Dan Dessau, U. Colorado: Laser ARPES studies of the cuprates.

Delayed Blog from Tuesday Experimental Discussions
Laser ARPES studies of the cuprates.

Dan Dessau, U. Colorado

In his well-prepared and carefully worded talk, Dan told us about his
new laser ARPES data for Bi2212. He started by comparing the normal state
data for the spectral function along nodal direction,
obtained with 6eV laser and with higher energy synchrotron light sources.
The quasiparticle peak is much more narrow in the laser ARPES data.
Dan extracted the fermionic self-energy from his data. T0 a blogger, it looks
quadratic in frequency at small frequencies (as in a Fermi liquid). At higer frequencies, it crosses over to near-linear behavior.
He next presented his data on the temperature dependence of the Fermi velocity
along nodal direction. His temperature dependence is linear in T
and is quite strong -- velocity at 300K is about 35% larger than the
value extrapolated to T=0. Finally, Dan discussed isotope effect.
He argued that he and his collaborators didn't find a large isotope
effect on the high energy incoherent states. This result is in disagreement with earlier
measurements by the group at Berkeley. Dan, however, found a 3meV isotope
effect on the position of the kink in the dispersion below T_c. He argue that this indicated that that the kink may be due to interaction with phonons.

In the discussion following the talk, A. Millis asked why the absolute magnitude of his Fermi velocity is larger than in measurements by other groups. Dan replied that this may be due to calibration, and that one should focus on relative variation of $v_F$ with $T$ rather than on absolute values. C. Varma
pointed that in the electronic scenario for the kink in a $d-$wave superconductor, the kink appears at $2\Delta$, and
asked whether Dan measured the isotope effect on the gap value. Dan answered that he didn't.