Events - Daily
November 12, 2012 >>
|Monday, November 12|
Biophysics Seminar Series - Jeremy England, MIT
Joseph Henry Room · 11:45 a.m.– 1:00 p.m.
Condensed Matter Seminar - Nuh Gedik, Massachusetts Institute of Technology, " Ultrafast Probing of Dirac Fermions in Topological Insulators"
The three-dimensional topological insulator (TI) is a new quantum phase of matter that exhibits quantum-Hall-like properties, even in the absence of an external magnetic field. Charge carriers on the surface of a TI behave like a two-dimensional gas of massless helical Dirac fermions for which the spin is ideally locked perpendicular to the momentum. In this talk, I will discuss recent experiments in which we used ultrafast laser pulses to directly visualize spin texture of topological surface electrons and their coupling to the bulk electrons. In order to obtain the spin texture, we performed novel time of flight based angle-resolved photoemission spectroscopy (ARPES) using circularly polarized laser pulses. The difference between the spectra obtained with right and left circularly polarized laser pulses enable simultaneous mapping of all three components of spin over the entire Dirac cone of a TI. We find that an idealized description of helical Dirac fermions only applies within a small energy window about the Dirac point, beyond which strong textural deformations occur. In order to visualize the coupling of topological surface electrons to other excitations, we recorded the temporal evolution of ARPES spectra in response to photoexcitation by a separate ultrashort laser pulse. This yields a movie of evolution of electronic band structure with femtosecond time resolution. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature.
PCTS Seminar Room · 1:15 p.m.– 2:30 p.m.