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A P Mills

Publications and source records attributed to A P Mills.

13 recordsLinked to original sources
PubMed2005

Experiments with a high-density positronium gas.

We have created a high-density gas of interacting positronium (Ps) atoms by irradiating a thin film of nanoporous silica with intense positron bursts and measured the Ps lifetime using a new single-shot technique. When the positrons were compressed to 3.3 x 10(10) cm-2, the apparent intensity of the orthopositronium lifetime component was found to decrease by 33%. We believe this is due to a combination of spin exchange quenching and PS2 molecule formation associated with colliding pairs of oppositely polarized triplet positronium atoms. Our data imply an effective cross section for this process of 2.9 x 10(-14) cm-2.

Journal Article↗
PubMed2005

Strongly enhanced hole-phonon coupling in the metallic state of the dilute two-dimensional hole gas.

We have studied the temperature dependent phonon emission rate P(T) of a strongly interacting (r(s) > or =22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that P(T) changes from P(T) approximately T5 to P(T) approximately T7 above 100 mK, indicating a crossover from screened piezoelectric (PZ) coupling to screened deformation potential (DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is almost 20 times stronger than expected from theory. The density dependence of P(T) shows that it is easier to cool low-density 2D holes in GaAs than higher density 2D hole systems.

Journal Article↗
PubMed2004

Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime.

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.

Journal Article↗
PubMed2003

DNA fuel for free-running nanomachines.

We describe kinetic control of DNA hybridization: loop complexes are used to inhibit the hybridization of complementary oligonucleotides; rationally designed DNA catalysts are shown to be effective in promoting their hybridization. This is the basis of a strategy for using DNA as a fuel to drive free-running artificial molecular machines.

Base Sequence↗
PubMed2002

Quantum antidot formation and correlation to optical shift of gold nanoparticles embedded in MgO.

Quantum antidots are subnanometer scale vacancy clusters, the localized electronic structure of which can significantly alter the properties of a nanomaterial. We use positron spectroscopy to study vacancy clusters generated during the formation of gold nanoparticles via ion implantation in an MgO matrix. We observed that quantum antidots are associated with the nanoparticle surfaces after annealing in an O2 atmosphere, but not after annealing in a H2 atmosphere. In the former case, the presence of quantum antidots bound to the gold nanoparticles correlates with the redshift of the gold surface plasmon resonance, thus allowing an explanation for the redshift based on the transfer of electrons away from the metal particles.

Journal Article↗
PubMed2001

St. Vincent and the Grenadines.

Saint Vincent and the Grenadines conjures up images of a yachting haven and a quiet tourists' paradise. The conflicting demands of a growing population, a middle-income economy dominated by fishing, plantation agriculture and tourism, and environmental and social concerns, all contribute to marine stress on the limited, precious, but internationally important resources. While the vision exists to manage effectively coastal and offshore resources, the institutional, financial and social capital to achieve that vision is limited. Development of the fledgling partnerships between local communities, national governmental structures and the international research, government and donor organisations seems the best hope to conserve the environment and coastal livelihoods of the islands.

Animals↗
PubMed2000

A DNA-fuelled molecular machine made of DNA.

Molecular recognition between complementary strands of DNA allows construction on a nanometre length scale. For example, DNA tags may be used to organize the assembly of colloidal particles, and DNA templates can direct the growth of semiconductor nanocrystals and metal wires. As a structural material in its own right, DNA can be used to make ordered static arrays of tiles, linked rings and polyhedra. The construction of active devices is also possible--for example, a nanomechanical switch, whose conformation is changed by inducing a transition in the chirality of the DNA double helix. Melting of chemically modified DNA has been induced by optical absorption, and conformational changes caused by the binding of oligonucleotides or other small groups have been shown to change the enzymatic activity of ribozymes. Here we report the construction of a DNA machine in which the DNA is used not only as a structural material, but also as 'fuel'. The machine, made from three strands of DNA, has the form of a pair of tweezers. It may be closed and opened by addition of auxiliary strands of 'fuel' DNA; each cycle produces a duplex DNA waste product.

DNA↗
PubMed1999

Article for analog vector algebra computation.

We introduce the concept of an analog neural network represented by chemical operations performed on strands of DNA. This new type of DNA computing has the advantage that it should be fault tolerant and thus more immune to DNA hybridization errors than a Boolean DNA computer. We describe a particular set of DNA operations to effect the interconversion of electrical and DNA data and to represent the Hopfield associative memory and the feed-forward neural network of Rumelhart et al. We speculate that networks containing as many as 10(9) neurons might be feasible.

Animals↗
PubMed1997

Out-of-hours weekend scintigraphy: assessing/predicting the need.

We have assessed the potential impact of a regular half-day session on Saturday only, or Sunday only, and compared this with a whole weekend on-call service for lung scans. We predicted the effect of these services using the data gathered over 2 years (1992-94), looking at the results of lung scans and admission and discharge of patients. The on-call service in all three cases would be justified if resources from the savings on patient discharge and bed availability could be earmarked for the nuclear medicine service. The cost of introducing such an on-call service for the department would be Pounds 3000 per year per session at the weekend and up to Pounds 10,000 per year for a full weekend on-call service. The total cost to the hospital would be negligible.

Costs and Cost Analysis↗
PubMed1974

Comparative sensitivity and resistance of some strains of Pseudomonas aeruginosa and Pseudomonas stutzeri to antibacterial agents.

A comparison has been made of the sensitivities to various antibiotic and non-antibiotic substances of some strains of Pseudomonas aeruginosa and P. stutzeri, the latter including strains isolated from eye and other cosmetic products and from other sources. Whereas P. aeruginosa strains showed a high resistance to cetrimide and to benzalkonium chloride, the P. stutzeri strains were generally more sensitive to these and to chlorhexidine. The P. stutzeri strains were also more sensitive to the various antibiotics tested. The loss of the ability to transfer an R factor by two strains of P. aeruginosa caused no significant change in their drug sensitivity pattern.

Alkanes↗