Surface location of sodium atoms attached to He-3 nanodroplets

We have experimentally studied the electronic $3p\leftarrow 3s$ excitation of Na atoms attached to $^3$He droplets by means of laser-induced fluorescence as well as beam depletion spectroscopy. From the similarities of the spectra (width/shift of absorption lines) with these of Na on $^4$He droplets, we conclude that sodium atoms reside in a ``dimple'' on the droplet surface. The experimental results are supported by Density Functional calculations at zero temperature, which confirm the surface location of sodium on $^3$He droplets, and provide a microscopic description of the ``dimple'' structure.Comment: 4 pages, 5 figure

Vertically coupled double quantum rings at zero magnetic field

Within local-spin-density functional theory, we have investigated the `dissociation' of few-electron circular vertical semiconductor double quantum ring artificial molecules at zero magnetic field as a function of inter-ring distance. In a first step, the molecules are constituted by two identical quantum rings. When the rings are quantum mechanically strongly coupled, the electronic states are substantially delocalized, and the addition energy spectra of the artificial molecule resemble those of a single quantum ring in the few-electron limit. When the rings are quantum mechanically weakly coupled, the electronic states in the molecule are substantially localized in one ring or the other, although the rings can be electrostatically coupled. The effect of a slight mismatch introduced in the molecules from nominally identical quantum wells, or from changes in the inner radius of the constituent rings, induces localization by offsetting the energy levels in the quantum rings. This plays a crucial role in the appearance of the addition spectra as a function of coupling strength particularly in the weak coupling limit.Comment: 18 pages, 8 figures, submitted to Physical Review

Quantizing Majorana Fermions in a Superconductor

A Dirac-type matrix equation governs surface excitations in a topological insulator in contact with an s-wave superconductor. The order parameter can be homogenous or vortex valued. In the homogenous case a winding number can be defined whose non-vanishing value signals topological effects. A vortex leads to a static, isolated, zero energy solution. Its mode function is real, and has been called "Majorana." Here we demonstrate that the reality/Majorana feature is not confined to the zero energy mode, but characterizes the full quantum field. In a four-component description a change of basis for the relevant matrices renders the Hamiltonian imaginary and the full, space-time dependent field is real, as is the case for the relativistic Majorana equation in the Majorana matrix representation. More broadly, we show that the Majorana quantization procedure is generic to superconductors, with or without the Dirac structure, and follows from the constraints of fermionic statistics on the symmetries of Bogoliubov-de Gennes Hamiltonians. The Hamiltonian can always be brought to an imaginary form, leading to equations of motion that are real with quantized real field solutions. Also we examine the Fock space realization of the zero mode algebra for the Dirac-type systems. We show that a two-dimensional representation is natural, in which fermion parity is preserved.Comment: 26 pages, no figure

Thermally assisted quantum cavitation in solutions of 3He in 4He

We have investigated the quantum-to-thermal crossover temperature T* for cavitation in liquid helium mixtures up to 0.05 3He concentrations. With respect to the pure 4He case, T* is sizeably reduced, to a value below 50 mK for 3He concentrations above 0.02. As in pure 4He, the homogeneous cavitation pressure is systematically found close to the spinodal pressure.Comment: Typeset using Revtex, 9 pages and 4 figure

Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields

Vertically coupled double quantum rings submitted to a perpendicular magnetic field $B$ are addressed within the local spin-density functional theory. We describe the structure of quantum ring molecules containing up to 40 electrons considering different inter-ring distances and intensities of the applied magnetic field. When the rings are quantum mechanically strongly coupled, only bonding states are occupied and the addition spectrum of the artificial molecules resembles that of a single quantum ring, with some small differences appearing as an effect of the magnetic field. Despite the latter has the tendency to flatten the spectra, in the strong coupling limit some clear peaks are still found even when $B\neq 0$ that can be interpretated from the single-particle energy levels analogously as at zero applied field, namely in terms of closed-shell and Hund's-rule configurations. Increasing the inter-ring distance, the occupation of the first antibonding orbitals washes out such structures and the addition spectra become flatter and irregular. In the weak coupling regime, numerous isospin oscillations are found as a function of $B$.Comment: 27 pages, 11 figures. To be published in Phys. Rev.

Bound states of 3He at the edge of a 4He drop on a cesium surface

We show that small amounts of 3He atoms, added to a 4He drop deposited on a flat cesium surface at zero temperature, populate bound states localized at the contact line. These edge states show up for drops large enough to develop well defined surface and bulk regions together with a contact line, and they are structurally different from the well-known Andreev states that appear at the free surface and at the liquid-solid interface of films. We illustrate the one-body density of 3He in a drop with 1000 4He atoms, and show that for sufficiently large number of impurities, the density profiles spread beyond the edge, coating both the curved drop surface and its flat base and eventually isolating it from the substrate.Comment: 10 pages and 7 figures. Submitted to PR

Quantum cavitation in liquid 3^3He: dissipation effects

We have investigated the effect that dissipation may have on the cavitation process in normal liquid $^3$He. Our results indicate that a rather small dissipation decreases sizeably the quantum-to-thermal crossover temperature $T^*$ for cavitation in normal liquid $^3$He. This is a possible explanation why recent experiments have not yet found clear evidence of quantum cavitation at temperatures below the $T^*$ predicted by calculations which neglect dissipation.Comment: To be published in Physical Review B6

Equation of state for dense supernova matter

We provide an equation of state for high density supernova matter by applying a momentum-dependent effective interaction. We focus on the study of the equation of state of high-density and high-temperature nuclear matter containing leptons (electrons and neutrinos) under the chemical equilibrium condition. The conditions of charge neutrality and equilibrium under $\beta$-decay process lead first to the evaluation of the lepton fractions and afterwards the evaluation of internal energy, pressure, entropy and in total to the equation of state of hot nuclear matter for various isothermal cases. Thermal effects on the properties and equation of state of nuclear matter are evaluated and analyzed in the framework of the proposed effective interaction model. Since supernova matter is characterized by a constant entropy we also present the thermodynamic properties for isentropic case. Special attention is dedicated to the study of the contribution of the components of $\beta$-stable nuclear matter to the entropy per particle, a quantity of great interest for the study of structure and collapse of supernova.Comment: 23 pages, 15 figure