0000000000085524
AUTHOR
Manuel Barranco
Collective excitations of 3He clusters
Collective excitations of3He clusters are studied by treating the cluster as a quantum liquid drop. We have used the Random-Phase Approximation sum rules technique within a Density Functional Formalism. Results forL=2 to 10 surface modes and theL=0 volume mode are presented.
The response of metal clusters toq- andL-dependent external fields
We have calculated the static polarizability and mean excitation energy of metal clusters submitted toq-andL-dependent external fields ofjL(qr)YL0(Ω) type. Use has been made of an Extended Random-Phase Approximation which includes exchange and correlation effects within a local model, and of the spherical jellium model to describe the neutralizing positive background.
Dipole surface plasmon in K+N clusters
Abstract The technique of sum rules has been used to investigate the dipole surface plasmon for K + N clusters within a Density Functional Theory and the spherical jellium model. The role played by non-local effects is discussed comparing the results obtained from different functionals. Band-structure and core-polarization effects have been phenomenologically included in the calculation by means of an electron effective mass and a dielectric constant. Comparison with recent experimental data is presented.
Shell structure in mixed3He−4Hedroplets
Due to the immiscibility of ${}^{3}\mathrm{He}$ into ${}^{4}\mathrm{He}$ at very low temperatures, mixed helium droplets consist of a core of ${}^{4}\mathrm{He}$ atoms coated by a ${}^{3}\mathrm{He}$ layer whose thickness depends on the number of atoms of each isotope. When these numbers are such that the centrifugal kinetic energy of the ${}^{3}\mathrm{He}$ atoms is small and can be considered as a perturbation to the mean-field energy, a novel shell structure arises, with magic numbers different from these of pure ${}^{3}\mathrm{He}$ droplets. If the outermost shell is not completely filled, the valence atoms align their spins up to the maximum value allowed by the Pauli principle.
Mg impurity in helium droplets.
Within the diffusion Monte Carlo approach, we have determined the structure of isotopically pure and mixed helium droplets doped with one magnesium atom. For pure 4He clusters, our results confirm those of M. Mella et al. [J. Chem. Phys. 123, 054328 (2005)] that the impurity experiences a transition from a surface to a bulk location as the number of helium atoms in the droplet increases. Contrarily, for pure 3He clusters Mg resides in the bulk of the droplet due to the smaller surface tension of this isotope. Results for mixed droplets are presented. We have also obtained the absorption spectrum of Mg around the 3s3p 1P_1 <-- 3s2 1S_0 transition.
COLLECTIVE SPIN EXCITATIONS OF ALKALI-METAL CLUSTERS
The response function of alkali-metal clusters, modeled as jellium spheres, to dipole (L=1) and quadrupole (L=2) spin-dependent fields is obtained within the time-dependent local-spin-density approximation of density-functional theory. We predict the existence of low-energy spin modes of surface type, which are identified from the strength function. Their collectivity and evolution with size are discussed.
Structure and energetics of mixed4He-3He drops
Using a finite-range density functional, we have investigated the energetics and structural features of mixed helium clusters. The possibility of doping the cluster with a molecule of sulfur hexafluoride is also considered. It is seen that the repulsion introduced by the impurity strongly modifies the properties of the smallest drops. Although only a qualitative comparison is possible, the gross features displayed by our calculations are in agreement with recent experimental findings. {copyright} {ital 1997} {ital The American Physical Society}
Multipole response of $^3$He clusters
Ground state properties of normal 3He drops have been studied using either a correlated wave function in conjunction with a realistic potential of Aziz type1) or a mean-field description based on an effective potential 2,3). In general, an overall good agreement between both methods has been found. The second one has the advantage of being rather easily applicable to both static and dynamic calculations, although being less fundamental than the first one. In this work we are concerned with the description of the collective modes of normal 3He drops within the self-consistent Random-Phase Approximation (RPA), in which the same effective interaction is used to generate both the mean-field and…
Toward a density functional description of liquid pH2.
A finite-temperature density functional approach to describe the properties of parahydrogen in the liquid-vapor coexistence region is presented. The first proposed functional is zero-range, where the density-gradient term is adjusted so as to reproduce the surface tension of the liquid-vapor interface at low temperature. The second functional is finite-range and, while it is fitted to reproduce bulk pH2 properties only, it is shown to yield surface properties in good agreement with experiments. These functionals are used to study the surface thickness of the liquid-vapor interface, the wetting transition of parahydrogen on a planar Rb model surface, and homogeneous cavitation in bulk liquid…
Bulk-plasmon dispersion relations in metals
En utilisant une technique de la regle de somme dans l'approximation de la phase aleatoire etendue, on examine la relation de dispersion des plasmons en volume, en introduisant les effets de la correlation et de l'echange dans le modele du jellium. Les resultats obtenus sont compares aux resultats experimentaux. On souligne le role cle que jouent les effets de la correlation et de l'echange dans l'amelioration de l'accord entre la theorie et l'experience. On calcule egalement la polarisabilite statique en fonction de 9. Les formules peuvent etre facilement modifiees pour incorporer les effets de la structure de bandes (a travers une masse effective electronique intrabande) et les effets de …
Cavitation in 3He-4he Liquid Mixtures
Phase transitions under equilibrium conditions are experimentally well determined and take place in the coexistence regime. However, phase transitions do not always occur under equilibrium conditions. As the new phase forms, the free energy of the system is lowered, but the original phase can be held in a metastable state close to the equilibrium transition point. Although they are internally stable, in each case there exists another configuration that has a lower thermodynamical potential. The metastable state is separated from the stable state by a thermodynamic barrier. Due to statistical fluctuations in density or concentration, this barrier can be overcome as the result of the formatio…
A density functional for liquid3He
We present a density functional for the description of liquid3He properties at zero temperature in a mean field approximation. Its basic ingredients are a zero-range, particle- and spin-density dependent effective interaction of Skyrme type, and a long-range effective interaction of Lennard-Jones type supplemented with a weighted density approximation similar to the one used in the study of classical fluids, to phenomenologically account for short range correlations. After fixing the value of its parameters, the functional yields a good description of the equation of state and Landau parameters (spin symmetric and spin antisymmetric as well) from saturation to solidification densities. The …
Ca impurity in small mixed He-4-He-3 clusters
The structure of small mixed helium clusters doped with one calcium atom has been determined within the diffusion Monte Carlo framework. The results show that the calcium atom sits at the He-4-He-3 interface. This is in agreement with previous studies, both experimental and theoretical, performed for large clusters. A comparison between the results obtained for the largest cluster we have considered for each isotope shows a clear tendency of the Ca atom to reside in a deep dimple at the surface of the cluster for He-4 clusters, and to become fully solvated for He-3 clusters. We have calculated the absorption spectrum of Ca around the 4s4p <- 4s(2) transition and have found that it is bluesh…
Thermal nucleation of cavities in liquid helium at negative pressures
We have investigated the nucleation rate at which cavities are formed in $^{4}\mathrm{He}$ and $^{3}\mathrm{He}$ at negative pressures due to thermal fluctuations. To this end, we have used a density functional that reproduces the He liquid-gas interface along the coexistence line. The inclusion of thermal effects in the calculation of the barrier against nucleation results in a sizable decrease of the absolute value of the tensile strength above 1.5 K.
Cavitation of electron bubbles in liquid parahydrogen
Within a finite-temperature density functional approach, we have investigated the structure of electron bubbles in liquid parahydrogen below the saturated vapour pressure, determining the critical pressure at which electron bubbles explode as a function of temperature. The electron-parahydrogen interaction has been modelled by a Hartree-type local potential fitted to the experimental value of the conduction band-edge for a delocalized electron in pH(2). We have found that the pressure for bubble explosion is, in absolute value, about a factor of two smaller than that of the homogeneous cavitation pressure in the liquid. Comparison with the results obtained within the capillary model shows t…
Nucleation and cavitation in parahydrogen
We have used a density functional approach to investigate thermal homogeneous nucleation and cavitation in parahydrogen. The effect of electrons as seeds of heterogeneous cavitation in liquid parahydrogen is also discussed within the capillary model. (C) 2011 Elsevier B.V. All rights reserved.
High temperature giant dipole and isoscalar resonances
We present a systematic study of the Giant Dipole Resonance (GDR) at high temperatures (T≧4 MeV) in the framework of a semiclassical approximation that uses them1 andm3 RPA sum rules to estimate the GDR mean energy. We focus on the evolution withT of the collective nature of the GDR and of theL=0, 2, 3 and 4 isoscalar resonances. We find that the GDR remains particularly collective at highT, suggesting that it might be possible to observe it experimentally even at temperatures close to the maximum one a nucleus can sustain.
Dipole surface plasmon in large K N + clusters
The dipole surface plasmon forK N + clusters is analyzed using the RPA sum-rule technique within a semiclassical Density Functional Theory and the spherical jellium model. The theoretical frequencies are blue shifted as compared to the experimental ones. The discrepancies between theory and experiment are reduced when considering non-local energy contributions in the density functional and phenomenologically including atomic lattice effects by means of an electron effective mass and a static dielectric constant.
Elementary excitations in superfluidH3e-H4emixtures
We have studied the dynamic structure function of superfluid $^{3}\text{H}\text{e-}^{4}\text{H}\text{e}$ mixtures at zero temperature as a function of pressure and $^{3}\text{H}\text{e}$ concentration. Results obtained in the full random-phase approximation (RPA) plus density-functional theory and in a generalized Landau-Pomeranchuk approach are presented and compared with experiment. Analytic expressions for several sum rules of the dynamic structure functions have been determined, and have been used to obtain average energies of the collective excitations. In the RPA approach, the dispersion relation of the collective modes shows typical features of level repulsion between zero-soundlike …