Supercurrent flow through an effective double barrier structure

Supercurrent flow is studied in a structure that in the Ginzburg-Landau regime can be described in terms of an effective double barrier potential. In the limit of strongly reflecting barriers, the passage of Cooper pairs through such a structure may be viewed as a realization of resonant tunneling with a rigid wave function. For interbarrier distances smaller than $d_0=\pi\xi(T)$ no current-carrying solutions exist. For distances between $d_0$ and $2d_0$, four solutions exist. The two symmetric solutions obey a current-phase relation of $\sin(\Delta\varphi/2)$, while the two asymmetric solutions satisfy $\Delta\varphi=\pi$ for all allowed values of the current. As the distance exceeds $nd_0$, a new group of four solutions appears, each contaning $(n-1)$ soliton-type oscillations between the barriers. We prove the inexistence of a continuous crossover between the physical solutions of the nonlinear Ginzburg-Landau equation and those of the corresponding linearized Schr\"odinger equation. We also show that under certain conditions a repulsive delta function barrier may quantitatively describe a SNS structure. We are thus able to predict that the critical current of a SNSNS structure vanishes as $\sqrt{T'_c-T}$, where $T'_c$ is lower than the bulk critical temperature.Comment: 20 pages, RevTex, to appear in Phys. Rev. B, 6 figures on request at [email protected]

Decoherence in elastic and polaronic transport via discrete quantum states

Here we study the effect of decoherence on elastic and polaronic transport via discrete quantum states. The calculations are performed with the help of nonperturbative computational scheme, based on the Green's function theory within the framework of polaron transformation (GFT-PT), where the many-body electron-phonon interaction problem is mapped exactly into a single-electron multi-channel scattering problem. In particular, the influence of dephasing and relaxation processes on the shape of the electrical current and shot noise curves is discussed in detail under the linear and nonlinear transport conditions.Comment: 11 pages, 3 figure

AUTOMATED ELEMENTAL ANALYSIS USING ENERGY DISPERSIVE X-RAY FLUORESCENCE ANALYSIS

The application of energy dispersive x-ray fluorescence analysis to the measurements of air particulate samples has been described in several recent publications. Among its many advantages, the possibility of automatic analysis for large scale monitoring programs has not been widely discussed. We describe the completion of a program in which a total of 34,000 air particulate samples have been collected and analyzed over the past two years. An automatic photon-excited energy dispersive spectrometer was used to obtain elemental concentrations for 27 elements on each sample. The sensitivity of the system is adequate to detect quantities of 10 ngm/cm{sup 2} or less in a measurement time of approximately six minutes per sample. The precision and accuracy of the results will be compared to statistical analyses of the data set. Summaries of the results are presented and the implications for air particulate monitoring discussed

Large-scale measurement of airborne particulate sulfur

An aerosol sampling and analysis system is described that represents an integral approach to large-scale monitoring of airborne particulate matter. During our two-year period 34,000 size-fractionated samples were collected by automated dichotomous samplers characterized by a particle size cutpoint of 2.4 ..mu..m. The total mass of the particulate matter was measured by beta-particle attenuation, and the elemental composition, including sulfur, was determined by photon-excited x-ray fluorescence. The long-term performance of the system is reported. Potential systematic effects related to the sampling and analysis of sulfur particles are treated. Both the accuracy and precision of sulfur measurement are estimated to be 2 percent. While the x-ray attenuation correction required is typically only a few percent, a larger correction is required for a small fraction of the samples due to the migration of the sulfur into the filter. Laboratory and field experiments showed insignificant gaseous SO/sub 2/ conversion on the type of filters employed in the study. Preliminary data on the composition and the temporal and spatial distribution of the St. Louis aerosol are presented. The long-term (4 month average) sulfur data indicate that the background air masses arriving at St. Louis from the west and north were about 30 percent lower in particulate sulfur than those from the east and south. Short-term (6 hour average) data indicate that the effects of stationary SO/sub 2/ sources extend for long distances (at least 40 km), and are highly directional in character