Friday, June 07, 2019

Round-up of various links

I'll be writing more soon, but in the meantime, some items of interest:

  • A cute online drawing utility for making diagrams and flowcharts is available free at
  • There is more activity afoot regarding the report of possible Au/Ag superconductivity.  For example, Jeremy Levy has a youtube video about this topic, and I think it's very good - I agree strongly with the concerns about heterogeneity and percolation. The IIS group also has another preprint on the arxiv, this one looking at I-V curves and hysteresis in these Au/Ag nanoparticle films.  Based on my prior experience with various "resistive switching" systems and nanoparticle films, hysteretic current-voltage characteristics don't surprise me when biases on the scale of volts and currents on the scale of mA are applied to aggregated nanoparticles.  
  • Another group finds weird effects in sputtered Au/Ag films, and these have similar properties as those discussed by Prof. Levy.  
  • Another group finds apparent resistive superconducting transitions in Au films ion-implanted with Ag, with a transition temperature of around 2 K.  This data look clean and consistent - it would be interesting to see Meissner effect measurements here.  
  • For reference, it's worth noting that low temperature superconductivity in Au alloys is not particularly rare (pdf here from 1984, for example, or this more recent preprint).   
  • On a completely different note, I really thought this paper on the physics of suction cups was very cute.
  • Following up, Science had another article this week about graduate programs dropping the GRE requirement.
  • This is a very fun video using ball bearings to teach about crystals - just like with drought balls, we see that aspects of crystallinity like emergent broken symmetries and grain boundaries are very generic.

Saturday, May 25, 2019

Brief items

A number of interesting items:

Thursday, May 23, 2019


I review quite a few papers - not Millie Dresselhaus level, but a good number.  Lately, some of the electronic review systems (e.g.,, which is a front end for "Scholar One", a product of Clarivate) have been asking me if I want to "receive publons" in exchange for my reviewing activity. 

What are publons?  Following the wikipedia link above is a bit informative, but doesn't agree much with my impressions (which, of course, might be wrong).   My sense is that the original idea here was to have some way of recording and quantifying how much effort scientists were putting into the peer review process.  Reviewing and editorial activity would give you credit in the form of publons, and that kind of information could be used when evaluating people for promotion or hiring.   (I'm picturing some situation where a certain number of publons entitles you to a set of steak knives (nsfw language warning).)

The original idea now seems to have been taken over by Clarivate, who are the people that run Web of Science (the modern version of the science citation index) and produce bibliographic software that continually wants to be upgraded.  Instead of just a way of doing accounting of reviewing activity, it looks like they're trying to turn publons into some sort of hybrid analytics/research social network platform, like researchgate.  It feels like Clarivate is trying to (big surprise here in the modern age of social media) have users allow a bunch of data collection, which Clarivate will then find a way to monetize.  They are also getting into the "unique researcher identifier" game, apparently in duplication of or competition with orcid.

Maybe it's a sign of my advancing years, but my cynicism about this is pretty high.  Anyone have further insights into this?

Sunday, May 19, 2019

Magnets and energy machines - everything old is new again.

(Very) long-time readers of this blog will remember waaaay back in 2006-2007, when an Irish company called Steorn claimed that they had invented a machine, based on rotation and various permanent magnets, that allegedly produced more energy than it consumed.  I wrote about this herehere (complete with comments from Steorn's founder), and here.  Long story short:  The laws of thermodynamics were apparently safe, and Steorn is long gone.

This past Friday, the Wall Street Journal published this article (sorry about the pay wall - I couldn't find a non-subscriber link that worked), describing how Dennis Danzik, science and technology officer for Inductance Energy Corp, claims to have built a gizmo called the Earth Engine.  This gadget is a big rotary machine that claims it spins two 900 kg flywheels at 125 rpm (for the slow version), and generates 240V at up to 100A, with no fuel, no emissions, and practically no noise.  They claim to have run one of these in January for 422 hours generating an average 4.4 kW.  If you want, you can watch a live-stream of a version made largely out of clear plastic, designed to show that there are no hidden tricks. 

To the credit of Dan Neil, the Pulitzer-winning WSJ writer, he does state, repeatedly, in the article that physicists think this can't be right.  He includes a great quote from Don Lincoln:  "Perpetual motion machines are bunk, and magnets are the refuge of charlatans." 

Not content with just violating the law of conservation of energy, the claimed explanation relies on a weird claim that seemingly would imply a non-zero divergence of \(\mathbf{B}]) and therefore magnetic monopoles:  "The magnets IEC uses are also highly one-sided, or 'anisotropic,' which means that their field is stronger on one face than the other - say 85% North and 15% South". 

I wouldn't rush out and invest in these folks just yet.

Friday, May 17, 2019

Light emission from metal nanostructures

There are many ways to generate light from an electrically driven metal nanostructure.  

The simplest situation is just what happens in an old-school incandescent light bulb, or the heating element in a toaster.  An applied voltage \(V\) drives a current \(I\) in a wire, and as we learn in freshman physics, power \(IV\) is dissipated in the metal - energy is transferred into the electrons (spreading them out up to higher energy levels within the metal than in the undriven situation, with energy transfer between the electrons due to electron-electron interactions) and the disorganized vibrational jiggling of the atoms (as the electrons also couple to lattice vibrations, the phonons).  The scattering electrons and jiggling ions emit light (even classically, that's what accelerating charges do).  If we look on time scales and distance scales long compared to the various e-e and e-lattice scattering processes, we can describe the vibrations and electron populations as having some local temperature.  Light is just electromagnetic waves.  Light in thermal equilibrium with a system (on average, no net energy transfer between the light and the system) is distributed in a particular way generically called a black body spectrum.  The short version:  current heat metal structures, and hot structures glow.  My own group found an example of this with very short platinum wires.  

In nanostructures, things can get more complicated.  Metal nanostructures can support collective electronic modes called plasmons.  Plasmons can "decay" in different ways, including emitting photons (just like an atom in an excited state can emit a photon and end up in the ground state, if appropriate rules are followed).  It was found more than 40 years ago that a metal/insulator/metal tunnel junction can emit light when driven electrically.  The idea is, a tunneling electron picks up energy \(eV\) when going from one side of the junction to the other.   Some fraction of tunneling electrons deposit that energy into plasmon modes, and some of those plasmon modes decay radiatively, spitting out light with energy \(\hbar \omega \le eV\).

This same thing can happen in scanning tunneling microscopy.  There is a "tip mode" plasmon where the STM tip is above the conducting sample, and this can be excited electrically.  That tip plasmon can decay optically and spit out photons, as discussed in some detail here back in 1990. 

The situation is tricky, though.  When it comes down to atomic-scale tunneling and all the details, there are deep connections between light emission and shot noise.  Light emission is often seen at energies larger than \(eV\), implying that there can be multi-electron processes at work.  In planar tunneling structures, light emission can happen at considerably higher energies, and it really looks there like there is radiation due to the nonequilibrium electronic distribution.  It's a fascinating area - lots of rich physics.


Wednesday, May 08, 2019

Updated: CM/nano primer - aggregated posts

Here is an updated and slightly reorganized (since 2017) listing of posts I've made over the years trying to explain some key concepts in condensed matter and nanoscale physics.  Please feel free to suggest topics that should be added. 

What is temperature?

What is chemical potential?
What is mass?
Fundamental units and condensed matter

What are quasiparticles?
What is effective mass?
What is a phonon?
What is a plasmon?
What are magnons?
What are skyrmions?
What are excitons?
What is quantum coherence?
What are universal conductance fluctuations?
What is a quantum point contact?  What is quantized conductance?
What is tunneling?

What are steric interactions?
(effectively) What is the normal force?
(effectively) What is jamming?
(effectively) What is capillary action?
What are liquid crystals?
What is a phase of matter?
About phase transitions....
(effectively) What is mean-field theory?

About reciprocal space....  About spatial periodicity.
What is band theory?
What is a "valley"? 
What is a metal?
What is a bad metal?  What is a strange metal?
What is a Tomonaga-Luttinger liquid?

What is a crystal?
What is a time crystal?
What is spin-orbit coupling?
What is Berry phase?
What is (dielectric) polarization?

About graphene, and more about graphene
Why twisting materials is interesting

About noise, part onepart two (thermal noise)part three (shot noise)part four (1/f noise)
What is inelastic electron tunneling spectroscopy?
What is demagnetization cooling?
About memristors....
What is thermoelectricity?
What are "hot" electrons?

What is a functional?  (see also this)
What is density functional theory?  Part 2  Part 3

What are the Kramers-Kronig relations?
What is a metamaterial?
What is a metasurface?
What is the Casimir effect?

About exponential decay laws
About hybridization
About Fermi's Golden Rule

Monday, April 29, 2019

The 1993 Stanford physics qual

Graduate programs in physics (and other science and engineering disciplines) often have some kind of exam that students have to take on the path to doctoral candidacy.  Every place is a bit different.  When I was an undergrad there, Princeton had a two-tiered exam system, with "prelims" largely on advanced undergrad level material and "generals" on grad-course-level content.  Rice has an oral candidacy exam with subfield-specific expectations laid out in our graduate handbook.  Stanford, when I went there in fall of 1993, had a written "qual", two days, six hours each day, ostensibly on advanced undergrad level material. 

There are a couple of main reasons for exams like this:  (1) Assessment, so that students learn the areas where they need to improve their depth of knowledge; (2) Synthesis -  there are very few times in your scientific career when you really have to sit down and look holistically at the discipline.  Students really do learn in preparing for such exams.

I've written about this particular exam experience here.  Thanks to an old friend whose handwriting decorates some of the pages, here (pdf) is a copy of that exam (without the solutions).   Wow.  Brings flashbacks. 

Wednesday, April 24, 2019

Liquid droplets with facets

One essential concept in condensed matter physics is spontaneous symmetry breaking - the idea that the collective response of many components acting together results in a situation that has less symmetry than the underlying system.  Crystalline solids are a classic example.  The universe itself has (to high precision) "continuous translational symmetry" - the laws of physics governing some isolated system are the same as the laws of physics if you slide that system over a bit.  Space has continuous rotational symmetry - reorienting your isolated system doesn't change anything.  A collection of atoms, though, can assemble into a crystal, and the crystal structure itself has lower symmetry.  For, e.g., an electron within the crystal, there is now discrete translational symmetry, meaning that the electron's environment is the same if the crystal is shifted not by arbitrary amounts, but by precise multiples of the lattice spacing of the crystal.  Similarly, only specific discrete rotations of the lattice about particular axes leave the electron's environment unchanged.

We tend to think of liquids as not breaking continuous rotational or translational symmetry.  If you consider timescales long compared to the jostling motion of atoms or molecules in a liquid, all positions look about the same, as do all directions in space.  (It is possible to have intermediate situations, with liquids made from non-spherical molecules, and these can have directionality and local clustering.  Such liquid crystals are used in the display you're most likely using to read this.)  The lack of translational and rotational symmetry breaking in liquids is one reason that droplets (and bubbles) tend to be spherical.  If it is energetically expensive to have an interface between, say, oil in water, for a fixed volume of oil, then the lowest energy situation is to have a spherical oil droplet - that minimizes the surface area of interface.
Fig. 1 from Guttman et al., PNAS 113,  493-496 (2016).
Faceted oil droplets!

Yesterday I stumbled upon this paper, and that sent me down a literature rabbit-hole.   This Nature paper (archived version here) led me to this PNAS paper, where I grabbed Fig. 1 at right.  It turns out that it is possible to form faceted liquid droplets of certain oils (alkanes) in aqueous suspensions.  The outermost layer of the alkanes acts rather like a lipid membrane, and it is possible to sit at a temperature where that layer crystallizes (the molecules in it spontaneously break translational and rotational symmetry) while the bulk of the droplet remains a liquid.  What picks out the orientation of the resulting faceted shape?  Spontaneous symmetry breaking.  Tiny fluctuations or attributes of the local environment.  Wild!

Wednesday, April 17, 2019

Brief items, + "grant integrity"

As I have been short on time to do as much writing of my own as I would like, here are links to some good, fun articles:

  • Ryan Mandelbaum at Gizmodo has a very good, lengthy article about the quest for high temperature superconductivity in hydrogen-rich materials.
  • Natalie Wolchover at Quanta has a neat piece about the physics of synchronization
  • Adam Mann in Nat Geo has a brief piece pointing toward this PNAS paper arguing the existence of a really weird state of matter for potassium under certain conditions.  Basically the structure consists of a comparatively well-defined framework of potassium with 1D channels, and the channels are filled with (and leak in 3D) liquid-like mobile potassium atoms.  Weird.
Not fun:  US Senator Grassley is pushing for an "expanded grant integrity probe" of the National Science Foundation.   The stated issue is concern that somehow foreign powers may be able to steal the results of federally funded research.  Now, there are legitimate concerns about intellectual property theft, industrial espionage, and the confidentiality of the grant review process.  The disturbing bit is the rhetoric about foreign actors learning about research results, and the ambiguity about whether international students fall under that label in his or other eyes.  Vague wording like that also appeared in a DOE memo reported by Science earlier in the year.  International students and scholars are an enormous source of strength for the US, not a weakness or vulnerability.  Policy-makers need to be reminded of this, emphatically.

Tuesday, April 16, 2019

This week in the arxiv

A fun paper jumped out at me from last night's batch of preprints on the condensed matter arxiv.

arXiv:1904.06409 - Ivashtenko et al., Origami launcher
A contest at the International Physics Tournament asked participants to compete to see who could launch a standard ping-pong ball the highest using a launcher made from a single A4 sheet of paper (with folds).  The authors do a fun physics analysis of candidate folded structures.  At first, they show that one can use idealized continuum elasticity to come up with a model that really does not work well at all, in large part because the act of creasing the paper (a layered fibrous composite) alters its mechanical properties quite a bit.  They then perform an analysis based on a dissipative mechanical model of a folded crease matched with experimental studies, and are able to do a much better job at predicting how a particular scheme performs in experiments.  Definitely fun.

There were other interesting papers this week as well, but I need to look more carefully at them.

Monday, April 08, 2019

Brief items

A few brief items as I get ready to write some more about several issues:

  • The NY Times posted this great video about using patterned hydrophobic/hydrophilic surfaces to get bouncing water droplets to spin.  Science has their own video, and the paper itself is here.  
  • Back in January Scientific American had this post regarding graduate student mental health.  This is a very serious, complex issue, thankfully receiving increased attention.
  • The new Dark Energy Spectroscopic Instrument has had "first light." 
  • Later this week the Event Horizon Telescope will be releasing its first images of the supermassive black hole at the galactic center.
  • SpaceX is getting ready to launch a Falcon Heavy carrying a big communications satellite.  The landing for these things is pretty science-fiction-like!

Tuesday, April 02, 2019

The physics of vision

We had another great colloquium last week, this one by Stephanie Palmer of the University of Chicago.  One aspect of her research looks at the way visual information is processed.  In particular, not all of the visual information that hits your retina is actually passed along to your brain.  In that sense, your retina is doing a kind of image compression. 

Your retina and brain are actually anticipating, effectively extrapolating the predictable parts of motion.  This makes sense - it takes around 50 ms for the neurons in your retina to spike in response to a visual stimulus like a flash of light.  That kind of delay would make it nearly impossible to do things like catch a hard-thrown ball or return a tennis serve.  You are able to do these things because your brain is telling you ahead of time where some predictably moving object should be.  A great demonstration of this is here.  It looks like the flashing radial lines are lagging behind the rotating "second hand", but they're not.  Instead, your brain is telling you predictive information about where the second hand should be.

People are able to do instrumented measurements of retinal tissue, looking at the firing of individual neurons in response to computer-directed visual stimuli.  Your retina has evolved both to do the anticipation, and to do a very efficient job of passing along the predictable part of visualized motion while not bothering to pass along much noise that might be on top of this.  Here is a paper that talks about how one can demonstrate this quantitatively, and here (sorry - can't find a non-pay version) is an analysis about how optimized the compression is at tossing noise and keeping predictive power.  Very cool stuff.

Saturday, March 23, 2019

The statistical mechanics of money

Slow posting recently because of many real-life things going on after the March Meeting.  We had a very engaging colloquium this week by Victor Yakovenko, a condensed matter theorist from the University of Maryland.   A number of years ago, he got into "econophysics", applying insights from physics to the economy.  A great review is here

A classic example is in his highly cited paper, with the same title as this post.  Make some simple assumptions:  Money is a conserved quantity, and the rates of transactions don't depend on the financial direction of the transactions.  Take those assumptions, start with everyone having the same amount of money, and allow randomized transactions between pairs of people.  The long-time result is an exponential (Boltzmann-like) distribution of wealth - the probability of having a certain amount of money \(m\) is proportional to \(\exp(-m/\langle m \rangle)\), where \(\langle m \rangle \) is the average wealth, a monetary "temperature".  The take-away:  complete equality is unstable just because of entropy, the number of possible transactions.

Apparently similar arguments can be applied to income, because it would appear that you can describe the distribution of incomes in many countries as an exponential distribution (more than 90% of the population).  Basically, for a big part of the population, it seems like income distribution is dominated by these transactional dynamics, while the income distribution for the top 3-ish% of the population follows a power-law distribution, likely because that income comes from returns on investments rather than wages.  The universality is quite striking, largely independent of governmental policies on managing the economy.

Yakovenko would be the first to say not to over-interpret these results, but the power of statistical arguments familiar from physics is impressive.  Now all we have to do is figure out the statistical mechanics of people....

Thursday, March 07, 2019

APS March Meeting wrapup

I spent the lion's share of today talking w/ my collaborators.  This was great scientifically, but meant that I only went to a couple of talks. 

  • This one was pretty slick.  If you look at the conduction properties of a Josephson junction as a function of magnetic field through it, you see a Frauenhofer pattern as a function of the enclosed flux (see this pdf, fig 2).  In principle, taking the inverse Fourier transform of this should reveal the real-space current distribution as a function of the distance along the width of the junction.  This group made Josephson junctions using oriented thin pieces of WTe2.  When the current flowed along one direction, they found that the Josephson current was mostly flowing near the edges of the strip of material.  When current flowed along a different direction in the plane, the current distribution was much more uniform.  
  • Similarly evocative, this talk presented work using magnetic focusing plus scanning gate microscopy plus collimating contacts to look at the real-space paths of electrons in a graphene-hBN bilayer w/ a Moire superlattice.  They could then infer the shape of the Fermi surface in momentum-space, confirming that the Moire superlattice results in a roughly triangular (miniband) Fermi surface.  Cooler than my jargon-heavy description sounds.
  • I greatly regret that I was unable to attend the invited session in honor of Millie Dresselhaus.  If one of my readers who did make it could please describe it in a comment, I'd appreciate it.
  • One other random note:  I did actually speak to the APS person who was in charge of the trade show, and I asked what the heck was up with the two weird "pain relief" booths, which seemed borderline late-night-infomercial/much more like something you'd see at a cheesy shopping mall.  This was apparently an experiment in allowing local vendors in, and it sounds very unlikely that it'll ever happen again.  
If I missed a big story from the meeting, please let me know in the comments.

Wednesday, March 06, 2019

APS March Meeting Day 3

A handful of semi-random highlights (broken up by my conversations w/ colleagues and catching up on work-related issues):

  • Laura Heydermann from ETH spoke about "artificial" magnetic systems, where mesoscopic, lithographically patterned arrays of magnetic islands can yield rich response.  A couple of representative papers are here and here, and recently they've been moving into 3D fabrication and magnetically sensitive imaging.  Very neat stuff.  
  • Christian Glatti from Saclay showed a very interesting result, analogous to the ac Josephson effect, but in fractional quantum Hall edge-state tunneling.  The relevant paper, just out in Science, is here.  This idea is, measure electronic shot noise as a function of bias voltage.  Ordinarily this has a minimum at zero bias, and the noise sits at the Johnson-Nyquist level there.  Now shine microwaves of frequency f on the device.  With photon-assisted tunneling, the net result is a change in the noise that has kinks at voltages of +/- hf/e*, where h is Planck's constant, and e* is the effective charge of the low-energy excitations.  Do this in the fractional quantum Hall regime, and you see fractional charge.  
  • On a related topic, Michael Pepper from Cambridge showed a very recent result.  In quantum point contacts at very low charge carrier densities, they see quantized conductance at some very surprising rational fractions of the usual conductance quantum 2e2/h.  I still need to digest this.  
  • I spent much of the afternoon at the big Kavli Symposium, on topics spanning from unit cell all the way to biological cells.  All excellent speakers.  I won't try to summarize this - rather, when the talks become available streaming, I will put the link here.  (Claudia Felser did bring donuts for the audience to talk about topology, always a crowd-pleaser.)

APS March Meeting, Day 2

A random selection from Day 2:

  • Thomas Silva at NIST gave a fun talk about some experiments using the linac coherent light source.  Using pump/probe time-resolved x-ray diffraction, they discovered some surprising acoustic modes in thin, polycrystalline metal films, with systematics suggesting that they might be seeing localization of phonons due to scattering off grain boundaries.
  • Along those lines, Gang Chen of MIT spoke about seeing reductions in thermal conductivity due to phonon localization.  His group was working with semiconductor superlattices, with little ErAs nanodots embedded in a disordered way at the superlattice interfaces.  They see systematics in the thermal conductivity that suggest that they are seeing Anderson localization of the heat-carrying phonons.  
  • I stopped by the session on conveying physics to a popular audience, and caught most of the talk by Allison Eck chock full of advice for would-be science writers, and a skyped-in talk by Sean Carroll about podcasting.  The depressing truth: If I really want to expand my audience, I should probably join twitter.  (The problem is, that's a conversational medium and I don't see how I could do it well given everything else.)
  • Abe Nitzan gave a prize talk that was a nice overview of the last decade's work on understanding electrons, photons, and phonons in molecular junctions.
  • I spent much of the afternoon at this session about the copper oxide superconductors.  Dan Dessau's talk primarily about this paper showed the capabilities of a new technique in analyzing angle-resolved photoemission data, to figure out the actual spatial shape of Cooper pairs in these systems. My collaborator Ivan Bozovic spoke (similar to this), showing the power of his tremendous MBE growth approach, able to create epitaxially perfect materials smoothly and systematically spanning the whole doping range.  The other talks in the session were also very interesting.

Monday, March 04, 2019

APS March Meeting, Day 1

A few things I saw at the APS Meeting today, besides 10 inches of fresh, wet snow on the ground this morning (disclaimer:  for various reasons I was session-hopping quite a bit, so this is rather disjointed):
  • Ignacio Franco at Rochester spoke about some experiments (here) that I'd not remembered, where carefully controlled, intense femtosecond light pulses were used to turn on a transient current in SiO2, normally one of the best insulators out there.  The theory is interesting, and made me start thinking about possible opportunities in this area.
  • A focus topic session on 2D magnetic materials was extremely crowded - so much so that I literally couldn't get in the room for the first talk.  Interesting talks, including Yujun Deng from Fudan presenting this workMasaki Nakano from the University of Tokyo spoke about growing epitaxial films of V5Se8, a cousin of a material with which we've worked; and Boyi Zhou at Washington University in St. Louis presented this work, which seems to show nontrivial electronic conduction in (ordinarily Mott insulating) monolayer RuCl3 layered on graphene.  Lots of interesting activity going on here, many fun ideas.
  • Naomi Ginsberg at Berkeley talked about some impressive imaging techniques used to follow energy flow in complex materials.  Combining super-resolution methods, interferometry, and time-resolved techniques is a heck of an enabling technique!
  • Peter Abbamonte at Illinois presented some remarkable measurements using an angle-resolved electron energy loss technique (M-EELS) to look at the strange metal state of a cuprate superconductor.  The main result is that this material seems to support a very broad plasmon mode with a lot of properties that are inconsistent with what you'd expect in a Fermi liquid, and may make connection with more exotic pictures of strange metals.  
  • Wojciech Zurek's talk about the foundations of quantum mechanics (based on this article) was very engaging (and apparently in a superposition of all possible fonts), though again the room was so full that people were sitting on the floor in the aisles and lining three walls.  The session also was running about 10 minutes ahead of schedule, which definitely was not great for people who ended up missing the beginning of Zurek's talk or Rovelli's before it.
The unwieldy size of the meeting is increasingly clear, with lines in the restrooms, and local fastfood places unable to handle the lunch crowd.  

Sunday, March 03, 2019

APS March Meeting, Day 0

A brief summary of topics/reading material/things I learned today during DCMP and joint DCMP/DMP executive committee meetings:
  • As usual, this will be the biggest March Meeting ever, with 11500 registrants ahead of time.  This is still increasingly problematic in terms of organization and availability of sites.
  • New APS Strategic Plan
  • New APS report on the Impact of Industrial Physics on the US Economy
  • DOE Basic Energy Sciences report (pdf) on the impact of the BES at its 40th anniversary
  • The upcoming privatization of the US Strategic Helium Reserve looks depressingly unavoidable.  Sounds like changing this is a non-starter in Congress.

Michelle Simmons and Si-based quantum computing

A last tidbit before the March Meeting.

Earlier this week, Prof. Michelle Simmons came to Rice for our Chapman Lecture series and gave a great talk about her team's project to develop quantum computing in a silicon platform, with individual phosphorus donor atoms as the qubits.   This idea goes back more than twenty years to this proposal by Bruce Kane.   Actually implementing this approach requires overcoming many technical challenges, including positioning individual phosphorus atoms inside single-crystal Si with nearly atomic precision, and similarly fabricating control and read-out electrodes in registry with those.  

Prof. Simmons' group has made truly remarkable progress in this direction.  The key enabling technique is using a scanning tunneling microscope (STM) as a lithography tool.  Single-crystal Si surfaces are prepared in ultrahigh vacuum and terminated with a hydrogen.  The STM tip is then used to strip off the hydrogen atoms with atomic precision.  (This is a serial technique, and so scaling up to the production numbers of the present-day Si industry would require something different, but for now it's fine.)  Phosphine gas decomposes in a particular way when exposed to the dangling bonds left behind by stripping the hydrogen, placing P atoms in particular locations.  This approach can also be used to make highly conductive wires and gates by doping, enabling transport measurements through single dopant atoms.   Growing more single-crystal Si on top of the dopants without having the dopants move around is another success story, making possible 3D fabrication schemes.  With isotopically pure Si, encapsulating the donors can give long coherence times.

There are many competing platforms for possible quantum computer implementations, and this approach is undoubtedly difficult.  In terms of technical achievement, though, this effort has shown the power of sustained support - progress has been truly impressive.

Thursday, February 28, 2019

APS March Meeting 2019

Once more, it is that time of year, when (mostly) condensed matter physicists gather in ever-increasing numbers to give and watch talks, network, try to get cool swag at the tradeshow, and generally grouse about the poor quality and high price of convention center coffee.  The March Meeting this year is in Boston for the first time since I've been going 2012.  (Strangely, I was unable to find a list of all the March Meeting sites online.  Perhaps a reader knows the last time the meeting was in Boston.)  It's my third and last year as a DCMP member-at-large, so it will be interesting to hear what comes up at the business meetings this time.   As I have done in past years, I'll do my best to write up some of what I see and give my impressions of the conference, though I may be more concise compared to previous years.

Tuesday, February 19, 2019

Why twisting materials is interesting

Twisted bilayer graphene is a hot topic, with 32 preprints on the arxiv using those keywords just since the beginning of the year.  It's worth explaining for non-experts, why this system, comprising two atomic layers of graphene twisted relative to each other by some angle, is so interesting. 

Let's start w/ the basics.  In the (non-relativistic) quantum world, we talk about the wavefunction \(\psi(\mathbf{r},t)\) of a system.  The Schroedinger equation describes how the wavefunction evolves with time, and by solving it we can find the particular energy levels ("stationary states") for a given problem.  The magnitude-squared of the spatial wavefunction, \(|\psi(\mathbf{r},t)|^2\) gives the probability of finding the particle in a particular place at a particular time.  

The wavefunction a free particle with a well-defined momentum \(\mathbf{p}\) can be treated as a wave with a wavevector \(\mathbf{p}/\hbar \equiv \mathbf{k}\), and therefore a wavelength \(2 \pi \hbar/|\mathbf{p}|\).   Higher momentum = shorter wavelength = the wavefunction has more closely spaced wiggles.  The kinetic energy goes like \(p^{2}/2m\), as in classical nonrelativistic mechanics.   (Note that the magnitude-squared of such a wave is constant as a function of spatial position.  That is consistent with the uncertainty principle:  Knowing the momentum precisely means that the position could be anything.)  

Take a particle and stick it in an environment where the local potential energy varies periodically in space - ideally in a system so large that we can neglect boundary effects for now.  The classic example of this is an electron in a crystalline solid.   I've talked about this kind of spatial periodicity before.  There are a couple of ways to think about this situation.  We have replaced "continuous translational symmetry" (the environment of the particle is unchanged if we consider moving the particle anywhere) with "discrete translational symmetry" (now we have to move an integer number of lattice spacings to get back to the same environment for the particle).  Mathematically, the single-particle stationary states can still be labeled by a parameter \(\mathbf{k}\), but they're Bloch waves rather than plane waves, and the energy \(E(\mathbf{k})\) is no longer necessarily proportional to \((\hbar k)^{2}\) all the time.   Physically, when the naive spatial periodicity of the single-particle state matches up with the spatial periodicity (or some harmonic) of the lattice, it makes sense that there should be deviations from what we'd see with a free particle.  The result is "band structure", ranges of energy densely filled with allowed single-particle states, separated by "band gaps", ranges of energy in which there is no way to make a single-particle state and still satisfy the Schroedinger equation with the spatially periodic potential energy.

The particular spatial periodicity of the lattice determines the form of \(E(\mathbf{k})\).  For a hexagonal lattice like single-layer graphene, it turns out that there are two "Dirac points", and that near those special values of \(\mathbf{k}\), the form of \(E(k)\) looks like what is obeyed by photons in free space (!), with energy linearly dependent on \(k\).

The key point here:  if we can tune the spatial periodicity of the potential arbitrarily, we can create interesting forms of \(E(\mathbf{k})\).  That's really a neat idea.  Carefully growing stacked multilayers of semiconductors along one direction has been used to create "minibands" for optoelectronic devices.  Starting from a 2D surface state in copper, people have been able to put down patterns of CO molecules to create spatial periodicities in 2D, creating structures that look and act like graphene, or very recently even fractals.  People have also tried doing this by patterning semiconductor structures, but it's very hard to get sufficient uniformity so that disorder isn't a problem.

Stacking graphene layers with some relative twist angle is a great way to create a 2D modulation with excellent uniformity over large areas (many many lattice spacings).  This 2D modulation shows up because of the Moire pattern, which gives a spatially periodic coupling between the bands in each of the layers.  By tweaking the relative angle, the spatial pattern can be tuned.  By squishing on the bilayer, in principle the strength of the coupling can be tuned.  This kind of 2D modulation should be possible in principle in twisted bilayers of all kinds of stackable materials.

The situation is even more interesting once we start thinking about electron-electron interactions.

Another way to think of bands:  Start from the atomic energy levels of the individual, isolated constituent atoms.  The electronic levels of each atom are sharply defined.  All of the 4s orbitals, say, have the same energy.  If you think about possible electronic states, the "band" made out of isolated (localized to individual atoms) 4s orbitals is very narrow in energy.  If you built up some linear combination of those 4s orbitals that had a parameter \(\mathbf{k}\), the energy \(E(\mathbf{k})\) of that state would basically be independent of \(\mathbf{k}\).  That is, the band would be "flat".   Turn on hopping between atoms, and band broadens out in energy. 

If we throw in a bunch of electrons and ask what is the lowest energy state of the many-electron system, we can often get away with mostly neglecting electron-electron interactions.  Because of the Pauli Principle, we fill up the bands from the bottom up, and very often the (single-particle kinetic + lattice interactions) energy grows very rapidly, so much so that any electron-electron interactions are not very important.   (That's what happens in the periodic table - as you go to atoms containing more and more electrons, the kinetic energy grows fast enough that e-e interactions don't really disrupt the basic hydrogen-like s-p-d-f orbital structure of energy levels.)

In the twisted bilayers, it is possible to end up with some bands that are very flat - so flat that the typical electron-electron interaction energy is comparable or large compared to the bandwidth.  In these flat band situations, electron-electron interactions can end up being very important in determining the collective many-body state of the electrons.  That appears to be what people are seeing in the experiments mentioned previously.

The bottom line:  Twisted stacking is a great, robust way to create a lateral spatially modulated potential, and therefore (within particular geometric limits) a "designer" band structure.  The resulting bands can be very flat, so that electron-electron interaction effects (apparently) can lead to remarkable many-body responses, like the onset of superconductivity or magnetism. 

Sunday, February 10, 2019

More brief items

Some additional interesting links:

  • Another example of emergent universal behavior, as it is demonstrated that runners at the start of a marathon seem to collectively obey hydrodynamics, like a fluid.
  • The Voices of the Manhattan Project oral histories effort has a large number of interviews online.  It’s important for posterity that these were recorded before everyone involved is gone.
  • Maybe massive open online courses were not, in fact, the end of the traditional model of higher education.  Who could have foreseen this?
  • There are people arguing that the preprint arxiv model is a good path toward opens access.  This is definitely something I like, especially more than models involving authors paying thousands of dollars to for-profit publishers for open access journals.

Wednesday, February 06, 2019

Brief items

This is the absolute most dense time of the year in terms of administrative obligations, so posting is going to be a bit sparse.  In the meantime, here is a bit of interesting reading:

Scientific American has an interesting article about the fact that two independent means of assessing the Hubble constant (analysis of the cosmic microwave background on one hand; analysis of "standard candles" on the other) disagree well outside the estimated systematic uncertainties.

Kip Thorne posted a biographical reminiscence about John Wheeler on the arxiv.  I haven't read it yet, but it's in my queue.

Quanta Magazine had put up a very well done article about turbulence.  Good stuff.  I liked the animation.

Tuesday, January 29, 2019

Three brief book reviews

In the spirit of Peter Woit's latest post, I also wanted to offer up three miniature book reviews.

The New Science of Strong Materials: Or Why You Don't Fall Through the Floor by J. E. Gordon.  This book is a fine, accessible (minimal math) introduction to materials science by one of the people who created the field as a distinct discipline.  The first edition came out in 1968, so it is a bit of an historical journey.  For example, the author describes how just recently people were able to achieve the first transmission electron microscopy images that directly showed dislocations.  The only way they could do it was to image a material that was actually a crystal of Pt-containing organic complexes - the Pt has high electron density for imaging, and the organic ligands keep the Pt spatially separated by a large enough distance (a few nm) to resolve in the equipment of the day.  Quite a difference from the present state of the art.  Gordon wrote in an engaging style with a dry UK wit, and clearly had a genuine fondness for wood as an amazing, versatile composite material.  Should be required reading for undergrad mechanical and civil engineers who need to get a real physical picture for stress, strain, and ways to mitigate crack propagation.  A fun read.

How to Invent Everything: A Survival Guide for the Stranded Time Traveler by Ryan North.
I won't spoil the amusing conceit that's used as a frame for this remarkable, fun collection of bite-sized bits of knowledge.   Suffice it to say that, in the event of a global collapse of civilization, this will be a handy tome to have on hand, should you need to recreate, say, agriculture or printing or distillation or the steam engine.  The recurring theme is, there are many societal and technological advancements that the human race seemed to be curiously slow to figure out (like, many tens of thousands of years slower than could have been done).  Just the kind of fun you would expect from the person who brought us Dinosaur Comics.  It does have a bit of a Randall Munroe What If vibe, but it's distinctive.

Math with Bad Drawings: Illuminating the Ideas That Shape Our Reality by Ben Orlin.
This was also very enjoyable.  Parts of it made me think that "Condensed Matter with Bad Drawings" would be a great approach, except that now it would seem hopelessly derivative.  The book takes a free-wheeling path through math in our lives, with large, healthy doses (perhaps a bit lengthy) of statistics (lies and damn lies - what different statistical quantities are telling and not telling you) and economics.  It's well done, and I particularly liked the beginning sections that explain what math really looks and feels like to a mathematician; that really resonated, and I wish I could convey even half as well that aspect of how physicists look at and think about the world around us.

Friday, January 25, 2019

"Seeing" atoms

The power of modern transmission electron microscopy (TEM) is very impressive.  Often in TEM images at high magnification, you can see what looks like the atomic lattice, but that can be a bit illusory.  Because the scattering effects of individual atoms, especially light ones like carbon, can be very slight, often those images are looking at the result from scattering off columns of atoms, with the crystalline structure of the material helping greatly to produce a clean image.  With state of the art instrumentation and processing, however, it is possible to resolve single atoms, even in atomically thin, light materials like graphene.  This image, from a new ACS Nano paper by Lee et al. from Oxford University, is a great example of what is now possible, showing the reconfiguration of carbon bonds as a nanoscale graphene constriction is modified.  Pretty eye-popping.

Sunday, January 20, 2019

Frontiers of physics - an underappreciated point

In what branch of physics are the most extreme conditions reached?  If asked, I'm sure the vast majority of people would guess particle physics. Enormous machines (and they want bigger ones all the time) are used to accelerate particles up to a hairsbreadth below the speed of light and smash the particles into each other or into targets.  The energy densities in those collisions are enormous and by intent are meant to rival conditions in the earliest moments of the universe or in extreme astrophysical conditions.  Still, while the details are special (nature doesn't collide directed bunches of ultrarelativistic protons head on), the fact is that there are, or at least have been, naturally arising processes that approach those conditions.  

The fact is, condensed matter physics (CMP) and atomic/molecular/optical (AMO) physics are actually more extreme, reaching conditions that do not ever happen spontaneously, anywhere.  Now-common laboratory techniques in CMP and AMO can produce experimental conditions that, as far as we know, simply do not occur in nature without the direct intervention of intelligent beings.  

The particular condition I'm talking about is temperature.  As I discussed a little here, temperature is a parameter that tells us the direction that energy tends to flow when two systems (say a coffee cup and a coaster) are allowed to exchange energy via microscopic degrees of freedom that we don't track, like the kinetic jiggling of vibrating atoms in a solid.  When the cup and coaster are at the same temperature, there is no net flow of energy between them, even though some amount of energy is fluctuating back and forth all the time.  

The cosmic microwave background, the relic electromagnetic radiation left over from the early universe, is described by an intensity vs. frequency distribution that we would expect from radiation in thermal equilibrium with a system at a temperature of 2.726 K.  What this means is, if you had some lump of matter floating in interstellar space, and you waited a very long time, the temperature of that lump would eventually settle down to 2.726 K, absent other effects.  It would never be colder.

In CMP labs around the world, however, macroscopic lumps of matter are routinely cooled to temperatures far colder than this.  With a conventional dilution refrigerator (see here) it is possible to cool kgs of material down to milliKelvin temperatures.  Through demagnetization cooling, particularly of materials with nuclear magnetic moments, microKelvin temperatures may be reached.  In AMO labs, laser cooling techniques can get clouds of atoms down to nanoKelvin temperatures, though typically the number of atoms involved is far smaller.  Pretty amazing, when you think about it!

Tuesday, January 15, 2019

This week in the arxiv

Twisted bilayer graphene (TBLG): Is there anything it can't do?  Two recent papers have appeared on the arxiv that show that TBLG looks like a versatile platform for exploring the physics of electrons in comparatively flat bands.  Remember, flat bands as a function of (crystal) momentum = high effective mass = tendency toward localization = interaction effects have an easier time dominating the kinetic energy.  There was a big splash at the APS meeting last year when superconductivity was found in this system that had some resemblance to the phenomena seen in the high-Tc cuprates.

arxiv:1901.03520 - Sharpe et al., Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
In this new work the Goldhaber-Gordon group at Stanford shows that, in TBLG, at the right gate voltage (that is, the right filling of the rather flat bands), the system seems to develop spontaneous ferromagnetism, seen both through the appearance of hysteresis in the electrical resistance as a function of magnetic field, and through the onset of an anomalous Hall signature.  Through non-local transport effects (e.g., drive a current over here, measure a voltage over there) they see indications of edge currents, suggesting that topology is important here. 

arxiv:1901.03710 - Cao et al., Strange metal in magic-angle graphene with near Planckian dissipation
Another feature of strongly correlated electronic materials like the cuprate superconductors is "strange metallicity", when the temperature dependence of the electrical resistance is linear with T over a large range, in contrast with simple expectations of Fermi liquid theory.  There have been arguments that in the limit of very strong electron-electron scattering, a kind of hydrodynamics kicks in for the electrons, with universal bounds on charge diffusion (and hence the resistance).  These arguments are sometimes based on fairly exotic ideas.   Not everyone agrees with the proposed universality.  In this new paper, the MIT group shows that the TBLG system does show resistance similar in form and magnitude to this strange metallicity.

The broad idea of tuning band flatness by stacking and rotating 2d materials continues to show promise as a playground for looking at the competition between different possible ordered states.

Tuesday, January 08, 2019

Magnetic data storage - heat-assisted v microwave-assisted

(Ironically, given my recent missive about the importance of condensed matter beyond applications to information technology, here is a post about condensed matter in information technology.)

It may seem like solid-state drives - basically flash memory - have taken over data storage, particularly in phones, tablets, and laptops.  However, magnetic storage media, particularly in the form of hard drives, are still the main tools of choice for the cloud.  Magnetic storage is very robust and doesn't rely on charge staying put on tiny floating gates to hold your information.  Rather, in a hard drive the zeros and ones of your data are encoded as magnetic domains of particular orientation in a specially engineered thin film of material on a disk platter. 

The amount of research and engineering development that has gone into hard drives is amazing.  The read/write heads "fly" at nanometer separation over incredibly smooth magnetic surfaces.  The smaller you can make the magnetic domains and still manipulate and read them in a controlled manner, the higher the density of the information storage.  The timing and positioning stability required to store and retrieve terabytes per square inch is amazing.  Reading the data requires detecting the magnetic fields produced by the domains.  These days that's done using magnetoresistance, some component of the read head with an electrical resistance that changes depending on the local magnetic environment.  Giant magnetoresistance went from the laboratory to hard drive read heads to Stockholm for the 2007 Nobel Prize in Physics.  Its successor in read heads, tunneling magnetoresistance, now rules the roost.

On the physics side, there are many challenges for continued scaling.  Magnetic domains interact with each other.  To be reliable for storage, it's important that the magnetization \(\mathbf{M}\) of a domain remain fixed once it is set.  The energy scales associated with pinning a domain tend to scale with domain size, meaning that, all other things being equal, tinier domains are easier to flip (for writing, yay, for long term stability, boo).  In the extreme limit, thermal fluctuations can provide enough energy to reorient the magnetization, leading to superparamagnetism.  A number of years ago, IBM and others came up with ways to increase the coercive fields (and anisotropy energies) of the bits in disk media.  Still, as bits get smaller, it's more important to pin them down, but somehow still allow deliberate reorientation of \(\mathbf{M}\) for write operations.

This article spurred me to write a little about this, in part because of what it gets wrong.  Hard drive manufacturers have decided that the best plan is to pin down the bits firmly, and then during the write process, locally kick those bits hard enough to allow reorientation of \(\mathbf{M}\). 

One method that's been under consideration for a while, and is the favorite of Seagate, is HAMR - heat-assisted magnetic recording.  This requires some means of locally heating the disk media right under the write-head so that thermal energy is available to help the bit reorient.  The Seagate approach uses an embedded laser source and a plasmonic antenna to drive the local optical heating (this has nothing to do with an electrical discharge, despite the linked article at the start of this paragraph).

Western Digital is pursuing an alternative approach microwave-assisted magnetic recording (MAMR).  The idea there (video) is to use a local oscillator (one based on spin torque) to generate microwaves locally at the write head, with the frequency of those microwaves tuned to drive ferromagnetic resonance of the bit to flip it.

Tons of physics in all of this, and an enormous amount of engineering cleverness.  Magnetic data storage will be with us for a while yet.