Thursday, September 20, 2018

What’s in a name? CMP

At a recent DCMP meeting, my colleague Erica Carlson raised an important point:  Condensed matter physics as a discipline is almost certainly hurt relative to other areas, and in the eye of the public, by having the least interesting, most obscure descriptive name.  Seemingly every other branch of physics has a name that either sounds cool, describes the discipline at a level immediately appreciated by the general public, or both.  Astrophysics is astro-physics, and just sounds badass.  Plasma physics is exciting because, come on, plasma.  Biophysics is clearly the physics relevant to biology.  High energy or particle physics are descriptive and have no shortage of public promotion.  Atomic physics has a certain retro-future vibe.

In contrast, condensed matter, while accurate, really does not conjure any imagery at all for the general public, or sound very interesting.  If the first thing you have to do after saying “condensed matter” is use two or three sentences to explain what that means, then the name has failed in one of its essential missions.

So, what would be better alternatives?  “Quantum matter” sounds cool, but doesn’t really explain much, and leaves out soft CM.  The physics of everything you can touch is interesting, but prosaic.  Suggestions in the comments, please!

Friday, September 14, 2018

Recently on the arxiv

While it's been a busy time, a couple of interesting papers caught my eye:

arxiv:1808.07865 - Yankowitz et al., Tuning superconductivity in twisted bilayer graphene
This lengthy paper, a collaboration between the groups of Andrea Young at UCSB and Cory Dean at Columbia, is (as far as I know) the first independent confirmation of the result from Pablo Jarillo-Herrero's group at MIT about superconductivity in twisted bilayer graphene.  The new paper also shows how tuning the interlayer coupling via in situ pressure (a capability of the Dean lab) affects the phase diagram.  Cool stuff.

arXiv:1809.04637 - Fatemi et al., Electrically Tunable Low Density Superconductivity in a Monolayer Topological Insulator
arxiv:1809.04691 - Sajadi et al., Gate-induced superconductivity in a monolayer topological insulator
While I haven't had a chance to read them in any depth, these two papers report superconductivity in gated monolayer WTe2, a remarkable material already shown to act as a 2D topological insulator (quantum spin Hall insulator). 

Seems like there is plenty of interesting physics that is going to keep turning up in these layered systems as material quality and device fabrication processes continue to improve.

Tuesday, September 04, 2018

Looking back at the Schön scandal

As I mentioned previously, I've realized in recent weeks that many current students out there may never have heard of Jan Hendrik Schön, and that seems wrong, a missed opportunity for a cautionary tale about responsible conduct of research.  It's also a story that gives a flavor of the time and touches on other issues still current today - faddishness and competitiveness in top-level science, the allure of glossy publications, etc.  It ended up being too long for a blog post, and it seemed inappropriate to drag out over many posts, so here is a link to a pdf.  Any errors are mine and are probably the result of middle-aged memory.  After all, this story did start twenty years ago.  I'm happy to make corrections if appropriate.  update 9/9/18 - corrected typos and added a couple of sentences to clarify things.

Wednesday, August 29, 2018

Unidentified superconducting objects, again.

I've had a number of people ask me why I haven't written anything about the recent news and resulting kerfuffle (here, here, and here for example) in the media regarding possible high temperature superconductivity in Au/Ag nanoparticles.   The fact is, I've written before about unidentified superconducting objects (also see here), and so I didn't have much to say.  I've exchanged some email with the IIS PI back in late July with some questions, and his responses to my questions are in line with what others have said.   Extraordinary claims require extraordinary evidence.  The longer this goes on without independent confirmation, the more likely it is that this will fade away.

Various discussions I've had about this have, however, spurred me to try writing down my memories and lessons learned from the Schon scandal, before the inevitable passage of time wipes more of the details from my brain.  I'm a bit conflicted about this - it was 18 years ago, there's not much point in rehashing the past, and Eugenie Reich's book covered this very well.  At the same time, it's clear that many students today have never even heard of Schon, and I feel like I learned some valuable lessons from the whole situation.  It'll take some time to see if I am happy with how this turns out before I post some or all of it.  Update:  I've got a draft done, and it's too long for a blog post - around 9000 words.  I'll probably convert it to pdf when I'm happy with it and link to it somehow.

Friday, August 24, 2018

What is a Tomonaga-Luttinger Liquid?

I've written in the past (say here and here) about how we think about the electrons in a conventional metals as forming a Fermi Liquid.    (If the electrons didn't interact at all, then colloquially we call the system a Fermi gas.  The word "liquid" is shorthand for saying that the interactions between the particles that make up the liquid are important.  You can picture a classical liquid as a bunch of molecules bopping around, experiencing some kind of short-ranged repulsion so that they can't overlap, but with some attraction that favors the molecules to be bumping up against each other - the typical interparticle separation is comparable to the particle size in that classical case.)  People like Lev Landau and others had the insight that essential features of the Fermi gas (the Pauli principle being hugely important, for example) tend to remain robust even if one thinks about "dialing up" interactions between the electrons.  

A consequence of this is that in a typical metal, while the details may change, the lowest energy excitations of the Fermi liquid (the electronic quasiparticles) should be very much like the excitations of the Fermi gas - free electrons.  Fermi liquid quasiparticles each carry the electronic amount of charge, and they each carry "spin", angular momentum that, together with their charge, makes them act like tiny little magnets.  These quasiparticles move at a typical speed called the Fermi velocity.  This all works even though the like-charge electrons repel each other.

For electrons confined strictly in one dimension, though, the situation is different, and the interactions have a big effect on what takes place.  Tomonaga (shared the Nobel prize with Feynman and Schwinger for quantum electrodynamics, the quantum theory of how charges interact with the electromagnetic field) and later Luttinger worked out this case, now called a Tomonaga-Luttinger Liquid (TLL).  In one dimension, the electrons literally cannot get out of each other's way - the only kind of excitation you can have is analogous to a (longitudinal) sound wave, where there are regions of enhanced or decreased density of the electrons.  One surprising result from this is that charge in 1d propagates at one speed, tuned by the electron-electron interactions, while spin propagates at a different speed (close to the Fermi velocity).  This shows how interactions and restricted dimensionality can give collective properties that are surprising, seemingly separating the motion of spin and charge when the two are tied together for free electrons.

These unusual TLL properties show up when you have electrons confined to truly one dimension, as in some semiconductor nanowires and in single-walled carbon nanotubes.  Directly probing this physics is actually quite challenging.  It's tricky to look at charge and spin responses separately (though some experiments can do that, as here and here) and some signatures of TLL response can be subtle (e.g., power law responses in tunneling with voltage and temperature where the accessible experimentally reasonable ranges can be limited).   

The cold atom community can create cold atomic Fermi gases confined to one-dimensional potential channels.  In those systems the density of atoms plays the role of charge, and while some internal (hyperfine) state of the atoms plays the role of spin, and the experimentalists can tune the effective interactions.  This tunability plus the ability to image the atoms can enable very clean tests of the TLL predictions that aren't readily done with electrons.

So why care about TLLs?  They are an example of non-Fermi liquids, and there are other important systems in which interactions seem to lead to surprising, important changes in properties.  In the copper oxide high temperature superconductors, for example, the "normal" state out of which superconductivity emerges often seems to be a "strange metal", in which the Fermi Liquid description breaks down.  Studying the TLL case can give insights into these other important, outstanding problems.

Saturday, August 18, 2018

Phonons and negative mass

There has been quite a bit of media attention given to this paper, which looks at whether sound waves involve the transport of mass (and therefore whether they should interact with gravitational fields and produce gravitational fields of their own). 

The authors conclude that, under certain circumstances, sound wavepackets (phonons, in the limit where we really think about quantized excitations) rise in a downward-directed gravitational field.  Considered as a distinct object, such a wavepacket has some property, the amount of "invariant mass" that it transports as it propagates along, that turns out to be negative.

Now, most people familiar with the physics of conventional sound would say, hang on, how do sound waves in some medium transport any mass at all?  That is, we think of ordinary sound in a gas like air as pressure waves, with compressions and rarefactions, regions of alternating enhanced and decreased density (and pressure).  In the limit of small amplitudes (the "linear regime"), we can consider the density variations in the wave to be mathematically small, meaning that we can use the parameter \(\delta \rho/rho_{0}\) as a small perturbation, where \(\rho_{0}\) is the average density and \(\delta \rho\) is the change.  Linear regime sound usually doesn't transport mass.  The same is true for sound in the linear regime in a conventional liquid or a solid. 

In the paper, the authors do an analysis where they find that the mass transported by sound is proportional with a negative sign to \(dc_{\mathrm{s}}/dP\), how the speed of sound \(c_{\mathrm{s}}\) changes with pressure for that medium.  (Note that for an ideal gas, \(c_{\mathrm{s}} = \sqrt{\gamma k_{\mathrm{B}}T/m}\), where \(\gamma\) is the ratio of heat capacities at constant pressure and volume, \(m\) is the mass of a gas molecule, and \(T\) is the temperature.  There is no explicit pressure dependence, and sound is "massless" in that case.)

I admit that I don't follow all the details, but it seems to me that the authors have found that for a nonlinear medium such that \(dc_{\mathrm{s}}/dP > 0\), sound wavepackets have a bit less mass than the average density of the surrounding medium.  That means that they experience buoyancy (they "fall up" in a downward-directed gravitational field), and exert an effectively negative gravitational potential compared to their background medium.  It's a neat result, and I can see where there could be circumstances where it might be important (e.g. sound waves in neutron stars, where the density is very high and you could imagine astrophysical consequences).  That being said, perhaps someone in the comments can explain why this is being portrayed as so surprising - I may be missing something.

Tuesday, August 14, 2018

APS March Meeting 2019 - DCMP invited symposia, DMP focused topics

A reminder to my condensed matter colleagues who go to the APS March Meeting:  We know the quality of the meeting depends strongly on getting good invited talks, the 30+6 minute talks that either come all in a group (an "invited session" or "invited symposium") or sprinkled down individually in the contributed sessions.

Now is the time to put together nominations for these things.  The more high quality nominations, the better the content of the meeting.

The APS Division of Condensed Matter Physics is seeking nominations for invited symposia.  See here for the details.  The online submission deadline is August 24th!

Similarly, the APS Division of Materials Physics is seeking nominations for invited talks as part of their Focus Topic sessions.  The list of Focus Topics is here.  The online submission deadline for these is August 29th.