721 research outputs found
Signatures of electron correlations in the transport properties of quantum dots
The transition matrix elements between the correlated and
electron states of a quantum dot are calculated by numerical diagonalization.
They are the central ingredient for the linear and non--linear transport
properties which we compute using a rate equation. The experimentally observed
variations in the heights of the linear conductance peaks can be explained. The
knowledge of the matrix elements as well as the stationary populations of the
states allows to assign the features observed in the non--linear transport
spectroscopy to certain transition and contains valuable information about the
correlated electron states.Comment: 4 pages (revtex,27kB) + 3 figures in one file ziped and uuencoded
(postscript,33kB), to appear in Phys.Rev.B as Rapid Communicatio
Particle creation in an oscillating spherical cavity
We study the creation of massless scalar particles from the quantum vacuum
due to the dynamical Casimir effect by spherical shell with oscillating radius.
In the case of a small amplitude of the oscillation, to solve the infinite set
of coupled differential equations for the instantaneous basis expansion
coefficients we use the method based on the time-dependent perturbation theory
of the quantum mechanics. To the first order of the amplitude we derive the
expressions for the number of the created particles for both parametric
resonance and non-resonance cases.Comment: 8 pages, LaTeX, no figure
Wigner Molecules in Nanostructures
The one-- and two-- particle densities of up to four interacting electrons
with spin, confined within a quasi one--dimensional ``quantum dot'' are
calculated by numerical diagonalization. The transition from a dense
homogeneous charge distribution to a dilute localized Wigner--type electron
arrangement is investigated. The influence of the long range part of the
Coulomb interaction is studied. When the interaction is exponentially cut off
the ``crystallized'' Wigner molecule is destroyed in favor of an inhomogeneous
charge distribution similar to a charge density wave .Comment: 10 pages (excl. Figures), Figures available on request LaTe
Effect of oxygen plasma etching on graphene studied with Raman spectroscopy and electronic transport
We report a study of graphene and graphene field effect devices after
exposure to a series of short pulses of oxygen plasma. We present data from
Raman spectroscopy, back-gated field-effect and magneto-transport measurements.
The intensity ratio between Raman "D" and "G" peaks, I(D)/I(G) (commonly used
to characterize disorder in graphene) is observed to increase approximately
linearly with the number (N(e)) of plasma etching pulses initially, but then
decreases at higher Ne. We also discuss implications of our data for extracting
graphene crystalline domain sizes from I(D)/I(G). At the highest Ne measured,
the "2D" peak is found to be nearly suppressed while the "D" peak is still
prominent. Electronic transport measurements in plasma-etched graphene show an
up-shifting of the Dirac point, indicating hole doping. We also characterize
mobility, quantum Hall states, weak localization and various scattering lengths
in a moderately etched sample. Our findings are valuable for understanding the
effects of plasma etching on graphene and the physics of disordered graphene
through artificially generated defects.Comment: 10 pages, 5 figure
Transport properties of quantum dots in the Wigner molecule regime
The transport properties of quantum dots with up to N=7 electrons ranging
from the weak to the strong interacting regime are investigated via the
projected Hartree-Fock technique. As interactions increase radial order
develops in the dot, with the formation of ring and centered-ring structures.
Subsequently, angular correlations appear, signalling the formation of a Wigner
molecule state. We show striking signatures of the emergence of Wigner
molecules, detected in transport. In the linear regime, conductance is
exponentially suppressed as the interaction strength grows. A further
suppression is observed when centered-ring structures develop, or peculiar spin
textures appear. In the nonlinear regime, the formation of molecular states may
even lead to a conductance enhancement.Comment: 26 pages, 14 figures, Accepted for publication on New Journal of
Physic
Polariton Nanophotonics using Phase Change Materials
Polaritons formed by the coupling of light and material excitations such as
plasmons, phonons, or excitons enable light-matter interactions at the
nanoscale beyond what is currently possible with conventional optics. Recently,
significant interest has been attracted by polaritons in van der Waals
materials, which could lead to applications in sensing, integrated photonic
circuits and detectors. However, novel techniques are required to control the
propagation of polaritons at the nanoscale and to implement the first practical
devices. Here we report the experimental realization of polariton refractive
and meta-optics in the mid-infrared by exploiting the properties of low-loss
phonon polaritons in isotopically pure hexagonal boron nitride (hBN), which
allow it to interact with the surrounding dielectric environment comprising the
low-loss phase change material, GeSbTe (GST). We demonstrate
waveguides which confine polaritons in a 1D geometry, and refractive optical
elements such as lenses and prisms for phonon polaritons in hBN, which we
characterize using scanning near field optical microscopy. Furthermore, we
demonstrate metalenses, which allow for polariton wavefront engineering and
sub-wavelength focusing. Our method, due to its sub-diffraction and planar
nature, will enable the realization of programmable miniaturized integrated
optoelectronic devices, and will lay the foundation for on-demand biosensors.Comment: 15 pages, 4 figures, typos corrected in v
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