## Thursday, February 15, 2018

### Physics in the kitchen: Jamming

Last weekend while making dinner, I came across a great example of emergent physics.  What you see here are a few hundred grams of vacuum-packed arborio rice:
The rice consists of a few thousand oblong grains whose only important interactions here are a mutual "hard core" repulsion.  A chemist would say they are "sterically hindered".  An average person would say that the grains can't overlap.  The vacuum packing means that the whole ensemble of grains is being squeezed by the pressure of the surrounding air, a pressure of around 101,000 N/m2 or 14.7 pounds per in2.  The result is readily seen in the right hand image:  The ensemble of rice forms a mechanically rigid rectangular block.  Take my word for it, it was hard as a rock.

However, as soon as I cut a little hole in the plastic packaging and thus removed the external pressure on the rice, the ensemble of rice grains lost all of its rigidity and integrity, and was soft and deformable as a beanbag, as shown here.

So, what is going on here?  How come this collection of little hard objects acts as a single mechanically integral block when squeezed under pressure?  How much pressure does it take to get this kind of emergent rigidity?  Does that pressure depend on the size and shape of the grains, and whether they are deformable?

This onset of collective resistance to deformation is called jamming.  This situation is entirely classical, and yet the physics is very rich.  This problem is clearly one of classical statistical physics, since it is only well defined in the aggregate and quantum mechanics is unimportant.  At the same time, it's very challenging, because systems like this are inherently not in thermal equilibrium.  When jammed, the particles are mechanically hindered and therefore can't explore lots of possible configurations.   It is possible to map out a kind of phase diagram of how rigid or jammed a system is, as a function of free volume, mechanical load from the outside, and temperature (or average kinetic energy of the particles).   For good discussions of this, try here (pdf), or more technically here and here.   Control over jamming can be very useful, as in this kind of gripping manipulator (see here for video).

## Tuesday, February 13, 2018

### Rice Cleanroom position

In case someone out there is interested, Rice is hiring a cleanroom research scientist.  The official job listing is here.  To be clear:  This is not a soft money position.

The Cleanroom Facility at Rice University is a shared equipment facility for enabling micro- and nanofabrication research in the Houston metropolitan area. Current equipment includes deposition, lithography, etching and a number of characterization tools. This facility attracts users from the George R. Brown School of Engineering and the Wiess School of Natural Science and regional universities and corporations whose research programs require advanced fabrication and patterning at the micro- and nanoscale. A new state of the art facility is currently being constructed and is expected to be in operation in summer 2018. Additionally, with new initiatives in Molecular Nanotechnology, the Rice University cleanroom is poised to see significant growth in the next 5-10 years. This job announcement seeks a motivated individual who can lead, manage, teach and grow this advanced facility.

The job responsibilities of a Cleanroom Research Scientist include conducting periodic and scheduled maintenance and safety check of equipment and running qualification and calibration recipes. The incumbent will be expected to maintain the highest safety standards, author and update standard operation procedures (SOPs), maintain and calibrate processes for all equipment. The Cleanroom Research Scientist will help facilitate new equipment installation, contact vendors and manufacturers and work in tandem with them to resolve equipment issues in a timely and safe manner. Efficient inventory management of parts, chemicals and supplies will be required. The Cleanroom Scientist will also oversee personal one-to-one training of users. Additionally, the incumbent will help develop cleanroom laboratory short courses that provide lectures to small groups of students. The incumbent will also coordinate with technical staff members in Rice SEA (Shared Equipment Authority).

## Saturday, February 10, 2018

### This week in the arxiv

Back when my blogging was young, I had a semi-regular posting of papers that caught my eye that week on the condensed matter part of the arxiv.  As I got busy doing many things, I'd let that fall by the wayside, but I'm going to try to restart it at some rate.  I generally haven't had the time to read these in any detail, and my comments should not be taken too seriously, but these jumped out at me.

arxiv:1802.01045 - Sangwan and Hersam; Electronic transport in two-dimensional materials
If you've been paying any attention to condensed matter and materials physics in the last 14 years, you've noticed a huge amount of work on genuinely two-dimensional materials, often exfoliated from the bulk as in the scotch tape method, or grown by chemical vapor deposition.  This looks like a nice review of many of the relevant issues, and contains lots of references for interested students to chase if they want to learn more.

arxiv:1802.01385 - Froelich; Chiral Anomaly, Topological Field Theory, and Novel States of Matter
While quite mathematical (relativistic field theory always has a certain intimidating quality, at least to me), this also looks like a reasonably pedagogical introduction of topological aspects of condensed matter.  This is not for the general reader, but I'm hopeful that if I put in the time and read it carefully, I will gain a better understanding of some of the topological discussions I hear these days about things like axion insulators and chiral anomalies.

arXiv:1802.01339 - Ugeda et al.; Observation of Topologically Protected States at Crystalline Phase Boundaries in Single-layer WSe2
arXiv:1802.02999 - Huang et al.; Emergence of Topologically Protected Helical States in Minimally Twisted Bilayer Graphene
arXiv:1802.02585 - Schindler et al.; Higher-Order Topology in Bismuth
Remember back when people didn't think about topology in the band structure of materials?  Seems like a million years ago, now that a whole lot of systems (often 2d materials or interfaces between materials) seem to show evidence of topologically special edge states.   These are three examples just this week of new measurements (all using scanning tunneling microscopy as part of the tool-set, to image edge states directly) reporting previously unobserved topological states at edges or surface features.

## Sunday, February 04, 2018

### New readers: What is condensed matter physics? What is special about the nanoscale?

If you're a new reader, perhaps brought here by the mention of this blog in the Washington Post, welcome!   Great to have you here.  Just a couple of quick FAQs to get you oriented:

What is condensed matter physics?  Condensed matter (once known as "solid state) is a branch of physics that deals with the properties of matter consisting of large numbers of particles (usually atoms or (electrons+the rest of the atoms)) in "condensed" states like liquids and solids - basically the materials that make up an awful lot of the stuff you interact with all the time.  New properties can emerge when you bring lots of particles together.  See here for an example involving plastic balls, or here (pdf) for a famous essay about this general point.  Condensed matter physicists are often interested in identifying the different types of states or phases that can arise, and understanding transitions between those states (like how does water boil, or how does magnetism turn on in iron as its temperature is lowered from the melting point, or how does a ceramic copper oxide suddenly start letting electricity flow without resistance below some particular temperature).  Hard condensed matter typically deals with systems where quantum mechanics is directly important (electronic, magnetic, and optical properties of materials, for example), while soft condensed matter describes systems where the main actors (while quantum deep down like all matter) are not acting in a quantum way - examples include the jamming of grains of sand when you build a sand castle, or the spontaneous alignment of rod-like molecules in the liquid crystal display you're using to read this.

While particle physics tries to look at the tiniest bits of stuff, condensed matter hits on some of the same (literally the same concepts and math) deep ideas about symmetry, and often has direct implications for technologies that affect your daily life.   Understanding this stuff has given us things like the entire electronics industry, the telecommunications industry, and soon probably quantum computers.

A powerful concept in physics in general and condensed matter in particular is universality.  For example, materials built out of all kinds of different ingredients can be mechanically rigid solids; there is something universal about mechanical rigidity that makes it emerge independent of the microscopic details.  Another example:  Lots of very different systems (metallic lead; waxy crystals of buckyball molecules with some alkaline metal atoms in between; ceramic copper oxides; hydrogen sulfide gas under enormous pressure) can conduct electricity without resistance at low temperatures - why and how is superconductivity an emergent property?

What is special about the nanoscale?  Because it's about collective properties, traditional condensed matter physics often uses a lot of nice approximations to describe systems, like assuming they're infinite in extent, or at least larger than lots of physically important scales.   When you get down to the nanoscale (recall that a typical atom is something like 0.3 nanometers in diameter), a lot of the typical approximations can fail.  As the size of the material or system becomes small compared to the length scales associated with various physical processes, new things can happen and the properties of materials can change dramatically.  Tightly confined liquids can act like solids.  Colorless materials can look brilliantly chromatic when structured on small scales.  Two electrical insulators brought together can produce a nanoscale-thick metallic layer.   We now have different techniques for structuring materials on the nanoscale and for seeing what we're doing down there, where the building blocks are often far smaller than the wavelengths of light.  Investigations at the nanoscale are tied to some of the most active topics in condensed matter, and verge into the interdisciplinary boundaries with chemistry, biology, materials science, electrical engineering, and chemical engineering.   That, and it's fun.

Please browse around through the archives, and I hope you find it interesting.

## Friday, February 02, 2018

### Why science blogging still matters

Nature has a piece up about science blogging.  It's pretty much on target.  I'm a bit surprised that there wasn't more discussion of blogging vs. twitter vs. other social media platforms, or the interactions between blogs and formal journalism.

## Monday, January 29, 2018

### Photonics West

A significant piece of my research program is optics-related, and thanks to an invited talk, I'm spending a couple of days at the SPIE Photonics West meeting in San Francisco, a mix of topics from the very applied (that is, details of device engineering and manufacturing) to the fundamental.   It's fun seeing talks on subjects outside of my wheelhouse.

A couple of items of interest from talks so far today:

• Andrew Rickman gave a talk about integrated Si photonics, touching on his ideas on why, while it's grown, it hasn't taken off in the same crazy exponential way as Moore's Law(s) in the microelectronics world.  On the economic side, he made a completely unsurprising argument:  For that kind of enormous growth, one needs high volume manufacturing with very high yield, and a market that is larger than just optical telecommunications.  One challenge of Si-based photonics is that Si is an indirect band gap material, so that for many photonic purposes (including many laser sources and detectors) it needs to be integrated with III-V semiconductors like InP.  Similarly, getting optical signals on and off of chips usually requires integration with macroscopically large optical fibers.   His big pitch, presumably the basis for his recent founding of Rockley Photonics, is that you're better off making larger Si waveguides (say micron-scale, rather than the 220 nm scale, a standard size set by certain mode choices) - this allegedly gives you much more manufacturing dimensional fault tolerance, easier integration with both III-V and fiber, good integration with electroabsorption modulators, etc. One big market he's really interested in is cloud computing, where apparently people are now planning for the transition form 100 Gbs to 400 Gbs (!) for communication within racks and even on boards.  That is some serious throughput.
• Min Gu at Royal Melbourne Institute of Technology spoke about work his group has been doing trying to take advantage of the superresolution approach of STED microscopy, but for patterning.   In STED, a diffraction limited laser spot first illuminates a target area (with the idea of exciting fluorescence), and then a spot from a second laser source, in a mode that looks donut-shaped, also hits that location, depleting the fluorescence everywhere except at the location of the "donut hole".  The result is an optical imaging method with resolution at the tens of nm level.  Gu's group has done work combining the STED approach with photopolymerization to do optical 3d printing of tiny structures.  They've been doing a lot with this, including making gyroid-based photonic crystals that can act as helicity-resolved beamsplitters for circularly polarized light.  It turns out that you can make special gyroid structures so that they have broken symmetries so that these photonic crystals support topologically protected (!) modes analogous to Weyl fermions.
• Venky Narayanamurti gave a talk about how to think about research and its long-standing demarcation into "basic" and "applied".  This drew heavily from his recent book (which is now on my reading list).   The bottom line:  In hindsight, Vannevar Bush didn't necessarily do a good thing by intellectually partitioning science and engineering into "basic" vs. "applied".  Narayanamurti would prefer to think in terms of invention and discovery, defined such that "Invention is the accumulation and creation of knowledge that results in a new tool, device, or process that accomplishes a particular specific purpose; discovery is the creation of new knowledge and facts about the world."  Neither of these are scheduled activities like development.  Research is "an unscheduled quest for new knowledge and the creation of new inventions, whose outcome cannot be predicted in advance, and in which both science and engineering are essential ingredients."  He sounded a very strong call that the US needs to change the way it is thinking about funding of research, and held up China as an example of a country that is investing enormous resources in scientific and engineering research.

## Monday, January 22, 2018

### In condensed matter, what is a "valley", and why should you care?

One big challenge of talking about condensed matter physics to a general audience is that there are a lot of important physical concepts that don't have easy-to-point-to, visible consequences.  One example of this is the idea of "valleys" in the electronic structure of materials.

To explain the basic concept, you first have to get across several ideas:

You've heard about wave-particle duality.  A free particle in in quantum mechanics can be described by a wavefunction that really looks like a wave, oscillating in space with some spatial frequency ($k\ = 2 \pi$/wavelength).  Momentum is proportional to that spatial frequency ($p = \hbar k$), and there is a relationship between kinetic energy and momentum (a "dispersion relation") that looks simple.  In the low-speed limit, K.E. $= p^2/2m$, and in the relativistic limit, K.E. $= pc$.

In a large crystal (let's ignore surfaces for the moment), atoms are arranged periodically in space.  This arrangement has lower symmetry than totally empty space, but can still have a lot of symmetries in there.  Depending on the direction one considers, the electron density can have all kinds of interesting spatial periodicities.  Because of the interactions between the electrons and that crystal lattice, the dispersion relation $E(\mathbf{k})$ becomes direction-dependent (leading to spaghetti diagrams).  Some kinetic energies don't correspond to any allowed electronic states, meaning that there are "bands" in energy of allowed states, separated by gaps.  In a semiconductor, the highest filled (in the limit of zero temperature) band is called the valence band, and the lowest unoccupied band is called the conduction band.

Depending on the symmetry of the material, the lowest energy states in the conduction band might not be near where $|\mathbf{k}| = 0$.  Instead, the lowest energy electronic states in the conduction band can be at nonzero $\mathbf{k}$.  These are the conduction band valleys.  In the case of bulk silicon, for example, there are 6 valleys (!), as in the figure.
 The six valleys in the Si conduction band, where the axes  here show the different components of $\mathbf{k}$, and  the blue dot is at $\mathbf{k}=0$.

One way to think about the states at the bottom of these valleys is that there are different wavefunctions that all have the same kinetic energy, the lowest they can and still be in the conduction band, but their actual spatial arrangements (how the electron probability density is arranged in the lattice) differ subtly.

In the case of graphene, I'd written about this before.  There are two valleys in graphene, and the states at the bottom of those valleys differ subtly about how charge is arranged between the two "sublattices" of carbon atoms that make up the graphene sheet.  What is special about graphene, and why other some materials are getting a lot of attention, is that you can do calculations about the valleys using the same math that gets used when talking about spin, the internal angular momentum of particles.  Instead of being in one graphene valley or the other, you can write about having "pseudospin" up or down.

Once you start thinking of valley-ness as a kind of internal degree of freedom of the electrons that is often conserved in many processes, like spin, then you can consider all sorts of interesting ideas.  You can talk about "valley ferromagnetism", where available electrons all hang out in one valley.  You can talk about the "valley Hall effect", where carriers of differing valleys tend toward opposite transverse edges of the material.   Because of spin-orbit coupling, these valley effects can link to actual spin physics, and therefore are of interest for possible information processing and optoelectronic ideas.