Department of Physics, University of California, Berkeley
Materials Sciences Division, Lawrence Berkeley National Lab
Time-resolved ARPES opens a direct window into the temporal dynamics of nonequilibrium systems. Our research group has recently developed a time-resolved ARPES system with high energy and momentum resolution that is capable of directly monitoring the relaxation dynamics of quasiparticle populations in the cuprates. The emerging technique is providing some exciting results.
The quasiparticle spectrum in cuprate superconductors has a distinctive momentum dependence, in contrast to that of most conventional superconductors. Perhaps most notably, the superconducting gap in the cuprates has d-wave symmetry, coming to a maximum toward the first Brillouin zone faces and dropping to zero along the Brillouin zone diagonals in four nodes. Strangely, the nodal quasiparticle spectrum is sharp and relatively unaffected by perturbations or variations in temperature, while the gapped quasiparticle spectrum becomes increasingly broad and sensitive to such effects as the gap size increases.
A great deal of interest has been focused on learning more about these momentum-dependent characteristics. Why does the superconducting gap have this shape? Why are nodal quasiparticles so robust? Are there any connections between superconductivity and potentially competing phenomena in the cuprates?
Using time-resolved ARPES, our group has discovered momentum dependence in the relaxation rate of a pump-induced nonequilibrium quasiparticle population. The relaxation rate also depends on quasiparticle density, indicating a direct connection to Cooper pair recombination. Characterizing the momentum dependence of this relaxation rate may offer a new link between superconducting quasiparticles and the scattering vectors associated with charge or spin density waves. It may also provide insights into the general nature of high-temperature superconductivity.
Our group has also discovered that the nodal quasiparticle dispersion peak is not as robust as perhaps previously thought. Photoexcitation using an infrared pump beam induces a sharp decrease in the intensity of the nodal quasiparticle dispersion peak. The effect only appears below the superconducting critical temperature.
Tracking Cooper Pairs in a Cuprate Superconductor by Ultrafast Angle-Resolved Photoemission
Christopher L. Smallwood, James P. Hinton, Christopher Jozwiak, Wentao Zhang, Jake D. Koralek, Hiroshi Eisaki, Dung-Hai Lee, Joseph Orenstein, and Alessandra Lanzara
Science 336, 1137 (2012)
Nodal quasiparticle meltdown in ultrahigh-resolution pump-probe angle-resolved photoemission
J. Graf, C. Jozwiak, C. L. Smallwood, H. Eisaki, R. A. Kaindl, D-H. Lee, and A. Lanzara
Nature Physics 7, 805-809 (2011)