arXiv subjects

Francesco Ancilotto

Publications and source records attributed to Francesco Ancilotto.

At least 55 records · Page 3Linked to original sources

arXiv2014

Density functional theory of a trapped Bose gas with tunable scattering length: from weak-coupling to unitarity

We study an interacting Bose gas at T=0 under isotropic harmonic confinement within Density Functional Theory in the Local Density approximation. The energy density functional, which spans the whole range of positive scattering lengths up to the unitary regime (infinite scattering length), is obtained by fitting the recently calculated Monte Carlo bulk equation of state [Phys. Rev. A 89, 041602(R) (2014)]. We compare the density profiles of the trapped gas with those obtained by MC calculations. We solve the time-dependent Density Functional equation to study the effect of increasing values of the interaction strength on the frequencies of monopole and quadrupole oscillations of the trapped gas. We find that the monopole breathing mode shows a non-monotonous behavior as a function of the scattering length. We also consider the damping effect of three-body losses on such modes.

cond-mat.quant-gas↗

arXiv2014

Monte Carlo Simulations of the Unitary Bose Gas

We investigate the zero-temperature properties of a diluted homogeneous Bose gas made of $N$ particles interacting via a two-body square-well potential by perfor ming Monte Carlo simulations. We tune the interaction strength to achieve arbitrary positive values of the scattering length and compute by Monte Carlo quadrature the energy per particle $E/N$ and the condensate fraction $N_0/N$ of this system by using a Jastrow ansatz for the many-body wave function which avoids the formation of the self-bound ground-state and describes instead a (metastable) gaseous state with uniform den sity. In the unitarity limit, where the scattering length diverges while the range of the inter-atomic potential is much smaller than the average distance between atoms, we find a finite energy per particle ($E/N=0.70\ \hbar^2(6π^2n)^{2/3}/2m$, with $n$ th e number density) and a quite large condensate fraction ($N_0/N=0.83$).

cond-mat.quant-gas↗

arXiv2014

Dipolar Bose gas in highly anharmonic traps

By means of mean-field theory, we have studied the structure and excitation spectrum of a purely dipolar Bose gas in pancake-shaped trap where the confinement in the x-y plane is provided by a highly anharmonic potential resulting in an almost uniform confinement in the plane. We show that the stable condensates is characterized by marked radially structured density profiles. The stability diagram is calculated by independently varying the strength of the interaction and the trap geometry. By computing the Bogoliubov excitation spectrum near the instability line we show that soft "angular" rotons are responsible for the collapse of the system. The free expansion of the cloud after the trap is released is also studied by means of time-dependent calculations, showing that a prolate, cigar-shaped condensate is dynamically stabilized during the expansion, which would otherwise collapse. Dipolar condensates rotating with sufficiently high angular velocity show the formation of multiply-quantized giant vortices, while the condensates acquire a ring-shaped form.

cond-mat.quant-gas↗

arXiv2013

Supersolid structure and excitation spectrum of soft-core bosons in 3D

By means of a mean-field method, we have studied the zero temperature structure and excitation spectrum of a three-dimensional soft-core bosonic system for a value of the interaction strength that favors a crystal structure made of atomic nano-clusters arranged with FCC ordering. In addition to the longitudinal and transverse phonon branches expected for a normal crystal, the excitation spectrum shows a soft mode related to the breaking of gauge symmetry, which signals a partial superfluid character of the solid. Additional evidence of supersolidity is provided by the calculation of the superfluid fraction, which shows a first-order drop, from 1 to 0.4, at the liquid-supersolid transition and a monotonic decrease as the interaction strength parameter is increased. The conditions for the coexistence of the supersolid with the homogeneous superfluid are discussed, and the surface tension of a representative solid-liquid interface is calculated.

cond-mat.quant-gas↗

arXiv2011

Adsorption of rare-gas atoms on Cu(111) and Pb(111) surfaces by van der Waals-corrected Density Functional Theory

The DFT/vdW-WF method, recently developed to include the Van der Waals interactions in Density Functional Theory (DFT) using the Maximally Localized Wannier functions, is applied to the study of the adsorption of rare-gas atoms (Ne, Ar, Kr, and Xe) on the Cu(111) and Pb(111) surfaces, at three high-symmetry sites. We evaluate the equilibrium binding energies and distances, and the induced work-function changes and dipole moments. We find that, for Ne, Ar, and Kr on the Cu(111) surface the different adsorption configurations are characterized by very similar binding energies, while the favored adsorption site for Xe on Cu(111) is on top of a Cu atom, in agreement with previous theoretical calculations and experimental findings, and in common with other close-packed metal surfaces. Instead, the favored site is always the hollow one on the Pb(111) surface, which therefore represents an interesting system where the investigation of high-coordination sites is possible. Moreover, the Pb(111) substrate is subject, upon rare-gas adsorption, to a significantly smaller change in the work function (and to a correspondingly smaller induced dipole moment) than Cu(111). The role of the chosen reference DFT functional and of different Van der Waals corrections, and their dependence on different rare-gas adatoms, are also discussed.

cond-mat.str-el↗

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

Ground-state path integral Monte Carlo simulations of positive ions in $^4$He clusters: bubbles or snowballs?

The local order around alkali (Li$^+$ and Na$^+$) and alkaline-eath (Be$^+$, Mg$^+$ and Ca$^+$) ions in $^4$He clusters has been studied using ground-state path integral Monte Carlo calculations. We apply a criterion based on multipole dynamical correlations to discriminate between solid-like versus liquid-like behavior of the $^4$He shells coating the ions. As it was earlier suggested by experimental measurements in bulk $^4$He, our findings indicate that Be$^+$ produces a solid-like ("snowball") structure, similarly to alkali ions and in contrast to the more liquid-like $^4$He structure embedding heavier alkaline-earth ions.

cond-mat.other↗

arXiv2003

Intriguing examples of inhomogeneous broadening

Three problems are considered in which inhomogeneous broadening can yield unusual consequences. One problem involves the energy levels of atoms moving within nanopores of nearly cylindrical cross section. A second involves atomic or molecular motion in a quasi-one dimensional interstitial channel within a bundle of carbon nanotubes. The third problem involves motion within a groove between two nanotubes at the surface of such a bundle. In each case, the density of states at low energy is qualitatively different from that occurring in the perfectly homogeneous case.

cond-mat.mtrl-sci↗

arXiv2003

Bose-Einstein Condensation of Molecular Hydrogen in Nanotube Bundles

We evaluate the effects of heterogeneity on the density of states of H$_2$ molecules inside interstitial channels within bundles of carbon nanotubes. As temperature (T) falls, the density increases within those tubes having the greatest binding energy. At T ~ 10 mK, the molecules undergo Bose-Einstein condensation, exhibiting a singular heat capacity.

cond-mat.soft↗

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↗

arXiv2003

Magneto-Optics in parabolic three-dimensional quantum dots in magnetic fields of arbitrary direction

We generalize the Kohn's theorem to the case of parabolic three-dimensional quantum dots in magnetic fields of arbitrary direction. We show numerically that the exact resonance frequencies in the magneto-optical absorption of these dots are reproduced by the adiabatic time-dependent local spin density approximation theory (TDLSDA). We use TDLSDA to predict spin density excitations in the dots.

cond-mat↗

arXiv2002

Dynamics of liquid He-4 in confined geometries from Time-Dependent Density Functional calculations

We present numerical results obtained from Time-Dependent Density Functional calculations of the dynamics of liquid He-4 in different environments characterized by geometrical confinement. The time-dependent density profile and velocity field of He-4 are obtained by means of direct numerical integration of the non-linear Schrodinger equation associated with a phenomenological energy functional which describes accurately both the static and dynamic properties of bulk liquid He-4. Our implementation allows for a general solution in 3-D (i.e. no symmetries are assumed in order to simplify the calculations). We apply our method to study the real-time dynamics of pure and alkali-doped clusters, of a monolayer film on a weakly attractive surface and a nano-droplet spreading on a solid surface.

cond-mat↗

arXiv2001

Evidence concerning Drying behavior of Ne near the Cs surface

Using density functional (DF) and Monte Carlo methods, we have studied the properties of Ne adsorbed on a Cs surface, focusing on the region at and near saturated vapor pressure (SVP). In the case of Ne/Rb, the experimental data of Hess, Sabatini and Chan are consistent with the calculations based on an ab initio fluid-substrate potential, while in the Ne/Cs case there is indication that the potential is approx 9% too deep. In that case, the calculations yield partial drying behavior consistent with the experimental finding of depressed fluid density near the surface, above SVP. However, we find no evidence of a drying transition, a result consistent with a mean field calculation of Ebner and Saam.

cond-mat.stat-mech↗

arXiv2000

Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages and thermal decomposition

Adsorption of acetylene on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is a ``bridge'' configuration, where the C$_2$H$_2$ molecule is bonded to two Si atoms. Instead, ``pedestal'' configurations, recently proposed as the lowest-energy structures, are found to be much higher in energy and, therefore, can represent only metastable adsorption sites. We have calculated the surface formation energies for two different saturation coverages, namely 0.5 and 1 monolayer, both observed in experiments. We find that although, in general, the full monolayer coverage is favored, a narrow range of temperatures exists in which the 0.5 monolayer coverage is the most stable one, where the acetylene molecules are adsorbed in a $2\times 2$ structure. This result disagrees with the conclusions of a recent study and represents a possible explanation of apparently controversial experimental findings. The crucial role played by the use of a gradient-corrected density functional is discussed. Finally, we study thermal decomposition of acetylene adsorbed on Si(100) by means of finite-temperature Molecular Dynamics, and we observe an unexpected behavior of dehydrogenated acetylene molecules.

cond-mat↗

arXiv2000

Wetting transitions of He4 on alkali metal surfaces from density functional calculations

We have studied the wetting properties of He4 adsorbed on the surface of heavy alkali metals by using a non-local free-energy density-functional which describes accurately the surface properties of liquid He4 in the temperature range 0<T<3 K.Our results for He4 on the Cs surface give both the temperature dependence of the contact angle and the wetting temperature in good agreement with the experimental findings. For the He/Rb system we find that a wetting transition on the Rb surface occurs at T=1.4 K, whereas the experiments show either wetting down to T=0 or a wetting transition at T=0.3 K. We suggest that this disagreement is due either to an inaccuracy of the fluid-substrate potential used in our calculations or the consequence of substrate roughness, which is known to affect the Rb surface, and whose effect would be to lower the wetting temperature. The sensitivity of the wetting transition to the He-surface potential are stressed for the He/Rb surface, which may justify the controversial experimental findings for this system.

cond-mat↗

arXiv2000

Adsorption of benzene on Si(100) from first principles

Adsorption of benzene on the Si(100) surface is studied from first principles. We find that the most stable configuration is a tetra-$σ$-bonded structure characterized by one C-C double bond and four C-Si bonds. A similar structure, obtained by rotating the benzene molecule by 90 degrees, lies slightly higher in energy. However, rather narrow wells on the potential energy surface characterize these adsorption configurations. A benzene molecule impinging on the Si surface is most likely to be adsorbed in one of three different di-$σ$-bonded, metastable structures, characterized by two C-Si bonds, and eventually converts into the lowest-energy configurations. These results are consistent with recent experiments.

cond-mat↗

arXiv1999

Prewetting transitions of Ar and Ne on alkali metal surfaces

We have studied by means of Density-Functional calculations the wetting properties of Ar and Ne adsorbed on a plane whose adsorption properties simulate the Li and Na surfaces. We use reliable ab-initio potentials to model the gas-substrate interactions. Evidence for prewetting transitions is found for all the systems investigated and their wetting phase diagrams are calculated.

cond-mat↗

arXiv1998

A Real-Space Full Multigrid study of the fragmentation of Li11+ clusters

We have studied the fragmentation of Li11+ clusters into the two experimentally observed products (Li9+,Li2) and (Li10+,Li) The ground state structures for the two fragmentation channels are found by Molecular Dynamics Simulated Annealing in the framework of Local Density Functional theory. Energetics considerations suggest that the fragmentation process is dominated by non-equilibrium processes. We use a real-space approach to solve the Kohn-Sham problem, where the Laplacian operator is discretized according to the Mehrstellen scheme, and take advantage of a Full MultiGrid (FMG) strategy to accelerate convergence. When applied to isolated clusters we find our FMG method to be more efficient than state-of-the-art plane wave calculations.

cond-mat↗