On the ferroelectric nature of the cubic-rhombohedral phase transition in Pb1−xGexTe

Capacitance measurements at various fixed d.c. biases as a function of temperature are presented for junction diodes fabricated from alloys of Pb_1?xGe_x with x=0.03, x=0.45 and x=0.06. Strong capacitance maxima at the temperatures associated with a cubic-rhombohedral phase transition are observed. These peaks are the first reported direct evidence for the ferroelectric nature of the crystalline phase transition in the PbTe based alloys.     

Microsphaerins A–D, four novel benzophenone dimers with activity against MRSA from the fungus Microsphaeropsis sp.

Bioassay-guided isolation using an MRSA whole cell assay yielded four novel benzophenone dimers, microsphaerins A–D (1–4), from two Singapore isolates of the soil anamorph Microsphaeropsis sp. The structures of 1–4 were elucidated on the basis of spectral data and the structures of 1 and 4 were confirmed by X-ray crystallographic analyses.     

Reduced C*-algebras of Fell bundles over inverse semigroups

We construct a weak conditional expectation from the section C*-algebra of a Fell bundle over a unital inverse semigroup to its unit fibre. We use this to define the reduced C*-algebra of the Fell bundle. We study when the reduced C*-algebra for an inverse semigroup action on a groupoid by partial equivalences coincides with the reduced groupoid C*-algebra of the transformation groupoid, giving both positive results and counterexamples.     

Photochemistry of visual pigment chromophore models by ab initio molecular dynamics

Ab initio excited-state molecular dynamics calculations have been performed to study the effect of methyl substitution and chromophore distortion on the photoreaction of different four-double-bond retinal model chromophores. Randomly distributed starting geometries were generated by zero-point energy sampling; after Franck-Condon excitation the reaction was followed on the S1 surface. For determining the photoproduct and its configuration, a simplified approach--torsion angle following--is discussed and applied. We find that chromophore distortion significantly affects the outcome of the photoreaction: with dihedral angles taken from the rhodopsin-embedded 11-cis-retinal chromophore, the reaction rate of the model chromophore is increased by a factor of 3 compared to that of the relaxed chromophore. Also, the reaction proceeds in a completely stereoselective manner involving only the cis double bond and with a minimum quantum yield of 72%. Bond torsion is more effective than methyl substitution for fast and selective photochemistry, which is in agreement with photophysical measurements on rhodopsin analogues. We conclude that apart from the geometric distortions caused by the protein pocket it is not necessary to postulate other specific interactions between the protein and the chromophore to effect the selective and ultrafast photoreaction in rhodopsin.     

Gap duration discrimination for frequency-asymmetric gap markers: psychophysical and electrophysiological findings

This study investigated gap duration discrimination (GDD) for frequency-asymmetric gap markers, where one marker was a two-tone complex consisting of a primary tone and a secondary tone, and the other marker was the primary tone alone. Three experiments were undertaken to examine the order effect wherein performance is better when the two-tone marker is the leading marker than when it is the trailing marker. Experiment 1 demonstrated that GDD for frequency-asymmetric markers is intermediate between the boundaries of within-frequency-channel versus across-frequency-channel processing. Experiment 2 compared psychophysical performance with auditory brainstem responses (ABRs) elicited by the same stimuli. Whereas GDD thresholds were elevated for a complex trailing marker relative to a within-frequency-channel baseline, ABRs elicited by the complex marker were more robust. Experiment 3 tested the hypothesis that poor GDD performance with frequency-asymmetric markers is due to some form of nonenergetic, or informational, masking. The results did not support a role for informational masking conferred by synthetic listening; however, informational masking conferred by the occurrence of novel spectral events provided a parsimonious account. One possible interpretation is that the capacity to accurately encode a gap is undermined by the occurrence of novel spectral events that engage limited attentional resources.     

Frequency discrimination under conditions of comodulation masking release (L)

Masked detection thresholds can often be improved by introducing coherent masker amplitude modulation across frequency, a phenomenon referred to as comodulation masking release (CMR). While CMR can be large for detection, it is smaller for supra-threshold tasks, such as intensity discrimination. In this experiment, frequency discrimination for a 1000-Hz tone near threshold was found to be poorer in an amplitude-modulated than a steady bandpass noise. These results parallel previous findings for intensity discrimination. Although this study examined the relatively simple task of frequency discrimination, the results may have implications for more complex tasks, such as speech recognition in fluctuating noise.     

Pool resolution is NP-hard to recognize

A pool resolution proof is a dag-like resolution proof which admits a depth-first traversal tree in which no variable is used as a resolution variable twice on any branch. The problem of determining whether a given dag-like resolution proof is a valid pool resolution proof is shown to be NP-complete.     

Gap detection in modulated noise: across-frequency facilitation and interference

This study tested the hypothesis that a detection advantage for gaps in comodulated noise relative to random noise can be demonstrated in conditions of continuous noise and salient envelope fluctuations. Experiment 1 used five 25-Hz-wide bands of Gaussian noise, low-fluctuation noise, and a noise with increased salience of the inherent fluctuations (staccato noise). The bands were centered at 444, 667, 1000, 1500, and 2250 Hz, with the gap signal always inserted in the 1000-Hz band. Results indicated that a gap detection advantage existed in continuous comodulated noise only for Gaussian and staccato noise. Experiment 2 demonstrated that the advantage did not exist for gated presentation. This experiment also showed that the advantage bore some similarity to comodulation masking release. However, differences were also noted in terms of the effects of the number of flanking bands and the absence of a detection advantage in gated conditions. The detrimental effect of a gated flanking band was less pronounced for a comodulated band than for a random band. This study indicates that, under some conditions, a detection advantage for gaps carried by a narrow band of noise can occur in the presence of comodulated flanking bands of noise.