Impact of Fano and Breit-Wigner resonances in the thermoelectric properties of nanoscale junctions

We show that the thermoelectric properties of nanoscale junctions featuring states near the Fermi level strongly depend on the type of resonance generated by such states, which can be either Fano- or Breit-Wigner-like. We give general expressions for the thermoelectric coefficients generated by the two types of resonances and calculate the thermoelectric properties of these systems, which encompass most nanoelectronics junctions. We include simulations of real junctions where metalloporphyrins dithiolate molecules bridge gold electrodes and prove that for some metallic elements the thermoelectric properties show a large variability with respect to the position of the resonance near the Fermi level. We find that the thermopower and figure of merit are largely enhanced when the resonance gets close to the Fermi level and reach values higher than typical values found in other nanoscale junctions. The specific value and temperature dependence are determined by a series of factors such as the strength of the coupling between the state and other molecular states, the symmetry of the state, the strength of the coupling between the molecule and the leads and the spin-filtering behavior of the junction. © 2013 American Physical Society.The research presented in this paper was funded by the Spanish MICINN through Grant No. FIS2012-34858 and by the Marie Curie network NanoCTM. V.M.G.S. thanks the Spanish Ministerio de Economía y Competitividad for a Ramón y Cajal fellowship (RYC-2010-06053). R.R.F. thanks Consejería de Educación del Principado de Asturias for a Severo Ochoa grant (Grant No. BP11-069).Peer Reviewe