6 research outputs found
Kondo effect in triple quantum dots
Numerical analysis of the simplest odd-numbered system of coupled quantum
dots reveals an interplay between magnetic ordering, charge fluctuations and
the tendency of itinerant electrons in the leads to screen magnetic moments.
The transition from local-moment to molecular-orbital behavior is visible in
the evolution of correlation functions as the inter-dot coupling is increased.
Resulting novel Kondo phases are presented in a phase diagram which can be
sampled by measuring the zero-bias conductance. We discuss the origin of the
even-odd effects by comparing with the double quantum dot.Comment: 4 pages, 4 figure
Spin qubits in double quantum dots - entanglement versus the Kondo effect
We investigate the competition between pair entanglement of two spin qubits
in double quantum dots attached to leads with various topologies and the
separate entanglement of each spin with nearby electrodes. Universal behavior
of entanglement is demonstrated in dependence on the mutual interactions
between the spin qubits, the coupling to their environment, temperature and
magnetic field. As a consequence of quantum phase transition an abrupt switch
between fully entangled and unentangled states takes place when the dots are
coupled in parallel.Comment: 3 figure
Enhanced Conductance Through Side-Coupled Double Quantum Dots
Conductance, on-site and inter-site charge fluctuations and spin correlations
in the system of two side-coupled quantum dots are calculated using the
Wilson's numerical renormalization group (NRG) technique. We also show spectral
density calculated using the density-matrix NRG, which for some parameter
ranges remedies inconsistencies of the conventional approach. By changing the
gate voltage and the inter-dot tunneling rate, the system can be tuned to a
non-conducting spin-singlet state, the usual Kondo regime with odd number of
electrons occupying the dots, the two-stage Kondo regime with two electrons, or
a valence-fluctuating state associated with a Fano resonance. Analytical
expressions for the width of the Kondo regime and the Kondo temperature are
given. We also study the effect of unequal gate voltages and the stability of
the two-stage Kondo effect with respect to such perturbations.Comment: 11 pages, 12 figure
Correlation Effects in Side-Coupled Quantum Dots
Using Wilson's numerical renormalization group (NRG) technique we compute
zero-bias conductance and various correlation functions of a double quantum dot
(DQD) system. We present different regimes within a phase diagram of the DQD
system. By introducing a negative Hubbard U on one of the quantum dots, we
simulate the effect of electron-phonon coupling and explore the properties of
the coexisting spin and charge Kondo state. In a triple quantum dot (TQD)
system a multi-stage Kondo effect appears where localized moments on quantum
dots are screened successively at exponentially distinct Kondo temperatures.Comment: 13 pages, 10 figure
Relevance of quantum fluctuations in the Anderson-Kondo model
We study a localized spin coupled to an Anderson impurity to model the
situation found in higher transition metal or rare earth compounds like e.g.\
LaMnO or Gd monopnictides. We find that, even for large quantum numbers of
the localized spin, quantum fluctuations play an essential role for the case of
ferromagnetic coupling between the spin and the impurity levels. For
antiferromagnetic coupling, a description in terms of a classical spin is
appropriate
Microscopic mechanisms of dephasing due to electron-electron interactions
We develop a non-perturbative numerical method to study tunneling of a single
electron through an Aharonov-Bohm ring where several strongly interacting
electrons are bound. Inelastic processes and spin-flip scattering are taken
into account. The method is applied to study microscopic mechanisms of
dephasing in a non-trivial model. We show that electron-electron interactions
described by the Hubbard Hamiltonian lead to strong dephasing: the transmission
probability at flux is high even at small interaction strength. In
addition to inelastic scattering, we identify two energy conserving mechanisms
of dephasing: symmetry-changing and spin-flip scattering. The many-electron
state on the ring determines which of these mechanisms will be at play:
transmitted current can occur either in elastic or inelastic channels, with or
without changing the spin of the scattering electron.Comment: 11 pages, 16 figures Submitted to Phys. Rev.