106 research outputs found
Bipolarons in the Extended Holstein Hubbard Model
We numerically and analytically calculate the properties of the bipolaron in
an extended Hubbard Holstein model, which has a longer range electron-phonon
coupling like the Fr\" ohlich model. In the strong coupling regime, the
effective mass of the bipolaron in the extended model is much smaller than the
Holstein bipolaron mass. In contrast to the Holstein bipolaron, the bipolaron
in the extended model has a lower binding energy and remains bound with
substantial binding energy even in the large-U limit. In comparison with the
Holstein model where only a singlet bipolaron is bound, in the extended
Holstein model a triplet bipolaron can also form a bound state. We discuss the
possibility of phase separation in the case of finite electron doping.Comment: 5 pages, 3 figure
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
Spin-charge separation and simultaneous spin and charge Kondo effect
We study the spin-charge separation in a Kondo-like model for an impurity
with a spin and a charge (isospin) degree of freedom coupled to a single
conduction channel (the ``spin-charge'' Kondo model). We show that the spin and
charge Kondo effects can occur simultaneously at any coupling strength. In the
continuum (wide-band or weak coupling) limit, the Kondo screening in each
sector is independent, while at finite bandwidth and strong coupling the
lattice effects lead to a renormalization of the effective Kondo exchange
constants; nevertheless, universal spin and charge Kondo effects still occur.
We find similar behavior in the two-impurity Anderson model with positive and
negative electron-electron interaction and in the two-impurity
Anderson-Holstein model with a single phonon mode. We comment on the
applicability of such models to describe the conductance of deformable
molecules with a local magnetic moment.Comment: 13 pages, 11 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
Dissociation of a Hubbard--Holstein bipolaron driven away from equilibrium by a constant electric field
Using a variational numerical method we compute the time-evolution of the
Holstein-Hubbard bipolaron from its ground state when at t=0 the constant
electric field is switched on. The system is evolved taking into account full
quantum effects until it reaches a quasi-stationary state. In the zero-field
limit the current shows Bloch oscillations characteristic for the adiabatic
regime where the electric field causes the bipolaron to evolve along the
quasiparticle band. Bipolaron remains bound and the net current remains zero in
this regime. At larger electric fields the system enters the dissipative regime
with a finite steady-state current. Concomitantly, the bipolaron dissociates
into two separate polarons. By examining different parameter regimes we show
that the appearance of a finite steady-state current is inevitably followed by
the dissociation of the bipolaron.Comment: 9 pages, 7 figure
Scaling of the magnetic response in doped antiferromagnets
A theory of the anomalous scaling of the dynamic magnetic response
in cuprates at low doping is presented. It is based on the memory function
representation of the dynamical spin suceptibility in a doped antiferromagnet
where the damping of the collective mode is constant and large, whereas the
equal-time spin correlations saturate at low . Exact diagonalization results
within the t-J model are shown to support assumptions. Consequences, both for
the scaling function and the normalization amplitude, are well in agreement
with neutron scattering results.Comment: 4 pages, 4 figure
Even and odd-frequency pairing correlations in 1-D t-J-h model: a comparative study
An equal time version of odd-frequency pairing for a generalized model
is introduced. It is shown that the composite operators describing binding of
Cooper pairs with magnetization fluctuations naturally appear in this approach.
The pairing correlations in both BCS and odd-frequency channels are
investigated exactly in 1D systems with up to 16 sites. Our results indicate
that at some range of parameters odd-frequency correlations become comparable,
however smaller than BCS pairing correlations. It is speculated that the spin
and density fluctuations in the frustrated model lead to the enhancement of the
odd gap susceptibilities. 4 postscript figure files are attached at the bottom
of the tex file.Comment: 6 pages + 4 figure
- …