203 research outputs found
Localization and entanglement of two interacting electrons in a quantum-dot molecule
The localization of two interacting electrons in a coupled-quantum-dots
semiconductor structure is demonstrated through numerical calculations of the
time evolution of the two-electron wave function including the Coulomb
interaction between the electrons. The transition from the ground state to a
localized state is induced by an external, time-dependent, uniform electric
field. It is found that while an appropriate constant field can localize both
electrons in one of the wells, oscillatory fields can induce roughly equal
probabilities for both electrons to be localized in either well, generating an
interesting type of localized and entangled state. We also show that shifting
the field suddenly to an appropriate constant value can maintain in time both
types of localization.Comment: 4 pages, 4 figure
The interaction of oxygen with small gold clusters
Presented in this work are the results of a quantum chemical study of oxygen adsorption on small Aun and Auân (n=2,3) clusters. Density functional theory(DFT), second order perturbation theory (MP2), and singles and doubles coupled clustertheory with perturbative triples [CCSD(T)] methods have been used to determine the geometry and the binding energy of oxygen to Aun. The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD(T) calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD(T) predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD(T) results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and the sign of the errors [as compared to CCSD(T)] is a disappointing feature of the DFT results for the present system: it is not possible to give a simple rule for correcting the DFT predictions (e.g., a useful rule would be that DFT predicts stronger binding of O2 by about 0.3 eV). It is likely that the errors in DFT appear not because of gold, but because oxygen binding to a metal cluster is a particularly difficult problem.This work
was supported by AFOSR through a DURINT grant
On the possibility of using differential cross section measurements for the electronic excitation of adsorbates by an electron beam, to determine the adsorbate orientation
We show, by detailed electronâmolecule scattering calculations, that the angular dependence of electron energy loss spectra in which an adsorbate is electronically excited can be used to identify the orientation of the molecule with respect to the surface and the nature of the final states. The calculations are exploratory and were carried out for an H2 molecule. The transition amplitude for electronâmolecule scattering is calculated by using the Schwinger variational principle with two open channels. The effects of the surface were introduced through a semiquantitative model which treats the surface as a partly reflecting, flat mirror
Level Set Approach to Reversible Epitaxial Growth
We generalize the level set approach to model epitaxial growth to include
thermal detachment of atoms from island edges. This means that islands do not
always grow and island dissociation can occur. We make no assumptions about a
critical nucleus. Excellent quantitative agreement is obtained with kinetic
Monte Carlo simulations for island densities and island size distributions in
the submonolayer regime.Comment: 7 pages, 9 figure
Casimir effect: running Newton constant or cosmological term
We argue that the instability of Euclidean Einstein gravity is an indication
that the vacuum is non perturbative and contains a condensate of the metric
tensor in a manner reminiscent of Yang-Mills theories. As a simple step toward
the characterization of such a vacuum the value of the one-loop effective
action is computed for Euclidean de Sitter spaces as a function of the
curvature when the unstable conformal modes are held fixed. Two phases are
found, one where the curvature is large and gravitons should be confined and
another one which appears to be weakly coupled and tends to be flat. The
induced cosmological constant is positive or negative in the strongly or weakly
curved phase, respectively. The relevance of the Casimir effect in
understanding the UV sensitivity of gravity is pointed out.Comment: Final, slightly extended version, to appear in Classical and Quantum
Gravit
Polarization state of the optical near-field
The polarization state of the optical electromagnetic field lying several
nanometers above complex dielectric structures reveals the intricate
light-matter interaction that occurs in this near-field zone. This information
can only be extracted from an analysis of the polarization state of the
detected light in the near-field. These polarization states can be calculated
by different numerical methods well-suited to near--field optics. In this
paper, we apply two different techniques (Localized Green Function Method and
Differential Theory of Gratings) to separate each polarisation component
associated with both electric and magnetic optical near-fields produced by
nanometer sized objects. The analysis is carried out in two stages: in the
first stage, we use a simple dipolar model to achieve insight into the physical
origin of the near-field polarization state. In the second stage, we calculate
accurate numerical field maps, simulating experimental near-field light
detection, to supplement the data produced by analytical models. We conclude
this study by demonstrating the role played by the near-field polarization in
the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
Quantum Diffusion of H/Ni(111) through the Monte Carlo Wave Function Formalism
We consider a quantum system coupled to a dissipative background with many
degrees of freedom using the Monte Carlo Wave Function method. Instead of
dealing with a density matrix which can be very high-dimensional, the method
consists of integrating a stochastic Schrodinger equation with a non-hermitian
damping term in the evolution operator, and with random quantum jumps. The
method is applied to the diffusion of hydrogen on the Ni(111) surface below 100
K. We show that the recent experimental diffusion data for this system can be
understood through an interband activation process, followed by quantum
tunnelling.Comment: In press at Phys.Rev.Let
Modified f(R) gravity from scalar-tensor theory and inhomogeneous EoS dark energy
The reconstruction of f(R)-gravity is showed by using an auxiliary scalar
field in the context of cosmological evolution, this development provide a way
of reconstruct the form of the function f (R) for a given evolution of the
Hubble parameter. In analogy, f(R)-gravity may be expressed by a perfect fluid
with an inhomogeneous equation of state that depends on the Hubble parameter
and its derivatives. This mathematical equivalence that may confuse about the
origin of the mechanism that produces the current acceleration, and possibly
the whole evolution of the Hubble parameter, is shown here.Comment: 8 page
- âŠ