Mercredi 15 août 2012 à 10h30
Salle 234, Ecole de Physique

Validity of Wiedemann-Franz law in small molecular wires

Vinitha Balachandran, Universita degli Studi dell'Insubria

In this talk, I will discuss our investigations on finite-size effects of the Lorenz number in a molecular wire. Using Landauer-Buttiker formalism, we find that for sufficiently long wires there are two validity regimes of the Wiedemann-Franz (WF) law, the cotunneling and the sequential tunneling regimes, while in small systems only the first regime survives. We compare our results with the standard Kubo formalism and explain its failure to obtain the WF law in small systems. Furthermore, our studies on exponentially localized disordered wires show that the Lorenz number value $L_0 = (pi^2/3)(kB/e)^2$ predicted by the WF law is obtained only in the cotunneling regime. Also, the Lorenz number L exhibits a typical distribution at different temperatures corresponding to different tunneling processes. In particular, first-order tunneling results in a low value of L, whereas second-order tunneling recovers the universal value $L_0$. In the latter part of the talk, I will discuss our recent findings in the study of thermoelectric efficiency of systems with broken time-reversal symmetry.