Paul E. Sokol

Preparation and synthesis of Ag2Se nanowires produced by template directed synthesis

We have prepared silver selenide nanowires by a novel synthesis technique. The nanowires were produced through template directed synthesis, in which we used a porous alumina membrane as the template. Silver was deposited on one surface and in the pores close to that surface of the template, followed by electro-deposition of selenium into the pores. Silver selenide nanowires of high quality were produced inside the pores.

Study of H2 confined in the highly ordered pores of MCM-48

Abstract We present quasi-elastic neutron scattering measurements of the diffusion of H 2 adsorbed in porous MCM-48, a silica glass, with an ordered pore network. Quasi-elastic scattering of hydrogen adsorbed within the pore network was used to monitor the liquid–solid transitions of the absorbed fluid. Unlike other porous media of similar pore size, where supercooling of the liquid–solid transition by 3–4 K was observed, no suppression of the transition was observed for MCM-48. Furthermore, no hysteresis between freezing and melting is observed, as in other porous media. The diffusion constant for the confined liquid has been extracted from the measurements, and the hydrogen adsorbed in the MCM-48 has a slightly higher diffusion constant than bulk liquid H 2 .

Liquid Crystal Order in a Highly Restrictive Porous Glass

Abstract The thermodynamic and structural properties of octylcyanobiphenyl (8CB) confined to the highly restrictive and randomly interconnected pores of Vycor glass were studied via AC calorimetry, DSC, and small angle neutron scattering. The weakly first order nematic to isotropic phase transition is absent and replaced by a continuous evolution of local orientational ordering with decreasing temperature. The results are consistent with predictions from a model that treats the host media as a collection of noninteracting pore segments. The continuous smectic-A to nematic phase transition is also absent with no evidence of any smectic ordering. In addition, crystallization is replaced by a glass-like melting transition with indications of another melting transition at much lower temperatures.

Location of frame overlap choppers on pulsed source instruments

A detailed study has been performed to investigate the effect of frame overlap in a cold neutron chopper spectrometer. The basic spectrometer is defined by two high-speed choppers, one near the moderator to shape the pulse from the moderator, and one near the sample to define energy resolution. Using ray-tracing timing diagrams, we have observed that there are regions along the guide where the trajectories of neutrons with different velocities converge temporally at characteristic points along the spectrometer. At these points of convergence, a frame overlap chopper would be totally ineffective, allowing neutrons of all velocities to pass through. Conversely, at points where trajectories of different velocity neutrons are divergent, a frame overlap chopper is most effective. An analytical model to describe this behaviour has been developed, and leads us to the counterintuitive conclusion that the optimum position for a frame overlap chopper is as close to the initial chopper as possible. We further demonstrate that detailed Monte Carlo simulations produce results which are consistent with this model.

Inelastic neutron scattering of H2 in xerogel

The properties of molecular hydrogen adsorbed in Britesorb were studied through inelastic neutron scattering. We have measured both the rotational energy levels and the momentum distribution at bilayer and nearly full pore fillings. Splitting of the J=1 rotational energy levels is observed for molecular hydrogen adsorbed on the surface, while the rotational properties of the hydrogen adsorbed after monolayer completion is consistent with behavior in the bulk. Additionally, the measurement of the momentum distribution showed that the mean kinetic energy of the molecules in the bilayer is 88 K±7 K. The kinetic energy measured in the nearly full pore was 81 K±6 K, which is consistent with a simple model in which the behavior of the monolayer is dominated by the interaction with the surface of the pore wall but H2 adsorbed after monolayer completion is bulk like.

Smectic Order in a Porous Interconnected Substrate

Abstract Measurement of the specific heat and small angle neutron scattering studies for nCB liquid crystals confined to the 0.05 μm diameter pores of Millipore filters, a fibrous porous substrate, reveal that in spite of the interconnection and restrictive size, smectic layers are formed. The existence of this long range smectic order that increases with decreasing temperature is in contrast to findings in other porous geometries where a smectic phase is either absent or greatly suppressed by the confinement. For 9CB, we also study the possible effects of confinement on the critical behavior at the second order smectic-A to nematic phase transition.

Monte Carlo simulation of instrument response for direct geometry time-of-flight spectrometers

A full Monte Carlo simulation of sample scattering and the final flight path for direct geometry time-of-flight spectrometers has been developed. This allows the scattering from systems with both realistic and complex scattering geometries as well as realistic scattering functions to be modeled. This simulation, PULSCAT, interfaces with commonly available ray tracing programs, such as VITESS, that simulate the incident beam. Spectra with elastic and inelastic features resulting from scattering from isotropic scattering systems in addition to multiple scattering for amorphous scattering systems can be modeled with PULSCAT. The sample geometry used in the simulation is entered through a GUI interface. Due to the large flexibility in the input parameters for the sample, sample environment equipment can be included in the simulations allowing for scattering from ancillary equipment (such as from a standard orange cryostat) to be modeled. This makes PULSCAT a powerful tool for simulating systems and investigating spurious effects present in collected spectra.

Quantum evaporation reveals Bose condensate in superfluid helium

Superfluidity in liquid helium ranks as one of the most fascinating discoveries in physics this century Helium, which liquefies at 4–5 K, behaves as a conventional liquid just below the condensation point, but undergoes a phase transition when the temperature is lowered to about 2.2 K. In this new phase, known as the superfluid phase, both the viscosity and thermal conductivity of the liquid drop to zero.

A timely arrival in neutron scattering

The hydrogen bond occupies a central position in physics, chemistry and biology. It determines the properties of several important materials (water not the least of them), controls the dynamics of phase transitions in ferroelectric materials, and plays a crucial role in the replication of DNA. Therefore a variety of experimental techniques – neutron diffraction, quasi-elastic neutron scattering and inelastic neutron scattering – have been used to study the hydrogen bond.

Bose Condensate in Superfluid 4He and Momentum Distributions by Deep Inelastic Scattering

In 1938 London [1,2] offered an explanation of the observation earlier that year of superfluid behavior in liquid 4He when it is cooled below a critical temperature of 2.17 °K. He argued that the superfluid transition was analogous to the Bose condensation of an (ideal) gas of non-interacting atoms obeying the same Bose-Einstein spin-statistics relation as 4He atoms. This relation requires the many-atom wave function to be completely symmetric in the atomic coordinates, resulting in a preference for the atoms to occupy the same single-particle states. For a finite system of atoms the momenta are quantized in spacings proportional to the inverse of the system size. At high temperatures the fraction of atoms occupying any one of the momentum states also scales as the inverse of the size. However, as the temperature is reduced below a critical Bose condensation temperature a significant fraction of the atoms, independent of the system size, begins to occupy the zero-momentum state. The Bose condensate fraction of an ideal gas approaches one at zero temperature. For 4He, by analogy, at high temperatures in the normal fluid the condensate fraction should be zero, but as temperatures are reduced below the superfluid transition temperature the condensate fraction should rise to a non-zero value. The effect of the strong interactions among the (non-ideal) 4He atoms is to deplete the zero temperature condensate fraction from one in an ideal gas to a value much less than one for 4He. While the analogy between superfluidity and Bose condensation is imperfect, the concept of a Bose condensate in the superfluid phase has survived. A variety of increasingly sophisticated many-body calculations have predicted a condensate fraction of about 10 % at zero temperature in superfluid 4He at SVP. Because of the importance of superfluidity and the related phenomenon of superconductivity to condensed matter physics, this simple prediction has motivated a more than twenty year effort involving up to one hundred scientists to measure the Bose condensate fraction in 4He.