A. P. Mills

Using DNA to Power Nanostructures

DNA hybridization has been used to power a number of DNA-based nanostructures constructed out of DNA. Here some considerations that go into DNA-based motor design are briefly reviewed. The emphasis will be on the operation of toeholds, single-stranded sections of DNA that facilitate the process of strand removal during certain points in the operation of a DNA-based motor. Reaction kinetics measurements for toehold mediated strand exchange are reported. These measurements have served as a guide for choosing toehold lengths.

Positronium molecule formation, Bose–Einstein condensation and stimulated annihilation

Abstract Low energy positron production and storage may be used to make and observe some interesting many-positron, many-electron systems both in vacuum and in the presence of ordinary matter. One of the most interesting possibilities is to make an annihilation photon laser. One might start on the path to this laser by creating a high density positron burst and forming a dense gas of positronium atoms within a cavity in a solid. This will pave the way for subsequently demonstrating the near room temperature Bose–Einstein condensation of a dense gas of positronium and establishing conditions under which stimulated emission of annihilation photons can be observed.

Accumulator for the production of intense positron pulses

An intense pulsed positron source has been developed using a buffer gas trap to accumulate large numbers of positrons and create a dense plasma, which may then be bunched and spatially focused. Areal densities of more than 3×1010e+cm−2 have been achieved in a subnanosecond pulse producing an instantaneous positron current of more than 10mA. We describe various aspects of the device including a detection technique specifically developed for use with intense positron pulses. Two applications are also described as well as future experiments such as the formation of positronium molecules and the positronium Bose-Einstein condensate.

Single shot positron annihilation lifetime spectroscopy

Recent developments in positron trapping technology have made possible experimentation with dense interacting positronium gases. Along with these capabilities comes a need for suitable measurement techniques, and accordingly we have developed a method to measure positronium lifetimes from a single intense burst of positrons. Our method is based on recording the anode signal from a photomultiplier with a fast oscilloscope following a short-time positron burst which allows us to measure transitory effects as well as high density positronium interactions.

Can we measure the gravitational free fall of cold Rydberg state positronium

Abstract In this paper we examine the possibilities for detecting the free fall of Rydberg positronium atoms. In our scheme, cold positronium atoms are emitted from a “point” source and excited to the n=25 circular Rydberg state with L=n−1. The positronium atoms are allowed to travel horizontally 10 m in a field free vacuum and focused onto a detector using an elliptical Van der Waals mirror. A free fall distance of order 50 μm and a few detected atoms per hour are anticipated. Various extraneous influences on the positronium, such as collisions with residual gas atoms, Stark mixing in stray electric and magnetic fields, photoionization due to thermal radiation, and accelerations due to patch potentials are estimated.

Gene expression profiling diagnosis through DNA molecular computation

Gene expression profiling is the characterization of cells based on the level of gene activity represented by concentrations of complementary DNA reverse transcribed from messenger RNA. The spectrum of cDNA concentrations, the expression profile, is determined using a DNA microarray. Although this approach is valuable for research, a simpler scheme that would give answers on a shorter time-scale for clinical applications is needed. An Adleman DNA self-assembly computer that would use cDNA as input might be ideal for clinical cell discrimination and a neural network architecture would be appropriate for making the necessary classifications. Preliminary experimental results suggest that expression profiling should be feasible using a DNA neural network that acts directly on cDNA.

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.

Two-dimensional metal in a parallel magnetic field.

We have investigated the effect of an in-plane parallel magnetic field (B(axially) on two high mobility metallic-like dilute two-dimensional hole gas systems in GaAs quantum wells. The experiments reveal that, while suppressing the magnitude of the low temperature resistance drop, B(axially) does not affect E(a), the characteristic energy scale of the metallic resistance drop. The field B(c) at which the metallic-like resistance drop vanishes is dependent on both the width of the quantum well and the orientation of B(axially). It is unexpected that E(a) is unaffected by B(axially) up to B(c) despite the fact that the Zeeman energy at B(c) is roughly equal to E(a).

Polarization dependence of n=2 positronium transition rates to Stark-split n=30 levels via crossed-beam spectroscopy

We produce Rydberg Ps by a two-step laser excitation from and from to states of principal quantum level that are Stark split by a motionally induced electric field. Our measurements are largely free of first-order Doppler shifts such that we are able to investigate the impact of laser polarization on the population of the closely spaced Stark levels. We find a variation in the distribution that is primarily dependent on the IR laser polarization with respect to the direction of the motionally induced electric field. With the IR light polarized parallel to the electric field F, the ratio of excitation probability to the levels of maximal Stark splitting compared to that of excitation to the states of minimal Stark splitting is found to be 3.37 ± 0.51, whereas with the IR light polarized perpendicular to F, the excitation ratio is 0.87 ± 0.64. Our results agree with those of Wall et al (2015 Phys. Rev. Lett. 114 173001) obtained with n = 11 and will be useful in the preparation of high-n states of Ps for a variety of experiments, including measuring the interaction of Ps with gravity, in precision time-of-flight (TOF) energy spectroscopy, and precision optical spectroscopy of Ps.

Spin-polarization-induced tenfold magnetoresistivity of highly metallic two-dimensional holes in a narrow GaAs quantum well

We observe that an in-plane magnetic field (