24 research outputs found
Strong-correlation effects in Born effective charges
Large values of Born effective charges are generally considered as reliable
indicators of the genuine tendency of an insulator towards ferroelectric
instability. However, these quantities can be very much influenced by strong
electron correlation and metallic behavior, which are not exclusive properties
of ferroelectric materials. In this paper we compare the Born effective charges
of some prototypical ferroelectrics with those of magnetic, non-ferroelectric
compounds using a novel, self-interaction free methodology that improves on the
local-density approximation description of the electronic properties. We show
that the inclusion of strong-correlation effects systermatically reduces the
size of the Born effective charges and the electron localization lengths.
Furthermore we give an interpretation of the Born effective charges in terms of
band energy structure and orbital occupations which can be used as a guideline
to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure
STRUCTURAL ORDERING RELATED TO CHEMICAL BONDS IN RANDOM NETWORKS
No abstract availabl
Manufacturing‐tolerant compact red‐emitting laser diode designs for next generation applications
Quantum well laser diodes with low far-field divergence remain a requirement for many applications such as optical interconnects and data networks, pump sources and next generation holographic red–green–blue displays requiring compact, high power, visible light sources with high spatial and spectral coherence. Many designs exist, but the structure must be easy to grow reproducibly, which has commercial advantages. The authors' low far-field divergence design widens the vertical mode in such a way as to decrease the far-field divergence without significantly reducing the confinement factor, thus keeping threshold current lower. In this study, the authors calculate the sensitivity of their design, which has high refractive index mode expansion layers inserted in the cladding, to unintentional variations in layer thickness and composition during growth. They obtain consistency in measured far-fields for three wafers grown over an interval of a year, with a full-width-half-maximum vertical far-field divergence of 17° for a narrow design (Design A) and just under 13° for a very narrow design (Design B). They have demonstrated a useful, reproducible design, adding to the range of versatile semiconductor lasers available for every application