Dresden 2006 – scientific programme

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HL: Halbleiterphysik

HL 24: Ultra fast phenomena

HL 24.9: Talk

Tuesday, March 28, 2006, 17:15–17:30, POT 151

Ultrafast dynamics of the mid-infrared response of carbon nanotubes and graphite — •Christian Frischkorn, Tobias Kampfrath, Luca Perfetti, and Martin Wolf — Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin

We report on time-resolved measurements of low-energy excitations in 1- and 2-dimensional solids like carbon nanotubes and thin graphite films. The mid-infrared response of these systems has been obtained from time-resolved THz spectroscopy data in the 10 - 30 THz spectral range. For carbon nanotubes, we find essentially two processes governing an electronic current dynamics [1]. First, strongly bound excitons are the main photoproduct in large-band gap tubes and thus prevent a typical free-carrier response, while in small-gap and metallic tubes carrier localization due to defects is observed as manifested in a substantial dichroism. In these measurements, the reduced polarizability perpendicular to the tube axis is exploited. In the case of graphite, a 2-dimensional semimetal, our results show that strongly coupled optical phonons in the graphite layer dominate the ultrafast energy and transport relaxation dynamics after optical excitation [2]. These phonon modes heat up on a femtosecond time scale and cool down with a time constant of several picoseconds. Moreover, the observed pronounced increase in the Drude relaxation rate significantly originates from these few active lattice vibrations. Our findings for both carbon nanotubes and graphite are of fundamental importance for technological applications in nanoelectronics. - [1] PRL (submitted); [2] PRL 95, 187403 (2005).