Search Results (11 total)

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≃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.

We present neutron scattering measurements of the phonon-roton (P-R) modes of liquid ⁴He at saturated vapor pressure confined in 44  Å mean pore diameter gelsil in the wave vector range 0.4≤Q≤2.15  Å⁻¹. Layer modes, modes which propagate in the liquid layers adjacent to the porous media walls, were also observed at wave vectors in the roton region (Q≃1.95  Å⁻¹) but not at Q≲1.7  Å⁻¹. The first goal is to document the filling dependence of the dynamic response and of the P-R mode energies and widths more systematically than has been done in the past. As the gelsil is filled with ⁴He, the P-R and layer modes are first observed at a fractional filling of f=76% at low temperature (T=0.4  K). At fillings f=76%, the P-R mode energies lie below the bulk superfluid ⁴He values in the wave vector range 0.4≤Q≤1.7  Å⁻¹, especially at Q≃1.1  Å⁻¹, as observed in helium films. As filling is increased, the intensity in the P-R mode increases markedly and the P-R mode energies move toward bulk superfluid values taking bulk values at full filling. The second goal is to determine the temperature dependence of the intensity in the P-R modes in a media in which the superfluid-normal transition temperature T_c=1.92  K is independently known and lies well below the bulk liquid value T_λ=2.17  K. As temperature is increased, the intensity in the P-R and layer modes decreases. However, a well-defined P-R mode is observed at temperatures up to T≃2.15  K, above T_c=1.92  K. Since well-defined modes exist because there is Bose-Einstein condensation (BEC), this suggests that there is BEC above T_c, probably localized. Localized BEC appears to exist up to T≃T_λ.

We present neutron scattering measurements of the atomic momentum distribution n(k) in solid helium under a pressure p=41   bar (molar volume V_m=20.01±0.02  cm³/mol) and at temperatures between 80 and 500 mK. The aim is to determine whether there is Bose-Einstein condensation (BEC) below the critical temperature, T_c=200  mK, where a superfluid density has been observed. Assuming BEC appears as a macroscopic occupation of the k=0 state below T_c, we find a condensate fraction of n₀=(-0.10±1.20)% at T=80  mK and n₀=(0.08±0.78)% at T=120  mK, consistent with zero. The shape of n(k) also does not change on crossing T_c within measurement precision.

We present neutron-scattering measurements of the momentum distribution of liquid ³He-⁴He mixtures. The experiments were performed at wave vectors Q, 26⩽Q⩽29 Å⁻¹, on the MARI time-of-flight spectrometer at the ISIS Facility, Rutherford Appleton Laboratory, a spallation neutron source. Mixtures with ³He concentrations x between 0 and 20% were investigated both in the superfluid and normal phases. From the data, we extract the Bose-Einstein condensate fraction n₀ and the momentum distributions of ³He and ⁴He atoms. We find that n₀ increases somewhat above the pure ⁴He value when ³He is added; e.g., from n₀=(7.25±0.75)% at x=0 to (11±3)% at x=15–20 %. This agrees with predictions but is less than the only previous measurement. We find a ⁴He kinetic energy K₄ for pure ⁴He that agrees with previous determinations. K₄ decreases somewhat with increasing ³He concentration, less than observed previously and found in early calculations but in agreement with a more recent Monte Carlo calculation. The ³He response is not well reproduced by a Fermi-gas momentum distribution, n(k). Rather an n(k) having a small step height at the Fermi surface and a substantial high-momentum tail characteristic of a strongly interacting Fermi liquid provides a good fit. This n(k) is consistent with calculated n(k). Thus agreement between theory and experiment is obtained comparing n(k) in contrast to earlier findings based on comparing calculated and observed ³He kinetic energies.

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. As coverage is increased, additional lines of 1D lattices form along the grooves. This is followed by an incommensurate, 2D monolayer covering the whole nanotube bundle surface. The lattice constants of these 1D and 2D systems are largely independent of filling once a single 1D line is formed. No occupation of the interstitial channels between nanotubes is observed in the present sample.

Neutron scattering measurements of the fundamental excitations of liquid ⁴He confined in 44 Å pore diameter gelsil glass at pressures up to 40 bars in the wave vector range 0.4<Q<2.3  Å^-1 are reported. Above 25.3 bars and at low temperature (T=0.4  K) the characteristic phonon-roton mode of superfluid ⁴He is no longer observed as a well-defined mode in the phonon-maxon region (0.4<Q<1.6  Å^-1). Modes at wave vectors Q>1.6  Å^-1, especially the rotons, are observed up to complete solidification of all the liquid at a pressure of ∼40  bars where the roton vanishes. At and above a pressure of 35.1 bars, Bragg peaks are observed, indicating coexistence of liquid and solid in the pores at pressures 35≲P≲40  bars.

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=1.6  K. From the data, we determine the shape of the momentum distribution, n(k), of atoms in the solid and the leading final state contribution to the scattering. We show that n(k) in this highly anharmonic, quantum solid differs significantly from a Gaussian. The n(k) is more sharply peaked with larger occupation of low momentum states than in a Maxwell-Boltzmann distribution, as found in liquid ⁴He and predicted qualitatively by path integral Monte Carlo calculations. The atomic kinetic energy is 〈K〉=(24.25±0.30)  K. If n(k) is assumed to be Gaussian, as is usually the practice, a 〈K〉 10% smaller is obtained.

We describe the first measurements of the diffusion of broadband single-cycle optical pulses through a highly scattering medium. Using terahertz time-domain spectroscopy, we measure the electric field of a multiply scattered wave with a time resolution shorter than one optical cycle. This time-domain measurement provides information on the statistics of both the amplitude and phase distributions of the diffusive wave. We develop a theoretical description, suitable for broadband radiation, which adequately describes the experimental results.

We describe observations of the amplitude and phase of an electric field diffusing through a three-dimensional random medium, using terahertz time-domain spectroscopy. These measurements are spatially resolved with a resolution smaller than the speckle spot size and temporally resolved with a resolution better than one optical cycle. By computing correlation functions between fields measured at different positions and with different temporal delays, it is possible to obtain information about individual scattering events experienced by the diffusing field. This represents a new method for characterizing a multiply scattered wave.

We report quasielastic neutron scattering measurements performed on H₂ adsorbed on single-walled carbon nanotubes. These measurements indicate that no quasielastic component to the scattering is present below 30 K. A quasielastic component is present at 30 K, indicating the onset of mobility of the adsorbed H₂. This component to the scattering is well described by a liquidlike diffusion model, with a diffusion coefficient an order of magnitude greater than that of bulk liquid H₂. The observed diffusion is consistent with two-dimensional diffusion on Grafoil and indicates that the adsorbed H₂ is leaving the groove sites of the nanotube bundles before diffusing on the outer surface of the bundles.

The concept of the Fresnel zone is central to many areas of imaging. In tomographic imaging, the transverse spatial resolution can be limited by the size of the first Fresnel zone, usually defined only for monochromatic radiation. With the increasing prevalence of broadband tomographic imaging systems, a generalization of this concept is required. Here, a proposed generalization is described in the context of femtosecond optics, and experimentally verified using terahertz time-domain spectroscopy. Based on this definition, a simple zone plate design is demonstrated.