Energy scales and crystals in science fiction

Crystals are fascinating.  Somehow, for reasons that don't seem at all obvious at first glance, some materials grow in cool shapes as solids, with facets and obvious geometric symmetries.  This was early support for the idea of atoms, and it's no wonder at all that people throughout history have looked upon obviously crystalline materials as amazing, possibly connected with magical powers.

In science fiction (or maybe more properly science fantasy), crystals show up repeatedly as having special properties, often able to control or direct energies that seem more appropriate for particle physics.  In Star Trek, dilithium crystals are able to channel and control the flow of matter-antimatter reactions needed for warp drive, the superluminal propulsion system favored by the Federation and the Klingon Empire.  In Star Wars, kyber crystals are at the heart of lightsabers, and were also heavily mined by the Empire for use in the planet-killing main weapon of the Death Star.

In real life, though, crystals don't do so well in interacting with very high energy electromagnetic or particle radiation.  Yes, it is possible for crystals to scatter x-rays and high energy electrons - that's the way x-ray diffraction and electron diffraction work.  On very rare occasions, crystals can lead to surprising nuclear processes, such as all the iron atoms in a crystal sharing the recoil when an excited iron nucleus spits out a gamma ray, as in the Mossbauer Effect.   Much more typically, though, crystals are damaged by high energy radiation - if the energy scale of the photon or other particle is much larger than the few eV chemical energy scales that hold atoms in place or bind core electrons (say a few tens of eV), then the cool look and spatial arrangement of the atoms really doesn't matter, and atoms get kicked around.  The result is the creation of vacancies or interstitial defects, some of which can even act as "color centers", so that otherwise colorless Al2O3, for example, can take on color after being exposed to ionizing radiation in a reactor.

Ahh well.  Crystals are still amazing even if they can't propel starships faster than light.

(Happy new year to my readers!  I'm still trying to be optimistic, even if it's not always easy.)

Condensed matter and Christmas decorations - 'tis the season

Modern outdoor decorations owe quite a bit to modern science - polymers; metallurgy; electric power for the lighting, fans, sensors, and motors which make possible the motion-actuated inflatable Halloween decorations that scare my dog....  Condensed matter physics has, as in many areas of society, had a big impact on Christmas decorations that is so ubiquitous and pervasive that no one even thinks about it.  In particular, I'm thinking about the light emitting diode and its relative, the diode laser.  I'm pretty sure that Nick Holonyak and Shuji Nakamura never imagined that LEDs would pave the way for animated multicolor icicle decorations.  Likewise, I suspect that the inventors discussed here (including Holonyak) never envisioned laser projected holiday lighting.  So, the next time someone asks if any of this quantum stuff or basic research is useful, remember that these inherently quantum devices have changed the world in all kinds of ways that everyone sees but few observe. 

Materials and neuromorphic computing

(In response to a topic suggestion from the Pizza Perusing Physicist....)

Neuromorphic computing is a trendy concept aimed at producing computing devices that are structured and operate like biological neural networks.  

In standard digital computers, memory and logic are physically separated and handled by distinct devices, and both are (nearly always) based on binary states and highly regular connectivity.  That is, logic gates take inputs that are two-valued (1 or 0), and produce outputs that are similarly two-valued; logic gates have no intrinsic memory of past operations that they've conducted; memory elements are also binary, with data stored as a 1 or 0; and everything is organized in a regular, immutable pattern - memory registers populated and read by clocked, sequential logic gates via a bus.

Natural neural networks, on the other hand, are very different.  Each neuron can be connected to many others via synapses.  Somehow memory and logic are performed by the same neuronal components.   The topology of the connections varies with time - some connections are reinforced by repeated use, while others are demoted, in a continuous rather than binary way.  Information traffic involves temporal trains of pulses called spikes.  

All of these things can be emulated with standard digital computers.  Deep learning methods do this, with multiple layers playing the roles of neurons, and weighted links between nodes modeling the connections and strengths.  This is all a bit opaque and doesn't necessarily involve simulating the spiking dynamics at all.  Implementing neural networks via standard hardware loses some of the perceived benefits of biological neural nets, like very good power efficiency.

In the last few years, as machine learning and big data have become increasingly important, there has been a push to try to implement in device hardware architectures that look a lot more like the biological analogs.  To do this, you might want nonvolatile memory elements that can also be used for logic, and can have continuously graded values of "on"-ness determined by their history.  Resistive switching memory elements, sometimes called memristors (though that is a loaded term - see here and here), can fit the bill, as in this example.  Many systems can act as resistive switches, with conduction changes often set by voltage-driven migration of ions or vacancies in the material.

On top of this, there has been a lot of interest in using strongly correlated materials in such applications.  There are multiple examples of correlated materials (typically transition metal oxides) that undergo dramatic metal-insulator transitions as a function of temperature.  These materials then offer a chance to emulate spiking - driving a current can switch such a material from the insulating to the metallic state via local Joule heating or more nontrivial mechanisms, and then revert to the insulating state.  See the extensive discussion here.  

Really implementing all of this at scale is not simple.  The human brain involves something like 100,000,000,000 neurons, and connections run in three dimensions.  Getting large numbers of effective solid-state neurons with high connectivity via traditional 2D planar semiconductor-style fab (basically necessary if one wants to have many millions of neurons) is not easy, particularly if it requires adapting processing techniques to accommodate new classes of materials.

If you're interested in this and how materials physics can play a role, check out this DOE report and this recent review article.

Brief items

Here are some tidbits that came across my eyeballs this past week:

• I just ran into this article from early in 2019.  It touches on my discussion about liquids, and is a great example of a recurring theme in condensed matter physics.  The authors look at the vibrational excitations of liquid droplets on surfaces.  As happens over and over in physics, the imposition of boundary conditions on the liquid motion (e.g., wetting conditions on the surface and approximately incompressible liquid with a certain surface tension) leads to quantization of the allowed vibrations.  Discrete frequencies/mode shapes/energies are picked out due to those constraints, leading to a "periodic table" of droplet vibrations.  (This one looks moderately like atomic states, because spherical harmonics show up in the mode description, as they do when looking at atomic orbitals.)
• Another article from the past, this one from 2014 in the IEEE Spectrum.  It talks about how we arrived at the modern form for Maxwell's equations.  Definitely a good read for those interested in the history of physics.  Maxwell's theory was developing in parallel with what became vector calculus, and Maxwell's original description (like Faraday's intuition) was very mechanistic rather than abstract.
• Along those lines, this preprint came out recently promoting a graphical pedagogical approach to vector calculus.  The spirit at work here is that Feynman's graphical diagrammatic methods were a great way to teach people perturbative quantum field theory, and do perhaps a diagrammatic scheme for vector calc could be good.  I'm a bit of a skeptic - I found the approach by Purcell to be very physical and intuitive, and this doesn't look simpler to me.
• This preprint about twisted bilayer graphene and the relationship between superconductivity and strongly insulating states caught my eye, and I need to read it carefully.  The short version:  While phase diagrams showing superconductivity and insulating states as a function of carrier density make it tempting to think that SC evolves out of the insulating states via doping (as likely in the cuprates), the situation may be more complicated.

What is a liquid?

I wrote recently about phases of matter (and longer ago here).  The phase that tends to get short shrift in the physics curriculum is the liquid, and this is actually a symptom indicating that liquids are not simple things. 

We talk a lot about gases, and they tend to be simple in large part because they are low density systems - the constituents spend the overwhelming majority of their time far apart (compared to the size of the constituents), and therefore tend to interact with each other only very weakly.  We can even look in the ideal limit of infinitesimal particle size and zero interactions, so that the only energy in the problem is the kinetic energy of the particles, and derive the Ideal Gas Law.  

There is no such thing as an Ideal Liquid Law.  That tells you something about the complexity of these systems right there.

A classical liquid is a phase of matter in which the constituent particles have a typical interparticle distance comparable to the particle size, and therefore interact strongly, with both a "hard core repulsion" so that the particles are basically impenetrable, and usually through some kind of short-ranged attraction, either from van der Waals forces and/or longer-ranged/stronger interactions.  The kinetic energy of the particles is sufficiently large that they don't bond rigidly to each other and therefore move past and around each other continuously.  However, the density is so high that you can't even do very well by only worrying about pairs of interacting particles - you have to keep track of three-body, four-body, etc. interactions somehow.    

The very complexity of these strongly interacting collections of particles leads to the emergence of some simplicity at larger scales.  Because the particles are cheek-by-jowl and impenetrable, liquids are about as incompressible as solids.  The lack of tight bonding and enough kinetic energy to keep everyone moving means that, on average and on scales large compared to the particle size, liquids are homogeneous (uniform properties in space) and isotropic (uniform properties in all directions).  When pushed up against solid walls by gravity or other forces, liquids take on the shapes of their containers.  (If the typical kinetic energy per particle can't overcome the steric interactions with the local environment, then particles can get jammed.  Jammed systems act like "rigid" solids.)

Because of the constant interparticle collisions, energy and momentum get passed along readily within liquids, leading to good thermal conduction (the transport of kinetic energy of the particles via microscopic, untraceable amounts we call heat) and viscosity (the transfer of transverse momentum between adjacent rough layers of particles just due to collisions - the fluid analog of friction).  The lack of rigid bonding interactions means that liquids can't resist shear; layers of particles slide past each other.  This means that liquids, like gases, don't have transverse sound waves.   The flow of particles is best described by hydrodynamics, a continuum approach that makes sense on scales much bigger than the particles.   

Quantum liquids are those for which the quantum statistics of the constituents are important to the macroscopic properties.  Liquid helium is one such example.  Physicists have also adopted the term "liquid" to mean any strongly interacting, comparatively incompressible, flow-able system, such as the electrons in a metal ("Fermi liquid").  

Liquids are another example emergence that is deep, profound, and so ubiquitous that people tend to look right past it.  "Liquidity" is a set of properties so well-defined that a small child can tell you whether something is a liquid by looking at a video of it; those properties emerge largely independent of the microscopic details of the constituents and their interactions (water molecules with hydrogen bonds; octane molecules with van der Waals attraction; very hot silica molecules in flowing lava); and none of those properties are obvious if one starts with, say, the Standard Model of particle physics.  

General relativity (!) and band structure

Today we had a seminar at Rice by Qian Niu of the University of Texas, and it was a really nice, pedagogical look at this paper (arxiv version here).   Here's the basic idea.

As I wrote about here, in a crystalline solid the periodic lattice means that single-particle electronic states look like Bloch waves, labeled by some wavevector \(\mathbf{k}\), of the form \(u_{\mathbf{k}}(\mathbf{r}) \exp(i \mathbf{k}\cdot \mathbf{r})\) where \(u_{\mathbf{k}}\) is periodic in space like the lattice.  It is possible to write down semiclassical equations of motion of some wavepacket that starts centered around some spatial position \(\mathbf{r}\) and some (crystal) momentum \(\hbar \mathbf{k}\).  These equations tell you that the momentum of the wavepacket changes with time as due to the external forces (looking a lot like the Lorentz force law), and the position of the wavepacket has a group velocity, plus an additional "anomalous" velocity related to the Berry phase (which has to do with the variation of \(u_{\mathbf{k}}\) over the allowed  values of \(\mathbf{k}\)).

The paper asks the question, what are the semiclassical equations of motion for a wavepacket if the lattice is actually distorted a bit as a function of position in real space.  That is, imagine a strain gradient, or some lattice deformation.  In that case, the wavepacket can propagate through regions where the lattice is varying spatially on very long scales while still being basically periodic on shorter scales still long compared to the Fermi wavelength.

It turns out that the right way to tackle this is with the tools of differential geometry, the same tools used in general relativity.  In GR, when worrying how the coordinates of a particle change as it moves along, there is the ordinary velocity, and then there are other changes in the components of the velocity vector because the actual geometry of spacetime (the coordinate system) is varying with position.  You need to describe this with a "covariant derivative", and that involves Christoffel symbols.  In this way, gravity isn't a force - it's freely falling particles propagating as "straight" as they can, but the actual geometry of spacetime makes their trajectory look curved based on our choice of coordinates.

For the semiclassical motion problem in a distorted lattice, something similar happens.  You have to worry about how the wavepacket evolves both because of the local equations of motion, and because the wavepacket is propagating into a new region of the lattice where the \(u_{\mathbf{k}}\) functions are different because the actual lattice is different (and that also affects the Berry phase anomalous velocity piece).   Local rotations of the lattice can lead to an affective Coriolis force on the wavepacket; local strain gradients can lead to effective accelerations of the wavepacket.

(For more fun, you can have temporal periodicity as well.  That means you don't just have Bloch functions in 3d, you have Bloch-Floquet functions in 3+1d, and that's where I fell behind.)

Bottom line:  The math of general relativity is an elegant way to look at semiclassical carrier dynamics in real materials.  I knew that undergrad GR course would come in handy....

Recent results on the arxiv

Here are a few interesting results I stumbled upon recently:

• This preprint has become this Science paper that was published this week.  The authors take a cuprate superconductor (YBCO) and use reactive ion etching to pattern an array of holes in the film.  Depending on how long they etch, they can kill global superconductivity but leave the system such that it still behaves as a funny kind of metal (resistance decreasing with decreasing temperature), with some residual resistance at low temperatures.  The Hall effect in this metallic state produces no signal - a sign that the is balance between particle-like and hole-like carriers (particle-hole symmetry).  For magnetic field perpendicular to the film, they also see magnetoresistance with features periodic in flux through one cell of the pattern, with a periodicity that indicates the charge carriers have charge 2e.  This is an example of a "Bose metal".  Neat!  (The question about whether there are pairs without superconductivity touches on our own recent work.)
• This preprint was recently revised (and thus caught my eye in the arxiv updates).  In it, the authors are using machine learning to try to find new superconductors.  The results seem encouraging.  I do wonder if one could do a more physics-motivated machine learning approach (that is, something with an internal structure to the classification scheme and the actual weighting procedure) to look at this and other related problems (like identifying which compounds might be growable via which growth techniques).
• This preprint is not a condensed matter topic, but has gotten a lot of attention.  The authors look at a particular nuclear transition in 4He, and find a peculiar angular distribution for the electron-positron pairs that come out.  The reason this is of particular interest is that this paper by the same investigators looking at a nuclear transition in 8Be three years ago found something very similar.  If one assumes that there is a previously unobserved boson (a dark matter candidate perhaps) of some sort with a mass of around 17 MeV that couples in there, that could explain both results.  Intriguing, but it would be great if these observations were confirmed independently by a different group.  

Advice on proposal writing

Many many people have written about how to write scientific grant proposals, and much of that advice is already online.   Rather than duplicate that work, and recognizing that sometimes different people need to hear advice in particular language, I want to link to some examples.

• Here (pdf) is some advice straight from the National Science Foundation about how to write a compelling proposal.  It's older (2004) and a bit out of date, but the main points are foundational.
• This is a very good collection of advice that has been updated (2015) to reflect current practice about NSF.  
• Here are lecture notes from a course at Illinois that touched on this as well, generalizing beyond the NSF.

Fundamentally, sponsored academic research is an odd thing.  You are trying to convince an agency or foundation with finite (often very finite) resources that allocating some of their precious support to you will be a good thing.  Limiting the conversation to the often ill-named "basic research" (see here and the book therein for a discussion of "basic" vs "applied"), this means work where the primary products of the research are (i) fundamental advances in our understanding of some system or phenomena; (ii) personnel trained in scientific/engineering/research knowledge and skills; (iii) scholarly publications (and/or patents for more technology-focused topics) that report the results, with the intent of propagating the work to the community and having an impact. 

This last one has taken a pre-eminent position of importance because it's something that can be readily counted and measured.  There is a rough rule that many program officers in NSF and DOE will tell you; averaging over their programs, they get roughly one high impact paper per $100K total cost.  They would like more, of course. 

Talk with program officers before writing and submitting - know the audience.  Program officers (including foundational ones) tend to take real pride in their portfolios.  Everyone likes funding successful, high-impact, exciting, trend-setting work.  Still, particular program officers have areas of emphasis, in part so that there is not duplication of effort or support within an agency or across agencies.  (This is especially true in areas like high energy theory, where if you've got DOE funding, you essentially can't get NSF support, and vice versa.)  You will be wasting your time if you submit to the wrong program or pitch your idea to the wrong reviewing audience.   NSF takes a strong line that their research directions are broadly set by the researchers themselves, via their deep peer review process (mail-in reviews, in-person or virtual panel discussions) and workshops that define programmatic goals.  DOE likewise has workshops to help define major challenges and open questions, though my sense is that the department takes a more active role in delineating priorities.   The DOD is more goal-directed, with program officers having a great deal of sway on topics of interest, and the prospect that such research may transition closer to technology-readiness.  Foundations are idiosyncratic, but a common refrain is that they prefer to fund topics that are not already supported by federal agencies.

Think it through, and think like a referee.  When coming up with an idea, do your best to consider in some detail how you would actually pull this off.  How could you tell if it works?  What would the implications be of success?  What are the likely challenges and barriers?  If some step doesn't go as planned, is it a show-stopper, or are their other ways to go?  As an experimentalist:  Do you have the tools you need to do this?  How big a signal are you trying to detect?   Remember, referees are frequently asked to evaluate strengths and weaknesses of technical approach.  Better to have this in mind while at an early stage of the process.

Clearly state the problem, and explain the proposal's organization.  Reviewers might be asked to read several proposals in a short timeframe.  It seems like a good idea to say up front, in brief (like in a page or so):  What is the problem?  What are the open scientific/engineering questions you are specifically addressing?  What is your technical approach?  What will the results mean?  Then, explain the organization of the proposal (e.g., section 2 gives a more detailed introduction to the problem and open questions; section 3 explains the technical approach, including a timeline of proposed work; etc.).  This lets readers know where to find things. 

I'll confess:  I got this organizational approach by emulating the structure of an excellent proposal that I reviewed a number of years ago.  It was really terrific - clear; pedagogical, so that a non-expert in that precise area could understand the issues and ideas; very cleanly written; easy-to-read figures, including diagrams that really showed how the ideas would work.   Reviewing proposals is very helpful in improving your own.  Very quickly you will get a sense of what you think makes a good or bad proposal.  NSF is probably the most open to getting new investigators involved in the reviewing process. 

Don't wait until the last minute.  You know that classmate of yours from undergrad days, the one who used to brag about how they waited until the night before to blitz through a 20 page writing assignment?  Amazingly, some of these people end up as successful academics.  I genuinely don't know how they do it, because these days research funding is so competitive and proposals are detailed and complicated.  There are many little formatting details that agencies enforce now.  You don't want to get to an hour before the deadline and realize that all of your bibliographic references are missing a URL field.   People really do read sections like data management plans and postdoctoral mentoring plans - you can't half-ass them.   Also, while it is unlikely to sink a really good proposal, it definitely comes across badly to referees if there are missing or mislabeled references, figures, etc. 

I could write more, and probably will amend this down the line, but work calls and this is at least a start.

Rice Academy of Fellows 2020

As I had posted a year ago:  Rice has a university-wide competitive postdoctoral fellow program known as the Rice Academy of Fellows.   Like all such things, it's very competitive.  The new application listing has gone live here with a deadline of January 3, 2020.  Applicants have to have a faculty mentor, so in case someone is interested in working with me on this, please contact me via email.  We've got some fun, exciting stuff going on!

Sorry for the hiatus

My apologies for the unusually long hiatus in posts.  Proposal deadlines + department chair obligations + multiple papers in process made the end of October very challenging.   Later next week I expect to pick up again.  Suggested topics (in the comments?) are always appreciated.  I realize I've never written an advice-on-grant-proposal-writing post.  On the science side, I'm still mulling over the most accessible way to describe quantum Hall physics, and there are plenty of other "primer" topics that I should really write at some point.

If I hadn't been so busy, I would've written a post during the baseball World Series about how the hair of Fox Sports broadcaster Joe Buck is a study in anisotropic light scattering.  Viewed straight on, it's a perfectly normal color, but when lit and viewed from an angle, it's a weirdly iridescent yellow - I'm thinking that this really might have interesting physics behind it, in the form of some accidental structural color. 

More items of interest

This continues to be a very very busy time, but here are a few interesting things to read:

• "Voodoo fusion" - an article from the APS Forum on Physics and Society that pretty much excoriates all of the fusion-related startup efforts, basically saying that they're about as legitimate as Theranos.  Definitely an interesting read.  
• https://arxiv.org/abs/1910.06389 - This is Kenneth Libbrecht's tour de force monograph on snow crystals.   Talk about an example of emergence:  From the modest water molecule comes, when many of them get together, the remarkable, intricate structure of snowflakes, with highly complex six-fold rotational symmetry.
• Somehow this tenure announcement just showed up in my newsfeed.  It's very funny, and the titles and abstracts of his talks are in a similar vein.  Perhaps I need to start writing up my physics talks this way.  
• Beware of unintended consequences of ranking metrics.
• https://arxiv.org/abs/1910.05813 - insert snarky comment here about how this is a mathematical model for hiring/awards/funding/publication.

"Phase of matter" is a pretty amazing emergent concept

As we await the announcement of this year's physics Nobel tomorrow morning (last chance for predictions in the comments), a brief note:

I think it's worth taking a moment to appreciate just how amazing it is that matter has distinct thermodynamic phases or states.

We teach elementary school kids that there are solids, liquids, and gases, and those are easy to identify because they have manifestly different properties.  Once we know more about microscopic details that are hard to see with unaided senses, we realize that there are many more macroscopic states - different structural arrangements of solids; liquid crystals; magnetic states; charge ordered states; etc.

When we take statistical physics, we learn descriptively what happens.  When you get a large number of particles (say atoms for now) together, the macroscopic state that they take on in thermal equilibrium is the one that corresponds to the largest number of microscopic arrangements of the constituents under the given conditions.  So, the air in my office is a gas because, at 298 K and 101 kPa, there are many many more microscopic arrangements of the molecules with that temperature and pressure that look like a gas than there are microscopic arrangements of the molecules that correspond to a puddle of N2/O2 mixture on the floor. 

Still, there is something special going on.  It's not obvious that there should have to be distinct phases at all, and such a small number of them.  There is real universality about solids - their rigidity, resistance to shear, high packing density of atoms - independent of details.  Likewise, liquids with their flow under shear, comparative incompressibility, and general lack of spatial structure.  Yes, there are detailed differences, but any kid can recognize that water, oil, and lava all have some shared "liquidity".  Why does matter end up in those configurations, and not end up being a homogeneous mush over huge ranges of pressure and temperature?  This is called emergence, because while it's technically true that the standard model of particle physics undergirds all of this, it is not obvious in the slightest how to deduce the properties of snowflakes, raindrops, or water vapor from there.    Like much of condensed matter physics, this stuff is remarkable (when you think about it), but so ubiquitous that it slides past everyone's notice pretty much of the time.

Items of interest

As I struggle with being swamped this semester, some news items:

• Scott Aaronson has a great summary/discussion about the forthcoming google/John Martinis result about quantum supremacy.  The super short version:  There is a problem called "random circuit sampling", where a sequence of quantum gate operations is applied to some number of quantum bits, and one would like to know the probability distribution of the outcomes.  Simulating this classically becomes very very hard as the number of qubits grows.  The google team apparently just implemented the actual problem directly using their 53-qubit machine, and could infer the probability distribution by directly sampling a large number of outcomes.   They could get the answer this way in 3 min 20 sec for a number of qubits where it would take the best classical supercomputer 10000 years to simulate.  Very impressive and certainly a milestone (though the paper is not yet published or officially released).  This has led to some fascinating semantic discussions with colleagues of mine about what we mean by computation.  For example, this particular situation feels a bit to me like comparing the numerical solution to a complicated differential equation (i.e. some Runge-Kutta method) on a classical computer with an analog computer using op-amps and R/L/C components.  Is the quantum computer here really solving a computational problem, or is it being used as an experimental platform to simulate a quantum system?  And what is the difference, and does it matter?  Either way, a remarkable achievement.  (I'm also a bit jealous that Scott routinely has 100+ comment conversations on his blog.)
• Speaking of computational solutions to complex problems.... Many people have heard about chaotic systems and why numerical solutions to differential equations can be fraught with peril due to, e.g., rounding errors.  However, I've seen two papers this week that show just how bad this can be.  This very good news release pointed me to this paper, where it shows that even using 64 bit precision doesn't save you from issues in some systems.  Also this blog post points to this paper, which shows that n-body gravitational simulations have all sorts of problems along these lines.  Yeow.
• SpaceX has assembled their mammoth sub-orbital prototype down in Boca Chica.  This is going to be used for test flights up to 22 km altitude, and landings.  I swear, it looks like something out of Tintin or The Conquest of Space.  Awesome.
• Time to start thinking about Nobel speculation.  Anyone?

DOE Experimental Condensed Matter PI Meeting, day 3 and wrapup

On the closing day of the PI meeting, some further points and wrap-up:

• I had previously missed work that shows that electric field can modulate magnetic exchange in ultrathin iron (overview).
• Ferroelectric layers can modulate transport in spin valves by altering the electronic energetic alignment at interfaces.  This can result in some unusual response (e.g., the sign of the magnetoresistance can flip with the sign of the current, implying spin-diode-like properties).
• Artificial spin ices are still cool model systems.  With photoelectron emission microscopy (PEEM), it's possible to image ultrathin, single-domain structures to reveal their mangetization noninvasively.  This means movies can be made showing thermal fluctuations of the spin ice constituents, revealing the topological character of the magnetic excitations in these systems.  
• Ultrathin oxide membranes mm in extent can be grown, detached from their growth substrates, and transferred or stacked.  When these membranes are really thin, it becomes difficult to nucleate cracks, allowing the membranes to withstand large strains (several percent!), opening up the study of strain effects on a variety of oxide systems.
• Controlled growth of stacked phthalocyanines containing transition metals can generate nice model systems for studying 1d magnetism, even using conventional (large-area) methods like vibrating sample magnetometry.
• In situ oxide MBE and ARPES, plus either vacuum annealing or ozone annealing, has allowed the investigation of the BSCCO superconducting phase diagram over the whole range of dopings, from severely underdoped to so overdoped that superconductivity is completely suppressed.  In the overdoped limit, analyzing the kink found in the band dispersion near the antinode, it seems superconductivity is suppressed at high doping because the coupling (to the mode that causes the kink) goes to zero at large doping.  
• It's possible to grow nice films of C60 molecules on Bi2Se3 substrates, and use ARPES to see the complicated multiple valence bands at work in this system.  Moreover, by doing measurements as a function of the polarization of the incoming light, the particular molecular orbitals contributing to those bands can be identified.
• Through careful control of conditions during vacuum filtration, it's possible to produce dense, locally crystalline films of aligned carbon nanotubes.  These have remarkable optical properties, and with the anisotropy of their electronic structure plus ultraconfined character, it's possible to get exciton polaritons in these into the ultrastrong coupling regime.

Overall this was a very strong meeting - the variety of topics in the program is impressive, and the work shown in the talks and posters was uniformly interesting and of high quality. 

DOE Experimental Condensed Matter PI Meeting, Day 2

Among the things I heard about today, as I wondered whether newly formed Tropical Storm Imelda would make my trip home a challenge:

• In "B20" magnetic compounds, where the crystal structure is chiral but lacks mirror or inversion symmetry, a phase can form under some circumstances that is a spontaneous lattice of skyrmions.  By adding disorder through doping, it is possible to un-pin that lattice.
• Amorphous cousins of those materials still show anomalous Hall effect (AHE), even though the usual interpretation these days of the AHE is as a consequence of Berry phase in momentum space that is deeply connected to having a lattice.  It's neat to see that some Berry physics survives even when the lattice does not.
• There is a lot of interest in coupling surface states of topological insulators to ferromagnets, including using spin-orbit torque to switch the magnetization direction of a ferromagnetic insulator.
• You could also try to switch the magnetization of \(\alpha-Fe_{2}O_{3}\) using spin-orbit torques, but watch out when you try to cram too much current through a 2 nm thick Pt film.
• The interlayer magnetic exchange in van der Waals magnets continues to be interesting and rich.
• Heck, you could look at several 2D materials with various kinds of reduced symmetry, to see what kinds of spin-orbit torques are possible.
• It's always fun to find a material where there are oscillations in magnetization with applied field even though the bulk is an insulator.
• Two-terminal devices made using (Weyl superconducting) MoTe2 show clear magnetoresistance signatures, indicating supercurrents carried along the material edges.
• By side-gating graphene structures hooked up to superconductors, you can also make a superconducting quantum intereference device using edge states of the fractional quantum Hall effect.
• In similar spirit, coupling a 2D topological insulator (1T'-WTe2) to a superconductor (NbSe2) means it's possible to use scanning tunneling spectroscopy to see induced superconducting properties in the edge state.
• Just in time, another possible p-wave superconductor.
• In a special stack sandwiching a TI between two magnetic TI layers, it's possible to gate the system to break inversion symmetry, and thus tune between quantum anomalous Hall and "topological Hall" response.
• Via a typo on a slide, I learned of the existence of the Ohion, apparently the smallest quantized amount of Ohio.

DOE experimental condensed matter PI meeting, day 1

The first day of the DOE ECMP PI meeting was very full, including two poster sessions.  Here are a few fun items:

• Transition metal dichalcogenides (TMDs) can have very strongly bound excitons, and if two different TMDs are stacked, you can have interlayer excitons, where the electron and hole reside in different TMD layers, perhaps separated by a layer or two of insulating hBN.  Those interlayer excitons can have long lifetimes, undergo Bose condensation, and have interesting optical properties.  See here, for example.   
• Heterojunctions of different TMDs can produce moire lattices even with zero relative twist, and the moire coupling between the layers can strongly affect the optical properties via the excitons.
• Propagating plasmons in graphene can have surprisingly high quality factors (~ 750), and combined with their strong confinement have interesting potential.
• You can add AlAs quantum wells to the list of materials systems in which it is possible to have very clean electronic transport and see fractional quantum Hall physics, which is a bit different because of the valley degeneracy in the AlAs conduction band (that can be tuned by strain).
• And you can toss in WSe2 in there, too - after building on this and improving material quality even further.
• There continues to be progress in trying to interface quantum Hall edge states with superconductors, with the end goal of possible topological quantum computing.  A key question is understanding how the edge states undergo Andreev processes at superconducting contacts.
• Application of pressure can take paired quantum Hall states (like those at \(\nu = 5/2, 7/2\)) and turn them into unpaired nematic states, a kind of quantum phase transition.  
• With clever (and rather involved) designs, it is possible to make high quality interferometers for fractional quantum Hall edge states, setting the stage for detailed studies of exotic anyons.

DOE Experimental Condensed Matter PI meeting, 2019

The US Department of Energy's Basic Energy Sciences component of the Office of Science funds a lot of basic scientific research, and for the last decade or so had a tradition of regular gatherings of their funded principal investigators for a number of programs.  Every two years there has been a PI meeting for the Experimental Condensed Matter Physics program, and this year's meeting starts tomorrow.

These meetings are very educational (at least for me) and, because of their modest size, a much better networking setting than large national conferences.  In past years I've tried to write up brief highlights of the meetings (for 2017, see a, b, c; for 2015 see a, b, c; for 2013 see a, b).   I will try to do this again; the format of the meeting has changed to include more poster sessions, which makes summarizing trickier, but we'll see.

update:  Here are my write-ups for day 1, day 2, and day 3.

Faculty position at Rice - Astronomy

Faculty position in Astronomy at Rice University

The Department of Physics and Astronomy at Rice University invites applications for a tenure-track faculty position in astronomy in the general field of galactic star formation and planet formation, including exoplanet characterization. We seek an outstanding theoretical, observational, or computational astronomer whose research will complement and extend existing activities in these areas within the department. In addition to developing an independent and vigorous research program, the successful applicant will be expected to teach, on average, one undergraduate or graduate course each semester, and contribute to the service missions of the department and university. The department expects to make the appointment at the assistant professor level. A Ph.D. in astronomy/astrophysics or related field is required.

Applications for this position must be submitted electronically at http://jobs.rice.edu/postings/21236. Applicants will be required to submit the following: (1) cover letter; (2) curriculum vitae; (3) statement of research; (4) teaching statement; (5) PDF copies of up to three publications; and (6) the names, affiliations, and email addresses of three professional references. We will begin reviewing applications December 1, 2019. To receive full consideration, all application materials must be received by January 10, 2020. The appointment is expected to begin in July, 2020.

Rice University is an Equal Opportunity Employer with a commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status. We encourage applicants from diverse backgrounds to apply.

Faculty position at Rice - Theoretical Biological Physics

Faculty position in Theoretical Biological Physics at Rice University

As part of the Vision for the Second Century (V2C2), which is focused on investments in research excellence, Rice University seeks faculty members, preferably at the assistant professor level, starting as early as July 1, 2020, in all areas of theoretical biological physics. Successful candidates will lead dynamic, innovative, and independent research programs supported by external funding, and will excel in teaching at the graduate and undergraduate levels, while embracing Rice’s culture of excellence and diversity.

This search will consider applicants from all science and engineering disciplines. Ideal candidates will pursue research with strong intellectual overlap with physics, chemistry, biosciences, bioengineering, chemical and biomolecular engineering, or other related disciplines. Applicants pursuing all styles of theory and computation integrating the physical and life sciences are encouraged to apply.

For full details and to apply, please visit https://jobs.rice.edu/postings/21170.  Applicants should please submit the following materials: (1) cover letter, including the names and contact information for three references, (2) curriculum vitae, (3) research statement, and (4) statement of teaching philosophy. Application review will commence no later than October 15, 2019 and continue until the position is filled. Candidates must have a PhD or equivalent degree and outstanding potential in research and teaching. We particularly encourage applications from women and members of historically underrepresented groups who bring diverse cultural experiences and who are especially qualified to mentor and advise members of our diverse student population.

Rice University, located in Houston, Texas, is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status.

Big questions about condensed matter (part 3)

More questions asked by Ross McKenzie's son about the culture/history of condensed matter physics:

3.  What are the most interesting historical anecdotes?  What are the most significant historical events?  Who were the major players?

The first couple of these are hard to address in anything resembling an unbiased way.  For events that happened before I was in the field, I have to rely on stories I've read or things I've heard.  Certainly the discovery of superconductivity by Onnes is a good example - where they thought that they had an experimental problem with their wiring, until they realized that their voltmeter reading dropping to zero (trying to measure the voltage drop across some mercury in the presence of a known current) happened at basically the same temperature every time.  (Pretty good for 1911!).  Major experimental results very often have fun story components.  From my thesis adviser, I'd heard lots of stories about the discovery of superfluidity in 3He, including plugging a leaky vacuum flange using borax; thinking up the experiment while recovering from a broken leg skiing accident; the wee-hour phone call to the adviser.   He also told me a story about this paper, where he and Gerry Dolan came up with a very clever way to see tiny deviations away from a mostly linear current-voltage curve, an observation connected with weak localization that paved the way for a lot of mesoscopic physics work.   

There are fun theory stories, too.  Bob Laughlin figuring out the theory of the fractional quantum Hall effect while stuck in a trailer at Livermore because his clearance paperwork hadn't come through yet.

Other stories I've read in books.  Strong recommendations for Crystal Fire; the less popular/more scholarly Out of the Crystal Maze.  Ohl's discovery of the photovoltaic effect in silicon.  The story about how Bell Labs and IBM researchers may or may not have traded hints poolside in Las Vegas about how to get field-effect transistors really working.  Shockley's inability to manage people eventually resulting in Silicon Valley.  

These aren't necessarily the best anecdotes, but they have elements of interest.  I'm sure there are many out there who could tell fun stories.

As for the major players, it seems that everyone mentioned on Prof. McKenzie's post and in the comments are theorists.  That seems limiting.  It's fair to talk about theorists if you're concentrating on theoretical developments, but experimentalists have often opened up whole areas.  Onnes liquefied helium and discovered superconductivity.  Laue invented x-ray diffraction.  Brattain made transistors.  Nick Holonyak was an inventor of the light emitting diode, which has been revolutionary.   Binnig and Rohrer invented the STM.  Bednorz and Muller discovered the cuprates.