Monday, February 08, 2016

Brief news items

As I go on some travel, here are some news items that looked interesting to me:

  • Rumors are really heating up that LIGO has spotted gravity waves.  The details are similar to some things I'd heard, for what that's worth, though that may just mean that everyone is hearing the same rumors.  update:  Press conference coming (though they may just say that the expt is running well....)
  • The starship Enterprise is undergoing a refit.
  • This paper reports a photocatalytic approach involving asymmetric, oblong, core-shell semiconductor nanoparticles, plus a single Pt nanoparticle catalyst, that (under the right solution conditions) can give essentially 100% efficient hydrogen reduction - every photon goes toward producing hydrogen gas.   If the insights here can be combined with improved solution stability of appropriate nanoparticles, maybe there are ways forward for highly efficient water splitting or photo production of liquid fuels.
  • Quantum materials are like obscenity - hard to define, but you know it when you see it.


Tahir said...

Hi again, Doug. Sorry to keep pestering you, but I could not resist throwing in a (somewhat subjective) definition of 'quantum material', at least, 'electronic quantum material'. I would be interested to see if you (or any other readers) agree with or disagree with my perspectives.

Incidentally, I believe that the answer to this question is interlinked with the issues you discussed in your previous post, regarding the issue of when Kohn-Sham eigenstates constitute a qualitatively correct representation of the true electronic state of a system. More generally, when can we consider a non-interacting system a good approximation to the interacting system? The answer is, when the interactions are sufficiently 'weak' to be meaningfully treatable by perturbation theory on some reference non-interacting Hamiltonian. So basically, Kohn-Sham theory works when the system is adiabatically connected to some approximate model of non-interacting electronic states (apart from Pauli exclusion and Fermi statistics, of course). These could be, for example, non-interacting free electrons (as in a metallic system), or non-interacting localized Wannier-like orbitals (as in insulating systems). Then the reason that Kohn-Sham works is because the self-consistent solutions essentially constitute a 'renormalized' free electron quasiparticle or independent Wannier-orbital quasiparticle.

I posit the following speculative definition: electronic 'quantum matter' is any state that cannot be adiabatically connected to some non-interacting single electron system. For example, in the BCS state of superconductivity, the 'independent particle' excitations are not single electrons, but rather, entangled Cooper Pairs, which are bosonic particles. Or in the Fractional Quantum Hall effect, the relevant non-interacting approximation would be of composite fermions with fractional charges. And it is in these instances where Kohn-Sham theory, which implicitly assumes and requires that the relevant renormalized non-interacting 'quasiparticles' describing the system be ELECTRONS, will fail.

So then I guess to rigorously characterize electronic quantum matter would come down to answering two questions: 1) For an arbitrary many-electron quantum system, can we find some reference non-interacting system to connect it to adiabatically? 2) If we can do this, can we unambiguously characterize said non-interacting system's renormalized independent particle excitations as 'not electron quasiparticles'?

Anonymous said...

the 100% efficiency paper - where are the error bars/spread? how many is many particles? the si description of how the efficiency was calculated... i feel like this paper could have been a little more thoroughly reviewed

Anonymous said...

I'm with (the above) anonymous. The claims in the catalysis paper are really fishy--there are all sorts of embedded assumptions in the estimation.

Anonymous said...

Doug,to set the record straight: it's not gravity waves, but gravitational waves.
Big difference. The first sentences on the respective Wikipedia pages explains the difference rather pedagogical.

(Your mention of gravity waves slipped past me too as I read this post the day after you posted it. I was following the twitter feed of the press conference when I noticed some Nature reporter (?) mention the errors many people, including me and you, make in this.)

Anyway, Kudos to Ligo et al for having the press conference the day the paper is published, not long before. This is how it should be done.