Evidence for a Self-Bound Liquid State and the Commensurate-Incommensurate Coexistence in 2D 3^3He on Graphite

We made heat-capacity measurements of two dimensional (2D) $^3$He adsorbed on graphite preplated with monolayer $^4$He in a wide temperature range (0.1 $\leq T \leq$ 80 mK) at densities higher than that for the 4/7 phase (= 6.8 nm$^{-2}$). In the density range of 6.8 $\leq \rho \leq$ 8.1 nm$^{-2}$, the 4/7 phase is stable against additional $^3$He atoms up to 20% and they are promoted into the third layer. We found evidence that such promoted atoms form a self-bound 2D Fermi liquid with an approximate density of 1 nm$^{-2}$ from the measured density dependence of the $\gamma$-coefficient of heat capacity. We also show evidence for the first-order transition between the commensurate 4/7 phase and the ferromagnetic incommensurate phase in the second layer in the density range of 8.1 $\leq \rho \leq$ 9.5 nm$^{-2}$.Comment: 6 pages, 4 figure

Structural and dynamical properties of superfluid helium: a density functional approach

We present a novel density functional for liquid 4He, properly accounting for the static response function and the phonon-roton dispersion in the uniform liquid. The functional is used to study both structural and dynamical properties of superfluid helium in various geometries. The equilibrium properties of the free surface, droplets and films at zero temperature are calculated. Our predictions agree closely to the results of ab initio Monte Carlo calculations, when available. The introduction of a phenomenological velocity dependent interaction, which accounts for backflow effects, is discussed. The spectrum of the elementary excitations of the free surface and films is studied.Comment: 37 pages, REVTeX 3.0, figures on request at [email protected]

Boson gas in a periodic array of tubes

We report the thermodynamic properties of an ideal boson gas confined in an infinite periodic array of channels modeled by two, mutually perpendicular, Kronig-Penney delta-potentials. The particle's motion is hindered in the x-y directions, allowing tunneling of particles through the walls, while no confinement along the z direction is considered. It is shown that there exists a finite Bose- Einstein condensation (BEC) critical temperature Tc that decreases monotonically from the 3D ideal boson gas (IBG) value $T_{0}$ as the strength of confinement $P_{0}$ is increased while keeping the channel's cross section, $a_{x}a_{y}$ constant. In contrast, Tc is a non-monotonic function of the cross-section area for fixed $P_{0}$. In addition to the BEC cusp, the specific heat exhibits a set of maxima and minima. The minimum located at the highest temperature is a clear signal of the confinement effect which occurs when the boson wavelength is twice the cross-section side size. This confinement is amplified when the wall strength is increased until a dimensional crossover from 3D to 1D is produced. Some of these features in the specific heat obtained from this simple model can be related, qualitatively, to at least two different experimental situations: $^4$He adsorbed within the interstitial channels of a bundle of carbon nanotubes and superconductor-multistrand-wires Nb$_{3}$Sn.Comment: 9 pages, 10 figures, submitte

Dynamics of liquid He-4 in confined geometries from Time-Dependent Density Functional calculations

We present numerical results obtained from Time-Dependent Density Functional calculations of the dynamics of liquid He-4 in different environments characterized by geometrical confinement. The time-dependent density profile and velocity field of He-4 are obtained by means of direct numerical integration of the non-linear Schrodinger equation associated with a phenomenological energy functional which describes accurately both the static and dynamic properties of bulk liquid He-4. Our implementation allows for a general solution in 3-D (i.e. no symmetries are assumed in order to simplify the calculations). We apply our method to study the real-time dynamics of pure and alkali-doped clusters, of a monolayer film on a weakly attractive surface and a nano-droplet spreading on a solid surface.Comment: q 1 tex file + 9 Ps figure

MEASUREMENTS OF THE ATTENUATION OF FOURTH SOUND IN HELIUM II

L'atténuation du quatrième son a été mesuré dans la gamme de température allant de 0,8 K à Tλ sous pression de vapeur saturante dans une cavité torique remplie de poudre. Entre 1,3 K et Tλ l'atténuation est due au mouvement de la composante normale de l'hélium II. Au dessous de 1,8 K la limitation du libre parcours des phonons par les parois des capillaires joue un rôle prédominant.The attenuation of fourth sound in He II has been measured in the temperature region 0.8 < T < Tλ under saturated vapor pressure in an annular cavity filled with powder. In the temperature range between 1.3 and Tλ the attenuation is mainly caused by the slipping of the normal fluid component of He II. For T ≤ 1.8 K the limitation of the free paths of the phonons by the walls of the capillaries plays the dominant role

INELASTIC NEUTRON SCATTERING OF WATER AND ICE IN POROUS SOLIDS

L'arrangement intermoléculaire de l'eau à 80 K dans les gels de titane, les zéolites et le chlorhydrate d'alumine a été étudié par la diffusion inélastique des neutrons. Les mesures effectuées de 35 jusqu'à 250 meV, comprennent les fréquences des libration et déformation des molécules. Les spectres ont mis en évidence des différences avec la glace qui augmentent quand la taille des pores de l'adsorbat diminue. Ces effets sont attribués à une structure de l'eau moins organisée que celle de la glace et dans laquelle les liaisons hydrogène sont moins nombreuses.The intermolecular structure of sorbed water and its dependence on the surface and porous properties of the adsorbent has been studied by incoherent inelastic neutron scattering measurements of water at 80K in titania gels, Zeolite-A and aluminium chlorohydrate. An energy transfer range from ~35 to ~250 meV was covered which included the frequences of molecular librations and deformation mode. Spectra showed an increasing departure from that of bulk ice as the pore size and water uptake was reduced. These changes can be tentatively ascribed to a less ordered water structure where hydrogen bonding is restricted compared to that in bulk ice