337 research outputs found
Electron-beam propagation in a two-dimensional electron gas
A quantum mechanical model based on a Green's function approach has been used
to calculate the transmission probability of electrons traversing a
two-dimensional electron gas injected and detected via mode-selective quantum
point contacts. Two-dimensional scattering potentials, back-scattering, and
temperature effects were included in order to compare the calculated results
with experimentally observed interference patterns. The results yield detailed
information about the distribution, size, and the energetic height of the
scattering potentials.Comment: 7 pages, 6 figure
Probing the potential landscape inside a two-dimensional electron-gas
We report direct observations of the scattering potentials in a
two-dimensional electron-gas using electron-beam diffaction-experiments. The
diffracting objects are local density-fluctuations caused by the spatial and
charge-state distribution of the donors in the GaAs-(Al,Ga)As heterostructures.
The scatterers can be manipulated externally by sample illumination, or by
cooling the sample down under depleted conditions.Comment: 4 pages, 4 figure
Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor
At low Landau level filling of a two-dimensional electron system, typically
associated with the formation of an electron crystal, we observe local minima
in Rxx at filling factors nu=2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9.
Each of these developing fractional quantum Hall (FQHE) states appears only
above a filling factor-specific temperature. This can be interpreted as the
melting of an electron crystal and subsequent FQHE liquid formation. The
observed sequence of FQHE states follow the series of composite fermion states
emanating from nu=1/6 and nu=1/8
Repulsive Core of NN S-Wave Scattering in a Quark Model with a Condensed Vacuum
We work in a chiral invariant quark model, with a condensed vacuum,
characterized by only one parameter. Bound state equations for the nucleon and
Delta are solved in order to obtain an updated value of their radii and masses.
Nucleon-nucleon S-Wave scattering is studied in the RGM framework both for
isospin T=1 and T=0. The phase shifts are calculated and an equivalent local
potential, which is consistent with K-N scattering, is derived. The result is a
reasonable microscopic short range repulsion in the nucleon-nucleon
interaction.Comment: 23 pages in latex revtex, 4 Postscript figure
Excitons and charged excitons in semiconductor quantum wells
A variational calculation of the ground-state energy of neutral excitons and
of positively and negatively charged excitons (trions) confined in a
single-quantum well is presented. We study the dependence of the correlation
energy and of the binding energy on the well width and on the hole mass. The
conditional probability distribution for positively and negatively charged
excitons is obtained, providing information on the correlation and the charge
distribution in the system. A comparison is made with available experimental
data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well
structures, which indicates that trions become localized with decreasing
quantum well width.Comment: 9 pages, 11 figure
Landau Level Crossings and Extended-State Mapping in Magnetic Two-dimensional Electron Gases
We present longitudinal and Hall magneto-resistance measurements of a
``magnetic'' two-dimensional electron gas (2DEG) formed in modulation-doped
ZnCdMnSe quantum wells. The electron spin splitting is
temperature and magnetic field dependent, resulting in striking features as
Landau levels of opposite spin cross near the Fermi level. Magnetization
measurements on the same sample probe the total density of states and Fermi
energy, allowing us to fit the transport data using a model involving extended
states centered at each Landau level and two-channel conduction for spin-up and
spin-down electrons. A mapping of the extended states over the whole quantum
Hall effect regime shows no floating of extended states as Landau levels cross
near the Fermi level.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Baryon Current Matrix Elements in a Light-Front Framework
Current matrix elements and observables for electro- and photo-excitation of
baryons from the nucleon are studied in a light-front framework. Relativistic
effects are estimated by comparison to a nonrelativistic model, where we use
simple basis states to represent the baryon wavefunctions. Sizeable
relativistic effects are found for certain transitions, for example, to radial
excitations such as that conventionally used to describe to the Roper
resonance. A systematic study shows that the violation of rotational covariance
of the baryon transition matrix elements stemming from the use of one-body
currents is generally small.Comment: 32 pages, LaTeX, 10 postscript figures, uses epsf.sty; figures
uuencoded with uufiles (or available by request in .ps or hardcopy form
Valence Quark Spin Distribution Functions
The hyperfine interactions of the constituent quark model provide a natural
explanation for many nucleon properties, including the Delta-N splitting, the
charge radius of the neutron, and the observation that the proton's quark
distribution function ratio d(x)/u(x)->0 as x->1. The hyperfine-perturbed quark
model also makes predictions for the nucleon spin-dependent distribution
functions. Precision measurements of the resulting asymmetries A_1^p(x) and
A_1^n(x) in the valence region can test this model and thereby the hypothesis
that the valence quark spin distributions are "normal".Comment: 16 pages, 2 Postscript figure
Aharonov-Bohm signature for neutral excitons in type-II quantum dot ensembles
It is commonly believed that the Aharonov-Bohm (AB) effect is a typical
feature of the motion of a charged particle interacting with the
electromagnetic vector potential. Here we present a magnetophotoluminescence
study of type-II InP/GaAs self-assembled quantum dots, unambiguously revealing
the Aharonov-Bohm-type oscillations for neutral excitons when the hole ground
state changes its angular momentum from lh = 0 to lh = 1, 2, and 3. The hole
ring parameters derived from a simple model are in excellent agreement with the
structural parameters for this system.Comment: Revised version, 10 pages, 3 figure
The Decay : A Test for Potential Models
We use a simple perturbation theory argument and measurements of charmonium
leptonic widths to estimate the ratio
\mbox{} in the general context of non-
relativistic potential models. We obtain . We then apply
well known potential model formulas, which include lowest order QCD
corrections, to find . The central value for
in the 1992 Particle Data Tables then
leads to a (non relativistic) prediction keV. This prediction is in good agreement with a
recent measurement by the ARGUS collaboration, is consistent with a recent
measurement by the L3 collaboration but is significantly higher than several
earlier measurements and than previous theoretical estimates, which usually
assume . The correction to is estimated to be smaller
but nonnegligible for the system. Using the current central
measurement for we find keV. A rough estimate
of relativistic corrections reduces the expected two photon rates to about 8.8
keV and 0.52 keV for the and mesons respectively. Such
correctionsComment: Estimates of likely relativistic corrections to the results have been
adde
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