Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund's rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the 'dark exciton'. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin-orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

The growth of delaminations in polymer-matrix fibre composites under cyclic-fatigue loading in operational aircraft structures has always been a very important factor which has the potential to significantly affect the service-life of such structures. The recent introduction by the Federal Aviation Administration (FAA) of a 'slow growth' approach to the certification of composites has further focused attention on the experimental data and the analytical tools needed to assess the growth of delaminations under fatigue loads. Specific attention is given to the test and data-reduction procedures required to determine a 'valid' rate of fatigue crack growth (FCG), da/dN, versus the range of the energy release-rate, ΔG, (or the maximum energy release-rate, G max , in a cycle) relationship (a) to characterise and compare different types of composites, and (b) for designing and lifing in-service composite structures. Now, fibre-bridging may occur behind the tip of the advancing delamination and may cause very significant retardation of the FCG rate. Such retardation effects cannot usually be avoided when using the Mode I double-cantilever beam test to ascertain experimentally the fatigue behaviour of composites, so that a means of estimating a valid (i.e. ideally a 'retardation-free' or, at least, a very low-retardation) relationship is needed. The present paper presents a novel methodology, that is based on a variant of the Hartman-Schijve equation, to ascertain a valid, 'retardation-free', upper-bound FCG rate curves.

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The trends for such important parameters of male fertility as seminal volume and total sperm number were assessed in men living permanently in the Greater Athens area over a prolonged period of time. To this end, the records of three andrological laboratories employing the same method for semen evaluation were analysed retrospectively. Out of 23,850 men examined from 1977 to 1993 (17 years) for couple subfertility, a total of 2385 (10%) were selected for evaluation by a randomization procedure. Analysis of the data included (i) estimation of mean seminal volume and total sperm number per year, (ii) assessment of percentage frequency distribution of each seminal parameter and (iii) evaluation of seminal volume and total sperm number changes in relation to the year of observation and age of the subjects. A significant decrease (P < 0.01) of total sperm number was observed over the years with a mean (+/-SEM) of 154.3 +/- 19.2 x 10(6) at the beginning (1977), dropping to 130.1 +/- 13.3 x 10(6) in the final year (1993). Mean seminal volume was lower in the final year of observation, but its difference from the initial year value was not significant. Frequency distribution analysis showed a marked decline in the 240-400 x 10(6) sub-set of the range of sperm number values from 16.9 +/- 4.5% (1977) to 10.6 +/- 1.6% in the final year (P < 0.01). Multiple regression analysis of seminal volume, total sperm number, age and year of assessment revealed a significant decline of the two seminal parameters along the years of observation (P < 0.05 and P < 0.0001 respectively). Over the same period, a marked deterioration of some air pollution indices was observed in that area. It is concluded that in this racially and ethnically homogeneous sample of men, living under the same environmental conditions, a significant decline in seminal volume and total sperm number occurred over the 17 years of observation.