Cavities and tuning physics

I've written before about cavity quantum electrodynamics.  An electromagnetic cavity - a resonator of some kind, like your microwave oven chamber is for microwaves, or like an optical cavity made using nearly perfect mirrors - picks out what electromagnetic modes are allowed inside it.  In the language of photons, the "density of states" for photons in the cavity is modified from what it would be in free space.  Matter placed in the cavity, e.g. an atom, then interacts with that modified environment, even if the cavity is not being excited.  Instead of thinking about just the matter, or just the radiation by itself, in the cavity you need to include the light-matter interaction, and you can end up with states called polaritons that are combinations of matter + radiation excitations.  There are various flavors of polaritons, as there are different kinds of cavities as well as different kinds of matter (atoms vs. excitons, for example).

I just heard a nice talk by Angel Rubio about recent advances in applying cavity effects to both chemistry and materials properties.  For a recent discussion of the former, you can try here (pdf file).  Similar in spirit, there is a great deal of interest in using cavity interactions to modify the ground states (or excited states) of solid materials.  Resonantly altering phonons might allow tuning of superconductivity, for example.  Or, you could take a material like SrTiO3, which is almost a ferroelectric, and try to stabilize ferroelectricity.  Or, you could to take something that is almost a spin liquid and try to get it there by putting it in a cavity and pumping a little.

It's certainly interesting to ponder.  Achieving this in practice is very challenging, because getting matter-cavity couplings to be sufficiently large is not easy.  Never the less, the idea that you can take a material and potentially change something fundamental about its properties just by placing it in the right surroundings sounds almost magical.  Very cool to consider.

Condensed matter’s rough start

 I’m teaching undergrad solid-state for the first time, and it has served as a reminder of how condensed matter physics got off the ground.  I suspect that one reason CM historically had not received a lot of respect in the early years (e.g. Pauli declaring that solid-state physics is the physics of dirt) is that it began very much as a grab bag of empirical observations, with the knowledge that the true underpinnings were well out of reach at the time.  Remember the order of a few key discoveries:

• Dulong and Petit realize that the specific heats of solids are all about the same size near room temperature, in 1819.
• Seebeck discovers that applying a temperature gradient across metals generates a voltage, in 1822.
• Braun observes that a metal wire touching a lead sulfide crystal conducts much better in one current direction than the other (making what we now call a Schottky diode) in 1874.
• Hall finds that a magnetic field applied perpendicular to a current in a metal leads to a voltage mutually transverse to both the current and the field, in 1879. Two years later he also found that you can get a transverse voltage in a ferromagnet even in the absence of an applied magnetic field, the “anomalous Hall effect” which was not really understood until this century.
• Electrons are discovered in 1897.

A whole host of materials physics observations predate the discovery of the electron, let alone modern statistical physics and quantum mechanics.  The early days of condensed matter had a lot of handwaving.  The derivation of the Hall effect in the classical Drude picture (modeling electrons in a metal based on the kinetic theory of gases) was viewed as a triumph, even though it clearly was incomplete and got the sign wrong (!) for a bunch of materials.  (Can you imagine trying to publish a result today and saying, ‘sure, it’s the wrong sign half the time, but it has to be sort of correct’?)

That we now actually understand so much about the physics of materials is one of the great intellectual accomplishments of the species, and the fact that so much of the explanation has real elegance is worth appreciating.

News items for the new year

After I was not chosen to be Speaker of the US House of Representatives, I think it’s time to highlight some brief items:

• Here is a great blog post by a Rice grad alum, Daniel Gonzales, about flow to approach faculty searches.  I had written a fair bit on this a number of years ago, but his take is much fresher and up to date.
• My colleagues in Rice’s chem department have written a very nice obituary in PNAS for Bob Curl.
• It’s taken nearly 2000 years, but people seem to have finally figured out the reason why Roman concrete lasts hundreds to thousands of years, while modern concrete often starts crumbling after 30 years or so.
• Capabilities for quantum optomechanical widgets are improving all the time.  Now it’s possible to implement a model for graphene, following some exquisite fabrication and impressive measurement techniques. 
• From the math perspective, this is just f-ing weird.  For more info, see here.