Hidden symmetries in the energy levels of excitonic 'artificial atoms'

Quantum dots or 'artificial atoms' are of fundamental and technological interest--for example, quantum dots may form the basis of new generations of lasers. The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure). Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot. The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms. We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle. In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system. Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states.     

Mechanical milling assisted by electrical discharge

Mechanical milling is an effective technique for the preparation of fine metallic and ceramic powders and can also be used to drive a wide range of chemical reactions. Milling devices include planetary machines, attritors and vibrational mills; products include amorphous, nanocrystalline and quasicrystalline materials, supersaturated solid solutions, reduced minerals, high-surface-area catalysts and reactive chemicals. During milling, solid-solid, solid-liquid and solid-gas reactions are initiated through repeated deformation and fracture of powder particles. A separate materials synthesis and processing technique involves reacting a material in a gas atmosphere under an electrical discharge. Here we show that the application of low-current, high-voltage electrical impulses during milling can result in both faster reactions and new synthesis and processing routes. We demonstrate the effects of glow (cold) and spark (hot) discharge milling on particle fracture for brittle, low-conductivity materials and ductile metals. Glow discharge milling was found to promote solid-gas reactions whereas spark discharge milling promotes fast fracturing, recrystallization, mineral reduction and solid-solid reactions.     

The star-formation history of the Universe from the stellar populations of nearby galaxies

The determination of the star-formation history of the Universe is a key goal of modern cosmology, as it is crucial to our understanding of how galactic structures form and evolve. Observations of young stars in distant galaxies at different times in the past have indicated that the stellar birthrate peaked some eight billion years ago before declining by a factor of around ten to its present value. Here we report an analysis of the 'fossil record' of the current stellar populations of 96,545 nearby galaxies, from which we obtained a complete star-formation history. Our results broadly support those derived from high-redshift galaxies. We find, however, that the peak of star formation was more recent--around five billion years ago. We also show that the bigger the stellar mass of the galaxy, the earlier the stars were formed, which indicates that high- and low-mass galaxies have very different histories.     

Emergence of cooperation and evolutionary stability in finite populations

To explain the evolution of cooperation by natural selection has been a major goal of biologists since Darwin. Cooperators help others at a cost to themselves, while defectors receive the benefits of altruism without providing any help in return. The standard game dynamical formulation is the 'Prisoner's Dilemma', in which two players have a choice between cooperation and defection. In the repeated game, cooperators using direct reciprocity cannot be exploited by defectors, but it is unclear how such cooperators can arise in the first place. In general, defectors are stable against invasion by cooperators. This understanding is based on traditional concepts of evolutionary stability and dynamics in infinite populations. Here we study evolutionary game dynamics in finite populations. We show that a single cooperator using a strategy like 'tit-for-tat' can invade a population of defectors with a probability that corresponds to a net selective advantage. We specify the conditions required for natural selection to favour the emergence of cooperation and define evolutionary stability in finite populations.