Direct Probe of the Seebeck Coefficient in a Kondo-Correlated Single-Quantum-Dot Transistor

Abstract

We report on the first measurement of the Seebeck coefficient in a tunnel-contacted and gate-tunable individual single-quantum dot junction in the Kondo regime, fabricated using the electromigration technique. This fundamental thermoelectric parameter is obtained by directly monitoring the magnitude of the voltage induced in response to a temperature difference across the junction, while keeping a zero net tunneling current through the device. In contrast to bulk materials and single molecules probed in a scanning tunneling microscopy (STM) configuration, investigating the thermopower in nanoscale electronic transistors benefits from the electric tunability to showcase prominent quantum effects. Here, striking sign changes of the Seebeck coefficient are induced by varying the temperature, depending on the spin configuration in the quantum dot. The comparison with Numerical Renormalization Group (NRG) calculations demonstrate that the tunneling density of states is generically asymmetric around the Fermi level in the leads, both in the cotunneling and Kondo regimes. arXiv:1811.04219v1 [cond-mat.mes-hall]

Two- and three-terminal far-from-equilibrium thermoelectric nano-devices in the Kondo regime

“…Our theory also allows to explain the results shown in figure 4(a) of a recent experimental paper [29], albeit with a notable exception. These authors have found 'striking' sign changes of the Seebeck coefficients, in addition to the standard sign change at the particle-hole symmetric point.…”

“…These sign changes depend on the spin configuration of the quantum dot and are observed with increasing temperature. In the experiment [29], S in fact changes sign at three values of the gate voltage, with one of them at the particle-hole symmetric point. Two other gate voltages at which sign changes appear are related to consecutive energy levels crossing the chemical potential.…”

“…We emphasise that the typical asymmetry of the Kondo peak outside the particle-hole symmetry point is well reproduced by the present approach. The experimental data in figure 4(a) in [29], on the other hand, seem to show only positive (or negative), i.e. finite values of S around those special points at the lowest temperatures, with an apparent maximum (minimum) at the gate voltage where the theoretical Seebeck coefficient has a dip (peak).…”

“…Furthermore, it has been proposed [14,23] that nonlinear working conditions can favourably affect the performance of heat engines. Recent studies in this direction include [24][25][26][27][28][29] and have been reviewed recently [30].…”

Magnetic field dependence of the thermopower of Kondo-correlated quantum dots: Comparison with experiment

“…Recently, the Kondo effect has attracted attention in the context of the thermoelectric response of gatetunable semiconductor and molecular quantum dots, both experimentally [24][25][26] and theoretically [27][28][29][30][31][32][33] . Understanding the thermoelectric properties of such systems is important for using nanoscale thermoelectric elements to improve the energy efficiency of microelectronic devices [34][35][36][37] .…”