Condensed Matter Physics Seminar
Vikram Deshpande
Caltech
Tuesday, April 15, 2008
1:30 pm in Jadwin Hall, Room A09
Electrons, phonons and their interactions in carbon nanotubes
Abstract: Carbon nanotubes are clean systems for studying physical phenomena in one dimension (1D). Electrons in 1D at high density are known to form a Luttinger liquid. However, at low density and in the absence of disorder, the ground state is predicted to be a 1D Wigner crystal - an electron solid dominated by inter-electron Coulomb repulsion. I will describe our recent experiments with ultra-clean semiconducting carbon nanotubes in the few-electron regime. Using low-temperature Coulomb blockade spectroscopy in a magnetic field, we map out the exchange coupling as a function of electron (or hole) number [1] and find excellent agreement with a Wigner crystal model. At high electric fields, these suspended nanotube devices show striking negative differential conductance - a phenomenon attributed to non-equilibrium phonons. We obtain the first direct evidence of such "hot" phonons [2] using spatially-resolved Raman spectroscopy in conjunction with electrical transport. We directly observe the parabolic temperature profile expected for Joule heating in 1D; surprisingly, short (~2 micron) devices show no heating profile even at very high bias (1.5 V). These measurements reveal the mechanisms of hot-phonon decay, thermalization and thermal transport in these 1D systems [3].