We apply the Keldysh formalism in order to derive a current formula easy to use for a system with many sites, one of which is interacting. The main technical challenge is to deal with the lesser Green function. It turns out that, in the case of the left-right symmetry, the knowledge of the lesser Green function is not necessary and an exact current formula can be expressed in terms of retarded Green functions only. The application is done for a triangular interferometer which gives a good account of the Fano-Kondo effect. It is found that the interference effects, in the context of Kondo correlations, give rise to a point in the parameters space where the conductance is temperature-independent. We include a comparison with the results from the Ng's ansatz, which are less accurate, but can be used also in the absence of the above mentioned symmetry.
We investigate the equilibrium and non-equilibrium transport through a quantum dot in the Kondo regime, embedded between a normal metal and a topological superconductor supporting Majorana bound states at its end points. We find that the Kondo physics is significantly modified by the presence of the Majorana modes. When the Majorana modes are coupled, aside from the Kondo scale TK , a new energy scale T * TK emerges, that controls the low energy physics of the system. At low temperatures, the ac-conductance is suppressed for frequencies below T * , while the noise spectrum acquires a ∼ ω 3 dependence. At high temperatures, T TK , the regular logarithmic dependence in the differential conductance is also affected. Under non-equilibrium conditions, and in particular in the {T, B} → 0 limit, the differential conductance becomes negative. These findings indicate that the changes in transport may serve as clues for detecting the Majorana bound states in such systems. In terms of methods used, we characterize the transport by using a combination of perturbative and renormalization group approaches.
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.
We investigate theoretically the spectral and dynamical effects of the short-range exchange interaction between a single manganese (Mn) atom hosted by cylindrical CdTe quantum dots and its light-hole excitons or biexcitons. Our approach is based on the Kohn-Luttinger k · p theory and configuration interaction method, the dynamics of the system in the presence of intraband relaxation being derived from the von Neumann-Lindblad equation. The complex structure of the light-hole exciton absorption spectrum reveals the exchange-induced exciton mixing and depends strongly on the Mn position. In particular, if the Mn atom is closer to the edges of the cylinder, the bright and dark light-hole excitons are mixed by the hole-Mn exchange alone. Consequently, their populations exhibit exchange-induced Rabi oscillations which can be viewed as optical signatures of light-hole spin reversal. Similar results are obtained for mixed biexcitons, in this case the exchange-induced Rabi oscillations being damped by the intraband hole relaxation processes. The effect of light-hole heavy-hole mixing is also discussed.
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