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.