Sunday, October 17, 2021

Brief items - Sarachik, Feynman, NSF postdocs and more

 Here are several items of interest:

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

Sunday, October 10, 2021

The Purcell effect - still mind-blowing.

The Purcell effect is named after E. M. Purcell, a Nobel-winning physicist who also was a tremendous communicator, author of one of the great undergraduate textbooks and a famous lecture about the physical world from the point of view of, e.g., a bacterium.  I've written about this before here, and in a comment I include the complete (otherwise paywalled) text of the remarkable original "paper" that describes the effect.

When we calculate things like the Planck black-body spectrum, we use the "density of states" for photons - for a volume \(V\), we are able to count up how many electromagnetic modes are available with frequency between \(\nu\) and \(\nu + \mathrm{d}\nu\), keeping in mind that for each frequency, the electric field can be polarized in two orthogonal directions.  The result is \( (8\pi/c^3)\nu^2 \mathrm{d}\nu\) states per unit volume of "free space".

In a cavity, though, the situation is different - instead, there is, roughly speaking, one electromagnetic mode per the bandwidth of the cavity per the volume of the cavity.  In other words, the effective density of states for photons in the cavity is different than that in free space.  That has enormous ramifications:  The rates of radiative processes, even those that we like to consider as fundamental, like the rate at which electrically excited atoms radiatively decay to lower states state, can be altered in a cavity.  This is the basis for a lot of quantum optics work, as in cavity quantum electrodynamics.  Similarly, the presence of an altered (from free space) photon density of states also modifies the spectrum of thermal radiation from that cavity away from the Planck black-body spectrum.  

Consider an excited atom in the middle of such a cavity.  When it is going to emit a photon, how does it "know" that it's in a cavity rather than in free space, especially if the cavity is much larger than an atom?  The answer is, somehow through the electromagnetic couplings to the atoms that make up the cavity.  This is remarkable, at least to me.   (It's rather analogous to how we picture the Casimir effect, where you can think about the same physics either, e.g., as due to altering local vacuum fluctuations of the EM field in the space between conducting plates, or as due to fluctuating dipolar forces because of fluctuating polarizations on the plates.)

Any description of a cavity (or plasmonic structure) altering the local photon density of states is therefore really short-hand.  In that approximation, any radiative process in question is tacitly assuming that an emitter or absorber in there is being influenced by the surrounding material.  We just are fortunate that we can lump such complicated, relativistically retarded interactions into an effective photon density of states that differs from that in free space. 

Tuesday, October 05, 2021

Spin glasses and the Nobel

The Nobel Prize in physics this year was a bit of a surprise, at least to me.  As one friend described it, it's a bit of a Frankenprize, stitched together out of rather disparate components.  (Apologies for the slow post - work was very busy today.)  As always, it's interesting to read the more in-depth scientific background of the prize.  I was unfamiliar with the climate modeling of Manabe and Hasselmann, and this was a nice intro.

The other prize recipient was Giorgio Parisi, a statistical mechanician whose key cited contribution was in the theory of spin glasses, but was generalizable to many disordered systems with slow, many-timescale dynamics including things like polymers and neural networks.  

The key actors in a spin glass are excess spins - local magnetic moments that you can picture as little magnetic dipoles. In a generic spin glass, there is both disorder (as shown in the upper panel of the cartoon, spins - in this case iron atoms doped into copper - are at random locations, and that leads to a broad distribution of spin-spin interactions in magnitude and sign) and frustration (interactions such that flipping spin A to lower its interaction energy with spin B ends up raising the interaction energy with spin C, so that there is no simple configuration of spins that gives a global minimum of the interaction energy).  One consequence of this is a very complicated energy landscape, as shown in the lower panel of the cartoon.  There can be a very large number of configurations that all have about the same total energy, and flipping between these configurations can require a lot of energy such that it is suppressed at low temperatures.  These magnetic systems then end up having slow, "glassy" dynamics with long, non-exponential relaxations, in the same way that structural glasses (e.g., SiO2 glass) can get hung up in geometric configurations that are not the global energetic minimum (crystalline quartz, in the SiO2 case).  

The standard tools of statistical physics are difficult to apply to the glassy situation.  A key assumption of equilibrium thermodynamics is that, for a given total energy, a system is equally likely to be found in any microscopic configuration that has that total energy.  Being able to cycle through all those configurations is called ergodicity.  In a spin glass at low temperatures, the potential landscape means that the system can get easily hung up in a local energy minimum, becoming non-ergodic.  

An approach that Parisi took to this problem involved "replicas", where one considers the whole system as an ensemble of replica systems, and a key measure of what's going on is the similarity of configurations between the replicas.  Parisi himself summarizes this in this pretty readable (for physicists) article.  One of Parisi's big contributions was showing that the Ising spin glass model of Sherrington and Kirkpatrick is exactly solvable.

I learned about spin glasses as a doctoral student, since the interacting two-level systems in structural glasses at milliKelvin temperatures act a lot like a spin glass (TLS coupled to each other via a dipolar elastic interaction, and sometimes an electric dipolar interaction), complete with slow relaxations, differences between field-cooled and zero-field-cooled properties, etc.  

Parisi has made contributions across many diverse areas of physics.  Connecting his work to that of the climate modelers is a bit of a stretch thematically - sure, they all worry about dynamics of complex systems, but that's a really broad umbrella.  Still, it's nice to see recognition for the incredibly challenging problem of strongly disordered systems.

Sunday, October 03, 2021

Annual Nobel speculation thread

Once more, the annual tradition:  Who do people think will win the Nobel this year in physics or chemistry?  I have repeately and incorrectly suggested Aharonov and Berry for geometric phases.  There is a lot of speculation on social media about AspectZeilinger, and Clauser for Bell's inequality tests.  Social media speculation has included quantum cascade lasers as well as photonic bandgap/metamaterials. Other suggestions I've seen online have included superconducting qubits (with various combinations of people) and twisted bilayer graphene, though both of those may be a bit early.  


Tuesday, September 28, 2021

Science/tech consulting in creative arts

I've watched the first two episodes of the new adaptation of Foundation.  It surely looks gorgeous, though there are some script challenges (even apart from the challenge of trying to adapt an enormous book series that was always long on ideas and short on character development).   The issues I've spotted seem mostly to be ones of poor script editing for consistency.  (The emperor of the Galactic Empire says in the first episode that the imperial population is 8 trillion, and then in the second episode a character says that the core worlds alone have a population of 40 trillion.  The latter number is more reasonable, given the size of Asimov's empire.)  

Watching this, I again think it would be great fun to do scientific/technical consulting for TV, movies, and even books. I'm on the list for the Science and Entertainment Exchange, though all I've ever done is give a tiny bit of feedback to a would-be author.  (My expertise probably looks too narrow, and not living in southern California seems to be a major filter.)  

It feels like there are some similarities to the role of science in public policy.  In the creative productions, science can contribute (and these media can be a great way of getting scientific ideas out into the public), but in the end plot and what can practically be implemented will always drive the final product.  In policy, science and technical knowledge should definitely factor in when relevant, but fundamentally there are social and political factors that can overwhelm those influences in decision-making.  Now back to our regularly scheduled psychohistorical crisis....

Wednesday, September 22, 2021

DOE Experimental Condensed Matter PI meeting, Day 3

Here are some tidbits from the last day of this year's meeting.  (I didn't really get to see the other posters in my own poster session, so apologies for missing those.  For the curious:  the meeting attendees alternate between posters and 15 minute talks from year to year.)

  • It's been known for a while that combining magnetism with topological insulator materials can lead to a rich phase diagram.  Tuning composition is a powerful tool.  Likewise, the van der Waals nature of these systems mean that it's possible to look systematically through a family of related materials.
  • Tuning composition in flat-band kagome metals is also of interest.
  • I had not appreciated just how important specific crystal growth approaches (e.g., rapid quenching vs. slow annealed cooling) are to the properties of some magnetic/topological materials, such as Fe5GeTe2.  
  • Strain can be a powerful tool for tuning electronic topology in some materials such as ZrTe5, and driving certain phonon modes via laser offers the potential of controlled switching of topological properties.
  • Quantum oscillations (e.g., magnetization as a function of 1/H) are a conventional way to learn about Fermi surfaces, and it is always bizarre when that kind of response shows up in a correlated material that is nominally an insulator, or in thermal transport but not electrical transport.
  • Speaking of quantum oscillations in insulators, how about thermal transport in the spin liquid phase of \(\alpha\)-RuCl3?  Looks like some kind of bosonic edge mode is responsible.
  • If transition metal dichalcogenides are starting to bore you, perhaps you'd be more interested in trichalcogenides, which can be grown as individual 1D chains within carbon and boron nitride nanotubes.
Thanks to everyone for making the meeting enjoyable and informative, even if we couldn't get together in a random Marriott in Maryland this time.  

Tuesday, September 21, 2021

DOE Experimental Condensed Matter PI meeting, Day 2

 More highlights from the meeting.  Office hours for my class conflicted with a couple of the talks, so these are briefer than I would've liked.

  • It is possible to use coherent x-ray scattering to look at time variations in the domain structure of an antiferromagnet.  In the magnetic diffraction pattern there is speckle near the magnetic Bragg spots that bops around as the domain structure fluctuates.
  • Amorphous magnetic alloys can show some really wild spin textures.  
  • By growing a few nanometers of a paramagnetic metal, Bi2Ir2O7, on top of an insulating spin ice, Dy2Ti2O7, it's possible to get enough coupling that field-driven spin ice transitions can generate magnetoresistance signatures in the metal layer.
  • Square planar nickelates can look a lot like the copper oxide superconductors in terms of band dispersion and possible "strange" metallicity.
  • Some rare-earth intermetallic compounds can have an impressively rich magnetic phase diagram.
  • I learned that some pyrochlore iridates can exhibit a kind of topological metallic state with giant anomalous Hall response.
  • I had not previously appreciated how wild it is that one can engineer ferroelectric response in stacks of 2D materials that are not intrinsically ferroelectric, as in hBN or even WSe2.
  • Ultrasound attenuation can be a heck of a tool for looking at superconductivity and other electronic transport properties.
  • Strontium titanate remains a really interesting test case for understanding exotic superconductivity, with its superconducting dome as a function of doping, very low carrier density, and incipient ferroelectricity.  Phonons + paraelectric fluctuations + spin-orbit coupling appear to be the big players
  • A related system in the sense of near-ferroelectricity and low carrier density is at interfaces of KTaO3.
  • This experiment in graphene/hBN/graphene stacks (with encapsulating hBN and graphite top and bottom gates) is an extremely pretty, tunable demonstration of superfluidity of bilayer excitons, an effect previously seen in one limit in GaAs systems.

Monday, September 20, 2021

DOE Experimental Condensed Matter PI meeting, Day 1

Somehow I found the first day of the virtual meeting more exhausting than when we do these things in person, probably because I had to go teach in the middle of the event.  A sampling of highlights:

  • Rather than relying on relatively crude methods to create defect centers in diamond for quantum sensing (or qubit purposes), one can use chemistry to build transition metal complexes with designer ligand fields (and hence energy level structures), as demonstrated here.
  • I know understand better why it has historically been so difficult to demonstrate, experimentally, that the quasiparticles in the fractional quantum Hall effect obey fractional (anyonic) statistics.  In an interferometer, it's critical to use screening (from top and bottom electron gases that act like capacitor plates) to reduce Coulomb interactions between edge states and the bulk. Once that's done, you can see clear evidence of fractional (anyonic) phase slips.
  • Some truly exceptional people can still do research even while being a university president.  At very low energies in an Ising ferromagnet with an in-plane magnetic field, hyperfine interactions can lead to hybridization of magnetic levels and the formation of "electronuclear" spin excitations.
  • Ultraclean ABC-stacked graphene trilayers can show remarkably rich response, dominated by strong electron-electron interaction effects.
  • High quality crystal growth can drastically lower the defect densities in transition metal dichalcogenides.  That makes it possible to construct bilayers of WSe2, for example, that can host apparent excitonic condensates.  Similar physics can be seen in MoSe2/WSe2 bilayers, where it is clear that exciton-exciton interactions can be very strong.
  • Pulling and pushing on a sample can lead to elastocaloric effects (like when a rubber band cools upon being stretched), and these can reveal otherwise hidden properties and phase transitions.
More tomorrow.  (One fun idea from a colleague:  Perhaps the program officers have hidden a secret easter egg token somewhere in the Gather virtual poster area, and whoever finds it gets a bonus award supplement to support a summer undergrad....)

DOE Experimental Condensed Matter Physics PI meeting, 2021

Every two years, the US Department of Energy Experimental Condensed Matter Physics program has a principal investigator meeting, and I've written up highlights of these for a while (for 2019, see a, b, c;  2017, see abc; for 2015 see abc; for 2013 see ab).

The meetings have always been very educational for me.  They're a chance to get a sense of the breadth of the whole program and the research portfolio.  It is unfortunate that the covid pandemic has forced this year's meeting to be virtual.  I'll do my best to summarize some tidbits in posts over the next three days.

Friday, September 17, 2021

Moiré materials and the Mott transition

There are back-to-back papers in Nature this week, one out of Columbia and one out of Cornell, using bilayers of transition metal dichalcogenides to examine the Mott transition.  (Sorry for the brevity - I'm pressed for time right now, but I wanted to write something....)

As I described ages ago in here, imagine a lattice of sites, each containing one electron.  While quantum statistics would allow each site to be doubly occupied (thanks to spin), if the on-site electron-electron repulsion \(U\) is sufficiently strong (large compared to the kinetic energy scale \(t\) associated with hopping between neighboring sites), then the interacting system will be an insulator even though the non-interacting version would be a metal.  Moving away from this half-filling condition, you can get conduction, just as having an empty site allows those sliding tile puzzles to work.

As discussed here, in bilayers of 2D materials can lead to the formation of a moiré lattice, where the interlayer interactions result in an effective periodic array of potential wells.  The Columbia folks got a moiré pattern by using a 4-5 degree twisted bilayer of WSe2, while the Cornell folks instead used an aligned bilayer of MoTe2 and WSe2 (where the moiré comes from the differing lattice constants).  In both cases, you end up with a triangular moiré lattice (encapsulated in hBN to provide a clean charge environment and protection from the air).  

The investigators are able to tune the systems in multiple ways.  With overall gate voltage, they can capacitively tune the "filling", the ratio of number of "free" charges to number of moiré lattice sites.  By adjusting top gate vs. back gate, they can tune the vertical electric field across the bilayer, and that is a way of tuning interactions by pushing around localized wavefunctions for the lattice sites.  

Both groups find that they can tune in/out of a Mott insulating phase when they're at one carrier per moiré lattice site.  Interestingly, both groups see that the Mott transition is continuous (second-order) - there is no sudden onset of insulating response as a function of tuning either knob.  Instead, there appears to be quantum critical scaling, and regions of linear-in-\(T\) temperature dependence of the resistivity (a possible indicator of a strange metal) on either side of the insulating region.  The Cornell folks are able to do magnetic circular dichroism measurements to confirm that the transition does not involve obvious magnetic ordering. 

This is very pretty work, and it shows the promise of the moiré lattice approach for studying fundamental issues (like whether or not the Mott transition in a triangular lattice is continuous).  I'm sure that there will be much more to come in these and related systems.

Monday, September 06, 2021

What is the spin Hall effect?

The Hall Effect is an old (1879) story, told in first-year undergraduate physics classes for decades. Once students are told about the Lorentz force law, it's easy to make a handwave classical argument that something like the Hall Effect has to exist:  Drive a current in a conductor in the presence of a magnetic induction \(\mathbf{B}\).  Charged particles undergo a \(q \mathbf{v} \times \mathbf{B}\) force that pushes them transverse to their original \(\mathbf{v}\) direction.  In a finite slab of material with current perpendicular to \(\mathbf{B}\), the particles have to pile up at the transverse edge, leading to the development of a (Hall) voltage perpendicular to the direction of current flow and the magnetic induction.  You can measure the Hall voltage readily, and it's used for sensing magnetic fields, as well as figuring out charge carrier densities in materials.

The spin Hall effect, in contrast, is a much newer idea.  It was first proposed by Dyakonov and Perel in 1971 as an extrinsic effect (that is, induced by scattering from impurities in a material), and this was revisited in 1999 by Hirsch and others.  It's also possible to have an intrinsic spin Hall effect (proposed here and here) due just to the electronic structure of a material itself, not involving impurities.

Adapted from here.

So what is the SHE?  In some non-magnetic conductors, in the absence of any external magnetic field, a charge current (say in the \(+x\) direction) results in a build-up of electrons with spin polarized up (down) along the \(z\) direction along the positive (negative) \(y\) edge of the material, as shown in the bottom left drawing of the figure.  Note that there is no net charge imbalance or transverse voltage - just a net spin imbalance. 

The SHE is a result of spin-orbit coupling - it's fundamentally a relativistic effect (!).  While we static observers see only electric fields in the material, the moving charge carriers in their frame of reference see effective magnetic fields, and that affects carrier motion.  In the extrinsic SHE, scattering of carriers from impurities ends up having a systematic spin dependence, so that spin-up carriers are preferentially scattered one way and spin-down carriers are scattered the other.  In the intrinsic SHE, there ends up being a spin-dependent term in the semiclassical velocity that one would get from the band structure, because of spin-orbit effects.  (The anomalous Hall effect, when one observes a Hall voltage correlated with the magnetization of a magnetic conductor, is closely related.  The net charge imbalance shows up because the populations of different spins are not equal in a ferromagnet.)  The result is a spin current density \(\mathbf{J}_{\mathrm{s}}\) that is perpendicular to the charge current density \(\mathbf{J}_{\mathrm{c}}\), and is characterized by a (material-dependent) spin Hall angle, \(\theta_{\mathrm{SH}}\), so that \(J_{\mathrm{s}} = (\hbar/2e)\theta_{\mathrm{SH}}J_{\mathrm{c}}\).

There is also an inverse SHE:  if (appropriately oriented) spin polarized charge carriers are injected into a strong spin-orbit coupled non-magnetic metal (say along \(+x\) as in the bottom right panel of the figure), the result is a transverse (\(y\)-directed) charge current and transverse voltage build-up.  (It's this inverse SHE that is used to detect spin currents in spin Seebeck effect experiments.)

The SHE and ISHE have attracted a lot of interest for technological applications.  Generating a spin current via the SHE and using that to push around the magnetization of some magnetic material is called spin orbit torque, and here is a recent review discussing device ideas.

Wednesday, September 01, 2021

Rice University physics faculty search in experimental quantum science and technology

The Department of Physics and Astronomy at Rice University invites applications for tenure-track faculty positions in the broad area of experimental quantum science and technology. This encompasses quantum information processing, quantum sensing, quantum communication, quantum opto-mechanics, and quantum simulation in photonic, atomic/ionic, quantum-material, and other solid-state platforms. We seek outstanding scientists whose research will complement and extend existing activities in these areas within the Department and across the University. In addition to developing an independent and vigorous research program, the successful applicants will be expected to teach, on average, one undergraduate or graduate course each semester, and contribute to the service missions of the Department and University. The Department anticipates making appointments at the assistant professor level. A Ph.D. in physics or related field is required.

Beginning September 1, 2021, applications for this position must be submitted electronically at .

Applications for this position must be submitted electronically. Applicants will be required to submit the following: (1) cover letter; (2) curriculum vitae; (3) statement of research; (4) statement on teaching; (5) statement on diversity, mentoring, and outreach; (6) PDF copies of up to three publications; and (7) the names, affiliations, and email addresses of three professional references. Rice University, and the Department of Physics and Astronomy, are strongly 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.We will begin reviewing applications November 15, 2021. To receive full consideration, all application materials must be received by January 1, 2022. The expected appointment date is July, 2022.  

Rice University is an Equal Opportunity Employer with 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.

Saturday, August 28, 2021

What is the spin Seebeck effect?

Thermoelectricity is an old story, and I've also discussed it here.  Take a length of some conductor, and hold one end of that conductor at temperature \(T_{\mathrm{hot}}\), and hold the other end of that conductor at temperature \(T_{\mathrm{cold}}\).  The charge carriers in the conductor will tend to diffuse from the hot end toward the cold end.  However, if the conductor is electrically isolated, that can't continue, and a voltage will build up between the ends of the conductor, so that in the steady state there is no net flow of charge.  The ratio of the voltage to the temperature difference is given by \(S\), the Seebeck coefficient.  

It turns out that spin, the angular momentum carried by electrons, can also lead to the generation of voltages in the presence of temperature differences, even when the material is an insulator and the electrons don't move.  

Let me describe an experiment for you.  Two parallel platinum wires are patterned next to each other on the surface of an insulator.  An oscillating current at angular frequency \(\omega\) is run through wire A,  while wire B is attached to a voltage amplifier feeding into a lock-in amplifier.  From everything we teach in first-year undergrad physics, you might expect some signal on the lock-in at frequency \(\omega\) because the two wires are capacitively coupled to each other - the oscillating voltage on wire A leads to the electrons on wire B moving back and forth because they are influenced by the electric field from wire A.  You would not expect any kind of signal on wire B at frequency \(2 \omega\), though, at least not if the insulator is ideal.

However, if that insulator is magnetically interesting (e.g., a ferrimagnet, an antiferromagnet, some kinds of paramagnet), it is possible to see a \(2 \omega\) signal on wire B.  

In the spin Seebeck effect, a temperature gradient leads to a build-up of a net spin density across the magnetic insulator.  This is analogous to the conventional Seebeck effect - in a magnetically ordered system, there is a flow of magnons from the hot side to the cold side, transporting angular momentum along.  This builds up a net spin polarization of the electrons in the magnetic insulator.  Those electrons can undergo exchange processes with the electrons in the platinum wire B, and if the spins are properly oriented, this causes a voltage to build up across wire B due to the inverse spin Hall effect.  

So, in the would-be experiment, the ac current in wire A generates a temperature gradient between wire A and wire B that oscillates at frequency \(2 \omega\).  An external magnetic field is used to orient the spins in the magnetic insulator, and if the transported angular momentum points the right direction, there is a \(2 \omega \) voltage signal on wire B.   

I think this is pretty neat - an effect that is purely due to the quantum properties of electrons and would just not exist in the classical electricity and magnetism that we teach in intro undergrad courses.

(On writing this, I realized that I've never written a post defining the spin Hall and related effects. I'll have to work on that....  Sorry for the long delay between postings.  The beginning of the semester has been unusually demanding of my time.)

Thursday, August 12, 2021

More amazingly good harmonic oscillators

 Harmonic oscillators are key elements of the physicist's toolkit for modeling the world.  Back at the end of March I wrote about some recent results using silicon nitride membranes to make incredibly high quality (which is to say, low damping) harmonic oscillators.  (Remember, the ideal harmonic oscillator that gets introduced in undergrad intro physics is a mass on a spring, with no friction or dissipation at all.  An ideal oscillator would have a \(Q\) factor that is infinite, and it would keep ringing forever once started.) This past week, two papers appeared on the arxiv showing that it's possible to design networks of (again) silicon nitride beams that have resonances at room temperature (in vacuum) with \(Q > 10^{9}\).  

(a) A perimeter mode of oscillation. (b) a false-
color electron micrograph of such a device.
One of these papers takes a specific motif, a suspended polygon made from beams, supported by anchoring beams coming from its vertices, as shown in the figure.  The resonant modes with the really high \(Q\) factors are modes of the perimeter, with nodes at the vertices.  This minimizes "clamping losses", damping that occurs at anchoring points (where the strain tends to be large, and where phonons can leak vibrational energy out of the resonator and into whatever is holding it).  

The other paper gets to a very similar design, through a process that combines biological inspiration (spiderwebs), physics insight, and machine learning/optimization to really maximize \(Q\).  

With tools like this, it's possible to do quantum mechanics experiments  (that is, mechanics experiments where quantum effects are dominant) at or near room temperature with these.  Amazing.

Monday, August 09, 2021

Brief items

 It's been a busy week, so my apologies for the brevity, but here are a couple of interesting papers and sites that I stumbled upon:

  • Back when I first started teaching about nanoscience, I said that you'd really know that semiconductor quantum dots had hit the big time when you occasionally saw tanker trucks full of them going down the highway.  I think we're basically there.  Here is a great review article that summarizes the present state of the art.
  • Reaching back a month, I thought that this is an impressive piece of work.  They combine scanning tunneling microscopy, photoluminescence with a tunable optical source, and having the molecule sitting on a layer of NaCl to isolate it from the electronic continuum of the substrate.  The result is amazingly (to me) sharp spectral features in the emission, spatially resolved to the atomic scale.
  • The emergence of python and the ability to embed it in web pages through notebooks has transformative educational potential, but it definitely requires a serious investment of time and effort.  Here is a fluid dynamics course from eight years ago that I found the other day - hey, it was new to me.
  • For a more up-to-the-minute example, here is a new course about topology and condensed matter.  Now if I only had time to go through this.  The impending start of the new semester. 
  • This preprint is also an important one.  There have been some major reports in the literature about quantum oscillations (e.g., resistivity or magnetization vs. magnetic field ) being observed in insulators.  This paper shows that one must be very careful, since the use of graphite gates can lead to a confounding effect that comes from those gates rather than the material under examination.
  • This PNAS paper is a neat one.  It can be hard to grow epitaxial films of some "stubborn" materials, ones involving refractory metals (high melting points, very low vapor pressures, often vulnerable to oxidation).  This paper shows that instead one can use solid forms of precursor compounds containing those metals.  The compounds sublime with reasonably high vapor pressures, and if one can work out their decomposition properly, it's possible to grow nice films and multilayers of otherwise tough materials.  (I'd need to be convinced that the purity achieved from this comparatively low temperature approach is really good.)

Monday, August 02, 2021

Metallic water!

What does it take to have a material behave as a metal, from the physicist's perspective?  I've written about this before (wow, I've been blogging for a long time).  Fundamentally, there have to be "gapless" charge-carrying excitations, so that the application of even a tiny electric field allows those charge carriers to transition into states with (barely) higher kinetic energies and momenta.  

Top: a droplet of NaK 
alloy.  Bottom: That 
droplet coated with 
adsorbed water that 
has become a metal. 
From here.
In conventional band insulators, the electronic states are filled right up to the brim in an energy band.  Apply an electric field, and an electron has no states available into which it can go without somehow grabbing enough energy to make it all the way to the bottom of the next (conduction) band.  Since that band gap can be large (5.5 eV for diamond, 8.5 eV for NaCl), no current flows, and you have an insulator.

This is, broadly speaking, the situation in liquid water. (Even though it's a liquid, the basic concept of bands of energy levels is still helpful, though of course there are no Bloch waves as in crystalline solids.)  According to calculations and experiments, the band gap in ordinary water is about 7 eV.  You can dissolve ions in water and have those carry a current - that's the whole deal with electrolytes - but ordinarily water is not a conductor based on electrons.  It is possible to inject some electrons into water, and these end up "hydrated" or "solvated" thanks to interactions with the surrounding polar water molecules and the hydronium and hydroxyl ions floating around, but historically this does not result in a metal.  To achieve metallicity, you'd have to inject or borrow so many electrons that they could get up into that next band.

This paper from late last week seems to have done just that.  A few molecular layers of water adsorbed on the outside of a droplet of liquid sodium-potassium metal apparently ends up taking in enough electrons (\( \sim 5 \times 10^{21}\) per cc) to become metallic, as detected through optical measurements of its conductivity (including a plasmon resonance).   It's rather transient, since chemistry continues and the whole thing oxidizes, but the result is quite neat!

Friday, July 30, 2021

Workshop highlights: Spins, 1D topo materials from carbon, and more

 While virtual meetings can be draining (no breaks to go hiking; no grabbing a beer and catching up, especially when attendees are spread out across a 7 timezones), this workshop was a great way for me to catch up on some science that I'd been missing.  I can't write up everything (mea culpa), but here are a few experimental highlights:

  • Richard Berndt's group has again shown that shot noise integrated with STM is powerful, and they have used tunneling noise measurements to probe where and how spin-polarized transport happens through single radical-containing molecules on gold surfaces.
  • Katharina Franke's group has looked at what happens when you have a localized spin on the surface of a superconductor.  Exchange coupling can rip apart Cooper pairs and bind a quasiparticle in what are called Yu-Shiba-Rusinov states.  With STM, it is possible to map these and related phenomena spatially, and the states can also be tuned via tip height, leading to very pretty data.
  • Amazing polymers from here.
    Pavel Jelinek gave a talk with some really eye-popping images as well as cool science.  I had not realized before that in 1D conjugated systems (think polyacetylene) it is possible to see a topological transition as a function of length, between a conjugated state (with valence-band-like orbitals filled, and conduction-band-like orbitals empty) and another conjugated state that has an unpaired electron localized at each end (equivalent to surface states) with effectively band inversion (empty valence-band-like states above filled conduction-band-like states) in the middle.  You can actually make polymers (shown here) that show these properties and image the end states via STM.  
  • Latha Venkataraman spoke about intellectually related work.  Ordinarily, even with a conjugated oligomer, conductance falls exponentially with increasing molecular length.   However, under the right circumstances, you can get the equivalent topological transition, creating resonant states localized at the molecular ends, and over some range of lengths, you can get electronic conduction increasing with increasing molecular length.  As the molecule gets longer the resonances become better defined and stronger, though at even larger lengths the two end states decouple from each other and conductance falls again.
  • Jascha Repp did a really nice job laying out their technique that is AFM with single-charge-tunneling to give STM-like information for molecules on insulating substrates.  Voltage pulses are applied in sync with the oscillating tip moving into close proximity with the molecule, such that single charges can be added or removed each cycle.  This is detected through shifts in the mechanical resonance of the AFM cantilever due to the electrostatic interactions between the tip and the molecule.  This enables time-resolved measurements as well, to look at things like excited state lifetimes in individual molecules.
The meeting is wrapping up today, and the discussions have been a lot of fun.  Hopefully we will get together in person soon!

Monday, July 26, 2021

2021 Telluride workshop on Quantum Transport in Nanoscale Systems

This week I'm (virtually) attending this workshop, which unfortunately is zoom-based because of the ongoing pandemic and travel restrictions.  As I've mentioned in previous years, it's rather like a Gordon Conference, in that it's supposed to be a smaller meeting with a decent amount of pre-publication work.   I'll write up some highlights later, but for now I wanted to feature this image.  At left is the molecular structure of [5]triangulene, which can be assembled by synthetic chemistry methods and surface catalysis (in this case on Au(111) surface).  At right is an AFM image taken using a tip functionalized by a carbon monoxide molecule.  In case you ever doubted, those cartoons from chemistry class are sometimes accurate!

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 ).  Evaluation of applications will begin May 15 and continue until the position is filled. Applications for this position must be submitted electronically at  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.