  A couple of quantum info papers:

  • In this Nature paper, the google quantum AI team have used their 53 qubit chip to do proof-of-concept demonstrations of two different quantum error correction approaches.  Perhaps someone more knowledgable that me can chime in below in the comments about how the ratio of physical qubits to logical qubits depends on the fidelity and other properties of the physical qubits.  Basically, I'm wondering if, e.g., ion trap-based schemes would be able to make even better advantage out of the 1D error correction approach here.
  • Meanwhile, in China a large group has demonstrated a 66 qubit system similar in design to the google/Martinis approach.  

Tuesday, July 06, 2021

Infrastructure and competitiveness

With the recent passage in the US Senate of an authorization that would potentially boost certain scientific investments by the US, and the House of Representatives version passing its versions for NSF and DOE, talk of "competitiveness" is in the air.  It took a while, but it seems to have dawned on parts of the US Congress that it would be broadly smart for the country to invest more in science and engineering research and education.  (Note that authorizations are not appropriations - declaring that they want to increase investment doesn't actually commit Congress to actually spending the money that way.  A former representative from my area routinely voted for authorizations to double the NSF budget, and then did not support the appropriations, so that he could claim to be both pro-science and anti-spending.) 

Looking through my old posts on related topics, I came across this one from 2014, about investment in shared research equipment at universities and DOE labs.  Since then, the NSF's former National Nanotechnology Infrastructure Network has been replaced by the National Nanotechnology Coordinated Infrastructure organization, but the overall federal support for this fantastic resource has actually gone down in real dollars, since its annual budget is unchanged since then at $16M/yr.  As I wrote back in 2014, in an era when one high end transmission electron microscope can cost $8M or more, that seems like underinvestment if the goal is to maximize innovation by making top-flight shared research instruments available to the broadest cross-section of universities and businesses.   

I reiterate my suggestion:  Companies (google? Intel? Microsoft? SpaceX? Tesla? 3M? Dupont? IBM?) and wealthy individuals who truly want to have a more competitive science and engineering workforce and innovation base should consider establishing an endowed entity to support research equipment and staffing at universities.   A comparatively modest investment ($300M) could support more than the entire NNCI every year, in perpetuity.  

Sunday, June 27, 2021

Quantum coherence and classical yet quantum materials

Because I haven't seen this explicitly discussed anywhere, I think it's worth pointing out that everyday materials around us demonstrate some features of coherence and decoherence in quantum mechanics.

Quantum mechanics allows superposition states to exist - an electron can be in a state with a well-defined momentum, but that is a superposition of all possible position states along some wavefront.   As I mentioned here, empirically a strong measurement means coupling the system being measured to some large number of degrees of freedom, such that we don't keep track of the detailed evolution of quantum entanglement.  In my example, that electron hits a CCD detector and interacts locally with the silicon atoms in one particular pixel, depositing its charge and energy there and maybe creating additional excitations.  That "collapses" the state of the electron into a definite position.  This kind of measurement is a two-way street - a quantum system leaves its imprint on the state of the measuring apparatus, and the measurement changes the quantum system's state.

One fascinating aspect of the emergence of materials properties is that we can have systems that act both very classically (as I'll explain in a minute) and also very quantum mechanically at the same time, for different aspects of the material.  

If I have a piece of aluminum sitting in front of me (like the case of my laptop) that hunk of metal does not show up in a superposition of positions or orientations.  It surely seems to have a definite position and orientation, and if I looked closely at a given moment I would find the aluminum atoms arranged in crystal lattices, with clear atomic positions.  Somehow, the interactions of the aluminum with the broader environment have washed out the quantumness of the atomic positions.  (Volumes have been written about interpretations of quantum mechanics and "the measurement problem", as I touched on here.  In the many-worlds view, we live in a particular branch of reality, while there are other branches that correspond to other possible positions and orientations of the aluminum piece, one for each possible outcome of a positional or orientational measurement.  I'm not going to touch on the metaphysics behind how to think about this here, except to say that somehow the position of the aluminum empirically acts classically.)

What about the electrons in the piece of crystalline aluminum?  Well, we've learned about band structure.  The allowed quantum states of electrons in a periodic potential consists of bands of states.  Each of these states has an associated crystal momentum \(\hbar \mathbf{k}\), and there is some relationship between energy and crystal momentum, \(E(\mathbf{k})\).  There are values of energy between the bands that do not correspond to any allowed electronic quantum states in that periodic lattice.  In aluminum, the electronic states are filled up to states in the middle of a band.  (One can be more rigorous that this, but it's beside the point I'm trying to make.)  Interestingly, the electrons in those filled states energetically far away from the highest occupied states are coherent - they are wavelike and extended, and indeed the Bloch waves themselves are a direct consequence of quantum interference throughout the periodic lattice.  Why haven't these electrons somehow decohered into some classical situation?   If you imagine some dynamic interaction that would "measure" the location, say, of one of those electrons, you have to consider some final state in which the electron would end up.  Because all of the states at nearby energies are already occupied, and the electrons obey the Pauli Principle, there is no low-energy (on the scale of, say, the thermal energy available, \(k_{\mathrm{B}}T\)) path to decoherence.  You'd need much larger energy/higher momentum/shorter wavelength processes to reach those electrons and scatter them to empty final states (as in ARPES).

By that argument, though, the electrons that are energetically close to the Fermi level in metals should be vulnerable to decoherence - they have energetically nearby states into which they can be scattered, and a variety of comparatively low energy scattering processes (electron-electron scattering, electron-phonon scattering).   Is  that true?  Yes.  This is exactly why you can't see quantum interference effects in electrical conduction in metals at room temperature, but at low temperatures you can see interference effects like universal conductance fluctuations and understand the effects of decoherence on those effects quantitatively.

I find it remarkable that a piece of aluminum can show both the emergence of classical physics (the piece of aluminum is not spatially delocalized) while having quantum coherent degrees of within.  Understanding how to engineer robust quantum coherent systems despite the tendency toward environmental decoherence is key to future quantum information science and technology.

Wednesday, June 16, 2021

Nanoscale Views on the Scientific Sense podcast

I recently had the opportunity to be interviewed for the Scientific Sense podcast, available on a variety of platforms.  It was a fun discussion, and it's now available here (youtube link) or here (spotify link).  

Tuesday, June 15, 2021

Brief items


Some news items:

  • Big news yesterday was the announcement at Condensed Matter Theory Center conference (I'll put up the link to the talk when it arrives on the CMTC youtube channel) by Andrea Young that ABC-stacked trilayer graphene superconducts at particular carrier densities and vertically directed electric field levels.  There are actually two superconducting states, with quite different in-plane critical fields (suggesting different pairing states).  Note that there is no twisting or moiré superlattice here, which suggests that superconductivity in stacked graphene may be more generic than has been thought.  Here is a relevant article in Quanta magazine.
  • Here is a talk by Padmanabhan Balaram, about greed in the academic publishing industry.  Even open-access journals apparently have profit margins of 30-40% (!!).  Think about that when publishers claim that production costs and their amazing editorial experience really justify that authors pay $5K per open-access publication.  (Note to self:  get around to putting manuscripts up on the arxiv....)  The talk is also an indictment of fixation on publication metrics.
  • On a lighter note, my very talented classmate, Yale chem professor Patrick Holland with a song about Reviewer 3.  It's more mellow than another famous response to Reviewer 3.
  • I was going to write a blog post about the physics motivating the use of sticky substances on baseballs, only to discover that someone already wrote that piece.  The time is ripe for someone to try to go to the other extreme:  Some kind of miracle superomniphobic coating on the ball so that the no-slip condition for air at the surface is violated, and every pitch then travels more like a knuckleball.

Friday, June 11, 2021

The power of computational materials theory

With the growth of computational capabilities and the ability to handle large data volumes, it looks like we are entering a new era for the global understanding of material properties.  

As an example, let me highlight this paper, with the modest title, "All Topological Bands of All Stoichiometric Materials".  (Note that this is related to the efforts reported here two years ago.) These authors oversee the Topological Materials Database, and they have ground through the entire Inorganic Crystal Structure Database using electronic structure methods (density functional theory (see here, here, here) with VASP both with and without spin-orbit coupling) and an automated approach to checking for topologically nontrivial electronic bands.  This allows the authors to look at essentially all of the inorganic crystals that have reliable structural information and make a pass at characterizing whether there are topologically interesting features in their band structure.  The surprising conclusion is that almost 88% of all of these materials have at least one topologically nontrivial band somewhere (though it may be buried energetically far away from the electronic levels that affect charge transport, for example).  Considering that people didn't necessarily appreciate that there was such a thing as topological insulators until relatively recently, that's really interesting.  

This broad computational approach has also been applied by some of the same authors to look for materials with flat bands - these are systems where the electronic energy depends only very weakly on (crystal) momentum, so that interaction effects can be large compared to the kinetic energy.

The ability to do large-scale surveys of predicted material properties is an exciting development!

Sunday, May 30, 2021

Ask me something.

 I realized today that I had not had an open "Ask me something" post since December, 2018.  Seems like it's time - please have at it.

Sunday, May 23, 2021

What is disorder, to condensed matter physicists?

Condensed matter physicists throw around the term "disorder" quite a bit - what does this mean, and how is it quantified?  This is particularly important when worrying about comparatively delicate, exotic quantum states, as in the recent discussions of the challenge of experimentally observing emergent Majorana fermions at the interfaces between semiconductor nanowires and superconductors.  

Latent in the use of the word "disorder" is a contrast with "order".  One of the most powerful ideas in condensed matter is Bloch's theorem:  In (infinite) crystalline solids, the spatial periodicity of the arrangement of atoms in a lattice leads to the conservation of a quantity \(\hbar \mathbf{k}\), the crystal momentum, for the electrons.  The allowed energies of single-electron states in that lattice (neglecting electron-electron interaction effects) is then a function \(E(\mathbf{k})\), and it is possible to think about a wavepacket (blob) of electrons with some dominant \(\hbar \mathbf{k}\) propagating along, as discussed extensively here for example.   "Disorder" in this context is some break with perfect spatial periodicity, which breaks \(\mathbf{k}\) conservation - in the Drude picture, this is what causes electron trajectories to scatter and do a random, diffusive walk.  

Now, not all disorder is created equal.  In a metal like gold, there is a quantitative difference between having a dilute concentration of silver atoms substituted on gold sites, and alternately having the same concentration of vacancies on gold sites.  Surely the latter is somehow more disordered.  In quantum classes, we learn to think about scattering lengths, and in conductors one can ask the physically motivated question, how far would a wavepacket propagate between scattering events (a "mean free path", \(\ell\), compared to its dominant wavelength \(\lambda\)?  For a metal we can think of the product  \(k_{\mathrm{F}} \ell\), where \(k_{\mathrm{F}}\) is the Fermi wavevector, \(2 \pi/ \lambda_{\mathrm{F}}\).  A "good metal" has \( k_{\mathrm{F}} \ell >> 1 \).  When \(k_{\mathrm{F}} \ell\  < 1\), it doesn't make sense to think of propagating wavepackets anymore.  

In other contexts, it's more helpful to think of disorder explicitly as associated with an energy scale that I'll call \(\delta\).  Some sort of structural change in a material away from ordered perfection leads, on some length scale, to a shift in electronic energies by an amount of typical magnitude \(\delta\).  The question then becomes, how does \(\delta\) compare with other energy scales in the material?  The case above where \(k_{\mathrm{F}} \ell < 1\) roughly corresponds to \(\delta\) being comparable to the electronic bandwidth (the energetic extent of \(E(\mathbf{k})\).  When one wants to think about the effects of disorder on superconductors, an important ratio is \(\delta/\Delta\), where \(\Delta\) is the superconducting gap energy scale of the ordered case.   When one wants to think about the effects of disorder on some fragile emergent phase like a fractional quantum Hall state, then a relevant comparison is between \(\delta\) and the relevant energy scale associated with that state.  

TL/DR version:  "Disorder" is a catch-all term, and it is quantified by how strongly the system is perturbed away from some target ordered condition.  

It's worth remembering that some of the progenitors of modern physics thought that it would be impossible to learn much about the underlying physics of real materials because disorder would be too severe and too idiosyncratic (that is, that each kind of defect would have its own peculiar impacts).  That's why Pauli derisively said "Festkörperphysik ist eine Schmutzphysik" (solid-state physics is the physics of dirt).   Fortunately, we have been able to learn quite a bit, and disorder has its own beautiful results, even if it continues to be the bane of some problems.

Sunday, May 09, 2021

Catching up

As may be obvious from my pace of posting, the last couple of weeks have been very busy and intense for multiple reasons.  I hope that once the academic year really ends I can get back into more of a routine.

Two notable stories this week:

  • Two papers were published back-to-back in Science (here and here, with commentary here) that demonstrate (a) that comparatively macroscopic mechanical oscillators - drumheads - can be operated as true quantum objects (cooled down to the point where the thermal energy scale \(k_{\mathrm{B}}T\) is small compared to the quantum energy level spacing \(\hbar \omega\) (this has been done before); and that these resonators can be quantum mechanically entangled, so that the two have to be treated as a single quantum system when understanding measurements performed on each individually.   This can be used, in the case of the second paper, to allow clever measurement schemes that shift measurement back-action (see here for a nice tutorial) away from a target system, enabling precision measurements of the target better than standard quantum limits.  
  • IBM has demonstrated 300 mm wafer fabrication of integrated circuits with features and techniques for the upcoming "2 nm node".  As I've mentioned before, we have fully transitioned to the point where labeling new semiconductor manufacturing targets with a length scale is basically a marketing ploy - the transistors on this wafer do not have 2 nm channel lengths, and the wiring does not have 2 nm lines and spaces.  However, this is a very impressive technical demonstration of wafer-scale success in a number of new approaches, including triple-stacked nanosheet gate-all-around transistors.

Monday, April 26, 2021

Brief items

 As we careen toward the end of the spring semester, here are a few interesting links for perusal:

  • My colleagues at the Rice Center for Quantum Materials are running a mini-workshop this week about topology and correlations in condensed matter.
  • More broadly, there is a new site for all things quantum at Rice.  More news in the coming weeks....
  • Speaking of quantum, I thought that this paper was pretty impressive as a technical achievement.  The authors are able to cool a mechanical resonator (a suspended aluminum drumhead, essentially) down to 500 microKelvin (!), so cold that \(k_{\mathrm{B}}T\) is smaller than the harmonic oscillator energy levels - down to the quantum ground state for its center of mass motion.  As someone who built a nuclear demagnetization stage as part of my PhD, I have to respect achieving that temperature for a sample in vacuum.  Likewise, as someone who studied tunneling two-level systems in solids, it's impressive to see the logarithmic temperature dependence of sound speed in the aluminum extend smoothly down to below 1 mK.  
  • On a more general thermodynamic topic, this paper really surprised me. It's a review article about the existence of a dynamical crossover (the "Frenkel line") that exists above the critical temperature and pressure for a number of fluids - basically a separation into different regimes of response (not true phases per se).  Embarrassingly, I'd never heard of this, and I need to find the time to read up on it.
  • I'm late to the party on this, as it got quite a bit of press, but this paper is really interesting - special engineered light modes that are designed to propagate without distortion (though with attenuation) through scattering media.  There are many potential applications, such as medical imaging (with light or with ultrasound).
  • Anyone want a dinner plate-sized chip with 2.4 trillion transistors

Saturday, April 24, 2021

Lecturer position, Rice Physics & Astronomy

The Department of Physics and Astronomy at Rice University invites applications from recent Ph.D. graduates for a lecturer position in physics and astronomy, commencing July/August 2021.  Familiarity with and/or interest in physics education research, undergraduate teaching at the introductory level, pedagogy, and curricular issues is preferred. This is a non-tenure-track position for a two-year term with the possibility of reappointment for additional three-year terms.  This is a full-time, 9-month academic calendar position.  There would also be opportunities to develop innovative teaching methods and pursue independent research or collaborations with existing research programs (see web page https://physics.rice.edu/ ).  Evaluation of applications will begin May 15 and continue until the position is filled. Applications for this position must be submitted electronically at https://jobs.rice.edu/postings/26670.  Applicants should submit (1) a curriculum vitae, (2) a statement of teaching interests, (3) a statement on diversity and outreach, (4) a list of publications, and (5) the names, affiliations, and email addresses of three professional references.  Applicants must be eligible to work in the U.S. Rice University is committed to a culturally diverse intellectual community. In this spirit, we particularly welcome applications from all genders and members of historically underrepresented groups who exemplify diverse cultural experiences and who are especially qualified to mentor and advise all members of our diverse student population.

Rice University is an Equal Opportunity Employer with a commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status. We encourage applicants from diverse backgrounds to apply.

Thursday, April 15, 2021

NSF Workshop on Quantum Engineering Infrastructure

 I spent three afternoons this week attending a NSF workshop on Quantum Engineering Infrastructure.  This was based in part on the perceived critical need for shared infrastructure (materials growth, lithographic patterning, deposition, etching, characterization) across large swaths of experimental quantum information sciences, and the fact that the NSF already runs the NNCI, which was the successor of the NNIN.  There will end up being a report generated as a result of the workshop, hopefully steering future efforts.  (I was invited because of this post.)

The workshop was very informative, touching on platforms including superconducting qubits, trapped ions, photonic devices including color centers in diamond/SiC, topological materials, and spin qubits in semiconductors.  Some key themes emerged:

  • There are many possible platforms out there for quantum information science, and all of them will require very serious materials development to be ready for prime time.  People forget that our command of silicon comes after thousands of person-years worth of research and process development.  Essentially every platform is in its infancy compared to that.  
  • There is clearly a tension between the need for exploratory research, trying new processes at the onesy-twosy level, and the requirements for work at larger scale, which needs dedicated process expertise and control at a level not typically possible in a shared university facility.  Everyone also knows that progress is automatically slow if people have to travel off-site to some user facility to do part of their processing.  Some places are well situated - MIT, for example, has an exploratory fab facility here, and a dedicated 200 mm substrate superconducting circuit fab at Lincoln Labs.  Life is extra complicated when running an unusual process in some tool like a PECVD system or an etcher can "season" the gadget, leaving an imprint on subsequent process runs.
  • Whoever really figures out how to do wafer-scale heteroepitaxy of single-crystal diamond will either become incredibly rich or will be assassinated by DeBeers.  
  • Fostering a healthy relationship between industrial materials growers and academic researchers would be very important.  Industrial expertise can be fantastic, but there is not necessarily much economic incentive to work closely with academia compared with large-scale commercial pressures.  There may be a key role for government encouragement or subsidy.  
  • It's going to be increasingly challenging for new faculty to get started in some research topics at universities - the detailed process knowhow and the need to buildup expertise can be expensive and slow to acquire compared to the timescale of, e.g., promotion to tenure.  An improved network that supports, curates, and communicates process development expertise might be extremely helpful.

Thursday, April 08, 2021

"Fireside Chat" about Majoranas

Along with Zeila Zanolli, tomorrow (Friday April 9) I will be serving as a moderator for a "fireside chat" about Majorana fermions being given by Sergey Frolov and Vincent Mourik.   This is being done as a zoom webinar (registration info here), at 11am EDT.   Should be an interesting discussion - about 20 minutes of presentation followed by q & a.  

Update:  Here is a youtube link to a version that includes the intro talk piece from the second (April 16) chat, and the Q&A from both the April 9 and April 16 events.  Alas, this edit means that you miss my and Zelia's glittering introduction, but I bet you'll get over it.

Monday, April 05, 2021

Place your bets. Muon g-2....

Back in the early 20th century, there was a major advance in physics when people realized that particles like the electron have intrinsic angular momentum, spin, discussed here a bit.  The ratio between the magnetic dipole moment of a particle (think of this like the strength of a little bar magnet directed along the direction of the angular momentum) and the angular momentum is characterized by a dimensionless number, the g-factor.  (Note that for an electron in a solid, the effective g-factor is different, because of the coupling between electron spin and orbital angular momentum, but that's another story.)

For a free electron, the g-factor is a little bit larger than 2, deviating from the nice round number due to contributions of high-order processes.  The idea here is that apparently empty space is not so empty, and there are fluctuating virtual particles of all sorts, the interactions of which with the electron leading to small corrections related to high powers of (m/M), where m is the electron mass and M is the mass of some heavier virtual particle.   The "anomalous" g-factor of the electron has been measured to better than one part in a trillion and is in agreement with theory calculations involving contributions of over 12000 Feynman diagrams, including just corrections due to the Standard Model of particle physics.

A muon is very similar to an electron, but 220 times heavier.  That means that the anomalous g-factor of the muon is a great potential test for new physics, because any contributions from yet-undiscovered particles are larger than the electron case.  Technique-wise, measuring the g-factor for the muon is complicated by the fact that muons aren't stable and each decays into an electron (plus a muon neutrino and an electron antineutrino).  In 2006, a big effort at Brookhaven reported a result (from a data run that ended in 2001) that seems to deviate from Standard Model calculations by around 3 \(\sigma\).  

The experiment was moved from Brookhaven to Fermilab and reconstituted and improved, and on Wednesday the group will report their latest results from a new, large dataset.  The big question is, will that deviation from Standard Model expectations grow in significance, indicating possible new physics?  Or will the aggregate result be consistent with the Standard Model?   Stay tuned.

UpdateHere is the FNAL page that includes a zoom link to the webinar, which will happen at 10 AM CST on Wednesday, April 7.

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.

Friday, February 26, 2021

And more items of interest

 Here are some outreach/popularization tidbits:

Wednesday, February 24, 2021

Brief items

Here are a few items I came across in the last few days that may be of broader interest:

Sunday, February 21, 2021

Grad school admissions this year

Based on conversations with my colleagues at my institution and across the US, graduate program application rates in the US seem to be up quite a bit this year, including in physics and astronomy.  This is happening at the same time that many graduate programs are still working to handle the exceptional circumstances that arose due to the pandemic.  These include: 

  • lower graduation rates (as students are slower to graduate when there is increased uncertainty in the post-degree employment market, academic or otherwise); 
  • continued visa challenges with international students (e.g., students who have enrolled remotely from outside the US in fall '20 but have not yet been able to get here, and therefore may well need extra time to affiliate with a research advisor once they get to the US, presumably in the late spring or summer); 
  • restricted budgets to support existing and incoming students (especially at some public universities whose finances have been hardest hit by the pandemic-related economic fallout)
This whole mess increases the stress on graduate applicants by making an already fraught process even more competitive, in the sense of more people vying for fewer openings.  Graduate admissions is a complicated process driven very strongly by detailed needs that are often not visible to the applicant (e.g., if researchers in an area don't have a need for more students in a given year, something that may not be clear until January, admissions offers in that area are going to be limited).  I hope people know this, but it's worth stating explicitly:  Not getting admitted to a program is about the fit at the time between the needs of the program and the particular profile of the applicant, not a vote on anyone's worth as a scientist or person.  

For additional reference, here is the post I made last year about choosing a graduate program.

Sunday, February 14, 2021

Majoranas - a brief follow-up

As you can always tell by the frequency of my posting, work-related activities have been dominating my schedule of late.  In addition to the usual stuff (papers, proposals, the normal academic activities), this is the time of year when as department chair there are deadlines and activities associated with faculty and staff evaluations, departmental budgets, graduate admissions, teaching assignments for next year, etc.  Still, in the wake of this article from Wired and some breathless reactions in the news and social media, it's worth following up my prior post on the topic of solid state implementations of Majorana fermions and what the pending retraction of this paper means. 

There are two main issues.  First, it has become clear that it can be very challenging to achieve the experimental conditions needed to have clear, unambiguous evidence of Majorana quasiparticles in the superconductor/semiconductor nanowire architecture.  This is explained in detail here, for example.   The interface quality of the semiconductor and of the semiconductor/superconductor boundary is extremely important, as disorder can lead to various confounding effects.  Interfaces are notoriously challenging.  ("God created the bulk; surfaces were invented by the devil." - Pauli)  There is no reason to think that it is impossible to reach the cleanliness level needed to see Majoranas in this type of structure, but like many material-related problems, this seems like it will require even more effort. 

Second is the particular issue of data presentation in this paper and whether it was misleading.  I have not personally looked at this in depth, but others have (twitter thread).  Snipping out segments of gate voltage without making that clear, and only plotting a limited range of gate voltage (leaving out where the conductance exceeds what is supposed to be the limiting value), is problematic.  

It's important to separate these two issues.  The issues with this particular paper are not a reason to stop this experimental approach or give up trying to confirm Majoranas this way.  It's just hard, the community isn't there just yet, and this is a cautionary tale about triumphal press releases.

Tuesday, February 02, 2021

Bringing modern industrial nanofab to quantum computing

One big selling point of solid-state quantum computing platforms is the notion of scalability. The semiconductor industry has spent billions of dollars and millions of person-hours developing the capability of fabricating tens of billions of nanoscale electronic components in parallel with remarkable reliability.   Surely it's not crazy to think that this will be the key to creating large numbers of functioning qubits as well.

Like many ideas that look plausible at first glance, this becomes very complicated under greater scrutiny.  Many of the approaches that people have in mind for solid-state quantum computing are not necessarily compatible with the CMOS manufacturing processes that produced the chips powering the computer you're currently using.  Virtually all of the university groups working on these systems use university-type fabrication methods - electron beam lithography for patterning, lift-off processing, etc.  In contrast, industrial chip makers use very different processes: elaborate immersion photolithography, subtractive patterning, and a whole host of clever tricks that have driven forward the mass production of silicon nanoelectronics.  The situation gets even worse in terms of materials development if one considers attempts to use more exotic systems.  The most reasonable quantum computing platform to approach first, if one is worried about industrial compatibility is probably using spins in gate-defined quantum dots in silicon.  

A team from Delft and Intel has done just that, as shown in this preprint.  They successfully demonstrate basic single-qubit effects like Rabi oscillations in single spins in quantum dots (single-electron transistors) defined in FinFETs, which they have patterned across a full 300 mm wafer (!) of isotopically pure 28Si (to avoid decoherence issues associated with nuclear spin).  They present data (which I have not read carefully) about how reproducible the properties of the single-electron transistors are across the wafer.   
The contrast between Si quantum devices
produced through university fab(top) and 
elite industrial fab (bottom).

I think the figure here from their paper's supplementary material really shows the point in terms of fabrication methods.  At the top is a cross-sectional TEM image of a chain of quantum dot devices, where the bright lumpy features are the defining metal gates that were patterned by e-beam lithography and deposited by lift-off processing.  In contrast, at the bottom is a cross-sectional TEM of the nominally equivalent industrially made device.  Behold the result of the accumulation of decades of technique and experience.

Of course, they were able to do this because Intel decided that it was worth it to invest in developing the special purpose masks and the process flow necessary.   Universities ordinarily don't have access to the equipment or the specialists able to do this work.  This makes me wonder again, as I have several times over the years, whether it would have been worthwhile for DOE or NSF to have set up (perhaps with Intel or IBM as a public-private partnership) some fabrication hub that would actually give the broader university research community access to these capabilities and this expertise.   It would be very expensive, but it might have pushed technology farther ahead than having several "nanocenters" that don't necessarily have technology much different than what is available at the top two dozen university cleanrooms.  


Wednesday, January 27, 2021

Zero bias peaks - an example of the challenge of experimental (condensed matter) physics

The puzzle-solving aspect of experimental physics is one reason why it can fun, but also why it can be very challenging.  In condensed matter, for example, we have limited experimental tools and can only measure certain quantities (e.g., voltages, currents, frequencies)  in the lab, and we can only tune certain experimental conditions (e.g., temperature, applied magnetic field, voltages on electrodes).  Getting from there to an unambiguous determination of underlying physics can be very difficult.

For example, when measuring electronic conduction in nanostructures, often we care about the differential conductance, \(dI/dV\), as a function of the bias voltage \(V\) applied across the system between a source and a drain electrode.  In an ideal resistor, \(dI/dV\) is just a constant as a function of the bias.  "Zero bias" \( (V = 0) \) is a special situation, when the electronic chemical potential (the Fermi level, at \(T = 0\)) of the source and drain electrodes are the aligned.  In a surprisingly large number of systems, there is some feature in \(dI/dV\) that occurs at \(V= 0\).  The zero-bias conductance \( (dI/dV)(V=0)\) can be suppressed, or it can be enhanced, relative to the high bias limit.  These features are often called "zero bias anomalies", and there are many physical mechanisms that can produce them.  

For example, In conduction through a quantum dot containing an odd number of electrons, at sufficiently low temperatures there can be a zero-bias peak in the conductance due to the Kondo Effect, where magnetic processes lead to forward-scattering of electrons through the dot when the Fermi levels are aligned.  This Kondo resonance peak in \(dI/dV\) has a maximum possible height of \(2e^2/h\), and it splits into two peaks in a particular way as a magnetic field is applied.  In superconducting systems, Andreev processes can lead to zero bias peaks that have very different underlying physics, and different systematic dependences on magnetic field and voltage.

Zero bias anomalies have taken on a new significance in recent years because they are one signature that is predicted for solid-state implementations of Majorana fermions involving superconductors connected to semiconductor nanowires.   These exotic quasiparticles have topological properties that make them appealing as a possible platform for quantum computingObservations of zero bias anomalies in these structures have attracted enormous attention for this reason.  

The tricky bit is, it has become increasingly clear that it is extremely difficult to distinguish conclusively between "Majorana zero modes" and cousins of the Andreev features that I mentioned above.  As I mentioned in my last post, there is a whole session at the upcoming APS meeting about this, recent papers, and now a retraction of a major claim in light of new interpretation.  It's a fascinating challenge that shows just how tricky these experiments and their analysis can be!  This stuff is just hard.

(Posting will likely continue to be slow - this is the maximally busy time of the year as department chair....)

Monday, January 18, 2021

Brief items, new year edition

 It's been a busy time, but here are a few items for news and discussion:

  • President-Elect Biden named key members of his science team, and for the first time ever has elevated the role of Presidential Science Advisor (and head of the White House Office of Science and Technology Policy) to a cabinet-level position.  
  • The President-Elect has also written a letter to the science advisor, outlining key questions that he wants to be considered.  
  • There is talk of a "Science New Deal", unsurprisingly directed a lot toward the pandemic, climate change, and American technological competitiveness.
  • The webcomic SMBC has decided to address controversy head on, reporting "Congressman Johnson comes out against Pauli Exclusion."  This would have rather negative unintended consequences, like destabilizing all matter more complex than elementary particles....
  • This session promises to be an interesting one at the March APS meeting, as it goes right to the heart of how difficult it is to distinguish Majorana fermion signatures in superconductor/semiconductor hybrid structures from spurious electrical features.  I may try to write more about this soon.
  • This paper (arxiv version) is very striking.  Looking in the middle of a sheet of WTe2 (that is, away from where the topological edge states live), the authors see quantum oscillations of the resistance as a function of magnetic field that look a lot like Landau quantization, even though the bulk of the material is (at zero field) quite insulating.  I need to think more carefully about the claim that this argues in favor of some kind of emergent neutral fermions.
  • Being on twitter for four months has made me realize how reality-warping that medium is.  Reading about science on twitter can be incredibly wearing - it feels like seemingly everyone else out there is publishing in glossy journals, winning major prizes, and landing huge grants.  This is, of course, a selection effect, but I don't think it's healthy.
  • I do think twitter has driven blog traffic up a bit, but I actually wonder if occasionally posting blog links to /r/physics on reddit would be far more effective in terms of outreach.  When one of my posts ends up there, it gets literally 50x the page views than normal.  Still, I have an old-internet-user aversion to astroturfing.