  We present analytical and numerical investigation of spectral and transport
properties of a quadruple quantum-dot (QQD) structure which is one of the
popular low-dimensional systems in the context of fundamental quantum physics
study, future electronic applications and quantum calculations. The density of
states, occupation numbers and conductance of the structure were analyzed using
the nonequilibrium Green's functions in the tight binding approach and the
equation-of-motion method. In particular the anisotropy of hopping integrals
and on-site electron energies as well as the effects of the finite intra- and
interdot Coulomb interactions were investigated. It was found out that the
anisotropy of the kinetic processes in the system leads to the Fano-Feshbach
asymmetrical peak. We demonstrated that the conductance of QQD device has a
wide insulating band with steep edges separating triple-peak structures if the
intradot Coulomb interactions are taken into account. The interdot Coulomb
correlations between the central QDs result in the broadening of this band and
the occurrence of an additional band with low conductance due to the Fano
antiresonances. It was shown that in this case the conductance of the
anisotropic QQD device can be dramatically changed by tuning the anisotropy of
on-site electron energies.
