2008

Low-temperature scanning tunneling spectroscopy and first-principles calculations are used to characterize electron transport through vibronic states of C₆₀ molecules. This is achieved by positioning a C₆₀ molecule on top of a molecular self-assembled template on Au(111). In these conditions, conductance spectra are shown to reveal the dynamic Jahn-Teller effect of the C₆₀ molecule. This vibronic transport study helps in solving a long-standing debate [Phys. Rev. Lett. 74, 1875 (1995); Phys. Rev. Lett. 91, 196402 (2003)] on density-functional calculations of the C₆₀ electron-phonon coupling strength.

A dynamical method for inelastic transport simulations in nanostructures is compared to a steady-state method based on nonequilibrium Green's functions. A simplified form of the dynamical method produces, in the steady state in the weak-coupling limit, effective self-energies analogous to those in the Born approximation due to electron-phonon coupling. The two methods are then compared numerically on a resonant system consisting of a linear trimer weakly embedded between metal electrodes. This system exhibits an enhanced heating at high biases and long phonon equilibration times. Despite the differences in their formulation, the static and dynamical methods capture local current-induced heating and inelastic corrections to the current with good agreement over a wide range of conditions, except in the limit of very high vibrational excitations where differences begin to emerge.

We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic and vibrational symmetries in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states - namely those belonging to the most transmitting eigenchannels - need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.