Observation of the Purcell effect in high-index-contrast micropillar

We have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and Aluminium Oxide. Compared to the more widely studied GaAs/AlAs micropillars these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. We have studied a number of samples containing a low density of InGaAs/GaAs self assembled quantum dots in a cavity and here report observation of a three fold enhancement in the radiative lifetime of a quantum dot exciton state due to the Purcell effect

Oxide-apertured microcavity single-photon emitting diode

We have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet-oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity structures with doped contact layers, we demonstrate a single-photon emitting diode where current may be injected into a single dot

Astrophysical Probes of the Constancy of the Velocity of Light

We discuss possible tests of the constancy of the velocity of light using distant astrophysical sources such as gamma-ray bursters (GRBs), Active Galactic Nuclei (AGNs) and pulsars. This speculative quest may be motivated by some models of quantum fluctuations in the space-time background, and we discuss explicitly how an energy-dependent variation in photon velocity \delta c/ c \sim - E / M arises in one particular quantum-gravitational model. We then discuss how data on GRBs may be used to set limits on variations in the velocity of light, which we illustrate using BATSE and OSSE observations of the GRBs that have recently been identified optically and for which precise redshifts are available. We show how a regression analysis can be performed to look for an energy-dependent effect that should correlate with redshift. The present data yield a limit M \gsim 10^{15} GeV for the quantum gravity scale. We discuss the prospects for improving this analysis using future data, and how one might hope to distinguish any positive signal from astrophysical effects associated with the sources.Comment: 37 pages LaTeX, 9 eps figures included, uses aasms4.st

Thermomechanical characterization of Hastelloy-X under uniaxial cyclic loading

In most high-temperature engineering applications, components are subjected to complex combinations of thermal and mechanical loading during service. A number of viscoplastic constitutive models were proposed which potentially can provide mathematical descriptions of material response under such conditions. Implementation of these models into large finite element codes such as MARC has already resulted in much improved inelastic analysis capability for hot-section aircraft engine components. However, a number of questions remain regarding the validity of methods adopted in characterizing these constitutive models for particular high-temperature materials. One area of concern is that the majority of experimental data available for this purpose are determined under isothermal conditions. This is in contrast to service conditions which, as noted above, almost always involve some form of thermal cycling. The obvious question arises as to whether a constitutive model characterized using an isothermal data base can adequately predict material response under thermomechanical conditions. An experimental program was initiated within the HOST program to address this particular concern. The results of the most recent isothermal and thermomechanical experiments are described

Flavor and Spin Contents of the Nucleon in the Quark Model with Chiral Symmetry

A simple calculation in the framework of the chiral quark theory of Manohar and Georgi yields results that can account for many of the ''failures'' of the naive quark model: significant strange quark content in the nucleon as indicated by the value of $\sigma _{\pi N},$ the $\overline{u}$-$\overline{d}$ asymmetry in the nucleon as measured by the deviation from Gottfried sum rule and by the Drell-Yan process, as well as the various quark contributions to the nucleon spin as measured by the deep inelastic polarized lepton-nucleon scatterings.Comment: figure has been separated from tex file. No other changes. Preprint CMU-HEP94-3

Charge-transfer exciton in La2CuO4 probed with resonant inelastic x-ray scattering

We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the dispersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features was also examined. The observed charge transfer edge shift, along with the low dispersion of the first charge transfer excitation are attributed to the lattice motion being coupled to the electronic system. In addition, we observe a dispersionless feature at 1.8 eV, which is associated with a d-d crystal field excitation.Comment: 5 pages, 4 figure

Dark Matter in SuperGUT Unification Models

After a brief update on the prospects for dark matter in the constrained version of the MSSM (CMSSM) and its differences with models based on minimal supergravity (mSUGRA), I will consider the effects of unifying the supersymmetry-breaking parameters at a scale above M_{GUT}. One of the consequences of superGUT unification, is the ability to take vanishing scalar masses at the unification scale with a neutralino LSP dark matter candidate. This allows one to resurrect no-scale supergravity as a viable phenomenological model.Comment: 12 pages, 16 figures, To be published in the Proceedings of the 6th DSU Conference, Leon, Mexico, ed. D. Delepin

Supersymmetric photonic signals at LEP

We explore and contrast the single-photon and diphoton signals expected at LEP 2, that arise from neutralino-gravitino (e^+ e^- -> chi + gravitino -> gamma + E_miss) and neutralino-neutralino (e^+ e^- -> chi + chi -> gamma + gamma + E_miss) production in supersymmetric models with a light gravitino. LEP 1 limits imply that one may observe either one, but not both, of these signals at LEP 2, depending on the values of the neutralino and gravitino masses: single-photons for m_chi > Mz and m_gravitino < 3 x 10^-5 eV; diphotons for m_chi < Mz and all allowed values of m_gravitino.Comment: 11 pages, LaTeX, 4 figures (included). Shortened version to appear in Physical Review Letter

Dynamical Formation of Horizons in Recoiling D Branes

A toy calculation of string/D-particle interactions within a world-sheet approach indicates that quantum recoil effects - reflecting the gravitational back-reaction on space-time foam due to the propagation of energetic particles - induces the appearance of a microscopic event horizon, or `bubble', inside which stable matter can exist. The scattering event causes this horizon to expand, but we expect quantum effects to cause it to contract again, in a `bounce' solution. Within such `bubbles', massless matter propagates with an effective velocity that is less than the velocity of light in vacuo, which may lead to observable violations of Lorentz symmetry that may be tested experimentally. The conformal invariance conditions in the interior geometry of the bubbles select preferentially three for the number of the spatial dimensions, corresponding to a consistent formulation of the interaction of D3 branes with recoiling D particles, which are allowed to fluctuate independently only on the D3-brane hypersurface.Comment: 25 pages LaTeX, 4 eps figures include