We present the results for NMR studies of the orientational behavior
of thin films (bi-layer) of molecular hydrogen adsorbed on hexagonal BN.
The interest in these experiments is to determine the influence of
restricted geometries on the orientational or
dering of quantum rotors. The phase diagram for system is explored for
ortho-H_2 concentration 0.30
The electronic properties of quasicrystals and their crystalline
approximants have been studied by ^27Al nuclear magnetic resonance.
An anomalous temperature dependence of the spin-lattice relaxation rate
(T_1^-1), T_1^-1 \sim T^2, was observed in a broad
temperature range in i-AlCuFe, R-AlCuFe, i-AlCuRu, i-AlPdRe, and
\alpha-AlMnSi. This temperature dependence can be explained by the
presence of a pseudogap described by g(E) \sim |E-E_F|^1/2. This
type of density-of-state was predicted by a recent theoretical
calculation(C. Janot et. al., Phys. Rev. Lett. 72, 1674(1994))
and also was indicated by a recent photonemission measurement
(X. Wu et. al., Phys. Rev. Lett. 75, 4540(1995)) in i-AlPdMn.
Our NMR results gave the first experimental evidence that this special
electronic density-of-state is a common feature in quasicrystals. For all
these samples, reasonable width of the pseudogap has been obtained from
the temperature dependence of the spin-lattice relaxation rate. A weak
temperature dependence of the Knight shift K \sim T^1/2, as a result
of g(E) \sim |E-E_F|^1/2 is also consistent with our ^27Al
measurements.
Cadmium Zinc Telluride (CZT) crystals is an important material for
optoelectrical applications such as substrates for IR detectors and X-ray
and gamma-ray spectrometers. High Pressure Bridgman (HPB) and Vertical
Gradient Freeze (VGF) grown crystals were investigated using electrical and
optical characterization methods. The resistivity of HPB crystals compared
to VGF crystals is seven orders of magnitude higher. This finding seems to
be related to the total overpressure above the melt from which they were
grown. Low temperature photoluminescence is employed to investigate the
correlation of growth defects with electrical properties. Post-growth
treatment for restoring deviations from stoichiometry will also be
discussed.(The authors at Fisk would like to acknowledge support
provided by NASA under grant # NAGW-2925.)
The incorporation of chromium in wide bandgap semiconductors is becoming a
topic of interest in the development of efficient and compact tunable solid
state mid-infrared lasers operating at room temperature. In the search for
optimized procedures of doping that will raise the chromium concentration to
a level of 10^20 ions/cm^3, we have developed a diffusion process in
the temperature range of 750 -950 ^0C. However, optimization needs to
be made during this process to preserve the optical transparency of the
doped samples. In order to minimize optical losses two kinds of dopant
sources where investigated, CrSe vapor and sputtered chromium metal. The
experimental data obtained for these two cases will be discussed in terms of
dopant diffusivity, distribution of defects and crystal
homogeneity.(The authors at Fisk would like to acknowledge support
provided by NASA under grant # NAGW 2925 and by DOE under Grant #
NAG03-94SF20368.)
It has been demonstrated by us and others that ion implantation is a rather
versatile technique to synthesize nanophase materials such as metal colloids
and semiconductor quantum dots in near surface of various substrate
materials. Depending upon ion energy and ion dose used, nanocrystals can be
formed at different depth from the implanted surface and with different size
via proper thermal treatments. Currently, both optical characterization and
theoretical modeling have been conducted i) to understand how the implanted
ions and formed metallic nanocrystals modify the optical response of the
implanted silica; ii) to illustrate how and where the structural damage
which is created during ion implantation alters the dielectric function of
the silica host; and iii) to evaluate the effect of thermal treatment under
different annealing environment and temperature on the implanted silica
substrates. Based on the experimental results and model calculation,
potential photonic applications can be realized.
The ability to fabricate free standing semiconductor quantum wells
enables
the study of excitons and electrons under the textbook condition
of
infinite barriers, i. e., the 2-D particle in a box. We have selectively
etched compositionally modulated III-V heterostructures to produce
quantum
wells which are confined on both sides by vacuum. The material is
patterned so that the wells are suspended horizontally between vertical
supports. This structure is ideal for probing the local properties
of
solids, e. g., the interaction of quantum confined states with surface
or
interface states. The inherent high contrast ratio also allows the
construction of efficient mirrors for lasers. This is of particular
interest in that it allows the fabrication of a Fabry-Perot laser
cavity
with a linear dimension on the order of the spontaneous emission
wavelength
of the active medium. The satisfaction of this criterion limits
the
emission to a single mode and would substantially lower the emission
threshold. We have been able to achieve reproducible well widths
from
80-200 =C5 with variable spacings from 100 to 2000 =C5 without difficulty.
We present experimental C-V data obtained for GaAs/AlGaAs multiquantum well
structures with 4, 8 and 10 wells at low
temperatures indicating the presence of space charge inside the
structure. Low bias results for structures with the same parameters and different
number of wells indicate the presence of negative charge in the
last well
before the collector contact. At high bias, peaks in the capacitance are
observed which appear to be related to space charge generated by increased tunneling
from states above the barrier into the well states when resonances occur. Shifting in the bias at which the
peaks are observed as the
temperature increases indicates a decrease in the amount of space charge as
the temperature increases. This is probably due to an increase in the tunneling
rate out of the well for nonresonant biases leading to a lower space charge.
-Supported in part by NSF under grant# ECS94-12248
The temperature dependence of capacitance and AC conductance for
GaAs/AlGaAs
multiquantum well (MQW) structures with 4,8,16
and 32 wells has been studied at different bias and frequency ranges.
Data indicates that low temperature conductance is mostly
due to thermally assisted tunneling (TA) through the first excited state. At high
temperature thermionic emission (TE) current is dominant. Activation energies
for TA and TE are 120\pm15 meV and 170\pm10 meV respectively.
Capacitance changes from the geometrical value
at low temperature to the capacitance corresponding to a single period of the structure
at high temperature. Applied bias voltage reduces effective barrier height
and prevents accumulation of charge in quantum wells. At biases above 150 mV
(for 4 well sample) TE current dominates over the whole
temperature region with energy activation of 140\pm10 meV. As the bias is
increased the single period capacitance (saturation) drops until only the
geometrical capacitance is observed at a bias of 400 mV.
An equivalent circuit is proposed to explain the behavior observed.
-Supported in part by NSF under grant# ECS94-12248
The Hall and longitudinal transport properties of electrons in the Al
GaAs/GaAs/AlGaAs quantum well system are investigated within a
Green's function many body quantum approach [1]. The quantum well is
modeled with a variational function scheme [2] in an analytic fashion
in order to facilitate the implementation of the theoretical transport
approach. Scattering, such as polar optical, impurity, and acoustic
mechanisms have been incorporated in the investigation [3], including
up to 60 Landau levels. While the results employ experimental [4]
parameters, an interesting comparison is made between the calculations
including and excluding the electron-electron interactions. The results
will be presented for a magnetic field regime in the range of about 2
to 30 tesla for low to high electric field values. [1] Cai et al.
Phys. Rev. B Vol. 31, 4070 (1985). [2] S. Mori and T. Ando,
J. Phys. Soc. Jpn. Vol. 48, 865 (1980). [3] J. E. Hasbun,
APS Bull, Vol 41, 419 (1996). [4] Kim et al., J. Appl. Phys.
Vol. 76, 2863 (1994). *Supported by grants from SUWG, Res. Corp.
and the NSF-PSC.
We propose a new variational wavefunction
to describe many-body systems with spin-dependent correlations.
This wavefunction is constructed using spin
coherent states to represent the spin state of particles.
In this basis, states are parametrized by a continuous variable that
looks like a classical spin vector. Particularly attractive is that
correlation operators can be represented as simple integrals
over c-valued functions. These integrals can be
evaluated using standard Monte Carlo techniques.
Additionally, since these variables commute,
commutator terms in Fermi hypernetted chain (FHNC) equations do not appear.
Explicit Monte Carlo calculations on liquid ^3 He
using this wavefunction
give very good agreement
with standard methods.
Applications to systems in condensed matter
and nuclear physics are discussed.
Deposition of high quality zinc sulfide (ZnS) thin film over a large area is
required if it is to be effectively used in electroluminescent devices, solar
cells, and other optoelectronic devices. Of all deposition techniques,
chemical bath deposition (CBD) is the least costly technique that meets
the above requirements. Recently it is found that the growth of ZnS film,
of thickness less than 100 nm in a single dip, by CBD is facilitated by the
use of ammonia and hydrazine as complexing agents. Here we report that the
thickness of the deposited ZnS film can be increased if ammonium salt is used
as a buffer. We also present an analytical study to explain our results
and to further understand the ZnS growth process in CBD.
Below gap excitation of photoluminscence (PL) in device quality
a-Si:H has been studied using an intense, tunable Free Electron
Laser (FEL). The range of the exciting light from the FEL is
between .7 and 1.2 eV which is well below the optical gap of
approximately 1.8 eV. The PL was measured in the range of .7 to
1.8 eV at a temperature of 77K. Results will be compared to
previous measurements using other infrared sources at higher
energies.
With the experi\-mental observation of phase conjugated light foci of
linear extensions well below the classical diffraction limit observed
by Bozhevolnyi, Keller, and Smolyaninov[Opt. Lett. 19,
1601 (1994)], inclusion of near-field compo\-nents in the theoretical
description of optical phase conjugation has received more
attention[G. S. Agarwal and S. D. Gupta, Opt. Commun. 119, 591 (1995)].
Because of the small spatial range of the optical near-field from the
source, a substantial part of the phase conjugation process related to
the near-field components is bound to take place in the surface region
of the nonlinear medium. Experimentally, the
use of a thin film as the nonlinear medium has already drawn
attention[P. Günter, in Notions and Perspectives of Nonlinear
Optics, ed. O. Keller (World Scientific, Singapore, 1996)].
As a natural consequence we exam\-ine a 2-D system consisting of a thin
film (quantum well) deposited on a sub\-strate, with the near-field
probe placed in the vacuum outside the film, by use of Fourier
analysis. To ensure that the description covers all components of the
near-field that can actually be conjugated, we avoid usage of standard
approximations such as the slowly varying enve\-lope approximation, the
dipole approximation, the dielectric concept, near-normal incidence of
the probe, and the requirement of strong pump fields.
We have used photoelectron spectroscopy to observe thermal
isomerization in cesium-halide cluster anions. In many of the
(CsX)_nCs_m^- (X = Cl, Br, I; n = 2-7; m = 0, 1) systems we have
studied, small changes in the source nozzle temperature produce
dramatic changes in the distribution of cluster isomers. When specific
isomers are selectively photodepleted, isomer interconversion quickly
reestablishes the thermal isomer distribution, even though the
clusters are isolated in a cluster beam.
[CC.02] NMR charaterization of the pseudogap at the Fermi level in Quasicrystals
X.-P. Tang, S. K. Wonnell, E. A. Hill, Y. Wu (Department of Physics and Astronomy,University of North Carolina, Chapel Hill, North Carolina 27599), S. J. Poon (Department of Physics, University of Virginia, Charlottesville, Virginia 22901)
[CC.03] Properties of Melt Grown CdZnTe Crystals as Related to Their Growth Conditions
Davis Swanson, Kuo-Tong Chen, Henry Chen, Arnold Burger (Center for Photonic Materials and Devices, Department of Physics, Fisk University), Yi-Gao Sha, Ching-Hua Su, Sandor Lehoczky (NASA/Marshall Space Flight Center)
[CC.04] Chromium Diffusion-doping in ZnS and ZnSe Crystals.
Troy Journigan, Kuo-Tong Chen, Henry Chen, Arnold Burger (Center for Photonic Materials and Devices, Department of Physics, Fisk University), Kathleen Schaffers, Ralph Page, Steve Payne (Lawrence Livermore National Laboratory)
[CC.05] Optical Characterization and Modeling of Metal Ion Implanted Optical Grade Silica
Z.Y. Gu, D.O. Henderson, A. Ueda, Y.S. Tung, Jinli Chen, R. Mu (Chemical Physics Laboratory, Fisk University, Nashville, TN 37208), W.C. White, R.A. Zuhr, Jane G. Zhu (Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6057)
[CC.06] Free Standing Quantum Wells
M. D. Williams (Center for Microelectronics and Photonics, Department= of Physics, Clark Atlanta University, Atlanta, GA)
[CC.07] Space Charge Analysis in Multiquantum Well Structures via Capacitance Measurements
S. G. Matsik, V. G. Silvestrov, A. G. U. Perera (Georgia State University), H. C. Liu (NRC-Canada)
[CC.08] Temperature Dependence of Capacitance and AC Conductance in MQW Structures
V. G. Silvestrov, S. G. Matsik, A. G. U. Perera (Georgia State University), H. C. Liu (NRC-Canada)
[CC.09] Electronic transport in a quantum well in the presence of electric and magnetic fields
J. E. Hasbun (State University of West Georgia)
[CC.10] Coherent State Wavefunction for Systems with Spin-Dependent Correlations
John W. Lawson (Department of Mathematical Sciences, Clemson University)
[CC.11] Deposition of zinc sulfide thin films by chemical bath process
Isaiah O. Oladeji, Lee Chow (Department of Physics, University of Central Florida, Orlando, FL 32816)
[CC.12] Photoluminescence of a-Si:H using a Free Electron Laser
Glennys A. Mensing, E. Hurt, J. Gilligan, N. Tolk (Vanderbilt University), P. C. Taylor (University of Utah)
[CC.13] Microscopic Description of Optical Near-Field Phase Conjugation
Torsten Andersen, Ole Keller (Institute of Physics, Aalborg University, Aalborg, Denmark.)
[CC.14] Thermal Isomerization in Isolated Cesium-Halide Clusters
Louis Bloomfield, Fredrik Fatemi, Darius Fatemi (Department of Physics, University of Virginia, Charlottesville, VA 22903)