14 research outputs found
Thermal entanglement in a triple quantum dot system
We present studies of thermal entanglement of a three-spin system in
triangular symmetry. Spin correlations are described within an effective
Heisenberg Hamiltonian, derived from the Hubbard Hamiltonian, with
super-exchange couplings modulated by an effective electric field. Additionally
a homogenous magnetic field is applied to completely break the degeneracy of
the system. We show that entanglement is generated in the subspace of doublet
states with different pairwise spin correlations for the ground and excited
states. At low temperatures thermal mixing between the doublets with the same
spin destroys entanglement, however one can observe its restoration at higher
temperatures due to the mixing of the states with an opposite spin orientation
or with quadruplets (unentangled states) always destroys entanglement. Pairwise
entanglement is quantified using concurrence for which analytical formulae are
derived in various thermal mixing scenarios. The electric field plays a
specific role -- it breaks the symmetry of the system and changes spin
correlations. Rotating the electric field can create maximally entangled qubit
pairs together with a separate spin (monogamy) that survives in a relatively
wide temperature range providing robust pairwise entanglement generation at
elevated temperatures.Comment: 9 pages, 5 figures, accepted in Eur. Phys. J.
Dark States and Transport through Quantum Dots
We consider current through triple and quadruple quantum dot systems in an in-plane electric eld and in the sequential tunneling regime. The electric eld breaks symmetry of the system and can trap electron in a dark state in which current ow can completely be blocked. Consequently rotating the electric eld, one can observe current oscillations and blockades due to dark state
Dark States and Transport through Quantum Dots
We consider current through triple and quadruple quantum dot systems in an in-plane electric field and in the sequential tunneling regime. The electric field breaks symmetry of the system and can trap electron in a dark state in which current flow can completely be blocked. Consequently rotating the electric field, one can observe current oscillations and blockades due to dark state
This content has been downloaded from IOPscience. Please scroll down to see the full text. Effect of assisted hopping on thermopower in an interacting quantum dot Effect of assisted hopping on thermopower in an interacting quantum dot
Abstract We investigate the electrical conductance and thermopower of a quantum dot tunnel coupled to external leads described by an extension of the Anderson impurity model which takes into account the assisted hopping processes, i.e., the occupancy-dependence of the tunneling amplitudes. We provide analytical understanding based on scaling arguments and the Schrieffer-Wolff transformation, corroborated by detailed numerical calculations using the numerical renormalization group method. The assisted hopping modifies the coupling to the two-particle state, which shifts the Kondo exchange coupling constant and exponentially reduces or enhances the Kondo temperature, breaks the particlehole symmetry, and strongly affects the thermopower. We discuss the gatevoltage and temperature dependence of the transport properties in various regimes. For a particular value of the assisted hopping parameter we find peculiar discontinuous behaviour in the mixed-valence regime. Near this value, we find very high Seebeck coefficient. We show that, quite generally, the thermopower is a highly sensitive probe of assisted hopping and Kondo correlations