Search for large extra dimensions in dielectron and diphoton production

We report a search for effects of large extra spatial dimensions in pp collisions at a center-of-mass energy of 1.8 TeV with the D0 detector, using events containing a pair of electrons or photons. The data are in good agreement with the expected background and do not exhibit evidence for large extra dimensions. We set the most restrictive lower limits to date, at the 95% C.L. on the effective Planck scale between 1.0 and 1.4 TeV for several formalisms and numbers of extra dimensions.     

Ergodicity-breaking transition and high-frequency response in a simple free-energy landscape

We present a simple dynamical model described by a Langevin equation in a piecewise parabolic free-energy landscape, modulated by a temperature-dependent overall curvature. The zero-curvature point marks a transition to a phase with broken ergodicity. The frequency-dependent response near this transition is reminiscent of observations near the glass transition.     

Distribution of OH in synthetic and natural quartz crystals

The relative intensities of different OH vibration bands have been studied in synthetic quartz crystals with total OH concentration varying by more than a factor of 30 by means of infrared absorption measurements at 78°K. Local variations of OH concentration also have been measured for different directions in these samples and in two crystals of natural quartz from Brazil. Integral absorptions of the different bands in the 3400 cm^?1 region were all found to increase linearly with total OH concentration. Therefore, mechanical Q could be related to any one band of the spectrum as well as to the total OH concentration. Since for natural quartz no relation between total OH concentration and mechanical Q seems to exist, the effect of OH defects characteristic for synthetic quartz must be high, while the OH defects characteristic for natural quartz (i.e., rock crystals and smoky quartz) have a small effect on the mechanical Q.OH concentrations in the Z- and +X-zones were found to be lower near the edge than near the seed. The concentration of OH varies in the order Z < + X ? ?X, suggesting that OH acts as a charge compensator for metal impurities, notably Al⁺³. In natural quartz, the OH concentrations near the edge were also lower than in the interior parts. The overall differences for different directions were much smaller than in the case of synthetic quartz.     

Optical probing of a fractionally charged quasihole in an incompressible liquid

In photoluminescence spectroscopy of a low-mobility two-dimensional electron gas subjected to a quantizing magnetic field, we observe an anomaly around nu=1 / 3 at a very low temperature (0.1 K) and an intermediate electron density (0.9 x 10(11) cm(-2)). The anomaly is explained as due to perturbation of the incompressible liquid at the Laughlin state due to close proximity of a localized charged exciton which creates a fractionally charged quasihole in the liquid. The anomaly of approximately 2 meV can be destroyed by applying a small thermal energy of approximately 0.2 meV that is enough to close the quasihole energy gap.     

Effect of ion implantation on quantum well infrared photodetectors

Ion implantation is a postgrowth processing technique which, when combined with annealing, can be used to tune the absorption wavelength of quantum well devices. We have implanted and annealed, three different quantum well infrared photodetector structures, and measured the absorption spectra of the samples by Fourier transform spectroscopy. The peak absorption wavelength shift of each structure has been calculated as a function of diffusion length by simulating the diffusion processes. We found different diffusion rates for the structures and attribute this to different numbers of as-grown defects. Our results indicate that agglomeration of single defects into defect clusters limits the ability of ion implantation to tune the wavelength of a structure with a higher number of as-grown defects. Thus, a structure with the lowest number of as-grown defects is most useful for fabricating a multi-color quantum well photodetector by ion implantation, because in this case ion implantation can enhance the diffusion rate considerably leading to large red- shift in peak absorption wavelength.     

Improved optimal variational method to analyze optical rib waveguides

We present an improved optimal variational method (Vopt) to overcome the assumption of field separability in the two orthogonal directions. The improved Vopt method is shown to give accurate results with less computational effort. The method is applied to rib waveguides and the results are compared with those obtained using the computationally intensive finite-element method.     

The effect of gate leakage on the noise figure of AlGaN/GaN HEMTs

The effect of gate leakage on the noise figure of AlGaN/GaN high electron mobility transistor (HEMTs) is explored. It is shown that these devices have a sizable amount of gate leakage that cannot be ignored when measuring their noise performance. Measurements across a single sample have more than 1 dB of variation in minimum noise figure. We will show this variation is because of gate leakage. A modified van der Ziel model is used to predict this large variation and allows easy noise figure prediction of HEMT and MESFET devices.     

Narrow wall axial-slot-coupled T junction between rectangular andcircular waveguides

An analysis is presented of a T junction between rectangular and circular waveguides coupled through a long axial slot in the narrow wall of the rectangular waveguide. A moment method with entire orthogonal basis functions is used for deriving expressions for the elements of the scattering matrix. The effect of the wall thickness is taken into account by treating the coupling slot as a short section of a rectangular waveguide and matching the boundary conditions at the interfaces. The conditions satisfied by the scattering matrix of the junction are verified. A comparison between theoretical and experimental results on variation of coupling with frequency is presented