  Quantum dots are model systems for quantum thermoelectric behavior because of
the ability to control and measure the effects of electron-energy filtering and
quantum confinement on thermoelectric properties. Interestingly, nonlinear
thermoelectric properties of such small systems can modify the efficiency of
thermoelectric power conversion. Using quantum dots embedded in semiconductor
nanowires, we measure thermovoltage and thermocurrent that are strongly
nonlinear in the applied thermal bias. We show that most of the observed
nonlinear effects can be understood in terms of a renormalization of the
quantum-dot energy levels as a function of applied thermal bias and provide a
theoretical model of the nonlinear thermovoltage taking renormalization into
account. Furthermore, we propose a theory that explains a possible source of
the observed, pronounced renormalization effect by the melting of Kondo
correlations in the mixed-valence regime. The ability to control nonlinear
thermoelectric behavior expands the range in which quantum thermoelectric
effects may be used for efficient energy conversion.
