Radiation kinetics in electronic materials Semiannual report

Degradation and recovery of minority carriers in solar cells including lithiu

DEFECT MOBILITY AND REACTION: DIFFUSIONAL AND RATE THEORY FORMULATIONS IN ONE AND THREE DIMENSIONS.

The migration of defects, leading to eventual reaction, is analyzed on the basis of diffusion theory and compared with chemical rate theory. A diffusional treatment is carried out in detail and in special cases of particular interest. Correlations between interstitials and vacancies, typical of metals following electron irradiation, are examined. Trapping by impurities and dislocations, thermal conversion, and high temperature bombardment are also treated. In each case, formulations are developed along both one- and three-dimensional diffusion models. These alternative results are compared with experiments, particularly annealing in copper irradiated by electrons at low temperature. The results support the conclusion that the defect which migrates in Stage I diffuses in three dimensions

Magnetocaloric effect in two-dimensional spin-1/2 antiferromagnets

The magnetocaloric effect is studied at the transition to saturation in the antiferromagnetic spin-1/2 Heisenberg model on the simplest two-dimensional lattices, namely the square and the triangular lattice. Numerical results are presented for the entropy which are consistent with identical universal properties. However, the absolute values of the entropy are bigger on the geometrically frustrated triangular lattice than on the non-frustrated square lattice, indicating that frustration improves the magnetocaloric properties.Comment: 2 pages, 2 figures included, to appear in Physica B (proceedings of SCES'05

Novel magnetic phases in a Gd2Ti2O7 pyrochlore for a field applied along the [100] axis

We report on longitudinal and transverse magnetisation measurements performed on single crystal samples of Gd2Ti2O7 for a magnetic field applied along the [100] direction. The measurements reveal the presence of previously unreported phases in fields below 10 kOe in an addition to the higher-field-induced phases that are also seen for H//[111], [110], and [112]. The proposed H-T phase diagram for the [100] direction looks distinctly different from all the other directions studied previously.Comment: 4 pages, 5 figure

Bridging the Information Literacy Gap with Clickers

A Class Performance System (CPS) is applied as an assessment tool to assist in determining the level of information literacy knowledge already possessed by adult students enrolled in a graduate Educational Leadership and Technology program. The CPS data collected reveals that students possess varying degrees of technology expertise and information literacy skills. Study results suggest that clickers facilitate differentiated information literacy pedagogy and increase engagement for graduate students. © 2009 Elsevier Inc. All rights reserved

Spin Dynamics of NiCl2−4SC(NH2)2Ni Cl_2-4SC(NH_2)_2 in the Field-Induced Ordered Phase

$Ni Cl_2$-$4SC(NH_2)_2$ (known as DTN) is a spin-1 material with a strong single-ion anisotropy that is regarded as a new candidate for Bose-Einstein condensation (BEC) of spin degrees of freedom. We present a systematic study of the low-energy excitation spectrum of DTN in the field-induced magnetically ordered phase by means of high-field electron spin resonance measurements at temperatures down to 0.45 K. We argue that two gapped modes observed in the experiment can be consistently interpreted within a four-sublattice antiferromagnet model with a finite interaction between two tetragonal subsystems and unbroken axial symmetry. The latter is crucial for the interpretation of the field-induced ordering in DTN in terms of BEC.Comment: 4 pages, 3 figure

Magnetic excitations in dipolar pyrochlore antiferromagnet Gd2_2Sn2_2O7_7

The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet $\rm Gd_2Sn_2O_7$ is studied by means of the electron spin resonance. In the ordered phase ($T_N = 1$ K), we have detected three gapped resonance modes. Their values agree well with the developed spin-wave theory which takes into account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy and the long-range dipolar interactions. The theory also predicts a fourth lowest-frequency gap, which lies beyond the experimental range of frequencies, but determines the exponential decrease of the specific heat at low temperature.Comment: 8 pages, 5 figure

Magnetocaloric effect in pyrochlore antiferromagnet Gd2Ti2O7

An adiabatic demagnetization process is studied in Gd2Ti2O7, a geometrically frustrated antiferromagnet on a pyrochlore lattice. In contrast to conventional paramagnetic salts, this compound can exhibit a temperature decrease by a factor of ten in the temperature range below the Curie-Weiss constant. The most efficient cooling is observed in the field interval between 120 and 60 kOe corresponding to a crossover between saturated and spin-liquid phases. This phenomenon indicates that a considerable part of the magnetic entropy survives in the strongly correlated state. According to the theoretical model, this entropy is associated with a macroscopic number of local modes remaining gapless till the saturation field. Monte Carlo simulations on a classical spin model demonstrate good agreement with the experiment. The cooling power of the process is experimentally estimated with a view to possible technical applications. The results for Gd2Ti2O7 are compared to those for Gd3Ga5O12, a well-known material for low temperature magnetic refrigeration.Comment: 6 pages, 4 figures, accepted versio