Electron spin resonance indication of the ferromagnet-spin glass transition in amorphous FeMn alloys

We find that ESR linewidth measurements of amorphous FeMn alloys indicate the ferromagnet-spin glass transition temperature without a scaling analysis. Comparison with neutron scattering data shows the importance of studying the long-wavelength limit.     

Structure and bonding in narrow gap semiconductors

Many trends in the behavior of elements and their compounds are immediately visible by comparison with their position in the periodic table. The most obvious trend is that the elements to the left and in the lower part of the periodic table are metals while the rest are covalently-bonded semiconductors or insulators; the coordination number of the covalently-bonded materials is generally given by the 8-N rule (N is the column number). The next clear trend comes in the consideration of binary compounds; here we find that the horizontal separation of the two elements in the periodic table is a good indication of their ionicity. For example, this explains the trends in the forbidden energy gap and the crystal structure along an isoelectronic series (e.g. KBr, Case, ZnSe, GaAs, Ge).     

Lie algebras and representations of continuous groups

It is shown that every finitely generated continuous group has a subgroup generated by its infinitesimal transformations. This subgroup has a group algebra which is the Lie algebra of the group. By obtaining complete systems in the Lie algebra and complete rectangular arrays, it is shown that these can yield matrix representations of the continuous group. Illustrative examples are given for the rotation groups and for the full linear groups. It would seem that all the finite motion representations can be obtained by these methods, including spin representations of rotation groups. But the completeness of the method is not here demonstrated.     

Angular distribution of photoluminescence as a probe of bose condensation of trapped excitons

Recent experiments on two-dimensional exciton systems have shown that excitons collect in shallow in-plane traps. We find that Bose condensation in a trap results in a dramatic change of the exciton photoluminescence (PL) angular distribution. The long-range coherence of the condensed state gives rise to a sharply focused peak of radiation in the direction normal to the plane. By comparing the PL profile with and without Bose condensation, we provide a simple diagnostic for the existence of a Bose condensate. The PL peak has strong temperature dependence due to the thermal order parameter phase fluctuations across the system. The angular PL distribution can also be used for imaging vortices in the trapped condensate. Vortex phase spatial variation leads to destructive interference of PL radiation in certain directions, creating nodes in the PL distribution that imprint the vortex configuration.