Spinning superfluid helium-4 nanodroplets

We have studied spinning superfluid $^4$He nanodroplets at zero temperature using Density Functional theory. Due to the irrotational character of the superfluid flow, the shapes of the spinning nanodroplets are very different from those of a viscous normal fluid drop in steady rotation. We show that when vortices are nucleated inside the superfluid droplets, their morphology, which evolves from axisymmetric oblate to triaxial prolate to two-lobed shapes, is in good agreement with experiments. The presence of vortex arrays confers to the superfluid droplets the rigid-body behavior of a normal fluid in steady rotation, and this is the ultimate reason of the surprising good agreement between recent experiments and the classical models used for their description.Comment: 5 pages, 3 figur

Vortex arrays in nanoscopic superfluid helium droplets

We have studied the appearance of vortex arrays in a rotating helium-4 nanodroplet at zero temperature within density functional theory. Our results are compared with those for classical rotating fluid drops used to analyze the shape and vorticity in recent experiments [L.F. Gomez et al., Science 345, 906 (2014)], where vortices have been directly seen in superfluid droplets for the first time. In agreement with the experiments, we have found that the shape of the droplet changes from pseudo-spheroid, oblate-like for a small number of vortices to a peculiar "wheel-like" shape, delimited by nearly flat upper and lower surfaces, when the number of vortices is large. Also in agreement with the experiments, we have found that the droplet remains stable well above the stability limit predicted by classical theories.Comment: 5 pages, 5 figure

The onset of nanoscale dissipation in superfluid He-4 at zero temperature: the role of vortex shedding and cavitation

Two-dimensional flow past an infinitely long cylinder of nanoscopic radius in superfluid He-4 at zero temperature is studied by time-dependent density functional theory. The calculations reveal two distinct critical phenomena for the onset of dissipation: 1) vortex-antivortex pair shedding from the periphery of the moving cylinder and 2) appearance of cavitation in the wake, which possesses similar geometry as observed experimentally for fast moving micrometer-scale particles in superfluid He-4. Vortex pairs with the same circulation are occasionally emitted in the form of dimers, which constitute the building blocks for the Benard-von Karman vortex street structure observed in classical turbulent fluids and Bose-Einstein condensates. The cavitation induced dissipation mechanism should be common to all superfluids that are self-bound and have a finite surface tension, which include the recently discovered self-bound droplets in ultracold Bose gases.Comment: 5 pages, 6 figure

Vertically coupled quantum dots in the local spin-density functional theory

We have investigated the structure of double quantum dots vertically coupled at zero magnetic field within local spin-density functional theory. The dots are identical and have a finite width, and the whole system is axially symmetric. We first discuss the effect of thickness on the addition spectrum of one single dot. Next, we describe the structure of coupled dots as a function of the interdot distance for different electron numbers. Addition spectra, Hund's rule and molecular-type configurations are discussed. It is shown that self-interaction corrections to the density functional results do not play a very important role in the calculated addition spectra.Comment: Typeset using Revtex, 14 pages and 12 Postscript figures, to be published in Phys. Rev.

Far-infrared spectroscopy of nanoscopic InAs rings

We have employed time-dependent local-spin density theory to analyze the far-infrared transmission spectrum of InAs self-assembled nano-rings recently reported [A. Lorke et al, cond-mat/9908263 (1999)]. The overall agreement between theory and experiment is good, which on the one hand confirms that the experimental peaks indeed reflect the ring-like structure of the sample, and on the other hand, asseses the suitability of the theoretical method to describe such small nanostructures. The addition energies of one- and two-electron rings are also reported and compared with the corresponding capacitance spectra.Comment: Typeset using Revtex, 7 pages and 8 Postscript figure

Wave-vector dependence of spin and density multipole excitations in quantum dots

We have employed time-dependent local-spin density functional theory to analyze the multipole spin and charge density excitations in GaAs-AlGaAs quantum dots. The on-plane transferred momentum degree of freedom has been taken into account, and the wave-vector dependence of the excitations is discussed. In agreement with previous experiments, we have found that the energies of these modes do not depend on the transferred wave-vector, although their intensities do. Comparison with a recent resonant Raman scattering experiment [C. Sch\"uller et al, Phys. Rev. Lett {\bf 80}, 2673 (1998)] is made. This allows to identify the angular momentum of several of the observed modes as well as to reproduce their energies.Comment: 14 pages in REVTEX and 14 postscript figure

Wave-vector dependence of spin and density multipole excitations in quantum dots

We have employed time-dependent local-spin density functional theory to analyze the multipole spin and charge density excitations in GaAs-AlGaAs quantum dots. The on-plane transferred momentum degree of freedom has been taken into account, and the wave-vector dependence of the excitations is discussed. In agreement with previous experiments, we have found that the energies of these modes do not depend on the transferred wave-vector, although their intensities do. Comparison with a recent resonant Raman scattering experiment [C. Sch\"uller et al, Phys. Rev. Lett {\bf 80}, 2673 (1998)] is made. This allows to identify the angular momentum of several of the observed modes as well as to reproduce their energies.Comment: 14 pages in REVTEX and 14 postscript figure

Vortex arrays in a rotating superfluid He-4 nanocylinder

Within Density Functional theory, we investigate stationary many-vortex structures in a rotating $^4$He nanocylinder at zero temperature. We compute the stability diagram and compare our results with the classical model of vortical lines in an inviscid and incompressible fluid. Scaling the results to millimeter-size buckets, they can be compared with experiments on vortex arrays conducted in the past. Motivated by recent experiments that have used atomic impurities as a means of visualizing vortices in superfluid $^4$He droplets, we have also considered the formation of chains of xenon atoms along a vortex line and the interaction between xenon atoms inside the same vortex and on different neighboring vortex lines

Breakup of quantum liquid filaments into droplets

We have investigated how the Rayleigh-Plateau instability of a filament made of a 41K-87Rb self-bound mixture may lead to an array of identical quantum droplets, with typical breaking times which are shorter than the lifetime of the mixture. If the filament is laterally confined -- as it happens in a toroidal trap -- and atoms of one species are in excess with respect to the optimal, equilibrium ratio, the droplets are immersed into a superfluid background made by the excess species which provides global phase coherence to the system, suggesting that the droplets array in the unbalanced system may display supersolid character. This possibility has been investigated by computing the non-classical translational inertia coefficient. The filament may be a reasonable representation of a self-bound mixture subject to toroidal confinement when the bigger circle radius of the torus is much larger than the filament radius.Comment: 11 pages, 8 figure