Truncation method for Green's functions in time-dependent fields

We investigate the influence of a time dependent, homogeneous electric field on scattering properties of non-interacting electrons in an arbitrary static potential. We develop a method to calculate the (Keldysh) Green's function in two complementary approaches. Starting from a plane wave basis, a formally exact solution is given in terms of the inverse of a matrix containing infinitely many 'photoblocks' which can be evaluated approximately by truncation. In the exact eigenstate basis of the scattering potential, we obtain a version of the Floquet state theory in the Green's functions language. The formalism is checked for cases such as a simple model of a double barrier in a strong electric field. Furthermore, an exact relation between the inelastic scattering rate due to the microwave and the AC conductivity of the system is derived which in particular holds near or at a metal-insulator transition in disordered systems.Comment: to appear in Phys. Rev. B., 21 pages, 3 figures (ps-files

Spin Susceptibility of a 2D Electron System in GaAs towards the Weak Interaction Region

We determine the spin susceptibility $\chi$ in the weak interaction regime of a tunable, high quality, two-dimensional electron system in a GaAs/AlGaAs heterostructure. The band structure effects, modifying mass and g-factor, are carefully taken into accounts since they become appreciable for the large electron densities of the weak interaction regime. When properly normalized, $\chi$ decreases monotonically from 3 to 1.1 with increasing density over our experimental range from 0.1 to $4\times10^{11} cm^{-2}$. In the high density limit, $\chi$ tends correctly towards $\chi\to 1$ and compare well with recent theory.Comment: Submitted to Physical Review

Affective expression among preterm infants of varying levels of biological risk

This investigation examines the association between risk status and the quality of emotional arousal and regulation among preterm infants in the second year of life. The behavior of 55 preterm infants stratified into three risk groups by severity and chronicity of respiratory illness was recorded during the procedures of the Strange Situation. Measures of emotional responsiveness included temporal and intensity features of facial and vocal expressions as well as concurrent activity with toys. Significant relationships between neonatal risk status and these expressive and regulatory features were observed. Infants in the High-Risk group (N = 16) differed from healthy Low-Risk infants (N = 23) and from those in the Moderate-Risk group (N = 16). The High-Risk infants showed a greater sensitivity to distress arousal at low levels of stress and less ability to modulate distress once aroused. High-Risk infants also demonstrated significantly less adaptive play with toys than the other preterm infants. In short, this study suggests that, when placed under stress, High-Risk preterm infants in their second year of life become more distressed and demonstrate less ability to recover from this distress and effectively re-engage their environment than preterm infants born at lower risk.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26744/1/0000296.pd

Circular polarization dependent study of the microwave photoconductivity in a two-dimensional electron system

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasi-optical setup in which linear and any circular polarization can be produced in-situ. The microwave induced resistance oscillations and the zero resistance regions are notedly immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations only occur near the cyclotron resonance absorption where an unprecedented large resistance response is observed.Comment: 5 pages, 3 figure

Dynamical scaling of the quantum Hall plateau transition

Using different experimental techniques we examine the dynamical scaling of the quantum Hall plateau transition in a frequency range f = 0.1-55 GHz. We present a scheme that allows for a simultaneous scaling analysis of these experiments and all other data in literature. We observe a universal scaling function with an exponent kappa = 0.5 +/- 0.1, yielding a dynamical exponent z = 0.9 +/- 0.2.Comment: v2: Length shortened to fulfil Journal criteri

Scalar-Tensor Cosmological Models

We analyze the qualitative behaviors of scalar-tensor cosmologies with an arbitrary monotonic $\omega(\Phi)$ function. In particular, we are interested on scalar-tensor theories distinguishable at early epochs from General Relativity (GR) but leading to predictions compatible with solar-system experiments. After extending the method developed by Lorentz-Petzold and Barrow, we establish the conditions required for convergence towards GR at $t\rightarrow\infty$. Then, we obtain all the asymptotic analytical solutions at early times which are possible in the framework of these theories. The subsequent qualitative evolution, from these asymptotic solutions until their later convergence towards GR, has been then analyzed by means of numerical computations. From this analysis, we have been able to establish a classification of the different qualitative behaviors of scalar-tensor cosmological models with an arbitrary monotonic $\omega(\Phi)$ function.Comment: uuencoded compressed postscript file containing 41 pages, with 9 figures, accepted for publication in Physical Review

Methods for Assessing Child and Family Outcomes in Early Childhood Special Education Programs

Although many concerns have been raised about methods of assessing outcomes in early childhood special education programs, professionals in the field are nevertheless faced with the need to select appropriate instruments for evaluating child and family outcomes as the result of intervention. A conference to address the current assessment needs of professionals was convened. This paper summarizes this conference, in which five prominent individuals in the field of early childhood special education gave specific recommendations for one child and one family outcome measure which would be applicable to a range of handicapped children between birth and age 5 being served in typical early intervention programs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68510/2/10.1177_027112148600600202.pd

Anti-glomerular basement membrane disease superimposed on membranous nephropathy: a case report and review of the literature

<p>Abstract</p> <p>Introduction</p> <p>Anti-glomerular basement membrane disease is a rare autoimmune disorder characterized by pulmonary hemorrhage, crescentic glomerulonephritis and the presence of circulating anti-glomerular basement membrane antibodies. The simultaneous occurrence of both anti-glomerular basement membrane disease and membranous nephropathy is rare.</p> <p>Case presentation</p> <p>A 59-year-old Hispanic man presented with acute onset of nausea and vomiting and was found to have renal insufficiency. Work-up included a kidney biopsy, which revealed anti-glomerular basement membrane disease with underlying membranous nephropathy. He was treated with emergent hemodialysis, intravenous corticosteroids, plasmapheresis, and cyclophosphamide without improvement in his renal function.</p> <p>Conclusion</p> <p>Simultaneous anti-glomerular basement membrane disease and membranous nephropathy is very rare. There have been 16 previous case reports in the English language literature that have been associated with a high mortality and morbidity, and a very high rate of renal failure resulting in hemodialysis. Co-existence of membranous nephropathy and anti-glomerular basement membrane disease may be immune-mediated, although the exact mechanism is not clear.</p

Morphological Transformation from Galaxy Harassment

Galaxy morphologies in clusters have undergone a remarkable transition over the past several billion years. Distant clusters at $z \sim 0.4$ are filled with small spiral galaxies, many of which are disturbed and show evidence of multiple bursts of star-formation. This population is absent from nearby clusters where spheroidals comprise the faint end of the luminosity function. Our numerical simulations follow the evolution of disk galaxies in a rich cluster owing to encounters with brighter galaxies and the cluster's tidal field---galaxy harassment. After a bursting transient phase, they undergo a complete morphological transformation from "disks" to"spheroidals". We examine the remnants and find support for our theory in detailed comparisons of the photometry and kinematics of the spheroidal galaxies in clusters. Our model naturally accounts for the intermediate age stellar population seen in these spheroidals as well as the trend in dwarf to giant ratio with cluster richness. The final shapes are typically prolate and are flattened primarily by velocity anisotropy. Their mass to light ratios are in the range 3---8 in good agreement with observations.Comment: Final version, to be published in the Ap.J. 1998, video edition with the "Galaxy Harassment" movi