  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.
