Condensed-Matter Physics & Materials Science Seminar

Nuh Gedik
(California Institute of Technology)

Abstract: Ultrafast optical spectroscopy has long been used with
great success to generate and probe non-equilibrium electronic
excitations with femtosecond time resolution. The spatial resolution
in these techniques, however, is limited to micron scales by the
diffraction limit and structural dynamics can only be inferred
indirectly. I will report direct measurements of structural dynamics
with atomic scale spatial resolution by using ultrafast electron
diffraction (UED). In UED, a femtosecond laser pulse is split into
two, the first part is used to induce structural change and the
second part is used to generate ultrafast high energy electron
packets via photoelectric effect. Recording the diffraction pattern
of these electron packets at different times after the
photo-excitation of the sample provides a movie of the laser induced
structural change with sub-picosecond temporal and sub-Angstrom
spatial resolution. I will discuss recent experiments where we used
UED to study lattice dynamics in cuprate superconductors in response
to photo-excitation of the charge carriers. Above a certain threshold
laser intensity, we observe a direct conversion between two
structures with different c axis lattice constants indicating a
non-equilibrium structural phase transition. Strikingly, this
threshold happens around 0.1 photons absorbed per Cu site, very close
to the chemically doped carriers needed to induce superconductivity.
Besides the ability of directly coupling into the structural
dynamics, extreme sensitivity of electrons enables the study of
interfaces and nano-scale specimens by UED. I will also discuss
measurements of structure and dynamics of nano-scale assemblies of
interfacial water adsorbed on different substrates.