Thermoelectricity of near-resonant tunnel junctions and their relation to Carnot efficiency

The resonant tunneling model is the simplest for describing electronic transport through nanoscale objects like individual molecules. A complete understanding includes not only charge but also thermal and their intricate interplay. Key linear response observables are electrical conductance G Seebeck coefficient S. Here we present experiments on unspecified tunnel junctions molecular that uncover correlations between $G$ $S$, in particular rigid boundaries $S(G)$. We find these can be consistently understood by single-level model, with excellent match to experiments. In this framework, measuring $I(V)$ $S$ a given junction provides access full thermoelectric characterization of system. remarkable result without targeted chemical design, expose conversion efficiencies which close Carnot limit. This insight allows provide design rules optimized efficiency.