Optimized Methodology for the Calculation of Electrostriction from First-Principles.

In this work a new method for the calculation of the electrostrictive properties of materials using density functional theory is presented. The method relies on the thermodynamical equivalence, in a dielectric, of the quadratic mechanical responses (stress or strain) to applied electric stimulus (electric or polarization fields) to the strain or stress dependence of its dielectric susceptibility or stiffness tensors. Comparing with current finite-field methodologies for the calculation of electrostriction, it is demonstrated that this presented methodology offers significant advantages of efficiency, robustness, and ease of use. These advantages render tractable the high throughput theoretical investigation into the largely unknown electrostrictive properties of materials, and the microscopic origins of giant electrostriction.

Electronic and excitonic properties of ultrathin (In,Ga)N layers: the role of alloy and monolayer width fluctuations.

We present an atomistic theoretical analysis of the electronic and excitonic properties of ultrathin, monolayer thick wurtzite (In,Ga)N embedded in GaN. Our microscopic investigation reveals that (i) alloy fluctuations within the monolayer lead to carrier localization effects that dominate the electronic and optical properties of these ultrathin systems and that (ii) excitonic binding energies in these structures exceed the thermal energy at room temperature, enabling excitonic effects to persist even at elevated temperatures. Our theoretical findings are consistent with, and provide an explanation for, literature experimental observations of (i) broad photoluminescence linewidth and (ii) excitonic effects contributing to the radiative recombination process at elevated temperatures. When accounting for small structural inhomogeneities, such as local thickness fluctuations of one monolayer, "indirect" excitons may be found, with electrons and holes independently localized in different spatial positions. This result also provides further arguments for experimentally observed effects such as (i) non-exponential decay curves in time dependent photoluminescence spectra and (ii) the "S"-shape temperature dependence of the photoluminescence peak energies. Overall, our results provide fundamental understanding, on an atomistic level, of the electronic and optical properties of ultrathin, quasi 2D (In,Ga)N monolayers embedded in GaN, and offer guidance for the tailoring of their properties for potential future device applications.

Screening for depression in a geriatric rehabilitation sample.

OBJECTIVE: To examine the utility of a short depression inventory (SDI) for use with elderly medical inpatients. METHOD: Subjects were 132 consecutive admissions to the inpatient geriatric rehabilitation unit at MetroHealth Medical Center, a general medical teaching hospital in Cleveland, Ohio. Sixty of the original sample qualified for inclusion in the final sample. Within the first five days of admission, all subjects were administered the Mini-Mental Status Examination (MMSE). Ss who scored 20 or above were asked to complete the SDI and the GDI. A clinical interview used DSM-III-R criteria to establish the diagnosis of major depression. The order of presentation of questionnaire or interview was varied to control for the effects of presentation order. RESULTS: When an optimal cut-off score of 4 or above on the SDI was used, the sensitivity and specificity were 100% and 85% respectively. Using the GDS, sensitivity and specificity were 100% and 90%. CONCLUSIONS: Using the SDI, impressive sensitivity and specificity were achieved. The SDI was also essentially equivalent to the GDI psychometrically. The SDI appears to have good promise as a depression screen in medical populations.

Differences between the lower social classes: some new trends.

From a four-year sample of 7407 patients, this study sought to determine whether characteristics of patients in Social Class IV are closer to those of Class V, as frequently assumed, or more similar to those of Class III. The implications of social class differences for newer forms of treatment, and newly emerging types of therapists, were also examined. Results suggest that Classes IV and V should be considered separately for both research and clinical planning purposes.