Dipolar interactions and origin of spin ice in ising pyrochlore magnets

Recent experiments suggest that the Ising pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 display qualitative properties of the nearest-neighbor "spin ice" model. We discuss the dipolar energy scale present in both these materials and discuss how spin-ice behavior can occur despite the presence of long-range dipolar interactions. We present results of numerical simulations and a mean field analysis of Ising pyrochlore systems. Based on our quantitative theory, we suggest that the spin-ice behavior in these systems is due to long-range dipolar interactions, and that the nearest-neighbor exchange in Dy2Ti2O7 is antiferromagnetic.     

Zeeman studies of the 0.839eV emission in SI GaAs:Cr

The multi-structured zero phonon transitions of the 0.839eV emission observed in SI GaAs:Cr have been investigated by Zeeman measurements. Magnetic splittings for the directions [111], [110] and [100] as well as angular dependence studies in the (110) plane show that the centre has a symmetry axis along [111] with a small orthorhombic distortion. The emissions cannot be due to the isolated [100] Jahn-Teller distorted Cr²⁺ ions observed in magnetic resonance but to [111] centres such as the (Cr²⁺-donor) pairs as suggested by White.     

Effect of substrate temperatures and annealing on optically detected magnetic resonance in glow-discharge amorphous silicon

Optically detected magnetic resonance experiments have been performed at 2 K in glow-discharge amorphous silicon with different substrate temperatures. Effects of annealing and illumination on ODMR have been examined. A broad line is interpreted in terms of a three-centre bond model.     

Donor-acceptor nature of the blue self-activated emission in ZnS crystals

Optically detected magnetic resonance in ZnS shows that the blue self-activated emission is due to the recombination of an electron trapped at a donor state with a hole trapped at an A-centre acceptor state.     

Observation of bound exciton and distant pair resonances in amorphous phosphorus

For the first time in an amorphous semiconductor magnetic resonance of a bound exciton is reported. ODMR investigations of amorphous phosphorous show that the luminescence consists of two bands, a low energy emission which is consistent with triple exciton recombination at IVAPs and a high energy emission due to electron-hole recombination at distant IVAPs. The triplet exciton has g = 2.13 ± 0.05, and the distant pair resonance occurs at g = 2.0 ± 0.02.     

Spin dependent conductivity and photovoltage in ZnSe:Mn2+ Schottky barrier diodes

Light emitting ZnSe:Mn²⁺ Schottky barrier diodes have been investigated by magnetic resonance and for the first time a deep centre resonance, Mn²⁺, has been observed by monitoring the forward and reverse conductivity. In the same experiments a donor resonance was also observed as an increase in conductivity. Using two ohmic contacts the Mn²⁺ resonance and a hole centre resonance, most probably a (V_Zn-Cl) acceptor, were recorded by monitoring the photoconductivity. The Mn²⁺ could also be detected in the photovoltaic signal. These experiments suggest new techniques to investigate deep centres in semiconductors.     

Optical detection of electron spin resonance in CdTe

We report the optical detection of electron spin resonance in p-type CdTe at 1.7 K in optical pumping conditions. The Overhauser shift of the electronic resonance, of the order of 45 G, is related to the sign of the electron g-factor g¹. We measure $\text{g}{}^1\text{= -1.59±0.02}$. Using this g¹ value and the previous results on the Knight shift, we deduce the value of the electron wavefunction on Cd in CdTe, which is consistent with the value in CdS.     

On the role of the dangling bond as a radiative centre in a-Si:H

A comparison of recent ODMR results a-Si : H with previously reported ESR and LESR measurements suggests that a principal radiative electron centre has g=2.0055 and may therefore be identified as the dangling board. Our results suggest that the dangling bond (DB) can participate in two distinct PL transitions, centred near 1.25 eV and 0.9 eV, which are present to a measurable extent in all samples examined, in the medium-to-high quantum efficiency range (η_PL ～ 0.1?1.0). This conclusion is in contrast with the generally accepted role of the dangling bond in a-Si : H as primarily a non-radiative recombination centre, but is consistent with a variety of data from other experimental techniques.     

Exact scalar field cosmologies in a higher derivative theory

We obtain exact cosmological solutions of a higher derivative theory described by the Lagrangian L=R+2αR ² in the presence of interacting scalar field. The interacting scalar field potential required for a known evolution of the FRW universe in the framework of the theory is obtained using a technique different from the usual approach to solve the Einstein field equations. We follow here a technique to determine potential similar to that used by Ellis and Madsen in Einstein gravity. Some new and interesting potentials are noted in the presence of R ² term in the Einstein action for the known behaviours of the universe. These potentials in general do not obey the slow rollover approximation.