Thermopower of a Kondo Spin-Correlated Quantum Dot

The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations, the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model.

Figure 1

SEM photograph of the quantum dot and the central part of the $20\mu \mathrm{m}$ long heating channel. The labels ${\mathrm{V}}_{1,2}$ and ${\mathrm{I}}_{1,2}$ indicate the 2DEG areas with Ohmic contacts.

Figure 2

Grey-scale plot of the differential conductance as a function of the QD potential $(\propto V_{\mathrm{E}})$ and the externally applied bias voltage across the QD ($V_{\mathrm{SD}}$). Alternating regimes of low and high conductance are observed between each successive conductance peak within the CB diamonds (dashed lines). Inset: Bias-voltage ($V_{\mathrm{SD}}$) depending traces of the differential conductivity of the dot for $V_{\mathrm{E}}=-1.08\mathrm{V}$ and $V_{\mathrm{E}}=-1.21\mathrm{V}$ [indicated by arrows].

Figure 3

Comparison of the thermovoltage measurement ($V_{\mathrm{T}}$), the conductance (G), and the calculated thermopower $S_{\mathrm{Mott}}$ as expected from the Mott relation [Eq. (2), where $E\propto (-e{V}_{\mathrm{E}})$] as a function of applied gate voltage $V_{\mathrm{E}}$. The horizontal bars indicate the regions of spin correlations within the gate-voltage range of strong coupling of the quantum dot to the leads ($V_{\mathrm{E}}>-1.1\mathrm{V}$).

Figure 4

(a) Thermovoltage signal for various lattice temperatures at constant $\Delta T$. The curves are normalized to the value at $V_{\mathrm{E}}=-1.8\mathrm{V}$. At high temperatures, the spin contribution to the thermopower between two CB peaks decreases and the oscillating CB substructure of the thermopower reappears (indicated by the arrow). (b),(c) Temperature dependent (differential) conductance of the zero bias resonance as a function of bias voltage ($V_{\mathrm{SD}}$) and QD potential ($V_{\mathrm{E}}$).