476 research outputs found
Long-Range Energy-Level Interaction in Small Metallic Particles
We consider the energy level statistics of non-interacting electrons which
diffuse in a -dimensional disordered metallic conductor of characteristic
Thouless energy We assume that the level distribution can be written
as the Gibbs distribution of a classical one-dimensional gas of fictitious
particles with a pairwise additive interaction potential
We show that the interaction which is consistent with the known correlation
function of pairs of energy levels is a logarithmic repulsion for level
separations in agreement with Random Matrix Theory. When
vanishes as a power law in with exponents and for
and 3, respectively. While for the energy-level
interaction is always repulsive, in three dimensions there is long-range level
attraction after the short-range logarithmic repulsion.Comment: Saclay-s93/014 Email: [email protected] [2017: missing
figure included
Theory of scanning gate microscopy
A systematic theory of the conductance measurements of non-invasive (weak
probe) scanning gate microscopy is presented that provides an interpretation of
what precisely is being measured. A scattering approach is used to derive
explicit expressions for the first and second order conductance changes due to
the perturbation by the tip potential in terms of the scattering states of the
unperturbed structure. In the case of a quantum point contact, the first order
correction dominates at the conductance steps and vanishes on the plateaus
where the second order term dominates. Both corrections are non-local for a
generic structure. Only in special cases, such as that of a centrally symmetric
quantum point contact in the conductance quantization regime, can the second
order correction be unambiguously related with the local current density. In
the case of an abrupt quantum point contact we are able to obtain analytic
expressions for the scattering eigenfunctions and thus evaluate the resulting
conductance corrections.Comment: 19 pages, 7 figure
Universal Quantum Signatures of Chaos in Ballistic Transport
The conductance of a ballistic quantum dot (having chaotic classical dynamics
and being coupled by ballistic point contacts to two electron reservoirs) is
computed on the single assumption that its scattering matrix is a member of
Dyson's circular ensemble. General formulas are obtained for the mean and
variance of transport properties in the orthogonal (beta=1), unitary (beta=2),
and symplectic (beta=4) symmetry class. Applications include universal
conductance fluctuations, weak localization, sub-Poissonian shot noise, and
normal-metal-superconductor junctions. The complete distribution P(g) of the
conductance g is computed for the case that the coupling to the reservoirs
occurs via two quantum point contacts with a single transmitted channel. The
result P(g)=g^(-1+beta/2) is qualitatively different in the three symmetry
classes. ***Submitted to Europhysics Letters.****Comment: 4 pages, REVTeX-3.0, INLO-PUB-94032
Semiclassical Theory of Time-Reversal Focusing
Time reversal mirrors have been successfully implemented for various kinds of
waves propagating in complex media. In particular, acoustic waves in chaotic
cavities exhibit a refocalization that is extremely robust against external
perturbations or the partial use of the available information. We develop a
semiclassical approach in order to quantitatively describe the refocusing
signal resulting from an initially localized wave-packet. The time-dependent
reconstructed signal grows linearly with the temporal window of injection, in
agreement with the acoustic experiments, and reaches the same spatial extension
of the original wave-packet. We explain the crucial role played by the chaotic
dynamics for the reconstruction of the signal and its stability against
external perturbations.Comment: 4 pages, 1 figur
Shot noise in the chaotic-to-regular crossover regime
We investigate the shot noise for phase-coherent quantum transport in the
chaotic-to-regular crossover regime. Employing the Modular Recursive Green's
Function Method for both ballistic and disordered two-dimensional cavities we
find the Fano factor and the transmission eigenvalue distribution for regular
systems to be surprisingly similar to those for chaotic systems. We argue that
in the case of regular dynamics in the cavity, diffraction at the lead openings
is the dominant source of shot noise. We also explore the onset of the
crossover from quantum to classical transport and develop a quasi-classical
transport model for shot noise suppression which agrees with the numerical
quantum data.Comment: 4 pages, 3 figures, submitted to Phys.Rev.Let
Lifetime of the first and second collective excitations in metallic nanoparticles
We determine the lifetime of the surface plasmon in metallic nanoparticles
under various conditions, concentrating on the Landau damping, which is the
dominant mechanism for intermediate-size particles. Besides the main
contribution to the lifetime, which smoothly increases with the size of the
particle, our semiclassical evaluation yields an additional oscillating
component. For the case of noble metal particles embedded in a dielectric
medium, it is crucial to consider the details of the electronic confinement; we
show that in this case the lifetime is determined by the shape of the
self-consistent potential near the surface. Strong enough perturbations may
lead to the second collective excitation of the electronic system. We study its
lifetime, which is limited by two decay channels: Landau damping and
ionization. We determine the size dependence of both contributions and show
that the second collective excitation remains as a well defined resonance.Comment: 18 pages, 5 figures; few minor change
Growth and optical properties of GaN/AlN quantum wells
We demonstrate the growth of GaN/AlN quantum well structures by
plasma-assisted molecular-beam epitaxy by taking advantage of the surfactant
effect of Ga. The GaN/AlN quantum wells show photoluminescence emission with
photon energies in the range between 4.2 and 2.3 eV for well widths between 0.7
and 2.6 nm, respectively. An internal electric field strength of
MV/cm is deduced from the dependence of the emission energy on the well width.Comment: Submitted to AP
Contribution of polycyclic aromatic hydrocarbon ionization to neutral gas heating in galaxies: model versus observations
[Abridged] The ionization of polycyclic aromatic hydrocarbons (PAHs), by
ultraviolet (UV) photons from massive stars is expected to account for a large
fraction of the heating of neutral gas in galaxies. Evaluation of this
proposal, however, has been limited by our ability to directly compare
observational diagnostics to the results of a molecular model describing PAH
ionization. The objective of this article is to take advantage of the most
recent values of molecular parameters derived from laboratory experiments and
quantum chemical calculations on PAHs and provide a detailed comparison between
modeled values and observational diagnostics for the PAH charge state and the
heating efficiency for PAHs. Despite the use of a simple analytical model, we
obtain a good agreement between model results and observational diagnostics
over a wide range of radiation fields and physical conditions, in environments
such as star-forming regions, galaxies, and protoplanetary disks. In addition,
we found that the modeled photoelectric heating rates by PAHs are close to the
observed cooling rates given by the gas emission. These results show that PAH
ionization is the main source of neutral gas heating in these environments. The
results of our photoelectric heating model by PAHs can thus be used to assess
the contribution of UV radiative heating in galaxies (vs shocks, for instance).
We provide the empirical formulas fitted to the model results, and the full
python code itself, to calculate the heating rates and heating efficiencies for
PAHs.Comment: Accepted for publication in Astronomy and Astrophysic
Scanning gate experiments: from strongly to weakly invasive probes
An open resonator fabricated in a two-dimensional electron gas is used to
explore the transition from strongly invasive scanning gate microscopy to the
perturbative regime of weak tip-induced potentials. With the help of numerical
simulations that faithfully reproduce the main experimental findings, we
quantify the extent of the perturbative regime in which the tip-induced
conductance change is unambiguously determined by properties of the unperturbed
system. The correspondence between the experimental and numerical results is
established by analyzing the characteristic length scale and the amplitude
modulation of the conductance change. In the perturbative regime, the former is
shown to assume a disorder-dependent maximum value, while the latter linearly
increases with the strength of a weak tip potential.Comment: 11 pages, 7 figure
Embedding method for the scattering phase in strongly correlated quantum dots
The embedding method for the calculation of the conductance through
interacting systems connected to single channel leads is generalized to obtain
the full complex transmission amplitude that completely characterizes the
effective scattering matrix of the system at the Fermi energy. We calculate the
transmission amplitude as a function of the gate potential for simple
diamond-shaped lattice models of quantum dots with nearest neighbor
interactions. In our simple models we do not generally observe an interaction
dependent change in the number of zeroes or phase lapses that depend only on
the symmetry properties of the underlying lattice. Strong correlations separate
and reduce the widths of the resonant peaks while preserving the qualitative
properites of the scattering phase.Comment: 11 pages, 3 figures. Proceedings of the Workshop on Advanced
Many-Body and Statistical Methods in Mesoscopic Systems, Constanta, Romania,
June 27th - July 2nd 2011. To appear in Journal of Physics: Conference Serie
- …