Causality-based criteria for a negative refractive index must be used with care

Using the principle of causality as expressed in the Kramers-Kronig relations, we derive a generalized criterion for a negative refractive index that admits imperfect transparency at an observation frequency $\omega$. It also allows us to relate the global properties of the loss (i.e. its frequency response) to its local behaviour at $\omega$. However, causality-based criteria rely the on the group velocity, not the Poynting vector. Since the two are not equivalent, we provide some simple examples to compare the two criteria.Comment: slightly longer version of published PR

The Effects of Interrupted Enrollment on Graduation from College: Racial, Income, and Ability Differences

We present a multiple spells-competing risks model of stopout, dropout, reenrollment, and graduation behavior. We find that students who experience an initial stopout are more likely to experience subsequent stopouts (occurrence dependence) and be less likely to graduate. We also find evidence of the impact of the length of an initial spell on the probability of subsequent events (lagged duration dependence). We simulate the impacts of race, family income, and high school performance on student behavior and show that there are often very large differences between unadjusted rates of student outcomes and adjusted rates. Differences in student performance often ascribed to race are shown to be the result of income, age at entry, and high school performance.

Some Components of Sediment Oxygen Demand in Lake Erie Sediments

Research on which this report is based was financed in part by the U. S, Department of Interior as authorized by the Water Resources and Development Act of 1978, (P. L. 95-467).(print) 40 leaves : ill. ; 28 cm.Item lacks publicaton date. Date supplied from WorldCat bibliographic recordWe have examined SOD of Lake Erie sediments with and without macrobenthic infauna, and with high and low macrobial activity, and have modeled the penetration of oxygen into sediments, since the depth of oxygen penetration is not easily measured. SOD of western and central basin sediments (95% silt clay, 3% organic carbon, 70-80% water content) ranges from 1.25-2.5 um 02/hr in laboratory microcosms. When tubificid oligochaetes are added to laboratory microcosms (30,000 m^-2 ), SOD rises to 1.5-3.5 um 02/hr. SOD is greater than the simple sum of organism respiration plus sediment SOD. The extra enhanced demand is due to enhanced microbial activity, the transport of 02 to greater depths in the sediment, and to the transport by feeding of FeS to the sediment-water interface. Enhanced demand over sediment plus respiration values appear to be proportional to the number of oligochaetes present, which would implicate FeS transport as a major factor in enhanced demand. Thus, tubificids enhance the rate of organic decay not only through aiding the transport of dissolved oxygen, but also by transporting reduced sulphur to be oxidized to SO4^2- at the sediment-water interface. Sediment sterilization techniques were not successful; these techniques probably result in the release of additional oxygen demanding substances as a result of sterilization. There is a pattern of decreasing SOD for a period of about 10-14 days after the start of an experiment until an equilibrium SOD is reached. This is due to the liberation of bacterial nutrients when sediments are added to microcosms and mixed. Thus experiments done soon after introduction of sediments into an SOD apparatus are likely to be in error by as much as a factor of two.Disclaimer -- Contents -- Abstract -- Figures -- Tables -- Background -- Conclusions and Recommendations -- Introduction -- Review of Related Research -- Geochemical Considerations -- Methods -- Results -- Bibliograph

Room-Temperature Continuous-Wave Vertical-Cavity Single-Quantum-Well Microlaser Diodes

Room-temperature continuous and pulsed lasing of vertical-cavity, single-quantum-well, surface-emitting microlasers is achieved at ~983nm. The active Ga[sub][0-8]In[sub][0-2]As single quantum well is 100 [angstroms] thick. These microlasers have the smallest gain medium volumes among lasers ever built. The entire laser structure is grown by molecular beam epitaxy and the microlasers are formed by chemically assisted ion-beam etching. The microlasers are 3-50-μm across. The minimum threshold currents are 1.1 mA (pulsed) and 1.5 mA (CW)

Stress relaxation in F-actin solutions by severing

Networks of filamentous actin (F-actin) are important for the mechanics of most animal cells. These cytoskeletal networks are highly dynamic, with a variety of actin-associated proteins that control cross-linking, polymerization and force generation in the cytoskeleton. Inspired by recent rheological experiments on reconstituted solutions of dynamic actin filaments, we report a theoretical model that describes stress relaxation behavior of these solutions in the presence of severing proteins. We show that depending on the kinetic rates of assembly, disassembly, and severing, one can observe both length-dependent and length-independent relaxation behavior

Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

Vertical-cavity electrically driven lasers with three GaInAs quantum wells and diameters of several μm exhibit room-temperature pulsed current thresholds as low as 1.3mA with 958 nm output wavelength

Massive creation of entangled exciton states in semiconductor quantum dots

An intense laser pulse propagating in a medium of inhomogeneously broadened quantum dots massively creates entangled exciton states. After passage of the pulse all single-exciton states remain unpopulated (self-induced transparency) whereas biexciton coherence (exciton entanglement) is generated through two-photon transitions. We propose several experimental techniques for the observation of such unexpected behavior

Decomposing the Sources of Earnings Inequality: Assessing the Role of Reallocation

This paper exploits longitudinal employer-employee matched data from the U.S. Census Bureau to investigate the contribution of worker and firm reallocation to changes in earnings inequality within and across industries between 1992 and 2003. We find that factors that cannot be measured using standard cross-sectional data, including the entry and exit of firms and the sorting of workers across firms, are important sources of changes in earnings distributions over time. Our results also suggest that the dynamics driving changes in earnings inequality are heterogeneous across industries.inequality, linked employer-employee data, sorting

Decomposing the Sources of Earnings Inequality Assessing the Role of Reallocation

This paper uses matched employer-employee data from the Longitudinal Employer Household Dynamics database to investigate the contribution of worker and firm reallocation to within industry changes in wage inequality between 1992 and 2003. We find that the entry and exit of firms and the sorting of workers and firms based on underlying worker "skills" are important determinants of changes in industry earnings distributions over time. Our results suggest that the underlying dynamics of earnings inequality are complex and are due to factors that cannot be measured in standard crosssectional data.

Studies of Diffuse Interstellar Bands. V. Pairwise Correlations of Eight Strong DIBs and Neutral Hydrogen, Molecular Hydrogen, and Color Excess

We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and EB-V . The DIBs are centered at \lambda\lambda 5780.5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson\^as correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width of \lambda 5780.5 is better correlated with column densities of H than with E(B-V) or H2, confirming earlier results based on smaller datasets. We show the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not well enough correlated with each other to suggest they come from the same carrier. We further conclude that these eight DIBs are more likely to be associated with H than with H2, and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest they arise from different molecules found in diffuse H regions with very little H (molecular fraction f<0.01). Of the 133 stars with available data in our study, there are three with significantly weaker \lambda 5780.5 than our mean H-5780.5 relationship, all of which are in regions of high radiation fields, as previously noted by Herbig. The correlations will be useful in deriving interstellar parameters when direct methods are not available. For instance, with care, the value of N(H) can be derived from W{\lambda}(5780.5).Comment: Accepted for publication in The Astrophysical Journal; 37 pages, 11 figures, 6 table