Correlated electrons and transport in a quantum point contact and in a double-quantum-dot system

A problem of electronic correlations is considered for two specific mesoscopic systems: the quantum point contact (QPC) and the double quantum dot (2QD) system. The systems are described using a generalized Anderson Hamiltonian. We show that charge fluctuations are relevant for electronic transport. In the QPC a local accumulation of charge and the dynamical Coulomb blockade effect lead to the 0.7 structure in the conductance characteristics. The evolution of the conductance with a magnetic field and in non-equilibrium situations is presented as well. The double quantum dot is studied in the approach, in which correlations within the 2QD are treated exactly, whereas the coupling of the 2QD to the leads is considered in the approximation valid at temperatures above the Kondo temperature. We analyse the evolution of the gate voltage dependence of the spin correlation functions and the conductance with the change of the interdot hopping. For the hopping parameter greater than a threshold value of the on-dot repulsion the physics of the device is dominated by the ground state eigenstates of the 2QD and antiferromagnetic correlations in the case of the doubly occupied 2QD. With a decrease of the interdot hopping repulsion below the threshold we observe a significant reduction of the antiferromagnetic coupling between the dots together with an enhanced occupation of the triplet states.