Sunday, November 28, 2021

LEDs - condensed matter/nanostructures having real impact

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

Image from here.

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

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

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

Sunday, November 21, 2021

Hanle magnetoresistance - always more to learn....

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

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

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

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

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

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

Hanle magnetoresistance idea, 
adapted from here.

This correction is called the Hanle magnetoresistance.  

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

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

Saturday, November 13, 2021

The community of department chairs

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

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

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

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

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

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

Saturday, November 06, 2021

The noise is the signal

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

Adapted from Fig. 1 of this paper.
The experiment is quite pretty, looking at disordered thin films of TiN, with a macroscopic superconducting transition temperature of \(T_{c} =\) 2.95 K.  With the shot noise measurement, the experimenters see enhanced noise at low applied voltages consistent with pairing (with pairs being transported presumably by the process of Andreev reflection).  The interesting point is that this enhanced noise persists up to temperatures as high as 2.7 times \(T_{c}\), despite the fact that the tunneling conductance \(dI/dV\) shows no sign of a gap or pseudogap up there.  This implies that superconductivity in this material dies as \(T\) exceeds \(T_{c}\) not because the pairing between electrons falls apart, but instead because of the loss of the global coherence needed for the superconducting state.  That's an exciting result.  

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

Sunday, October 24, 2021

The physics of ornithopters

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

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

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

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

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.