RESUMEN
We report the development of a novel method to determine the thermopower of atomic-sized gold contacts at low temperature. For these measurements a mechanically controllable break junction (MCBJ) system is used and a laser source generates a temperature difference of a few kelvins across the junction to create a thermo-voltage. Since the temperature difference enters directly into the Seebeck coefficient S = -ΔV/ΔT, the determination of the temperature plays an important role. We present a method for the determination of the temperature difference using a combination of a finite element simulation, which reveals the temperature distribution of the sample, and the measurement of the resistance change due to laser heating of sensor leads on both sides next to the junction. Our results for the measured thermopower are in agreement with recent reports in the literature.
RESUMEN
Current rectification, i.e., induction of dc current by oscillating electromagnetic fields, is demonstrated in molecular junctions at an optical frequency. The magnitude of rectification is used to accurately determine the effective oscillating potentials in the junctions induced by the irradiating laser. Since the gap size of the junctions used in this study is precisely determined by the length of the embedded molecules, the oscillating potential can be used to calculate the plasmonic enhancement of the electromagnetic field in the junctions. With a set of junctions based on alkyl thiolated molecules with identical HOMO-LUMO gap and different lengths, an exponential dependence of the plasmonic field enhancement on gap size is observed.