Magnetic field dependence of the thermopower of Kondo-correlated quantum dots

Recent experiments have measured the signatures of the Kondo effect in the zero-field thermopower of strongly correlated quantum dots [Svilans et al., Phys. Rev. Lett. 121, 206801 (2018); Dutta et al., Nano Lett.19, 506 (2019)]. They confirm the predicted Kondo-induced sign change in the thermopower, upon increasing the temperature through a gate-voltage dependent value T-1 greater than or similar to T-K, where T-K is the Kondo temperature. Here, we use the numerical renormalization group (NRG) method to investigate the effect of a finite magnetic field B on the thermopower of such quantum dots. We show that, for fields B exceeding a gate-voltage dependent value B-0, an additional sign change takes place in the Kondo regime at a temperature T-0 (B >= B-0) > 0 with T-0 < T-1. The field B-0 is comparable to, but larger than, the field B-c at which the zero-temperature spectral function splits in a magnetic field. The validity of the NRG results for B-0 are checked by comparison with asymptotically exact higher-order Fermi-liquid calculations [Oguri and Hewson, Phys. Rev. B 97, 035435 (2018)]. Our calculations clarify the field-dependent signatures of the Kondo effect in the thermopower of Kondo-correlated quantum dots and explain the recently measured trends in the B-field dependence of the thermoelectric response of such systems [Svilans et al., Phys. Rev. Lett. 121, 206801 (2018)].