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

Gate-Controlled Electron Spin Resonance in a GaAs/AlGaAs Heterostructure

The electron spin resonance (ESR) of two-dimensional electrons is investigated in a gated GaAs/AlGaAs heterostructure. We found that the ESR resonance frequency can be turned by means of a gate voltage. The front and back gates of the heterostructure produce opposite g-factor shift, suggesting that electron g-factor is being electrostatically controlled by shifting the equilibrium position of the electron wave function from one epitaxial layer to another with different g-factors

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

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

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

A time predefined variable depth search for nurse rostering

This paper presents a variable depth search for the nurse rostering problem. The algorithm works by chaining together single neighbourhood swaps into more effective compound moves. It achieves this by using heuristics to decide whether to continue extending a chain and which candidates to examine as the next potential link in the chain. Because end users vary in how long they are willing to wait for solutions, a particular goal of this research was to create an algorithm that accepts a user specified computational time limit and uses it effectively. When compared against previously published approaches the results show that the algorithm is very competitive

Neonatal intervention

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68335/2/10.1177_027112148300300105.pd