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.