Entanglement in a three spin system controlled by electric and magnetic field

We show influence of electric field and magnetic flux on spin entanglement in an artificial triangular molecule build of coherently coupled quantum dots. In a subspace of doublet states an explicit relation of concurrence with spin correlation functions and chirality is presented. The electric field modifies super-exchange correlations, shifts many-electron levels (the Stark effect) as well as changes spin correlations. For some specific orientation of the electric field one can observe monogamy, for which one of the spins is separated from two others. Moreover, the Stark effect manifests itself in different spin entanglement for small and strong electric fields. A role of magnetic flux is opposite, it leads to circulation of spin supercurrents and spin delocalization.Comment: 7 pages, 8 figure

Current Correlations in a Quantum Dot Ring: A Role of Quantum Interference

We present studies of the electron transport and circular currents induced by the bias voltage and the magnetic flux threading a ring of three quantum dots coupled with two electrodes. Quantum interference of electron waves passing through the states with opposite chirality plays a relevant role in transport, where one can observe Fano resonance with destructive interference. The quantum interference effect is quantitatively described by local bond currents and their correlation functions. Fluctuations of the transport current are characterized by the Lesovik formula for the shot noise, which is a composition of the bond current correlation functions. In the presence of circular currents, the cross-correlation of the bond currents can be very large, but it is negative and compensates for the large positive auto-correlation functions.Comment: 10 pages, 8 figures, minor improvment

Theoretical Studies of Quantum Interference in Electronic Transport Through Carbon Nanotubes

We performed studies of coherent electronic transport through a single walled carbon nanotube. In the calculations multiple scattering on the contacts and interference processes were taken into account. Conductance is a composition of contributions from different channels. We studied also spin--dependent transport in the system with ferromagnetic electrodes. The magnetoresistance is large and shows large oscillations, it can be even negative in some cases.Comment: 6 pages, 4 figures, oral presentation at the XXXI International School on the Physics of Semiconducting Coumpounds, Jaszowiec 2002, to appear in the Acta Physica Polonic

Fano versus Kondo Resonances in a Multilevel "Semi-Open" Quantum Dot

Linear conductance across a large quantum dot via a single level e_0 with large hybridization to the contacts is strongly sensitive to quasi-bound states localized in the dot and weakly coupled to e_0. It oscillates with the gate voltage due to interference of the Fano type. At low temperature and Coulomb blockade, Kondo correlations damp the oscillations on an extended range of gate voltage values, by freezing the occupancy of the e_0 level itself. As a consequence, antiresonances of Fano origin are washed out. The results are in good correspondence with experimental data for a large quantum dot in the semi-open regime.Comment: 4 eps figures, RevTex format, revised version, to appear in Phys. Rev. Letter

Stochastic particle acceleration in flaring stars

The acceleration of electrons by the Fermi-Parker mechanisms in a quasistationary turbulent plasma of dimension l, mean magnetic field strength B, and mean number density n are considered. The electrons suffer radiative and ionization losses and have a scattering mean free path that increases linearly with their momentum. Analytic solutions for the steady-state electron energy spectra are presented. The spectra are characterized by an exponential cutoff above a given momentum determined by the synchrontron or the confinement time, depending on the physical characteristics of the accelerating region