Ultrafast charge-carrier dynamics in low-dimensional solids

Christian Frischkorn

Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin

The question of how charge carriers interact with their surroundings and dissipate their excess energy after excitation is of fundamental and technological importance. Energy and current relaxation of charge carriers determine the performance of electronic circuits at ever higher clock frequencies and smaller device dimensions. We have addressed these issues in the low-dimensional solids graphite and carbon nanotubes using ultrashort broadband THz pulses of up to 40 THz.

In the case of the quasi-2-dimensional solid graphite, our results show that the laser-excited electrons lose more than 90% of their excess energy within the first 500 fs after photoexcitation. Strongly coupled optical phonons in the graphite layer dominate the ultrafast electronic energy and transport relaxation. These phonon modes heat up on this subpicosecond time scale and subsequently 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.

For carbon nanotubes as a prototypical 1-dimensional solid, we find essentially two processes governing the electronic current dynamics. 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 on a 100-nm length scale due to defects is observed as manifested in a substantial dichroism. In the latter measurements, the reduced polarizability perpendicular to the tube axis is exploited. Our findings might point to limitations in the performance of both graphite- and carbon nanotube-based electronic circuits.