Effect of multilayer barriers on the optical properties of GaInNAs single quantum-well structures grown by metalorganic vapor phase epitaxy

We report on the effects of combined strain-compensating and strain-mediating layers of various widths on the optical properties of 1.3 μm GaInNAs∕GaAs single quantum well structures grown by metalorganic vapor phase epitaxy (MOVPE). While the emission wavelength of GaInNAs∕GaAs quantum wells can be redshifted by the adoption of strain-compensated GaNAs layers, the material quality is degraded by the increased stress at the well∕barrier interface. This detrimental effect can be cured by inserting a strain-mediating InGaAs layer between them. Contrary to what is expected, however, the emission wavelength is blueshifted by the insertion of the InGaAs layer, which is attributed to the reduced N incorporation due to the improved interface quality. Our results indicate that the optical properties of MOVPE-grown GaInNAs∕GaAs quantum wells can be optimized in quantum efficiency and emission wavelength by combination of strain-compensating and strain-mediating layers with suitable characteristics

Spectroscopic characterization of 1.3µm GaInNAs quantum-well structures grown by metal-organic vapor phase epitaxy

We report optical studies of high-quality 1.3 μm strain-compensated GaInNAs/GaAs single-quantum-well structures grown by metalorganic vapor phase epitaxy. Photoluminescence excitation (PLE) spectroscopy shows clearly the electronic structure of the two-dimensional quantum well. The transition energies between quantized states of the electrons and holes are in agreement with theoretical calculations based on the band anti-crossing model in which the localized N states interact with the extended states in the conduction band. We also investigated the polarization properties of the luminescence by polarized edge-emission measurements. Luminescence bands with different polarization characters arising from the electron to heavy-hole and light-hole transitions, respectively, have been identified and verify the transition assignment observed in the PLE spectrum

Observation of Parity Violation in the Omega-minus -> Lambda + K-minus Decay

The alpha decay parameter in the process Omega-minus -> Lambda + K-minus has been measured from a sample of 4.50 million unpolarized Omega-minus decays recorded by the HyperCP (E871) experiment at Fermilab and found to be [1.78 +/- 0.19(stat) +/- 0.16(syst)]{\times}10^{-2}. This is the first unambiguous evidence for a nonzero alpha decay parameter, and hence parity violation, in the Omega-minus -> Lambda + K-minus decay.Comment: 10 pages, 7 figure

The glassy response of solid He-4 to torsional oscillations

We calculated the glassy response of solid He-4 to torsional oscillations assuming a phenomenological glass model. Making only a few assumptions about the distribution of glassy relaxation times in a small subsystem of otherwise rigid solid He-4, we can account for the magnitude of the observed period shift and concomitant dissipation peak in several torsion oscillator experiments. The implications of the glass model for solid He-4 are threefold: (1) The dynamics of solid He-4 is governed by glassy relaxation processes. (2) The distribution of relaxation times varies significantly between different torsion oscillator experiments. (3) The mechanical response of a torsion oscillator does not require a supersolid component to account for the observed anomaly at low temperatures, though we cannot rule out its existence.Comment: 9 pages, 4 figures, presented at QFS200

A renormalizable SO(10) GUT scenario with spontaneous CP violation

We consider fermion masses and mixings in a renormalizable SUSY SO(10) GUT with Yukawa couplings of scalar fields in the representation 10 + 120 + 126 bar. We investigate a scenario defined by the following assumptions: i) A single large scale in the theory, the GUT scale. ii) Small neutrino masses generated by the type I seesaw mechanism with negligible type II contributions. iii) A suitable form of spontaneous CP breaking which induces hermitian mass matrices for all fermion mass terms of the Dirac type. Our assumptions define an 18-parameter scenario for the fermion mass matrices for 18 experimentally known observables. Performing a numerical analysis, we find excellent fits to all observables in the case of both the normal and inverted neutrino mass spectrum.Comment: 16 pages, two eps figure

Nanofabrication by magnetic focusing of supersonic beams

We present a new method for nanoscale atom lithography. We propose the use of a supersonic atomic beam, which provides an extremely high-brightness and cold source of fast atoms. The atoms are to be focused onto a substrate using a thin magnetic film, into which apertures with widths on the order of 100 nm have been etched. Focused spot sizes near or below 10 nm, with focal lengths on the order of 10 microns, are predicted. This scheme is applicable both to precision patterning of surfaces with metastable atomic beams and to direct deposition of material.Comment: 4 pages, 3 figure

SUSY GUT Model Building

I discuss an evolution of SUSY GUT model building, starting with the construction of 4d GUTs, to orbifold GUTs and finally to orbifold GUTs within the heterotic string. This evolution is an attempt to obtain realistic string models, perhaps relevant for the LHC. This review is in memory of the sudden loss of Julius Wess, a leader in the field, who will be sorely missed.Comment: 24 pages, 14 figures, lectures given at PiTP 2008, Institute for Advanced Study, Princeton, to be published in the European Physical Journal

Electrically pumped WSe2-based light-emitting van der Waals heterostructures embedded in monolithic dielectric microcavities

Vertical stacking of atomically thin layered materials opens new possibilities for the fabrication of heterostructures with favorable optoelectronic properties. The combination of graphene, hexagonal boron nitride and semiconducting transition metal dichalcogenides allows fabrication of electroluminescence (EL) devices, compatible with a wide range of substrates. Here, we demonstrate a full integration of an electroluminescent van der Waals heterostructure in a monolithic optical microcavity made of two high reflectivity dielectric distributed Bragg reflectors (DBRs). Owing to the presence of graphene and hexagonal boron nitride protecting the WSe2 during the top mirror deposition, we fully preserve the optoelectronic behaviour of the device. Two bright cavity modes appear in the EL spectrum featuring Q-factors of 250 and 580 respectively: the first is attributed directly to the monolayer area, while the second is ascribed to the portion of emission guided outside the WSe2 island. By embedding the EL device inside the microcavity structure, a significant modification of the directionality of the emitted light is achieved, with the peak intensity increasing by nearly two orders of magnitude at the angle of the maximum emission compared with the same EL device without the top DBR. Furthermore, the coupling of the WSe2 EL to the cavity mode with a dispersion allows a tuning of the peak emission wavelength exceeding 35 nm (80 meV) by varying the angle at which the EL is observed from the microcavity. This work provides a route for the development of compact vertical-cavity surface-emitting devices based on van der Waals heterostructures

HyperCP: A high-rate spectrometer for the study of charged hyperon and kaon decays

The HyperCP experiment (Fermilab E871) was designed to search for rare phenomena in the decays of charged strange particles, in particular CP violation in $\Xi$ and $\Lambda$ hyperon decays with a sensitivity of $10^{-4}$. Intense charged secondary beams were produced by 800 GeV/c protons and momentum-selected by a magnetic channel. Decay products were detected in a large-acceptance, high-rate magnetic spectrometer using multiwire proportional chambers, trigger hodoscopes, a hadronic calorimeter, and a muon-detection system. Nearly identical acceptances and efficiencies for hyperons and antihyperons decaying within an evacuated volume were achieved by reversing the polarities of the channel and spectrometer magnets. A high-rate data-acquisition system enabled 231 billion events to be recorded in twelve months of data-taking.Comment: 107 pages, 45 Postscript figures, 14 tables, Elsevier LaTeX, submitted to Nucl. Instrum. Meth.