Al0.48In0.52As/Ga0.47In0.53As resonant tunnelling diodes with large current peak/valley ratio

Resonant tunnelling diodes consisting of an MBE-grown AlInAs barrier on each side of a lattice-matched GaInAs well have been fabricated. Current peak/valley ratios up to 7.1 and 39 have been measured at 300 K and 77 K, respectively. These represent the largest values reported to date in any double-barrier, single-well device. The enhanced peak/valley ratio has been attributed to thick barriers (72 Å) and wide, undoped GaInAs spacer layers (400 Å)     

A conservative bound on the estimation error covariance matrix in the presence of correlated driving noise and correlated discrete measurement noise

In an earlier paper, the conservative and minimal bound to the crosscorrelation terms between estimation error and a random forcing function was presented. That bound was found to be a particular linear combination of the estimation error covariance and the forcing function covariance involving a free scalar parameter. The bound was then substituted for the cross-correlation terms in the differential equation for the estimation error covariance matrix in order to approximate its behavior between discrete measurement times. The time history of the free parameter which minimized a linear combination of the elements of the estimated covariance matrix at the next measurement time was found as the noniterative solution to an optimal control problem with a matrix state.In this paper, necessary and sufficient conditions are presented for the problem of minimizing a linear combination of the elements of the approximated estimation error covariance at the end of an interval in which are linearly incorporated a finite number of discrete vector measurements corrupted by white and/or correlated measurement noise. Although the determination of the optimal trajectory in general requires iteration, a particularly simple algorithm is presented. Numerical results are presented for the case of a satellite in a highly elliptic orbit about a model Earth.     

Frequency-dependent ultrasonic differentiation of normal and diffusely diseased liver

The attenuation coefficient in 38 pathologically graded in vitro liver specimens was measured over a frequency range from 1.25-8 MHz and fitted to the power law model. The attenuation in the normal group (n = 17) exhibited a frequency dependence of the form 0.399f1.139; in the mild disease group (n = 13), it exhibited a dependence of the form 0.395f1.212; and in the moderate/severe disease group (n = 8), it exhibited a dependence of the form 0.391f1.325. Using a Student's t test, it is shown that, due to these differences in the frequency dependence, the statistical significance level at which the null hypothesis regarding the difference between the mean attenuation slopes of any two of these categories is rejected, is a strong function of frequency in the range of 1-4 MHz. The significance level relating to the difference between the normal and moderate/severe disease group is more than one order of magnitude better than the other categories. In all cases, no substantial improvement occurs beyond 4 MHz. It is also shown that attenuation slope values at 3 MHz confirm in vivo literature results obtained via different techniques.     

Efficient power transfer through small apertures

Efficient power transfer through a small slot in a thin conducting screen separating two resonant cavities was experimentally achieved by empirically optimising the natural resonant frequency of the slot and the circuit coupling factors. The design of a power combining circuit based on the use of slot coupled microstrip sources is discussed in terms of an electrical equivalent circuit which confirms that a high power transfer efficiency is possible with such an arrangement.<>     

Synthesis of D- and L-myo-inositol 1,2,4,6-tetrakisphosphate, regioisomers of myo-inositol 1,3,4,5 tetrakisphosphate: activity against Ins(1,4,5)P3 binding proteins

We report here the synthesis of D- and L-myo-inositol 1,2,4,6-tetrakisphosphate 3a and 3b and the racemic modification 3ab. Racemic myo-inositol 1,2,4,6-tetrakisphosphate 3ab was synthesised from DL-1,2,4,6-tetra-O-allyl-myo-inositol 9ab. Benzylation and de-allylation provided the tetraol 11ab, which was phosphitylated in the presence of bis(benzyloxy)diisopropylaminophosphine and 1H-tetrazole, then oxidised to give the fully protected 1,2,4,6-tetrakisphosphate 13ab. Hydrogenolysis of 13ab and purification of product by ion exchange chromatography gave racemic myo-inositol 1,2,4,6-tetrakisphosphate 3ab, which showed no demonstrable agonism or antagonism for Ca2+ release at 200 microM in permeabilised hepatocytes. The chiral derivatives, D-3a and L-myo-inositol 1,2,4,6-tetrakisphosphate 3b were synthesised from 5-O-benzyl-1,4,6-tri-O-p-methoxybenzyl-myo-inositol 19ab, which was resolved using R-(-)-O-acetylmandelic acid providing two diastereoisomers 21 and 22 which were separated and deacylated to give the corresponding enantiomers. Further transformations gave the corresponding chiral 1,2,4,6-tetraols which were phosphitylated, oxidised, deprotected and purified as for the racemic mixture. The enantiomeric tetrakisphosphates 3a and 3b were evaluated for inhibition of the metabolic enzymes inositol 1,4,5-trisphosphate 5-phosphatase and 3-kinase in comparison with the enantiomers of another synthetic regioisomer D- and L-myo-inositol 1,2,4,5-tetrakisphosphate. Both D- and L-myo-inositol 1,2,4,6-tetrakisphosphate inhibit 5-phosphatase with an IC50 value of 3.8 microM and 14 microM, repectively. However, both enantiomers were poorly recognised by the 3-kinase enzyme, with IC50 values greater than 100 microM. The enantiomers of the 1,2,4,5-tetrakisphosphate showed the same relative pattern of activity towards the two enzymes but were more potent against 5-phosphatase (0.47 microM and 3 microM respectively).