Thursday, February 9, 2012

Is there a dynamical particle-hole asymmetry in the cuprates?

There is a very interesting preprint Dynamical particle-hole asymmetry in the cuprate superconductors by Sriram Shastry.
Most theories of the metallic state (including Fermi liquid and marginal Fermi liquid theories) assume/assert/embody that adding electrons or holes to the ground state requires the same amount of energy. In a similar vein quasi-particle peaks in spectral functions should be symmetric.

There are two important exceptions to theories which assert dynamical particle-hole symmetry:

The hidden Fermi liquid theory of Anderson

The extremely correlated Fermi liquid theory of Shastry

Shastry points out that there is already some experimental evidence for asymmetry. The figure below [taken from a Science paper by Pasupathy et al. about a different issue] shows the differential conductivity [roughly proportional to the local density of states] from an STM measurement on an overdoped sample of BSCCO. The relevant point is that the background has a significant slope. If there was particle-hole symmetry the background would be flat, as it is for simple metals.
This slope is much larger than the small asymmetry expected from band structure that arises due to the proximity of the Fermi energy to a van Hove singularity.
In light of the theoretical issues discussed above this seems to be an important result and needs to be checked.

There are a few puzzles.
1. Earlier tunneling experiments do not see such a large asymmetry.
2. As the doping decreases the asymmetry does not increase, which is what I would have expected, since the system is effectively becoming more correlated.
3. The above data show no sign of a van Hove singularity.

I thank Jure Kokalj for helpful discussions about this topic.


  1. You're just dancing around the edges of this thing when you should be going for the jugular. Seriously, you are wasting your time and ours.

  2. A more trivial explanation of competing order was proposed by Hu and Seo PRB 73, 094523, (2006). Also, I'm not convinced of your points 1 and 3 -- earlier tunneling data is well described by vHs scenario Wei et al, PRB 57, 3650 (1998). J. E. Hirsch also weighed in (PRB 1999)

  3. You are also losing sight of the fundamentally two component nature of the phenomenon.

  4. @kT Okay fine, we're all wasting time and loosing sight, etc., etc. Why you make these anonymous comments while promoting the work of one set of authors mystifies me.

  5. Sorry, I don't buy into Shastry's extremely correlated Fermi liquid theory simply because it completely ignores the now implicit two component perspective of the phenomenon. Sure, it's nice of you are trying to rescue Fermi liquid theory in this regard, but that is simply no longer possible with new results that are now in hand. Good luck continuing to flail on this, but I've got better and more productive things to think about nowadays.

    I could point you to a variety of modern authors and papers, I only bring this group up because RIXS has become a preferred spectroscopic method of examining what we now know to be the relevant high energy states.

  6. For some further insight into the entire spectrum of behavior across the doping phase diagram, I can suggest a series of papers by Kristjen Haule and collaborators, here, here and here. This field is rapidly evolving, but I can recommend that you read anything Kristjen Haule is involved with.