Spatially Resolved Thermodynamics of the Partially Ionized Exciton Gas in GaAs.

We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.

Semicentralised supply and treatment systems: integrated infrastructure solutions for fast growing urban areas.

Currently, the development of the world population is characterised by two trends: absolute population growth and rapid urbanisation. Especially rapid urbanisation, taking place in Asia, Latin America and Africa, poses major pressure on the affected regions. The development of e.g. Asian countries today is stamped by a combination of urbanisation with high economic growth rates. Conventional centralised infrastructure of supply, treatment and disposal of water is not able to cope with the new challenges arising from these, in history incomparable, high growth rates. Therefore new approaches to infrastructure supply and treatment systems are required - for ecological, sociocultural and economic reasons. The semicentralised approach, focusing on integrated water supply and treatment structures for wastewater and waste on the neighbourhood level, offers one possible solution to the challenges imposed by rapid urbanisation and growing resource needs. The change from centralised to semicentralised supply and treatment systems will minimise the grave discrepancy between the rapid urban growth and the provision of supply and treatment infrastructure. Integrated semicentralised supply and treatment systems face the challenge of growing amounts of wastewater and solid waste combined with rising needs of water for private households and industrial use. The semicentralised approach offers a wide range of flexibility in implementation, energy self-sufficient operation, enormous saving potentials in water demands through intra-urban water reuse and further more advantages in comparison to centralised sectored solutions as practised today.