arXiv subjects

Manuel Barranco

Publications and source records attributed to Manuel Barranco.

At least 37 records · Page 2Linked to original sources

arXiv2017

Density Functional Theory of doped superfluid liquid helium and nanodroplets

During the last decade, density function theory (DFT) in its static and dynamic time dependent forms, has emerged as a powerful tool to describe the structure and dynamics of doped liquid helium and droplets. In this review, we summarize the activity carried out in this field within the DFT framework since the publication of the previous review article on this subject [M. Barranco et al., J. Low Temp. Phys. 142, 1 (2006)]. Furthermore, a comprehensive presentation of the actual implementations of helium DFT is given, which have not been discussed in the individual articles or are scattered in the existing literature. This is an Accepted Manuscript of an article published on August 2, 2017 by Taylor & Francis Group in Int. Rev. Phys. Chem. 36, 621 (2017), available online: http://dx.doi.org/10.1080/0144235X.2017.1351672

physics.chem-ph↗

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2015

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.

cond-mat.mes-hall↗

arXiv2014

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.

cond-mat.mes-hall↗

arXiv2012

Desorption of alkali atoms from 4He nanodroplets

The dynamics following the photoexcitation of Na and Li atoms located on the surface of helium nanodroplets has been investigated in a joint experimental and theoretical study. Photoelectron spectroscopy has revealed that excitation of the alkali atoms via the (n+1) -> ns transition leads to the desorption of these atoms. The mean kinetic energy of the desorbed atoms, as determined by ion imaging, shows a linear dependence on excitation frequency. These experimental findings are analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet combined with a Bohmian dynamics description of the desorbing atom. This hybrid method reproduces well the key experimental observables. The dependence of the observables on the impurity mass is discussed by comparing the results obtained for the 6Li and 7Li isotopes. The calculations show that the desorption of the excited alkali atom is accompanied by the creation of highly non-linear density waves in the helium droplet that propagate at supersonic velocities.

physics.atm-clus↗

arXiv2010

Excited electron-bubble states in superfluid helium-4: a time-dependent density functional approach

We present a systematic study on the excited electron-bubble states in superfluid helium-4 using a time-dependent density functional approach. For the evolution of the 1P bubble state, two different functionals accompanied with two different time-development schemes are used, namely an accurate finite-range functional for helium with an adiabatic approximation for electron versus an efficient zero-range functional for helium with a real-time evolution for electron. We make a detailed comparison between the quantitative results obtained from the two methods, which allows us to employ with confidence the optimal method for suitable problems. Based on this knowledge, we use the finite-range functional to calculate the time-resolved absorption spectrum of the 1P bubble, which in principle can be experimentally determined, and we use the zero-range functional to real-time evolve the 2P bubble for several hundreds of picoseconds, which is theoretically interesting due to the break down of adiabaticity for this state. Our results discard the physical realization of relaxed, metastable 2P electron-bubbles

cond-mat.other↗

arXiv2010

Evolution of the excited electron bubble in liquid 4He and the appearance of fission-like processes

We have studied the evolution of an excited electron bubble in superfluid 4He for several tens of picoseconds combining the dynamics of the liquid with an adiabatic evolution for the electron. The path followed by the excited bubble in its decay to the ground state is shown to strongly depend on pressure. While for pressures below 1 bar the 1P excited electron bubble has allowance for radiatively decay to the deformed ground state, evolving then non-radiatively towards the ground state of the spherical electron bubble, we have found that above 1 bar two distinct baby bubbles appear in the course of the dynamical evolution, pointing to a different relaxation path in which the electron may be localized in one of the baby bubbles while the other collapses, allowing for a pure radiationless de-excitation. Our calculations are in agreement with experiments indicating that relaxed 1P bubbles are only observed for pressures smaller than a critical one, of the order of 1 bar, and that above this value the decay of the excited bubble has to proceed differently. A similar analysis carried out for the 2P bubble shows that the adiabatic approximation fails at an early stage of its dynamical evolution due to the crossing of the 2P and 1F states.

cond-mat.other↗

arXiv2008

Magnesium doped helium nanodroplets

We have studied the structure of $^4$He droplets doped with magnesium atoms using density functional theory. We have found that the solvation properties of this system strongly depend on the size of the $^4$He droplet. For small drops, Mg resides in a deep surface state, whereas for large size drops it is fully solvated but radially delocalized in their interior. We have studied the $3s3p$ $^1$P$_1 \leftarrow 3s^2$ $^1$S$_0$ transition of the dopant, and have compared our results with experimental data from laser induced fluorescence (LIF). Line broadening effects due to the coupling of dynamical deformations of the surrounding helium with the dipole excitation of the impurity are explicitly taken into account. We show that the Mg radial delocalization inside large droplets may help reconcile the apparently contradictory solvation properties of magnesium as provided by LIF and electron-impact ionization experiments. The structure of $^4$He drops doped with two magnesium atoms is also studied and used to interpret the results of resonant two-photon-ionization (R2PI) and LIF experiments. We have found that the two solvated Mg atoms do not easily merge into a dimer, but rather form a weakly-bound state due to the presence of an energy barrier caused by the helium environment that keep them some 9.5 Å apart, preventing the formation of the Mg$_2$ molecule. From this observation, we suggest that Mg atoms in $^4$He drops may form, under suitable conditions, a soft ``foam''-like aggregate rather than coalesce into a compact metallic cluster. Our findings are in qualitative agreement with recent R2PI experimental evidences. We predict that, contrarily, Mg atoms adsorbed in $^3$He droplets do not form such metastable aggregates.

cond-mat.other↗

arXiv2007

Absorption spectrum of Ca atoms attached to $^4$He nanodroplets

Within density functional theory, we have obtained the structure of $^4$He droplets doped with neutral calcium atoms. These results have been used, in conjunction with newly determined {\it ab-initio} $^1Σ$ and $^1Π$ Ca-He pair potentials, to address the $4s4p$ $^1$P$_1 \leftarrow 4s^2$ $^1$S$_0$ transition of the attached Ca atom, finding a fairly good agreement with absorption experimental data. We have studied the drop structure as a function of the position of the Ca atom with respect of the center of mass of the helium moiety. The interplay between the density oscillations arising from the helium intrinsic structure and the density oscillations produced by the impurity in its neighborhood plays a role in the determination of the equilibrium state, and hence in the solvation properties of alkaline earth atoms. In a case of study, the thermal motion of the impurity within the drop surface region has been analyzed in a semi-quantitative way. We have found that, although the atomic shift shows a sizeable dependence on the impurity location, the thermal effect is statistically small, contributing by about a 10% to the line broadening. The structure of vortices attached to the calcium atom has been also addressed, and its effect on the calcium absorption spectrum discussed. At variance with previous theoretical predictions, we conclude that spectroscopic experiments on Ca atoms attached to $^4$He drops will be likely unable to detect the presence of quantized vortices in helium nanodrops.

cond-mat.soft↗

arXiv2006

Optical response of two-dimensional few-electron concentric double quantum rings: A local-spin-density-functional theory study

We have investigated the dipole charge- and spin-density response of few-electron two-dimensional concentric nanorings as a function of the intensity of a perpendicularly applied magnetic field. We show that the dipole response displays signatures associated with the localization of electron states in the inner and outer ring favored by the perpendicularly applied magnetic field. Electron localization produces a more fragmented spectrum due to the appearance of additional edge excitations in the inner and outer ring.

cond-mat.mes-hall↗

arXiv2006

Spin-orbit effects in GaAs quantum wells: Interplay between Rashba, Dresselhaus, and Zeeman interactions

The interplay between Rashba, Dresselhaus and Zeeman interactions in a quantum well submitted to an external magnetic field is studied by means of an accurate analytical solution of the Hamiltonian, including electron-electron interactions in a sum rule approach. This solution allows to discuss the influence of the spin-orbit coupling on some relevant quantities that have been measured in inelastic light scattering and electron-spin resonance experiments on quantum wells. In particular, we have evaluated the spin-orbit contribution to the spin splitting of the Landau levels and to the splitting of charge- and spin-density excitations. We also discuss how the spin-orbit effects change if the applied magnetic field is tilted with respect to the direction perpendicular to the quantum well.

cond-mat.mes-hall↗

arXiv2006

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.

cond-mat.mes-hall↗

arXiv2006

Spin-orbit effects on the Larmor dispersion relation in GaAs quantum wells

We have studied the relevance of spin-orbit coupling to the dispersion 00009 relation of the Larmor resonance observed in inelastic light scattering and electron-spin resonance experiments on GaAs quantum wells. We show that the spin-orbit interaction, here described by a sum of Dresselhaus and Bychkov-Rashba terms, couples Zeeman and spin-density excitations. We have evaluated its contribution to the spin splitting as a function of the magnetic field $B$, and have found that in the small $B$ limit, the spin-orbit interaction does not contribute to the spin splitting, whereas at high magnetic fields it yields a $B$ independent contribution to the spin splitting given by $2(λ_R^2-λ_D^2)$, with $λ_{R,D}$ being the intensity of the Bychkov-Rashba and Dresselhaus spin-orbit terms.

cond-mat.mes-hall↗

arXiv2006

Electron bubbles in liquid helium: infrared-absorption spectrum

Within Density Functional Theory, we have calculated the energy of the transitions from the ground state to the first two excited states in the electron bubbles in liquid helium at pressures from zero to about the solidification pressure. For $^4$He at low temperatures, our results are in very good agreement with infrared absorption experiments. Above a temperature of $\sim 2$ K, we overestimate the energy of the $1s-1p$ transition. We attribute this to the break down of the Franck-Condon principle due to the presence of helium vapor inside the bubble. Our results indicate that the $1s-2p$ transition energies are sensitive not only to the size of the electron bubble, but also to its surface thickness. We also present results for the infrared transitions in the case of liquid $^3$He, for which we lack of experimental data.

cond-mat.soft↗

arXiv2005

Ground state structure and conductivity of quantum wires of infinite length and finite width

We have studied the ground state structure of quantum strips within the local spin-density approximation, for a range of electronic densities between $\sim$ 5$\times10^4$ and 2$\times10^6$ cm$^{-1}$ and several strengths of the lateral confining potential. The results have been used to address the conductance $G$ of quantum strips. At low density, when only one subband is occupied, the system is fully polarized and $G$ takes a value which is close to 0.7(2e$^2/h$), decreasing with increasing electron density in agreement with experiments. At higher densities the system becomes paramagnetic and $G$ takes a value near (2e$^2/h$), showing a similar decreasing behaviour with increasing electron density. In both cases, the physical parameter that determines the value of the conductance is the ratio $K/K_0$ of the compressibility of the system over the free one.

cond-mat.mes-hall↗

arXiv2003

Probing vortices in 4He nanodroplets

We present static and dynamical properties of linear vortices in 4He droplets obtained from Density Functional calculations. By comparing the adsorption properties of different atomic impurities embedded in pure droplets and in droplets where a quantized vortex has been created, we suggest that Ca atoms should be the dopant of choice to detect vortices by means of spectroscopic experiments.

cond-mat.soft↗

arXiv2001

Quantized vortices in mixed 3He-4He drops

Using density functional theory, we investigate the structure of mixed $^3$He$_{N_3}$-$^4$He$_{N_4}$ droplets with an embedded impurity (Xe atom or HCN molecule) which pins a quantized vortex line. We find that the dopant+vortex+$^4$He$_{N_4}$ complex, which in a previous work [F. Dalfovo {\it et al.}, Phys. Rev. Lett. {\bf 85}, 1028 (2000)] was found to be energetically stable below a critical size $N_{\rm cr}$, is robust against the addition of $^3$He. While $^3$He atoms are distributed along the vortex line and on the surface of the $^4$He drop, the impurity is mostly coated by $^4$He atoms. Results for $N_4=500$ and a number of $^3$He atoms ranging from 0 to 100 are presented, and the binding energy of the dopant to the vortex line is determined.

cond-mat↗

arXiv2001

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.

cond-mat↗