Search Results (11 total)

We have measured the complex transmission coefficient of a two-dimensional photonic crystal slab for propagation perpendicular to the slab, using terahertz time-domain spectroscopy. At high frequencies, where the lattice parameter exceeds the wavelength, we find that the group delay is equal to that of empty space despite the fact that the volume-weighted average dielectric of the slab is ε_ave∼3.8. In contrast to most other examples of anomalous group delay effects, this unusual phase transparency persists over a very broad spectral bandwidth, demonstrating that such anomalies do not require the presence of a nearby isolated resonance. Our results are consistent with finite-difference time-domain simulations.

We study the polarization of antihyperon in lepton induced reactions such as e⁺e^-→H̅ +X and l+p→l^′+H̅ +X with polarized beams using different models for spin transfer in high energy fragmentation processes. We compare the results with the available data and those for hyperons. We make predictions for future experiments.

We describe the first measurements of the diffusion of broadband single-cycle optical pulses through a highly scattering medium. Using terahertz time-domain spectroscopy, we measure the electric field of a multiply scattered wave with a time resolution shorter than one optical cycle. This time-domain measurement provides information on the statistics of both the amplitude and phase distributions of the diffusive wave. We develop a theoretical description, suitable for broadband radiation, which adequately describes the experimental results.

We describe observations of the amplitude and phase of an electric field diffusing through a three-dimensional random medium, using terahertz time-domain spectroscopy. These measurements are spatially resolved with a resolution smaller than the speckle spot size and temporally resolved with a resolution better than one optical cycle. By computing correlation functions between fields measured at different positions and with different temporal delays, it is possible to obtain information about individual scattering events experienced by the diffusing field. This represents a new method for characterizing a multiply scattered wave.

In this paper, the complete high-order perturbation formulas of electron-paramagnetic-resonance (EPR) parameters (zero-field splittings and g factors) for 3d² ion in trigonal tetrahedral clusters have been derived from a cluster approach. In these formulas, not only the contribution due to crystal-field mechanism, but also the contributions due to charge-transfer mechanism (which is neglected in the crystal-field theory) are included. From these formulas, the g shifts Δg_∥(=g_∥-g_e), Δg_⊥(=g_⊥-g_e) and the zero-field splittings D for various V³⁺ centers in 4H- and 6H-SiC crystals are reasonably explained. It is found that in the studies of EPR parameters for 3d² ions (particularly, in the case of ions having high valence state) in strong covalency crystals, both the contribution due to crystal-field mechanism and that due to charge-transfer mechanism should be taken into account.

The dynamic behavior of the supercurrent in mesoscopic Josephson junctions interacting with various nonclassical electromagnetic fields (NEMF) is investigated. It is shown that for squeezed number states, squeezed states, and a superposition of two coherent states, the supercurrent exhibits a collapse and revival (CR) phenomenon that is well known in the context of quantum optics. For coherent states and number states, this phenomenon does not occur. This CR phenomenon, which is due to the granular nature of the field, is also absent if the field is considered classically. The dynamic behavior of the supercurrent fluctuation is also studied and it is found that, generally, when the fluctuation is large, the supercurrent exhibits collapses, and, when the fluctuation is small, the supercurrent exhibits revivals. Finally, the amplitude of current-voltage steps for various NEMF’s are studied in some detail.

The microstructure of Sr-free bismuth cuprates, i.e., Bi₂(La_1-xCa_1+x)CuO_6.5+δ and (Bi_2-xPb_x)LaCaCuO_6.5+δ, was determined by means of x-ray diffraction and electron diffraction (ED). ED analysis revealed that the two types of Sr-free compounds possess a monoclinic superstructural modulation, which is similar to Bi₂Sr₂CuO_6+δ. In the Bi₂(La_1-xCa_1+x)CuO_6.5+δ system, the modulation wavelength along the b and c axes increased markedly with the Ca substitution for La. At the same time this substitution facilitated the commensurate-to-incommensurate transition. For the (Bi_2-xPb_x)LaCaCuO_6.5+δ system, a similar change in the modulation wave vector was also observed. The dependence of the modulation vector on chemical composition can be best interpreted by the crystal-misfit model. In addition, Raman scattering analysis of the Bi₂(La_1-xCa_1+x)CuO_6.5+δ system showed that the vibrational properties of the oxygen atoms in the Bi-O bonding also changed with the increase in the modulation wavelength, and this behavior can be regarded as a consequence of the structural relaxation caused by the enhancement of the degree of crystal fit.

We have measured the cross section for production of ψ and ψ^′ in p̅ and π^- interactions with Be, Cu, and W targets in experiment E537 at Fermilab. The measurements were performed at 125 GeV/c using a forward dimuon spectrometer in a closed geometry configuration. The gluon structure functions of the p̅ and π^- have been extracted from the measured dσ / dx_F spectra of the produced ψ's. From the p̅ W data we obtain, for p̅ , xG(x)=(2.15±0.7)[1-x]^(6.83±0.5)[1+(5.85±0.95)x]. In the π^- case, we obtain, from the W and the Be data separately, xG(x)=(1.49±0.03)[1-x]^(1.98±0.06) (for π^-W), xG(x)=(1.10±0.10)[1-x]^(1.20±0.20) (for π^-Be).

Raman measurements were performed on single-phase samples of Bi_2.1Sr_1.9-xLa_xCuO_y (0≤x≤1.0) and Bi_2-xPb_xSr₂CuO_y (0.05≤x≤0.6). Significant modifications of the spectra, becoming more pronounced with increasing x, are noted with respect to the undoped Bi₂Sr₂CuO_y in the energy ranges of 450–465 cm^-1 and 610–650 cm^-1. The experimental results provide important data related to the origin of the incommensurate modulation structure and its relation to the superconductivity of the Bi₂Sr₂CuO_y system.

The production of the J / ψ resonance in 125-GeV/c p̅ and φ^- interactions with Be, Cu, and W targets has been measured. The cross section per nucleon for J / ψ production is suppressed in W interactions relative to the lighter targets, especially at large values of Feynman x, which is opposite to the expectation from the various explanations of the European Muon Collaboration effect. Models incorporating modifications of the gluon structure functions in heavy targets show qualitative agreement with the data.

The cross section for the reaction p̅ N→μ⁺μ^-X with muon pairs in the mass range 4<M<9 GeV/c² and x_F>0 was measured to be σ=0.104±0.005±0.008 nb/nucleon. The distributions dσ / dx_F and M³dσ / dM were compared to the QCD-improved Drell-Yan model and to calculations including first-order QCD corrections, with use of deep-inelastic structure functions. Excellent agreement with the data was obtained if the calculations were multiplied by factors of 2.45 and 1.41, respectively.