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.