On the damped nonlinear evolution equation wtt = σ(w)xx − ywt

Initial boundary value problems for the damped nonlinear wave equation w_tt = σ(w)_xx ? yw_t arise in several areas of applied mathematics and, in particular, in studies of shearing flow in a nonlinear viscoelastic fluid; the problems of global existence and nonexistence of smooth solutions have been extensively studied in the strictly hyperbolic case σ′(δ) ? ε > 0, ?δ?R¹ as well as in the case where σ′(0) > 0 and the initial data are chosen so small that σ′(w) > 0 for as long as a smooth solution w(x, t) exists. In this paper the global nonexistence problem is studied for the cases σ′(0) = 0 and σ′(0) > 0 but σ′(δ) < 0 for ¦δ¦ sufficiently large and growth estimates which are valid on the maximal interval of existence of a sufficiently smooth solution are derived.     

A comparison of T2*-weighted magnitude and phase imaging for measuring the arterial input function in the rat aorta following intravenous injection of gadolinium contrast agent

The arterial input function (AIF) is important for quantitative MR imaging perfusion experiments employing Gd contrast agents. This study compared the accuracy of T(2)*-weighted magnitude and phase imaging for noninvasive measurement of the AIF in the rat aorta. Twenty-eight in vivo experiments were performed involving simultaneous arterial blood sampling and MR imaging following Gd injection. In vitro experiments were also performed to confirm the in vivo results. At 1.89 T and TE=3 ms, the relationship between changes in 1/T(2)* in blood (estimated from MR signal magnitude) and Gd concentration ([Gd]) was measured to be approximately 19 s(-1) mM(-1), while that between phase and [Gd] was approximately 0.19 rad mM(-1). Both of these values are consistent with previously published results. The in vivo phase data had approximately half as much scatter with respect to [Gd] than the in vivo magnitude data (r(2)=.34 vs. r(2)=.17, respectively). This is likely due to the fact that the estimated change in 1/T(2)* is more sensitive than the phase to a variety of factors such as partial volume effects and T(1) weighting. Therefore, this study indicates that phase imaging may be a preferred method for measuring the AIF in the rat aorta compared to T(2)*-weighted magnitude imaging.     

Unitarily Inequivalent Representations in Algebraic Quantum Theory

It has been maintained that the physical content of a model of a system is completely contained in the C∗-algebra of quasi-local observables that is associated with the system. The reason given for this is that the unitarily inequivalent representations of are physically equivalent. But, this view is dubious for at least two reasons. First, it is not clear why the physical content does not extend to the elements of the von Neumann algebras that are generated by representations of . It is shown here that although the unitarily inequivalent representations of are physically equivalent, the extended representations are not. Second, this view detracts from special global features of physical systems such as temperature and chemical potential by effectively relegating them to the status of fixed parameters. It is desirable to characterize such observables theoretically as elements of the algebra that is associated with a system rather than as parameters, and thereby give a uniform treatment to all observables. This can be accomplished by going to larger algebras. One such algebra is the universal enveloping von Neumann algebra, which is generated by the universal representation of ; another is the direct integral of factor representations that are associated with the set of values of the global features. Placing interpretive significance on the von Neumann algebras mentioned earlier sheds light on the significance of unitarily inequivalent representations of , and it serves to show the limitations of the notion of physical equivalence.     

Pronounced enhancement of the lower critical field and critical current deep in the superconducting state of PrOs4Sb12

We have observed an unexpected enhancement of the lower critical field H(c1)(T) and the critical current I(c)(T) deep in the superconducting state below T approximately 0.6 K (T/T(c) approximately 0.3) in the filled skutterudite heavy fermion superconductor PrOs(4)Sb(12). From a comparison of the behavior of H(c1)(T) with that of the heavy fermion superconductors U(1-x)Th(x)Be(13) and UPt(3), we speculate that the enhancement of H(c1)(T) and I(c)(T) in PrOs(4)Sb(12) reflects a transition into another superconducting phase that occurs below T/T(c) approximately 0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs(4)Sb(12) near T/T(c) approximately 0.3 that correlate with the features we have observed in H(c1)(T) and I(c)(T).     

Combining energetic and informational masking for speech identification

This study examined combinations of energetic and informational maskers in speech identification. Speech targets and maskers (speech or noise) were processed and filtered into sets of 15 narrow frequency bands. The target was the sum of eight randomly selected bands. More masking occurred for speech maskers than for spectrally matched noise maskers regardless of whether the masker bands overlapped the target bands. The greater effect of the speech maskers was interpreted as due to informational masking. When the masker was comprised of nonoverlapping bands of speech, the addition of bands of noise overlapping the speech masker, but not the speech target, reduced the overall amount of masking. Surprisingly, presenting the noise to the ear contralateral to the target and masker produced an even greater release from masking. The contralateral noise was apparently sufficient to cause a slight change in the image of the ipsilateral speech masker, possibly pulling it away from the target enough to allow the focus of attention on the target. This finding is consistent with the interpretation that in some conditions small binaural differences may be sufficient to cause, or significantly strengthen, the perceptual segregation of sounds.     

Temporary threshold shift in bottlenose dolphins (Tursiops truncatus) exposed to mid-frequency tones

A behavioral response paradigm was used to measure hearing thresholds in bottlenose dolphins before and after exposure to 3 kHz tones with sound exposure levels (SELs) from 100 to 203 dB re 1 microPa2 s. Experiments were conducted in a relatively quiet pool with ambient noise levels below 55 dB re 1 microPa2/Hz at frequencies above 1 kHz. Experiments 1 and 2 featured 1-s exposures with hearing tested at 4.5 and 3 kHz, respectively. Experiment 3 featured 2-, 4-, and 8-s exposures with hearing tested at 4.5 kHz. For experiment 2, there were no significant differences between control and exposure sessions. For experiments 1 and 3, exposures with SEL=197 dB re 1 microPa2 s and SEL > or = 195 dB re 1 microPa2 s, respectively, resulted in significantly higher TTS4 than control sessions. For experiment 3 at SEL= 195 dB re 1 microPa2 s, the mean TTS4 was 2.8 dB. These data are consistent with prior studies of TTS in dolphins exposed to pure tones and octave band noise and suggest that a SEL of 195 dB re 1 microPa2 s is a reasonable threshold for the onset of TTS in dolphins and white whales exposed to midfrequency tones.     

Where is the shot noise of a quantum point contact?

Reznikov et al. [Phys. Rev. Lett. 75, 3340 (1995)]] have presented definitive observations of nonequilibrium noise in a quantum point contact. Especially puzzling is the "anomalous" peak structure of the excess noise measured at constant current; to date it remains unexplained. We show that their experiment directly reveals the deep link between conservation principles in the electron gas and its low-dimensional, mesoscopic behavior. The keys to that connection are gauge invariance and the compressibility sum rule. These are central not only to the experiment of Reznikov et al., but to the very nature of all mesoscopic transport.     

Phenolic glycosides from the stem bark of Albizzia julibrissin

From the stem bark of Albizzia julibrissin DURAZZ (Leguminosae), two new phenolic glycosides (albibrissinosides A and B) were isolated. Their structures were determined by spectroscopic analysis. The albibrissinoside B was found to be a radical scavenger on the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical.