Light-induced superconductivity

The physics of systems driven out of equilibrium remains a frontier topic, and as new techniques are enabled involving ultrafast lasers, exciting developments have been coming along.  Light-induced (possible) superconductivity is one example that has gotten a lot of attention lately.  Several years ago, the group of Andrea Cavalleri started with a non-superconducting copper oxide material closely related to the high-T_c superconductors.  By smacking this system with a light pulse intended to disrupt an intervening phase ("stripe order", a kind of spontaneous modulation of the charge density in the material into stripes), they were able to get the material to have an optical response that looked just like that of a superconducting cuprate, at least on the picosecond timescale.

Around this same time, Cavalleri and others pointed out that carefully tailored light pulses could also be created that would couple to particular vibrational modes of crystals.  That way, again on the picosecond timescale, one could imagine reaching into a crystal and distorting it transiently.   The Max Planck group made use of this approach in a very deliberate way.  There is a trend toward higher superconducting transition temperatures in the cuprate superconductors as particular bonds within the lattice are distorted due to the overall crystal structure.  What these folks did was hit a cuprate, YBa₂Cu₃O_6.5, with a pulse designed to transiently distort the bonds even more in the favorable-for-superconductivity direction.  Again, they found an optical response that indicated, below T_c, strengthened superconductivity, and above T_c, optical signatures similar to that of superconductivity all the way up to room temperature!  Subsequent ultrafast x-ray diffraction measurements indicated that the lattice really was distorting as desired in those experiments.  

The very recent attention has resulted from this paper, where this group has again optically driven some lattice modes, inducing signatures in the optical response that look very much like superconductivity well above T_c, this time in K₃C₆₀.  Interestingly, in this case when the material is already superconducting, it doesn't seem like the optical pulse enhances the superconductivity.

All of this is very cool, though it's important to remember that these are transient effects, and on the timescales so far it is extremely difficult to perform any other measurements (e.g., non-optical ones, like magnetometry) that would independently test for superconductivity.  Still, this is an impressive strategy, and beyond nonequilibrium physics it strongly suggests that greater control over material structure (than what we have so far been able to achieve) could pay enormous dividends.