More recent hot topics

Still working on proposals, so this will be brief.  However, as mentioned in my previous post, I did want to add in some topics/open questions that are fairly hot right now:

• Topological insulators - These materials are nominally band insulators, in that they have a filled band of electronic states, an energy gap, and an empty conduction band.  However, unlike ordinary band insulators, these have an odd number of states that live at their surface that exist in the band gap.  Because of strong spin-orbit coupling, the spin of a carrier in one of these surface states is locked in a particular orientation relative to the carrier's momentum.  That tends to suppress large angle scattering by ordinary disorder, since ordinary disorder scattering doesn't flip spin.  One big question is, can these materials be grown in such a way that the bulk really is insulating?  During crystal growth, it is energetically cheap to form point defects in many of these materials that act as dopants, leading to serious bulk conduction.  Recent work has found materials (e.g., Bi₂Te₂Se) that are less problematic, but no one has (to my knowledge) figured out a way to grow really insulating thin films that retain the cool surface state properties.  A second question is, can one actually employ these surface states for anything useful?
• Gating in strongly correlated materials - pioneered by Iwasa's group in Japan, there has been a boon in using ionic liquids (essentially molten organic salts) as electrolytes in gating experiments.  These liquids allow one to obtain surface charge densities comparable to those possible in chemical doping, on the order of one charge per unit cell on the surface.  That's enough to do interesting physics in strongly correlated materials (e.g., gating an insulating copper oxide layer into superconductivity).  How far can one push this?  Can this technique be used to develop a transistor based on the Mott metal-insulator transition?  
• Quantum computing - this isn't new, of course, but there seems to be more and more work going on toward making some form of solid state, scalable quantum computer.  Which of the competing approaches will win out?  Spins, with their amazingly long coherence times in isotopically pure Si and diamond?  Superconducting flux or charge qubits?  It does not look like there is any fundamental reason why you couldn't have quantum computers, but it's an enormous technical challenge.
• Optomechanics - there are a number of groups out there having lots of fun looking at micro- or nanoelectromechanical systems and coupling them to optics.  This lets you do optical cooling methods to put the mechanical systems into low-occupation quantum states; this lets you entangle the light with with mechanical system; etc.  What are the ultimate limits here?  Could this usher in a new style of precision measurement, or lead to new quantum information manipulations?
• Plasmonics - we're firmly out of the stage now where every weird metal nanostructure with a plasmon resonance was netting a high profile paper.  Instead, people are looking at using plasmons to confine light to deep subwavelength scales, for super-tiny optical emitters, detectors, etc.  How small a laser can one make using plasmonics?  What other quantum optics tricks can one play with these tools?  Can plasmonic effects be engineered to improve photovoltaics significantly, or photocatalysis?

There are many more, of course.  Suggestions and discussion, as always, are appreciated.