1,297 research outputs found
The Anderson impurity model with a narrow-band host: from orbital physics to the Kondo effect
A particle-hole symmetric Anderson impurity model with a metallic host of
narrow bandwidth is studied within the framework of the local moment approach.
The resultant single-particle spectra are compared to unrestricted
Hartree-Fock, second order perturbation theory about the noninteracting limit,
and Lanczos spectra by Hofstetter and Kehrein. Rather accurate analytical
results explain the spectral evolution over almost the entire range of
interactions. These encompass, in particular, a rationale for the four-peak
structure observed in the low-energy sector of the Lanczos spectra in the
moderate-coupling regime. In weak coupling, the spectral evolution is governed
by orbital effects, while in the strong coupling Kondo limit, the model is
shown to connect smoothly to the generic Anderson impurity with a flat and
infinitely wide hybridization band.Comment: 17 pages, 7 figure
Coherence scale of the two-dimensional Kondo Lattice model
A doped hole in the two-dimensional half-filled Kondo lattice model with
exchange J and hopping t has momentum (pi,pi) irrespective of the coupling J/t.
The quasiparticle residue of the doped hole, Z_{(\pi, \pi)}, tracks the Kondo
scale, T_K, of the corresponding single impurity model. Those results stem from
high precision quantum Monte Carlo simulations on lattices up to 12 X 12.
Accounting for small dopings away from half-filling within a rigid band
approximation, this result implies that the effective mass of the charge
carriers at the Fermi level tracks 1/T_K or equivalently that the coherence
temperature T_{coh} \propto T_K. This results is consistent with the large-N
saddle point of the SU(N) symmetric Kondo lattice model.Comment: 4 pages, 4 figure
Kondo Resonance Decoherence by an External Potential
The Kondo problem, for a quantum dot (QD), subjected to an external bias, is
analyzed in the limit of infinite Coulomb repulsion by using a consistent
equations of motion method based on a slave-boson Hamiltonian. Utilizing a
strict perturbative solution in the leads-dot coupling, T, to T^4 and T^6
orders, we calculate the QD spectral density and conductance, as well as the
decoherent rate that drive the systemm from the strong to the weak coupling
regime. Our results indicate thet the weak coupling regime is reached for
voltages larger than a few units of the Kondo temperature.Comment: 5 figure
Metamagnetism and Lifshitz Transitions in Models for Heavy Fermions
We investigate metamagnetic transitions in models for heavy fermions by
considering the doped Kondo lattice model in two dimensions. Results are
obtained within the framework of dynamical mean field and dynamical cluster
approximations. Universal magnetization curves for different temperatures and
Kondo couplings develop upon scaling with the lattice coherence temperature.
Furthermore, the coupling of the local moments to the magnetic field is varied
to take into account the different Land\'e factors of localized and itinerant
electrons. The competition between the lattice coherence scale and the Zeeman
energy scale allows for two interpretations of the metamagnetism in heavy
fermions: Kondo breakdown or Lifshitz transitions. By tracking the
single-particle residue through the transition, we can uniquely conclude in
favor of the Lifshitz transition scenario. In this scenario, a quasiparticle
band drops below the Fermi energy which leads to a change in topology of the
Fermi surface.Comment: 8 pages, 7 figure
Influence of Correlated Hybridization on the Conductance of Molecular Transistors
We study the spin-1/2 single-channel Anderson impurity model with correlated
(occupancy dependent) hybridization for molecular transistors using the
numerical renormalization-group method. Correlated hybridization can induce
nonuniversal deviations in the normalized zero-bias conductance and, for some
parameters, modestly enhance the spin polarization of currents in applied
magnetic field. Correlated hybridization can also explain a gate-voltage
dependence to the Kondo scale similar to what has been observed in recent
experiments.Comment: 4 pages, 5 figure
Interactions and magnetic moments near vacancies and resonant impurities in graphene
The effect of electronic interactions in graphene with vacancies or resonant
scatterers is investigated. We apply dynamical mean-field theory in combination
with quantum Monte Carlo simulations, which allow us to treat
non-perturbatively quantum fluctuations beyond Hartree-Fock approximations. The
interactions narrow the width of the resonance and induce a Curie magnetic
susceptibility, signaling the formation of local moments. The absence of
saturation of the susceptibility at low temperatures suggests that the coupling
between the local moment and the conduction electrons is ferromagnetic
Entanglement at the boundary of spin chains near a quantum critical point and in systems with boundary critical points
We analyze the entanglement properties of spins (qubits) attached to the
boundary of spin chains near quantum critical points, or to dissipative
environments, near a boundary critical point, such as Kondo-like systems or the
dissipative two level system. In the first case, we show that the properties of
the entanglement are significantly different from those for bulk spins. The
influence of the proximity to a transition is less marked at the boundary. In
the second case, our results indicate that the entanglement changes abruptly at
the point where coherent quantum oscillations cease to exist. The phase
transition modifies significantly less the entanglement.Comment: 5 pages, 4 figure
Anderson-like impurity in the one-dimensional t-J model: formation of local states and magnetic behaviour
We consider an integrable model describing an Anderson-like impurity coupled
to an open -- chain. Both the hybridization (i.e. its coupling to bulk
chain) and the local spectrum can be controlled without breaking the
integrability of the model. As the hybridization is varied, holon and spinon
bound states appear in the many body ground state. Based on the exact solution
we study the state of the impurity and its contribution to thermodynamic
quantities as a function of an applied magnetic field. Kondo behaviour in the
magnetic response of the impurity can be observed provided that its parameters
have been adjusted properly to the energy scales of the holon and spinon
excitations of the one-dimensional bulk.Comment: 32 pages, 11 figure
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