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

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.

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

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