Volume 217, Issue 8

A short review is given of recent progress in understanding the demixing transition in a mesoscopic gaseous cloud of bosonic and fermionic alkali atoms under the effect of their mutual repulsions at zero temperature. The mixture is confined inside a magnetic trap exerting external harmonic potentials on the atoms and the bosonic component is taken in a Bose-Einstein condensed state while the fermions are in a spin-polarized state. Main emphasis is placed on the role of the confinement: in a mesoscopic system (i) the phase transition becomes smooth as a function of the driving parameter, which is the boson-fermion s -wave scattering length; and (ii) various configurations of the phase-separated gaseous cloud are allowed, including some very exotic ones in an anisotropic trap. The transition inside a spherical trap leads to an egg-type structure, with the bosonic condensate being surrounded by a shell of fermions, and is best identified from a sharp upturn of the frequency of low-lying monopolar and dipolar oscillations of fermionic character. This dynamical signature of the transition appears when the overlap between the two components has already started to decrease, but is still quite appreciable. A system of specific experimental interest is the mixture of 6 Li and 7 Li isotopes, whose parameters could be driven towards the phase transition.

Large crystallites of high purity zeolite rho have been synthesized by controlled monitoring of the aging and heating period of the mother gel. The microwave conductivity of Rb-rho samples doped with up to 20Rb atoms p.u.c. was measured over the temperature range 15–300K. Confirmation that the conductivity was due to rubidium doped zeolite rho and not decomposition products or residual rubidium metal was provided by XRD, TEM, and microwave loss measurements on zeolite supported silver particles, the latter providing the first experimental confirmation of the validity of the quasistatic analysis for highly conducting particles. At low concentrations of rubidium dopant the observed microwave responses were dominated by dielectric effects. In the sample Rb 17 /Rb-rho, a strongly temperature-dependent electronic contribution to the conductivity was observed above ~150K. In Rb 20 /Rb-rho, conductivities in the range 1.5–2.3 Sm −1 were observed between 15 and 300K. The residual conductivity at 15K indicates that the sample is indeed metallic, but the values of conductivity measured are low in comparison to conventional metals.

Over a wide range of densities we have systematically investigated the single particle motion in liquid sodium in a series of quasielastic neutron scattering experiments. Our experimental results are in excellent agreement with predictions from molecular dynamics simulations and Mode-Coupling-Theory. The results allow to determine the microscopic mechanisms that control the single particle motion on atomic time- and length- scale and to relate these mechanisms to the values of the macroscopic diffusion coefficient at different densities. We can clearly distinguish three different density regimes where respectively three different coupling mechanisms between diffusive motion and collective modes dominate: At high density, particle diffusion is hindered and the main contribution to forward motion of a particle comes from a coupling to longitudinal density modes; however, at slightly reduced density the motion of the particle is coupled to transverse shear modes which cause an enhancement of the diffusive motion and hence of the diffusion coefficient. Only at very low density, the system gradually returns to particle dynamics which is controlled by binary collisions as predicted by simple hard sphere theories.

We explore extreme multielectron ionization of (Xe) n molecular clusters resulting in the formation of highly charged Xe k + , k =8–32, ions in ultraintense laser fields (intensity I = 10 16 –10 19 W cm −2 ), which is driven by a compound, sequential-parallel, inner-outer ionization mechanism. A computational and theoretical study is advanced for the three fundamental processes of electron fs dynamics, which involve the barrier suppression of inner ionization of the constituents, the formation of an energetic electron-positive ion charged plasma within the cluster and the outer ionization of unbound electrons from the cluster. New features of the formation, characteristics, response and dynamics of the electron-positive ion charged plasma in molecular clusters in ultraintense laser fields were explored, providing novel information on a transient (1–100 fs) metallic state in finite chemical systems.

The chemical potential of metal atoms in isolated clusters has been investigated in dependence of cluster size by means of molecular beam experiments. For that purpose the heat of condensation, which is released during a cluster deposition experiment, is measured with novel microcalorimeters. The experimentally determined heat of condensation enables one to determine the formation enthalpies of the isolated particles. Additional temperature dependent measurements gives insight in the melting behaviour of the isolated systems. The investigations allow one to discuss the size-dependence of the chemical potential of metal atoms in small particles in comparison to predictions on the basis of a simple thermodynamic droplet model.

Deflections of semiconductor cluster beams in an inhomogeneous electric field deliver enhanced polarizabilities for cold isolated Ga N As M clusters with odd number of atoms n = N + M up to n =17. One speculated that the high polarizabilities result from their electronic open shell structure. This was qualitatively confirmed by optical absorption cross section measurements, but the enhancement of polarizability is too large to be explained through a Kramers–Kronig-relation as purely electronic effect. Moreover, quantum chemical calculations gave permanent dipole moments of some tenths of a Debye for certain compositions. The clusters are too heavy to identify these dipole moments by beam broadening effects, but we will show in the present analytic investigation within classical top theory, that a slight reversible-adiabatic alignment of the clusters dipole moment in the electric field increases the average beam deflection and the derived polarizability values. This effect was up to now not taken into account and it offers a consistent explanation of the Ga N As M polarizabilities by small permanent dipole moments of some tenths of a Debye and a reasonable rotational temperature between 2 and 12K.

Strong collision falloff curves for barrierless recombination reactions at low temperatures are calculated taking into account transitional modes only. Specific rate constants k ( E , J ) from statistical adiabatic channel/classical trajectory (SACM/CT) calculations are used. Broadening factors of the falloff curves are found to depend only weakly on the temperature. A systematic analysis of the influence of the centrifugal barriers E 0 ( J ), which are governed by the potential energy surface of the bond energies E 0 , and of the number of transitional modes is made. Guidelines for estimating center broadening factors and shape functions for the broadening factors are given.

X-ray diffraction, small angle X-ray scattering and preliminary inelastic X-ray scattering measurements for a metallic fluid up to the supercritical region have been carried out using synchrotron radiation at SPring-8. We obtained the structure factor, S ( k ), and the pair distribution functions, g ( r ), for expanded fluid Hg from the liquid to the dense vapour region by X-ray diffraction measurements. Change of the local structure in the metal–insulator transition in fluid Hg is discussed. To obtain information on the density fluctuation in fluid Hg near the critical point, small angle X-ray scattering measurements were carried out at SPring-8. In addition to the static structures, the dynamic structure factor of expanded fluid Hg was obtained for the first time using inelastic X-ray scattering technique newly established at a synchrotron radiation facility of the third generation source. Preliminary results of the first experiment are presented.