Monday, August 13, 2007

M. Norman, 8/9, "What is a Fermi Arc ?" P. Hirschfeld

Delayed Blog Posting of M.R. Norman talk 8/9/07 “What are Fermi Arcs?”

Definition of Fermi arcs observed in underdoped cuprates, particularly Bi-2212: spectral function in pseudogap state near (pi,0) with k on Fermi surface is pulled back from Fermi level after normalizing by Fermi function. Thus max of A(k,w) is at finite w<0, style=""> On the other hand, as one goes towards the node, this pullback disappears, and maximum is now centered at w=0, indicating gapless normal metal like excitations, and the dispersion through the Fermi surface is observed.

Define the point in k-space on the Fermi surface where the gapped character disappears and the gapless character begins as the endpoint of a Fermi “arc”. The distance in k-space along FS of this point from the node is called the length of the arc, and it has been measured by Kanigel et al Nat. Phys. 2006 to depend linearly on the temperature (note this is not a trivial effect of thermal smearing since f(w) has been divided out).

One can’t go all the way to zero, since SC interferes, but the implication is that the pseudogap state at T=0, if one could get there, is a nodal liquid. Mike quoted Taillefer’s result showing that the limiting T->0 thermal conductivity in the SC state is continuous across the underdoped Tc->0 transition as indirect support for this point of view.

One alternate point of view which has been discussed is question of hole pockets instead of arcs, discounted until recently since the “back side” of these pockets has never been seen. These should be weak because of matrix element effects, however, and apparently there is a report of their observation by Valla et al. Mike is skeptical, as this data has been discussed for some time but not published. Questions about relation with quantum oscillations observed by Taillefer postponed because this deserves session to itself.

As one goes through Tc, arc length is observed to collapse, i.e. length vs T goes like some power greater than one. Since we know nodal lifetime is collapsing (to some extent even in BSCCO) in SC state, could this simply be reflecting d-wave + lifetime effect?

Gap extracted by usual methods is flat in underdoped samples – 2-gap effect?

Question by blogger: Loram, Tallon pointed out recently (cond-mat that arclike physics could be obtained with large T-dependent scattering rate in pseudogap phase modelled with constant d-wave like pseudogap. Then as true gap gets small close to nodes, scattering smears spectral function into single peak centered at w=0, looks like arc. Millis points out this idea is not new. Norman responds by saying that models of this type may describe spectral function at Fermi surface, but they will fail elsewhere in Brilllouin zone.

Final point by blogger: there is an inconsistency somewhere in the comparison of the underdoped thermal conductivity story and the nodal liquid picture, since the value of the thermal conductivity, interpreted in the universal transport scenario, implies an extremely large gap slope, in contradiction to ARPES and Raman determinations of same quantity. “Universal” interpretation of this value may not apply.


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