Quantum liquids.

This article is dedicated to Roger A Cowley and his seminal contributions to our understanding of quantum liquids, both liquidHe andHe. Roger Cowley's neutron scattering measurements of the collective and independent particle response of liquidHe were made at Chalk River Laboratories in 1965-74 chiefly with A D B (Dave) Woods. They measured the phonon-roton (P-R) mode energy, intensity and width with new precision.

Determination of Dynamical Heterogeneity from Dynamic Neutron Scattering of Proteins.

Motional displacements of hydrogen (H) in proteins can be measured using incoherent neutron-scattering methods. These displacements can also be calculated numerically using data from molecular dynamics simulations. An enormous amount of data on the average mean-square motional displacement (MSD) of H as a function of protein temperature, hydration, and other conditions has been collected.

Excitations in the quantum liquid He: A review.

Progress made in measuring and interpreting the elementary excitations of superfluid and normal liquid [Formula: see text] in the past 25 years is reviewed. The goal is to bring up to date the data, calculations and our understanding of the excitations since the books and reviews of the early 1990s. Only bulk liquid [Formula: see text] is considered.

Excitations in Quantum Liquids.

Progress made in measuring and interpreting the elementary excitations of superfluid and normal liquid \4he in the past 25 years is reviewed. The goal is to bring up to date the data, calculations and our understanding of the excitations since the books and reviews of the early 1990s. Only bulk liquid \4he is considered.

Anomalous and anisotropic nanoscale diffusion of hydration water molecules in fluid lipid membranes.

We have studied nanoscale diffusion of membrane hydration water in fluid-phase lipid bilayers made of 1,2-dimyristoyl-3-phosphocholine (DMPC) using incoherent quasi-elastic neutron scattering. Dynamics were fit directly in the energy domain using the Fourier transform of a stretched exponential. By using large, 2-dimensional detectors, lateral motions of water molecules and motions perpendicular to the membranes could be studied simultaneously, resulting in 2-dimensional maps of relaxation time, τ, and stretching exponent, β.

Motional displacements in proteins: The origin of wave-vector-dependent values.

The average mean-square displacement, 〈r(2)〉, of H atoms in a protein is frequently determined using incoherent neutron-scattering experiments. 〈r(2)〉 is obtained from the observed elastic incoherent dynamic structure factor, S(i)(Q,ω=0), assuming the form S(i)(Q,ω=0) =exp(-Q(2)〈r(2)〉/3). This is often referred to as the Gaussian approximation (GA) to S(i)(Q,ω=0).

Evidence for a common physical origin of the Landau and BEC theories of superfluidity.

There are two renowned theories of superfluidity in liquid (4)He, quite different and each with specific domains of application. In the first, the Landau theory, superflow follows from the existence of a well-defined collective mode supported by dense liquid (4)He, the phonon-roton mode. In the second, superflow is a manifestation of Bose-Einstein condensation (BEC) and phase coherence in the liquid.

Long-time mean-square displacements in proteins.

We propose a method for obtaining the intrinsic, long-time mean square displacement (MSD) of atoms and molecules in proteins from finite-time molecular dynamics (MD) simulations. Typical data from simulations are limited to times of 1 to 10 ns, and over this time period the calculated MSD continues to increase without a clear limiting value. The proposed method consists of fitting a model to MD simulation-derived values of the incoherent intermediate neutron scattering function, I(inc)(Q,t), for finite times.

Intrinsic mean-square displacements in proteins.

The thermal mean-square displacement (MSD) of hydrogen in proteins and its associated hydration water is measured by neutron scattering experiments and used an indicator of protein function. The observed MSD as currently determined depends on the energy resolution width of the neutron scattering instrument employed. We propose a method for obtaining the intrinsic MSD of H in the proteins, one that is independent of the instrument resolution width.

Vibrational dynamics of hydrogen in proteins.

Biological macromolecules expand with increasing temperature and this dynamic expansion is associated with the onset of function. The expansion is typically characterized by the mean square vibrational displacement (MSD), of specific constituents such as hydrogen within the macromolecules. The increases with increasing temperature and the slope of versus temperature can increase significantly at a temperature T{D} identified as a dynamical transition.

Phonon-roton modes and localized Bose-Einstein condensation in liquid helium under pressure in nanoporous media.

We show, using inelastic neutron scattering, that liquid helium in porous media, two gelsils and MCM-41, supports a phonon-roton mode up to a pressure of 36-37 bars only. Modes having the highest energy ("maxons") broaden and become unobservable at the lowest pressures (p approximately 26 bars) while rotons survive to the highest pressure. By comparing with the superfluid density observed by Yamamoto and co-workers in gelsil, we propose that there is a Bose glass phase containing islands of BEC surrounding the superfluid phase.

Bose-Einstein condensation in solid 4He.

We present neutron scattering measurements of the atomic momentum distribution n(k) in solid helium under a pressure p=41 bar (molar volume Vm=20.01+/-0.02 cm3/mol) and at temperatures between 80 and 500 mK.

One-dimensional and two-dimensional quantum systems on carbon nanotube bundles.

We report the first measurement of the structure of 4He atoms adsorbed on bundles of single-walled carbon nanotubes. Neutron diffraction techniques and nanotube samples closed at the end were used. At low coverage, 4He forms a 1D, single line lattice along the grooves between two nanotubes on the surface of the nanotube bundles.

Excitations of metastable superfluid 4He at pressures up to 40 bars.

Neutron scattering measurements of the fundamental excitations of liquid 4He confined in 44 A pore diameter gelsil glass at pressures up to 40 bars in the wave vector range 0.4 View Article and Find Full Text PDF

Quantum momentum distribution and kinetic energy in solid 4He.

We present measurements of neutron scattering from solid 4He at high momentum transfer. The solid is held close to the melting line at molar volume 20.87 cm3/mol and temperature T=1.

Phonon-roton excitations in liquid 4He at negative pressures.

We present neutron scattering measurements of the phonon-roton excitations of superfluid 4He held at negative pressures from zero to -5 bar. The liquid was stretched to negative pressures by immersing it in the porous medium MCM-41. In the wave vector range 0.

Dynamics of quantum liquids in confinement, theory and experiment.

Liquid (4)He immersed in porous media such as aerogel, Vycor, and Geltech silica are excellent examples of bosons in disorder and confinement. Of special interest is the impact of disorder on Bose-Einstein condensation (BEC), on the elementary excitations of the superfluid and on their connection to the superfluid properties. Indeed, the modifications induced by disorder can be used to reveal the interdependence of BEC, the excitations and superfluidity.

Phonons, rotons, and layer modes of liquid 4He in aerogel.

We report the first observation of two-dimensional layer modes in both fully filled and partially filled aerogel. Using complementary high-energy resolution and high statistical precision neutron scattering instruments, and two different 87% porous aerogel samples, we show that the three-dimensional (3D) phonon-roton excitation energies and lifetimes of liquid 4He in aerogel are the same as in bulk 4He within current precision. The layer modes are the excitations that distinguish aerogel from the bulk rather than a difference in the 3D roton energy.

Dynamics of liquid 4He in vycor.

Using inelastic neutron scattering, we have observed well-defined phonon-roton ( p-r) excitations in superfluid 4He in Vycor over a wide wave-vector range, 0.3 View Article and Find Full Text PDF