  We investigate the electrical conductance and thermopower of a quantum dot
tunnel coupled to external leads described by an extension of the Anderson
impurity model which takes into account the assisted hopping processes, i.e.,
the occupancy-dependence of the tunneling amplitudes. We provide analytical
understanding based on scaling arguments and the Schrieffer-Wolff
transformation, corroborated by detailed numerical calculations using the
numerical renormalization group (NRG) method. The assisted hopping modifies the
coupling to the two-particle state, which shifts the Kondo exchange coupling
constant and exponentially reduces or enhances the Kondo temperature, breaks
the particle-hole symmetry, and strongly affects the thermopower. We discuss
the gate-voltage and temperature dependence of the transport properties in
various regimes. For a particular value of the assisted hopping parameter we
find peculiar discontinuous behaviour in the mixed-valence regime. Near this
value, we find very high Seebeck coefficient. We show that, quite generally,
the thermopower is a highly sensitive probe of assisted hopping and Kondo
correlations.
