Effect of unitary impurities in non-STM-types of tunneling in high-T_c superconductors

Based on an extended Hubbard model, we present calculations of both the local (i.e., single-site) and spatially-averaged differential tunneling conductance in d-wave superconductors containing nonmagnetic impurities in the unitary limit. Our results show that a random distribution of unitary impurities of any concentration can at most give rise to a finite zero-bias conductance (with no peak there) in spatially-averaged non-STM type of tunneling, in spite of the fact that local tunneling in the immediate vicinity of an isolated impurity does show a conductance peak at zero bias, whereas to give rise to even a small zero-bias conductance peak in the former type of tunneling the impurities must form dimers, trimers, etc. along the [110] directions. In addition, we find that the most-recently-observed novel pattern of the tunneling conductance around a single impurity by Pan et al. [Nature (London) 403,746 (2000)] can be explained in terms of a realistic model of the tunneling configuration which gives rise to the experimental results reported there. The key feature in this model is the blocking effect of the BiO and SrO layers which exist between the tunneling tip and the CuO_2 layer being probed.Comment: 9 pages, 7 ps-figures, to appear in Phys. Rev. B (Sep. 1, 2000); typos corrected, references added, figure 6 changed to expand the explanation on recent experimental measurements by S.H. Pan et al. [Nature (London) 403, 746 (2000)

A high flux source of cold strontium atoms

We describe an experimental apparatus capable of achieving a high loading rate of strontium atoms in a magneto-optical trap operating in a high vacuum environment. A key innovation of this setup is a two dimensional magneto-optical trap deflector located after a Zeeman slower. We find a loading rate of 6x10^9/s whereas the lifetime of the magnetically trapped atoms in the 3P2 state is 54s.Comment: 12 pages, 16 figure

The Design of the Monitoring System for the Thermal Effect of the Surry Nuclear Power Plant on the James River

The demand for electric power in the United States is expected to double every 10 years. As hydroelectric power plant sites reach their full capacity, the demand for electricity will be met by the development of fossil fuel and nuclear power plants. The average thermal efficiency of nuclear power plants is presently about 32%. Therefore, a significant amount of heat is not utilized. For large power plants, the once-through cooling method, in which water is withdrawn from an adjacent body of water and returned after being heated, is the most common one. However, the great amount of heat discharged into the water may result in changes in the physical and chemical properties as well as in the ecology due to the rise in temperature of the water. The objectives of this study is to determine the region of the James River estuary which will be affected by the thermal discharges of the Surry nuclear power plant located at Hog Island and the temperature distribution within that region. The area under study is shown in figure 1. The cooling water is pumped in from the James River at the right side of Hog Island and returned at the left side. The following is a progress report of the first year\u27s work on this project

X-ray Lags in PDS 456 Revealed by Suzaku Observations

X-ray reverberation lags from the vicinity of supermassive black holes have been detected in almost 30 AGN. The soft lag, which is the time delay between the hard and soft X-ray light curves, is usually interpreted as the time difference between the direct and reflected emission, but is alternatively suggested to arise from the direct and scattering emission from distant clouds. By analysing the archival Suzaku observations totalling an exposure time of ~ 770 ks, we discover a soft lag of $10\pm3.4$ ks at $9.58\times10^{-6}$ Hz in the luminous quasar PDS 456, which is the longest soft lag and lowest Fourier frequency reported to date. In this study, we use the maximum likelihood method to deal with non-continuous nature of the Suzaku light curves. The result follows the mass-scaling relation for soft lags, which further supports that soft lags originate from the innermost areas of AGN and hence are best interpreted by the reflection scenario. Spectral analysis has been performed in this work and we find no evidence of clumpy partial-covering absorbers. The spectrum can be explained by a self-consistent relativistic reflection model with warm absorbers, and spectral variations over epochs can be accounted for by the change of the continuum, and of column density and ionization states of the warm absorbers.Comment: accepted for publication in MNRA

Predicting magnetopause crossings at geosynchronous orbit during the Halloween storms

[1] In late October and early November of 2003, the Sun unleashed a powerful series of events known as the Halloween storms. The coronal mass ejections launched by the Sun produced several severe compressions of the magnetosphere that moved the magnetopause inside of geosynchronous orbit. Such events are of interest to satellite operators, and the ability to predict magnetopause crossings along a given orbit is an important space weather capability. In this paper we compare geosynchronous observations of magnetopause crossings during the Halloween storms to crossings determined from the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the magnetosphere as well to predictions of several empirical models of the magnetopause position. We calculate basic statistical information about the predictions as well as several standard skill scores. We find that the current Lyon-Fedder-Mobarry simulation of the storm provides a slightly better prediction of the magnetopause position than the empirical models we examined for the extreme conditions present in this study. While this is not surprising, given that conditions during the Halloween storms were well outside the parameter space of the empirical models, it does point out the need for physics-based models that can predict the effects of the most extreme events that are of significant interest to users of space weather forecasts

Coupling Between An Optical Phonon and the Kondo Effect

We explore the ultra-fast optical response of Yb_{14}MnSb_{11}, providing further evidence that this Zintl compound is the first ferromagnetic, under-screened Kondo lattice. These experiments also provide the first demonstration of coupling between an optical phonon mode and the Kondo effect.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Let