Monday, April 24, 2017

Quantum conduction in bad metals, and jphys+

I've written previously about bad metals.  We recently published a result (also here) looking at what happens to conduction in an example of such a material at low temperatures, when quantum corrections to conduction (like these) should become increasingly important.   If you're interested, please take a look at a blog post I wrote about this that is appearing on jphys+, the very nice blogging and news/views site run by the Institute of Physics.

Sunday, April 23, 2017

Thoughts after the March for Science

About 10000 people turned out (according to the Houston Chronicle) for our local version of the March for Science.   Observations:

  • While there were some overtly partisan participants and signs, the overarching messages that came through were "We're all in this together!", "Science has made the world a better place, with much less disease and famine, a much higher standard of living for billions, and a greater understanding of the amazingness of the universe.", "Science does actually provide factual answers to properly formulated scientific questions", and "Facts are not opinions, and should feed into policy decisions, rather than policy positions altering what people claim are facts."
  • For a bunch of people often stereotyped as humorless, scientists had some pretty funny, creative signs.  A personal favorite:  "The last time scientists were silenced, Krypton exploded!"  One I saw online:  "I can't believe I have to march for facts."
  • Based on what I saw, it's hard for me to believe that this would have the negative backlash that some were worrying about before the event.  It simply wasn't done in a sufficiently controversial or antagonistic way.  Anyone who would have found the messages in the first point above to be offensive and polarizing likely already had negative perceptions of scientists, and (for good or ill) most of the population wasn't paying much attention anyway.
So what now?

  • Hopefully this will actually get more people who support the main messages above to engage, both with the larger community and with their political representatives.  
  • It would be great to see some more scientists and engineers actually run for office.  
  • It would also be great if more of the media would get on board with the concept that there really are facts.  Policy-making is complicated and must take into account many factors about which people can have legitimate disagreements, but that does not mean that every statement has two sides.  "Teach the controversy" is not a legitimate response to questions of testable fact.  In other words, Science is Real
  • Try to stay positive and keep the humor and creativity flowing.  We are never going to persuade a skeptical, very-busy-with-their-lives public if all we do is sound like doomsayers.

Thursday, April 20, 2017

Ready-to-peel semiconductors!

Every now and then there is an article that makes you sit up and say "Wow!"  

Epitaxy is the growth of crystalline material on top of a substrate with a matching (or very close to it) crystal structure.  For example, it is possible to grow InAs epitaxially on top of GaSb, or SiGe epitaxially on top of Si.  The idea is that the lattice of the underlying material guides the growth of the new layers of atoms, and if the lattice mismatch isn't too bad and the conditions are right, you can get extremely high quality growth (that is, with nearly perfect structure).  The ability to grow semiconductor films epitaxially has given us a ton of electronic devices that are everywhere around us, including light emitting diodes, diode lasers, photodiodes, high mobility transistors, etc.   Note that when you grow, say, AlGaAs epitaxially on a GaAs substrate, you end up with one big crystal, all covalently bonded.  You can't readily split off just the newly grown material mechanically.  If you did homoepitaxy, growing GaAs on GaAs, you likely would not even be able to figure out where the substrate ended and the overgrown film began.

In this paper (sorry about the Nature paywall - I couldn't find another source), a group from MIT has done something very interesting.  They have shown that a monolayer of graphene on top of a substrate does not screw up overgrowth of material that is epitaxially registered with the underlying substrate.  That is, if you have an atomically flat, clean GaAs substrate ("epiready"), and cover it with a single atomic layer of graphene, you can grow new GaAs on top of the graphene (!), and despite the intervening carbon atoms (with their own hexagonal lattice in the way), the overgrown GaAs will have registry (crystallographic alignment and orientation) with the underlying substrate.  Somehow the short-ranged potentials that guide the overgrowth are able to penetrate through the graphene.  Moreover, after you've done the overgrowth, you can actually peel off the epitaxial film (!!), since it's only weakly van der Waals bound to the graphene.  They demonstrate this with a variety of overgrown materials, including a III-V semiconductor stack that functions as a LED.  

I found this pretty amazing.  It suggests that there may be real opportunities for using layered van der Waals materials to grow new and unusual systems, perhaps helping with epitaxy even when lattice mismatch would otherwise be a problem.  I suspect the physics at work here (chemical interactions from the substrate "passing through" overlying graphene) is closely related to this work from several years ago.

Wednesday, April 19, 2017

March for Science, April 22

There has been a great deal written by many (e.g., 1 2 3 4 5 6) about the upcoming March for Science.  I'm going to the Houston satellite event.  I respect the concern that such a march risks casting scientists as "just another special interest group", or framing scientists as a group as leftists who are reflexively opposed to the present US administration.  Certainly some of the comments from the march's nominal twitter feed are (1) overtly political, despite claims that the event is not partisan; and (2) not just political, but rather extremely so.

On balance, though, I think that the stated core messages (science is not inherently partisan; science is critical for the future of the country and society; policy making about relevant issues should be informed by science) are important and should be heard by a large audience.   If the argument is that scientists should just stay quiet and keep their heads down, because silence is the responsible way to convey objectivity, I am not persuaded.  

Friday, April 14, 2017

"Barocalorics", or making a refrigerator from rubber

People have spent a lot of time and effort in trying to control the flow and transfer of heat.  Heat is energy transferred in a disorganized way among many little degrees of freedom, like the vibrations of atoms in a solid or the motion of molecules in a gas.  One over-simplified way of stating how heat likes to flow:  Energy tends to be distributed among as many degrees of freedom as possible.  The reason heat flows from hot things to cold things is that tendency.  Manipulating the flow of heat then really all comes down to manipulating ways for energy to be distributed.

Refrigerators are systems that, with the help of some externally supplied work, take heat from a "cold" side, and dump that heat (usually plus some additional heat) to a "hot" side.  For example, in your household refrigerator, heat goes from your food + the refrigerator inner walls (the cold side) into a working fluid, some relative of freon, which boils.  That freon vapor gets pumped through coils; a fan blows across those coils and (some of) the heat is transferred from the freon vapor to the air in your kitchen.   The now-cooler freon vapor is condensed and pumped (via a compressor) and sent back around again.  

There are other ways to cool things, though, than by running a cycle using a working fluid like freon. For example, I've written before about magnetic cooling.  There, instead of using the motion of liquid and gas molecules as the means to do cooling, heat is made to flow in the desired directions by manipulating the spins of either electrons or nuclei.  Basically, you can use a magnetic field to arrange those spins such that it is vastly more likely for thermal energy to come out of the jiggling motion of your material of interest, and instead end up going into rearranging those spins.

Stretching a polymer tends to heat it, due to the barocaloric
effect.  Adapted from Chauhan et al., doi:10.1557/mre.2015.17 
It turns out, you can do something rather similar using rubber.  The key is something called the elasto-caloric or barocaloric effect - see here (pdf!) for a really nice review.  The effect is shown in the figure, adapted from that paper.   An elastomer in its relaxed state is sitting there at some temperature and with some entropy - the entropy has contributions due to the jiggling around of the atoms, as well as the structural arrangement of the polymer chains.  There are lots of ways for the chains to be bunched up, so there is quite a bit of entropy associated with that arrangement.  Roughly speaking, when the rubber is stretched out quickly (so that there is no time for heat to flow in or out of the rubber) those chains straighten, and the structural piece of the entropy goes down.  To make up for that, the kinetic contribution to the entropy goes up, showing up as an elevated temperature.  Quickly stretch rubber and it gets warmer.  A similar thing happens when rubber is compressed instead of stretched.  So, you could imagine running a refrigeration cycle based on this!  Stretch a piece of rubber quickly; it gets warmer (\(T \rightarrow T + \Delta T\)).  Allow that heat to leave while in the stretched state (\(T + \Delta T \rightarrow T\)).  Now release the rubber quickly so no heat can flow.  The rubber will get colder now than the initial \(T\); energy will tend to rearrange itself out the kinetic motion of the atoms and into crumpling up the polymer chains.  The now-cold rubber can be used to cool something.  Repeat the cycle as desired.  It's a pretty neat idea.  Very recently, this preprint showed up on the arxiv, showing that a common silicone rubber, PDMS, is great for this sort of thing.  Imagine making a refrigerator out of the same stuff used for soft contact lenses!  These effects tend to have rather limited useful temperature ranges in most elastomers, but it's still funky.


Monday, April 10, 2017

Shrinkage - the physics of shrink rays

It's a trope that's appeared repeatedly in science fiction:  the shrink ray, a device that somehow takes ordinary matter and reduces it dramatically in physical size.  Famous examples include Fantastic Voyage, Honey I Shrunk the Kids, Innerspace, and Ant Man.  This particular post was inspired partly by my old friend Rob Kutner's comic series Shrinkage, where tiny nanotech-using aliens take over the mind of the (fictitious) President, with the aim of turning the world into a radioactive garden spot for themselves.  (Hey Rob - your critters thrive on radioactivity, yet if they're super small, they're probably really inefficient at capturing that radiation.  Whoops.)  Coincidentally, this week there was an announcement about a film option for Michael Crichton's last book, in which some exotic (that is to say, mumbo jumbo) "tensor field" is used to shrink people.

It's easy to enumerate many problematic issues that should arise in these kinds of stories:
  • Do the actual atoms of the objects/people shrink?  
  • If so, even apart from how that's supposed to work, what do these people breathe?  (At least Ant Man has a helmet that could be hand-waved to shrink air molecules....)  Or eat/drink?
  • What about biological scaling laws?
  • If shrunken objects keep their mass, that means a lot of these movies don't work.  Think about that tank that Hank Pym carries on his keychain....  If they don't keep their mass, where does that leave the huge amounts of energy (\(mc^2\)) that would have to be accounted for?
  • How can these people see if their eyes and all their cones/rods become much smaller than the wavelength of light?
  • The dynamics of interacting with a surrounding fluid medium (air or water) are completely different for very small objects - a subject explored at length by Purcell in "Life at Low Reynolds Number". 
The only attempt I've ever seen in science fiction to discuss some kind of real physics that would have to be at work in a shrink ray was in Isaac Asimov's novel Fantastic Voyage II.   One way to think about this is that the size of atoms is set by a competition between the electrostatic attraction between the electrons and the nucleus, and the puffiness forced by the uncertainty principle.  The typical size scale of an atom is given by the Bohr radius, \( a_{0} \equiv (4 \pi \epsilon_{0} \hbar^{2})/(m_{\mathrm{e}}e^{2}) \), where \(m_{\mathrm{e}} \) is the mass of the electron, and e is the electronic charge.   Shrinking actual atoms would require rejiggering some fundamental natural constants.  For example, you could imagine shrinking atoms by cranking up the electronic charge (and hence the attractive force between the electron and the nucleus).  That would have all kids of other consequences, however - such as screwing up chemistry in a big way.  

Of course, if we want to keep the appearances that we see in movies and TV, then somehow the colors of shrunken objects have to remain what they were at full size.   That would require the typical energy scale for optical transitions in atoms, for example, to remain unchanged.  That is, the Rydberg \( \equiv  m_{\mathrm{e}}e^4/(8 \epsilon_{0}^2 h^3 c) \) would have to stay constant.  Satisfying these constraints is very tough.  Asimov's book takes the idea that the shrink ray messes with Plank's constant, and I vaguely recall some discussion about altering c as well.  

While shrinking rays (and their complement) are great fun in story-telling, they're much more in the realm of science fantasy than true science fiction....


Friday, March 31, 2017

Site recommendation: Inside Science

A brief post in a busy time:  If you like well-written, even-handed journalistic discussions of science, I strongly recommend checking out Inside Science, an editorially independent, non-profit science news service affiliated with the American Institute of Physics.   The writing is engaging and of consistently high quality, and I'm glad it's supported by underwriters so that it's not dependent on clicks/ad revenue.

Tuesday, March 28, 2017

What is Intel's Optane memory?

Intel has developed a new product, dubbed Optane, that is a memory technology that supposedly combined the speed of conventional DRAM with the nonvolatility of flash memory.   It would appear that the devices function as a form of "3d crosspoint" memory, where the functionality is all in a blob of material at the crossing point between two wires (a bit line and a word line).  Depending on some particular voltage pulse applied to the junction, the blob of material either has a high electrical resistance or a low electrical resistance, corresponding to the two different states of a binary bit.   The upsides here are that information is stored in a material property rather than as charge (making it non-volatile, probably radiation hard, probably uninfluenced by magnetic fields, etc.), and the devices can be packed very densely, since they need fewer transistors, etc. than conventional DRAM.

There are multiple different mechanisms to achieve that kind of response from the mysterious blob of material.  For example, you can have a material that changes structural phase under current flow, between two different structures with different electrical properties.  You can have a material where some redox chemistry takes place, switching on or off a conductive filament.  You can have a material where other redox chemistry takes place, along with the migration of oxygen vacancies, to create or destroy a conductive filament (as in the HP implementation of memristors, which I've written about before).  You could use magnetic data storage of some sort, with spin transfer torque driving switching of some giant magnetoresistive or tunneling magnetoresistive device.

Breathless articles like this one this week make some pretty bold claims for Optane's performance.  However, no one seems to know what it is.  There has been speculation.  Intel's CEO says it's based on actual bulk changes in the electrical properties of the mysterious material.  Intel categorically denies that it is based on phase changes, "memristor" approaches, or spin transfer torque.  

Well, now that they are actually shipping chips, it's only a very short matter of time before someone cuts one open and reverse-engineers what is actually in there.  So, we have only a little while to speculate wildly or place bets.  Please go for it in the comments, or chime in if you have an informed perspective!  Personally, I suspect it really is some form of bias-driven chemical alteration of material, whether this is called "memristor" in the HP sense of the word or not.  (Note that something rather analogous happened back when IBM and Intel switched to using "high-k" dielectrics in transistors.  They wouldn't say what material they'd come up with, and in the end it turned out to be (most commonly) hafnium oxynitrides.)

Wednesday, March 22, 2017

Hysteresis in science and engineering policy

I have tried hard to avoid political tracts on this blog, because I don't think that's why people necessarily want to read here.  Political flamewars in the comments or loss of readers over differences of opinion are not outcomes I want.  The recent proposed budget from the White House, however, inspires some observations.  (I know the President's suggested budget is only the very beginning of the budgetary process, but it does tell you something about the administration priorities.)

The second law of thermodynamics tell us that some macroscopic processes tend to run only one direction.  It's easier to disperse a drop of ink in a glass of water than to somehow reconstitute the drop of ink once the glass has been stirred.  

In general, the response of a system to some input (say the response of a ferromagnet to an applied magnetic field, or the deformation of a blob of silly putty in response to an applied stress) can depend on the history of the material.  Taking the input from A to B and back to A doesn't necessarily return the system to its original state.  Cycling the input and ending up with a looping trajectory of the system in response because of that history dependence is called hysteresis.  This happens because there is some inherent time scale for the system to respond to inputs, and if it can't keep up, there is lag.

The proposed budget would make sweeping changes to programs and efforts that, in some cases, took decades to put in place.   Drastically reducing the size and scope of federal agencies is not something that can simply be undone by the next Congress or the next President.  Cutting 20% of NIH or 17% of DOE Office of Science would have ripple effects for many years, and anyone who has worked in a large institution knows that big cuts are almost never restored.   Expertise at EPA and NOAA can't just be rebuilt once eliminated.  

People can have legitimate discussions and differences of opinion about the role of the government and what it should be funding.  However, everyone should recognize that these are serious decisions, many of which are irreversible in practical terms.   Acting otherwise is irresponsible and foolish.

Wednesday, March 15, 2017

APS March Meeting 2017 Day 3 - updated w/ guest post!

Hello readers - I have travel plans such that I have to leave the APS meeting after lunch today.  That means I will miss the big Kavli Symposium session.  If someone out there would like to offer to write up a bit about those talks, please email me or comment below, and I'd be happy to give someone a guest post on this.

Update:  One of my readers was able to attend the first two talks of the Kavli Symposium, by Duncan Haldane and Michael Kosterlitz, two of this year's Nobel laureates.  Here are his comments.  If anyone has observations about the remaining talks in the symposium, please feel free to email me or post in the comments below.
I basically ran from the Buckley Prize talk by Alexei Kitaev down the big hall where Duncan Haldane was preparing to talk.  When I got there it was packed full but I managed to squeeze into a seat in the middle section.  I sighted my postdoc near the back of the first section; he later told me he’d arrived 35 minutes early to get that seat.

I felt Haldane’s talk was remarkably clear and simple given the rarified nature of the physics behind it.  He pointed out that condensed matter physics really changed starting in the 1980’s, and conceptually now is much different than the conventional picture  presented in books like Ashcroft and Mermin’s Solid State Physics that many of us learned from as students.  One prevailing idea leading up to that time was that changes in the ground state must always be related to changes in symmetry.  Haldane’s paper on antiferromagnetic Heisenberg spin chains showed that the ground state properties of the chains were drastically different depending on whether  the spin at each site is integer (S=1,2,3,…) or half-integer (S=1/2, 3/2, 5/2 …) , despite the fact that the Hamiltonian has the same spherical symmetry for any value of S.  This we now understand on the basis of the topological classifications of the systems.  Many of these topological classifications were later systematically worked out by Xiao-Gang Wen who shared this year’s Buckley prize with Alexei Kitaev. Haldane flashed a link to his original manuscript on spin chains which he has posted on arXiv.org as https://arxiv.org/abs/1612.00076 , and which he noted was “rejected by many journals”.  He was also amused or bemused or maybe both by the fact that people referred to his ideas as “Haldane’s conjecture” rather than recognizing that he’d solved the problem.  He noted that once one understands that the topological classification determines many of the important properties it is obvious that simplified “toy models” can give deep insight into the underlying physics of all systems in the same class.  In this regard he singled out the AKLT model, which revealed how finite chains of spin S=1 have effective S=1/2 degrees of freedom associated with each end.  These are entangled with each other no matter how long the finite chain – a remarkable demonstration of quantum entanglement over a long distance.  This also is a simple example of the special nature of surface states or excitations in topological systems. 

Kosterlitz began by pointing out that the Nobel prize was effectively awarded for work on two distinct aspects of topology in condensed matter, and both of these involved David Thouless which led to his being awarded one-half of the prize, with the other half shared by Kosterlitz and Haldane.  He then relayed a bit about his own story: he started as a high energy physicist, and apparently did not get offered the position he wanted at CERN so he ended up at Birmingham, which turned out to be remarkably fortuitous.  There he teamed with Thouless and gradually switched his interests to condensed matter physics.  They wanted to understand data suggesting that quasi-two-dimensional films of liquid helium seemed to show a phase transition despite the expectation that this should not be possible.  He then gave a very professorial exposition of the Kosterlitz-Thouless (K-T) transition, starting with the physics of vortices, and how their mutual interactions involve a potential that depends on the logarithm of the distance.  The results point to a non-zero temperature above which the free energy favors free vortices and below which vortex-anti vortex pairs are bound. He then pointed out how this is relevant to a wide variety of two dimensional systems, including xy magnets, and also the melting of two-dimensional crystals in which two K-T transitions occur corresponding respectively to the unbinding of dislocations and disclinations.  
I greatly enjoyed both of these talks, especially since I have experimentally researched both spin chains and two-dimensional melting at different times in my career. 

APS March Meeting 2017 Day 2

Some highlights from day 2 (though I spent quite a bit of time talking with colleagues and collaborators):

Harold Hwang of Stanford gave a very nice talk about oxide materials, with two main parts.  First, he spoke about making a hot electron (metal base) transistor  (pdf N Mat 10, 198 (2011)) - this is a transistor device made from STO/LSMO/Nb:STO, where the LSMO layer is a metal, and the idea is to get "hot" electrons to shoot over the Schottky barrier at the STO/LSMO interface, ballistically across the metallic LSMO base, and into the STO drain.  Progress has been interesting since that paper, especially with very thin bases.  In principle such devices can be very fast. 

The second part of his talk was about trying to make free-standing ultrathin oxide layers, reminiscent of what you can see with the van der Waals materials like graphene or MoS2.  To do this, they use a layer of Sr3Al2O6 - that stuff can be grown epitaxially with pulsed laser deposition on nice oxide substrates like STO, and other oxide materials (even YBCO or superlattices) can be grown epitaxially on top of it. Sr3Al2O6 is related to the compound in Portland cement that is hygroscopic, and turns out to be water soluble (!), so that you can dissolve it and lift off the layers above it.  Very impressive.

Bharat Jalan of Minnesota spoke about growing BaSnO3 via molecular beam epitaxy.  This stuff is a semiconductor dominated by the Ba 5s band, with a low effective mass so that it tends to have pretty high mobilities.  This is an increasingly trendy new wide gap oxide semiconductor that could potentially be useful for transparent electronics.  

Ivan Bozovic of Brookhaven (and Yale) gave a very compelling talk about high temperature superconductors, specifically LSCO, based on having grown thousands of extremely high quality (as assessed by the width of the transition in penetration depth measurements) epitaxial films of varying doping concentrations.   Often people assert that the cuprates, when "overdoped", basically become more conventional BCS superconductors with a Fermi liquid normal state.  Bozovic presents very convincing evidence (from pretty much the data alone, without complex models for interpretation) that shows this is not right - that instead these materials are weird even in the overdoped regime, with systematic property variations that don't look much like conventional superconductors at all.  In the second part of his talk, he showed clear transport evidence for electronic anisotropy in the normal state of LSCO over the phase diagram, with preferred axes in the plane that vary with temperature and don't necessarily align with crystallographic axes of the material.  Neat stuff.   

Shang-Jie Yu at Maryland spoke about work on coherent optical manipulation of phonons.  In particular, previous work from this group looked at ensembles of spherical core-shell nanoparticles in solution, and found that they could excite a radial breathing vibrational mode with an optical pulse, and then measure that breathing in a time-resolved way with probe pulses.  Now they can do more complex pulse sequences to control which vibrations get excited - very cute, and it's impressive to me that this works even when working with an ensemble of particles with presumably some variation in geometry.


Monday, March 13, 2017

APS March Meeting 2017 Day 1

Some talks I saw today at the APS March Meeting in New Orleans:

John Martinis spoke about "quantum supremacy".  Quantum supremacy means achieving performance truly superior to classical situation - in Martinis' usage, the idea is to look at cross-correlations between different qubits, and compare with expectations for fully entangled/coherent systems, to assess how well you are able to set, entangle, and preserve the coherence of your quantum bits.

An optical analog:  Coherent light (laser pointer) incident on frosted glass results in a diffuse spot that is, when examined in detail, an incredibly complicated speckle pattern.  The statistics of that speckled light (correlations over different spatial regions) are very different than if you just had a defocused spot.  In his system, he is taking nine (superconducting, tunable transmon) qubits, where they can control both the coupling between neighboring bits and the energy of each bit.  They set the system in an initial state (injecting a known number of microwave photons into particular qubits); set the energies in a known but randomly selected way, turn on and off the neighbor couplings (25 ns timescale) for some number of cycles, and then look where the microwave photons end up, and take the statistics.  They find that they get good agreement with an error rate of 0.3%/qubit/cycle.  That's enough that they could conceivably do something useful.

As a demo, they use their qubits to model the Hofstadter butterfly problem - finding the energy levels of a 2d electronic system (on a hexagonal lattice, which maps to a 1d problem that they can implement w. their array of nine qubits).  They can get a nice agreement between theory and experiment.  Very impressive.  He  concluded w/ a warning not to believe all hype from qc investigators, including himself.  In general, the approach is basically brute force up to ~ 45 qubits or more (couple of hundred), to think about optimal control and feedback schemes before worrying about truly huge scaling.  The only downside to the talk was that it was in a room that was far too small for the audience.

Alex MacLeod gave a nice talk about using scanning near-field optical microscopy to study the metal-insulator transition in V2O3, as in this paper.   By performing cryogenic near-field scanning optical microscopy in ultrahigh vacuum (!), they measured scattered light from nanoscale scanning tip, giving local dielectric information (hence distinction between metal and insulator surroundings) with an effective spatial resolution that is basically the radius of curvature of the tip.   There is pattern formation at the metal-insulator transition because the two phases have different crystal structures (metal = corundum; insulator = monoclinic), and therefore the transition is a problem of constrained free energy minimization.  This generically leads to pattern formation in the mixed-phase regime.  They see a clear percolation transition in optical measurements, coinciding w/ long distance transport measurements - they really are seeing metallic domains.  Strangely, they find a temperature offset betw/ the structural transition (as seen through x-ray) vs the MIT.  The structural transition temperature is higher, and coincides with max anisotropy in the imaged patterns.  They also see pieces of persistent metallic state at low T, suggesting that some other frustration is going on to stabilize this.

Anatole von Lilienfeld of Basel gave an interesting talk about using machine learning techniques to get quantum chemistry information about small molecules faster and allegedly with better accuracy than full density functional theory calculations.  Basically you train the software on molecules that have been solved to some high degree of accuracy, parametrizing the molecules by their structure (a "Coulomb matrix" that takes into account the relative coordinates and effective charges of the ions) and/or bonding (a "bag of bonds" that takes into account two-body bonds).  Then the software can do a really good job interpolating quantum properties (HOMO-LUMO gaps, ionization potentials) of related molecules faster than you could calculate them in detail.  Impressive, but it seems like a powerful look-up table rather than providing much physical insight.

Melissa Eblen-Zayas gave a fun talk about trying to upgrade the typical advanced junior lab to include real elements of experimental design.  Best line:  "At times student frustration was palpable."

Dan Ralph gave a very compelling talk about the origins of spin-orbit torques in thin-film heterostructures.  I've written in the past about related work.   This was a particularly clear exposition, and went to new territory.  Traditionally, if you have a thin film of a heavy metal (tantalum, say), and you pass current through that film, at the upper (and lower) film surface you will accumulate spin density oriented in the plane and perpendicular to the charge current.  He made a clear argument that this is required because of the mirror symmetry properties of typical polycrystalline metal films.  However, if instead you work with a thin material with much lower symmetry (WTe2, for example) instead of the heavy metal, you can exert spin torques on adjacent magnetic overlayers as if the accumulated spin was out of the plane (which could be useful for certain device approaches).

Saturday, March 11, 2017

APS March Meeting 2017

Once again, it's that time of year when somewhat absurd numbers of condensed matter (and other) physicists gather together.  This time the festivities are in New Orleans.  I'll be at the meeting tomorrow (this will be my first time attending the business meetings as a member-at-large of the Division of Condensed Matter Physics, so that should be new and different)  through Wednesday afternoon.  As in previous years, I will do my best to write up some of the interesting things I learn about.  (If you're at the meeting and you don't already have a copy, now is the perfect time to swing by the Cambridge University Press exhibit at the trade show and pick up my book :-) )

Sunday, March 05, 2017

Career guidance and advice - aggregated posts

Similarly, over the years I have written several posts about (academic) career topics.  Google doesn't always pagerank these very highly (that is a form of peer review, I suppose), so here they are in one place.  Again, some should probably be rewritten and updated, but this is a start.

Advice on choosing a graduate school
Advice on choosing/finding a postdoc position
Guide to faculty searches, 2015 edition
Tenure - some advice
How to write a scientific paper
How to write a response to referees
About refereeing
How to carry on a scientific collaboration
About coauthorship
Things no one teaches you as part of your training
Lab habits and data management


Tuesday, February 28, 2017

CM/nano primer - aggregated posts

Over the years I've written quite a few posts that try to explain physics concepts relevant to condensed matter/nano topics.  I've thought about compiling some edited (more likely completely rewritten) version of these as a primer for science journalists.  Here are the originals, collected together in one meta-post, since many current readers likely never saw them the first time around.

What is temperature?
What is chemical potential?
What is mass?

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 metal?
What is a bad metal?  What is a strange metal?

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 crystal?
What is a time crystal?
What is spin-orbit coupling?
About graphene, and more about graphene
About noise, part one, part 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 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




Tuesday, February 21, 2017

In memoriam: Millie Dresselhaus

Millie Dresselhaus has passed away at 86.  She was a true giant, despite her diminutive stature.   I don't think anything I could write would be better than the MIT write-up linked in the first sentence.  It was great to have had the opportunity to interact with her on multiple occasions and in multiple roles, and both nanoscience in particular and the scientific community in general will be poorer without her enthusiasm, insights, and mentoring.  (One brief anecdote to indicate her work ethic:  She told me once that she liked to review on average something like one paper every couple of days.)

Metallic hydrogen?

There has been a flurry of news lately about the possibility of achieving metallic hydrogen in the lab.  The quest for metallic hydrogen is a fun story with interesting characters and gadgets - it would be a great topic for an episode of Nova or Scientific American Frontiers.   In brief faq form (because real life is very demanding right now):

Why would this be a big deal?  Apart from the fact that it's been sought for a long time, there are predictions that metallic hydrogen could be a room temperature superconductor (!) and possibly even metastable once the pressure needed to get there is removed.

Isn't hydrogen a gas, and therefore an insulator?  Sure, at ambient conditions.  However, there is very good reason to believe that if you took hydrogen and cranked up the density sufficiently (by squeezing it), it would actually become a metal.

What do you mean by a metal?  Do you mean a ductile, electrically conductive solid?  Yes on the electrically conductive part, at least.  From the chemistry/materials perspective, a metal often described a system where the electrons are delocalized - shared between many many ions/nuclei.  From the physics perspective (see here), a metal is a system where the electrons have "gapless excitations" - it's possible to create excitations of the electrons (moving an electron from a filled state to an empty state of different energy and momentum) down to arbitrarily low energies.  That's why the electrons in a metal can respond to an applied voltage by flowing as a current.

What is the evidence that hydrogen can become a metal at high densities?  Apart from recent experiments and strong theoretical arguments, the observation that Jupiter (for example) has a whopping magnetic field is very suggestive.

How do you get from a diatomic, insulating gas to a metal?  You squeeze.  While it was originally hoped that you would only need around 250000 atmospheres of pressure to get there, it now seems like around 5 million atmospheres is more likely.  As the atoms are forced to be close together, it is easier for electrons to hop between the atoms (for experts, a larger tight-binding hopping matrix element and broader bands), and because of the Pauli principle the electrons are squeezed to higher and higher kinetic energies.  Both trends push toward metal formation.

Yeah, but how do you squeeze that hard?  Well, you could use a light gas gun to ram a piston into a cylinder full of liquid hydrogen like these folks back when I was in grad school.  You could use a whopping pulsed magnetic field like a z-pinch to compress a cylinder filled with hydrogen, as suggested here (pdf) and reported here.  Or, you could put hydrogen in a small, gasketed volume between two diamond facets, and very carefully turn a screw that squeezes the diamonds together.  That's the approach taken by Dias and Silvera, which prompted the recent kerfuffle.  

How can you tell it's become a metal?  Ideally you'd like to measure the electrical conductivity by, say, applying a voltage and measuring the resulting current, but it can be very difficult to get wires into any of these approaches for such measurements.  Instead, a common approach is to use optical techniques, which can be very fast.  You know from looking at a (silvered or aluminized) mirror that metals are highly reflective.  The ability of electrons in a metal to flow in response to an electric field is responsible for this, and the reflectivity can be analyzed to understand the conductivity.

So, did they do it?  Maybe.  The recent result by Dias and Silvera has generated controversy - see here for example.   Reproducing the result would be a big step forward.  Stay tuned.

Sunday, February 12, 2017

What is a time crystal?

Recall a (conventional, real-space) crystal involves a physical system with a large number of constituents spontaneously arranging itself in a way that "breaks" the symmetry of the surrounding space.  By periodically arranging themselves, the atoms in an ordinary crystal "pick out" particular length scales (like the spatial period of the lattice) and particular directions.

Back in 2012, Frank Wilczek proposed the idea of time crystals, here and here, for classical and quantum versions, respectively.  The original idea in a time crystal is that a system with many dynamical degrees of freedom, can in its ground state spontaneously break the smooth time translation symmetry that we are familiar with.  Just as a conventional spatial crystal would have a certain pattern of, e.g., density that repeats periodically in space, a time crystal would spontaneously repeat its motion periodically in time.  For example, imagine a system that, somehow while in its ground state, rotates at a constant rate (as described in this viewpoint article).  In quantum mechanics involving charged particles, it's actually easier to think about this in some ways.  [As I wrote about back in the ancient past, the Aharonov-Bohm phase implies that you can have electrons producing persistent current loops in the ground state in metals.]

The "ground state" part of this was not without controversy.   There were proofs that this kind of spontaneous periodic groundstate motion is impossible in classical systems.  There were proofs that this is also a challenge in quantum systems.  [Regarding persistent currents, this gets into a definitional argument about what is a true time crystal.]

Now people have turned to the idea that one can have (with proper formulation of the definitions) time crystals in driven systems.  Perhaps it is not surprising that driving a system periodically can result in periodic response at integer multiples of the driving period, but there is more to it than that.  Achieving some kind of steady-state with spontaneous time periodicity and a lack of runaway heating due to many-body interacting physics is pretty restrictive.  A good write-up of this is here.  A theoretical proposal for how to do this is here, and the experiments that claim to demonstrate this successfully are here and here.   This is another example of how physicists are increasingly interested in understanding and classifying the responses of quantum systems driven out of equilibrium (see here and here).

Sunday, February 05, 2017

Losing a colleague and friend - updated

Blogging is taking a back seat right now.  I'm only posting because I know some Rice connections and alumni read here and may not have heard about this.  Here is a longer article, though I don't know how long it will be publicly accessible.

Update:  This editorial was unexpected (at least by me) and much appreciated.  There is also a memorial statement here.

Update 2:  The Houston Chronicle editorial is now behind a pay-wall.  I suspect they won't mind me reproducing it here:

"If I have seen further it is by standing on the shoulders of giants."

Isaac Newton was not the first to express this sentiment, though he was perhaps the most brilliant. But even a man of his stature knew that he only peered further into the secrets of our universe because of the historic figures who preceded him.

Those giants still walk among us today. They work at the universities, hospitals and research laboratories that dot our city. They explore the uncharted territory of human knowledge, their footsteps laying down paths that lead future generations.

Dr. Marjorie Corcoran was one of those giants. The Rice University professor had spent her career uncovering the unknown - the subatomic levels where Newton's physics fall apart. She was killed after being struck by a Metro light rail train last week.

Corcoran's job was to ask the big questions about the fundamental building blocks and forces of the universe. Why does matter have mass? Why does physics act the way it does?
She worked to understand reality and unveil eternity. To the layperson, her research was a secular contemplation of the divine.

Our city spent years of work and millions of dollars preparing for the super-human athletic feats witnessed at the Super Bowl. But advertisers didn't exactly line up to sponsor Corcoran - and for good reason. Anyone can marvel in a miraculous catch. It is harder to grasp the wonder of a subatomic world, the calculations that bring order to the universe, the research that hopes to explain reality itself.

Only looking backward can we fully grasp the incredible feats done by physicists like Corcoran.
"A lot of people don't have a very long timeline. They're thinking what's going to happen to them in the next hour or the next day, maybe the next week," Andrea Albert, one of Corcoran's former students, told the editorial board. "No, we're laying the foundation so that your grandkids are going to have an awesome, cool technology. I don't know what it is yet. But it is going to be awesome."

Houston is already home to some of the unexpected breakthroughs of particle physics. Accelerators once created to smash atoms now treat cancer patients with proton therapy.

All physics is purely academic - until it isn't. From the radio to the atom bomb, modern civilization is built on the works of giants.

But the tools that we once used to craft the future are being left to rust.

Federal research funding has fallen from its global heights. Immigrants who help power our labs face newfound barriers. Our nation shouldn't forget that Albert Einstein and Edward Teller were refugees.
"How are we going to foster the research mission of the university?" Rice University President David Leebron posed to the editorial board last year. "I think as we see that squeeze, you look at the Democratic platform or the Republican platform or the policies out of Austin, I worry about the level of commitment."

In a competitive field, Corcoran went out of her way to help new researchers. In a field dominated by men, she stood as a model for young women. And in a nation focused on quarterly earnings, her work was dedicated to the next generation.

Marjorie Corcoran was a giant. The world stands taller because of her.

Sunday, January 29, 2017

What is a crystal?

(I'm bringing this up because I want to write about "time crystals", and to do that....)

A crystal is a larger whole comprising a spatially periodic arrangement of identical building blocks.   The set of points that delineates the locations of those building blocks is called the lattice, and the minimal building block is called a basis.  In something like table salt, the lattice is cubic, and the basis is a sodium ion and a chloride ion.  This much you can find in a few seconds on wikipedia.  You can also have molecular crystals, where the building blocks are individual covalently bonded molecules, and the molecules are bound to each other via van der Waals forces.   Recently there has been a ton of excitement about graphene, transition metal dichalcogenides, and other van der Waals layered materials, where a 3d crystal is built up out of 2d covalently bonded crystals stacked periodically in the vertical direction.

The key physics points:   When placed together under the right conditions, the building blocks of a crystal spontaneously join together and assemble into the crystal structure.  While space has the same properties in every location ("invariance under continuous translation") and in every orientation ("invariance under continuous orientation"), the crystal environment doesn't.  Instead, the crystal has discrete translational symmetry (each lattice site is equivalent), and other discrete symmetries (e.g., mirror symmetry about some planes, or discrete rotational symmetries around some axes).   This kind of spontaneous symmetry breaking is so general that it happens in all kinds of systems, like plastic balls floating on reservoirs.  The spatial periodicity has all kinds of consequences, like band structure and phonon dispersion relations (how lattice vibration frequencies depend on vibration wavelengths and directions).

Wednesday, January 25, 2017

A book recommendation

I've been very busy lately, hence a slow down in posting, but in the meantime I wanted to recommend a book.  The Pope of Physics is the recent biography of Enrico Fermi from  Gino Segrè and Bettina Hoerlin.  The title is from Fermi's nickname as a young physicist in Italy - he and his colleagues (the "Via Panisperna boys", named for the address of the Institute of Physics in Rome) took to giving each other nicknames, and Fermi's was "the Pope" because of his apparent infallibility.  The book is compelling, gives insights into Fermi and his relationships, and includes stories about that wild era of physics that I didn't recall hearing before.   (For example, when trying to build the first critical nuclear pile at Stag Field in Chicago, there was a big contract dispute with Stone and Webster, the firm hired by the National Defense Research Council to do the job.  When it looked like the dispute was really going to slow things down, Fermi suggested that the physicists themselves just build the thing, and the put it together from something like 20000 graphite blocks in about two weeks.)

While it's not necessarily as page-turning as The Making of the Atomic Bomb, it's a very interesting biography that offers insights into this brilliant yet emotionally reserved person.  It's a great addition to the bookshelf.  For reference, other biographies that I suggest are True Genius:  The Life and Science of John Bardeen, and the more technical works No Time to be Brief:  A Scientific Biography of Wolfgang Pauli and Subtle is the Lord:  The Science and Life of Albert Einstein.

Monday, January 16, 2017

What is the difference between science and engineering?

In my colleague Rebecca Richards-Kortum's great talk at Rice's CUWiP meeting this past weekend, she spoke about her undergrad degree in physics at Nebraska, her doctorate in medical physics from MIT, and how she ended up doing bioengineering.  As a former undergrad engineer who went the other direction, I think her story did a good job of illustrating the distinctions between science and engineering, and the common thread of problem-solving that connects them.

In brief, science is about figuring out the ground rules about how the universe works.   Engineering is about taking those rules, and then figuring out how to accomplish some particular task.   Both of these involve puzzle-like problem-solving.  As a physics example on the experimental side, you might want to understand how electrons lose energy to vibrations in a material, but you only have a very limited set of tools at your disposal - say voltage sources, resistors, amplifiers, maybe a laser and a microscope and a spectrometer, etc.  Somehow you have to formulate a strategy using just those tools.  On the theory side, you might want to figure out whether some arrangement of atoms in a crystal results in a lowest-energy electronic state that is magnetic, but you only have some particular set of calculational tools - you can't actually solve the complete problem and instead have to figure out what approximations would be reasonable, keeping the essentials and neglecting the extraneous bits of physics that aren't germane to the question.

Engineering is the same sort of process, but goal-directed toward an application rather than specifically the acquisition of new knowledge.  You are trying to solve a problem, like constructing a machine that functions like a CPAP, but has to be cheap and incredibly reliable, and because of the price constraint you have to use largely off-the-shelf components.  (Here's how it's done.)

People act sometimes like there is a vast gulf between scientists and engineers - like the former don't have common sense or real-world perspective, or like the latter are somehow less mathematical or sophisticated.  Those stereotypes even comes through in pop culture, but the differences are much less stark than that.  Both science and engineering involve creativity and problem-solving under constraints.   Often which one is for you depends on what you find most interesting at a given time - there are plenty of scientists who go into engineering, and engineers can pursue and acquire basic knowledge along the way.  Particularly in the modern, interdisciplinary world, the distinction is less important than ever before.

Friday, January 13, 2017

Brief items

What with the start of the semester and the thick of graduate admissions season, it's been a busy week, so rather than an extensive post, here are some brief items of interest:

  • We are hosting one of the APS Conferences for Undergraduate Women in Physics this weekend.  Welcome, attendees!  It's going to be a good time.
  • This week our colloquium speaker was Jim Kakalios of the University of Minnesota, who gave a very fun talk related to his book The Physics of Superheroes (an updated version of this), as well as a condensed matter seminar regarding his work on charge transport and thermoelectricity in amorphous and nanocrystalline semiconductors.  His efforts at popularizing physics, including condensed matter, are great.  His other books are The Amazing Story of Quantum Mechanics, and the forthcoming The Physics of Everyday Things.  That last one shows how an enormous amount of interesting physics is embedded and subsumed in the routine tasks of modern life - a point I've mentioned before.   
  • Another seminar speaker at Rice this week was John Biggins, who explained the chain fountain (original video here, explanatory video here, relevant paper here).
  • Speaking of videos, here is the talk I gave last April back at the Pittsburgh Quantum Institute's 2016 symposium, and here is the link to all the talks.
  • Speaking of quantum mechanics, here is an article in the NY Review of Books by Steven Weinberg on interpretations of quantum.  While I've seen it criticized online as offering nothing new, I found it to be clearly written and articulated, and that can't always be said for articles about interpretations of quantum mechanics.
  • Speaking of both quantum mechanics interpretations and popular writings about physics, here is John Cramer's review of David Mermin's recent collection of essays, Why Quark Rhymes with Pork:  And other Scientific Diversions (spoiler:  I agree with Cramer that Mermin is wrong on the pronunciation of "quark".)  The review is rather harsh regarding quantum interpretation, though perhaps that isn't surprising given that Cramer has his own view on this.

Sunday, January 08, 2017

Physics is not just high energy and astro/cosmology.

A belated happy new year to my readers.  Back in 2005, nearly every popularizer of physics on the web, television, and bookshelves was either a high energy physicist (mostly theorists) or someone involved in astrophysics/cosmology.  Often these people were presented, either deliberately or through brevity, as representing the whole discipline of physics.  Things have improved somewhat, but the overall situation in the media today is not that different, as exemplified by the headline of this article, and noticed by others (see the fourth paragraph here, at the excellent blog by Ross McKenzie).

For example, consider Edge.org, which has an annual question that they put to "the most complex and sophisticated minds".   This year the question was, what scientific term or concept should be more widely known?  It's a very interesting piece, and I encourage you to read it.  They got responses from 206 contributors (!).   By my estimate, about 31 of those would likely say that they are active practicing physicists, though definitions get tricky for people working on "complexity" and computation.  Again, by my rough count, from that list I see 12-14 high energy theorists (depending on whether you count Yuri Milner, who is really a financier, or Gino Segre, who is an excellent author but no longer an active researcher) including Sabine Hossenfelder, one high energy experimentalist, 10 people working on astrophysics/cosmology, four working on some flavor of quantum mechanics/quantum information (including the blogging Scott Aronson), one on biophysics/complexity, and at most two on condensed matter physics.   Seems to me like representation here is a bit skewed.  

Hopefully we will keep making progress on conveying that high energy/cosmology is not representative of the entire discipline of physics....