arXiv subjects

Luciano Reatto

Publications and source records attributed to Luciano Reatto.

At least 19 recordsLinked to original sources

arXiv2025

Borophane as substrate for adsorption of He-4: A journey across dimensionality

In search of substrates for adsorption of He atoms allowing for novel quantum phases in restricted geometry we study the case of borophane. We focus on two allotropes of borophane, alpha-4H and Rect-2H. With a suitable Density Functional Theory we characterize the adsorption potential of a He atom on such crystalline substrates finding its corrugation, the preferential adsorption sites and the energy barrier between sites. Rect-2H borophane is particularly interesting due to thepresence of ridges in the adsorption potential with modest energy barriers in one direction of the basal plane and much higher barrier in the orthogonal direction, thus forming channels for motion of the adsorbed atoms. We study the adsorption of He-4 on Rect-2H borophane using Path Integral Monte Carlo simulations. In the first adsorbed layer the He-4 atoms are rather delocalized along a channel with no exchanges between channels. This strong anisotropy is present also in the first few additional adsorption layers of He-4 with presence of ordered and of disordered regions. In the second and the fifth layers at low temperature we find superfluidity on the length scale of the simulated systems. In the second layer the superfluidity is one-dimensional along the grooves. In the fifth layer the state is a strongly anisotropic two-dimensional superfluid at low coverage, with a crossover to an isotropic one at layer completion. Starting from the sixth layer the adsorbed He-4 film evolves toward a three-dimensional superfluid. Our main prediction is that Rect-2H borophane as a substrate will allow to probe 1D superfluidity in the second absorption layer, as well as the evolution from a 2D anisotropic superfluid to an isotropic one in the fifth layer, and eventually the onset of 3D superfluidity for higher coverages.

cond-mat.mes-hall↗

arXiv2024

Controlling quantum vortex dynamics and vortex-antivortex annihilation in Bose-Einstein condensates with optical lattices

Superfluids with strong spatial modulation can be experimentally produced in the area of cold atoms under the influence of optical lattices. Here we address $^{87}$Rb bosons at T=0 K in a flat geometry under the influence of a periodic potential with the Gross-Pitaevskii theory. The statics and dynamics of vortex excitations are studied in the case of one dimensional (1D) and of two dimensional (2D) optical lattices, as function of the intensity of the optical lattice. We compute how the vortex energy depends on the position of its core and the energy barrier that a vortex has to surmount in order to move in the superfluid. The dynamics of a vortex dipole, a pair of vortices of opposite chirality, differ profoundly from the case of a uniform superfluid. In the 1D case, when parallel ridges of density are present, the dynamics depends on the positions of the two vortices. If they are in the same channel between two ridges, then the two vortices approach each other until they annihilate each other in a short time. If the two vortices are in distinct channels the dipole undergoes a rigid translation but with a velocity depending on the intensity of the optical lattice and this translation velocity can even change sign with respect to the case of the uniform superfluid. Superimposed on this translation an oscillatory motion is also present. These oscillatory motions can be both longitudinal, i.e. along the channel, as well as transverse. In all cases the transverse motions are one-side, in the sense that the vortex core never crosses the equilibrium position nearest the starting position. In the case of the 2D lattices we study (square, triangular and honeycomb), the two vortices of a dipole move mainly by jumps between equilibrium positions and approach each other until annihilation.

cond-mat.quant-gas↗

arXiv2021

Phase diagram of SALR fluids on spherical surfaces

We investigate the phase diagram of a fluid of hard-core disks confined to the surface of a sphere and whose interaction potential contains a short-range attraction followed by a long-range repulsive tail (SALR). Based on previous works in the bulk we derive a stability criterion for the homogeneous phase of the fluid, and locate a region of instability linked to the presence of a negative minimum in the spherical harmonics expansion of the interaction potential. The inhomogeneous phases contained within this region are characterized using a mean-field density functional theory. We find several inhomogeneous patterns that can be separated into three broad classes: cluster crystals, stripes, and bubble crystals, each containing topological defects. Interestingly, while the periodicity of inhomogeneous phases at large densities is mainly determined by the position of the negative minimum of the potential expansion, the finite size of the system induces a richer behavior at smaller densities.

cond-mat.soft↗

arXiv2021

Vortices in the supersolid phase of dipolar Bose-Einstein condensates

Vortices are expected to exist in a supersolid but experimentally their detection can be difficult because the vortex cores are localized at positions where the local density is very low. We address here this problem by performing numerical simulations of a dipolar Bose-Einstein Condensate (BEC) in a pancake confinement at $T=0$ K and study the effect of quantized vorticity on the phases that can be realized depending upon the ratio between dipolar and short-range interaction. By increasing this ratio the system undergoes a spontaneous density modulation in the form of an ordered arrangement of multi-atom "droplets". This modulated phase can be either a "supersolid" (SS) or a "normal solid" (NS). In the SS state droplets are immersed in a background of low-density superfluid and the system has a finite global superfluid fraction resulting in non-classical rotational inertia. In the NS state no such superfluid background is present and the global superfluid fraction vanishes. We propose here a protocol to create vortices in modulated phases of dipolar BEC by "freezing" into such phases a vortex-hosting superfluid (SF) state. The resulting system, depending upon the interactions strengths, can be either a SS or a NS To discriminate between these two possible outcome of a "freezing" experiment, we show that upon releasing of the radial harmonic confinement, the expanding vortex-hosting SS shows tell-tale quantum interference effects which display the symmetry of the vortex lattice of the originating SF, as opposed to the behavior of the NS which shows instead a ballistic radial expansion of the individual droplets. Such markedly different behavior might be used to prove the supersolid character of rotating dipolar condensates.

cond-mat.quant-gas↗

arXiv2021

Localization versus inhomogeneous superfluidity: Submonolayer He-4 on fluorographene, hexagonal boron nitride, and graphene

We study a sub monolayer He-4 adsorbed on fluorographene (GF) and on hexagonal boron nitride (hBN) at low coverage. The adsorption potentials have been computed ab-initio with a suitable density functional theory including dispersion forces. The properties of the adsorbed He-4 atoms have been computed at finite temperature with path integral Monte Carlo and at T=0 K with variational path integral. From both methods we find that the lowest energy state of He-4 on GF is a superfluid. Due to the very large corrugation of the adsorption potential this superfluid has a very strong spatial anisotropy, the ratio between the largest and smallest areal density being about 6, the superfluid fraction at the lowest T is about 55%, and the temperature of the transition to the normal state is in the range 0.5-1 K. Thus, GF offers a platform for studying the properties of a strongly interacting highly anisotropic bosonic superfluid. At a larger coverage He-4 has a transition to an ordered commensurate state with occupation of 1/6 of the adsorption sites. This phase is stable up to a transition temperature located between 0.5 and 1~K. The system has a triangular order similar to that of He-4 on graphite. The lowest energy state of He-4 on hBN is an ordered commensurate state with occupation of 1/3 of the adsorption sites and triangular symmetry. A disordered state is present at lower coverage as a metastable state. In the presence of an electric field the corrugation of the adsorption potential is slightly increased but up to a magnitude of 1 V/Ang. the effect is small and does not change the stability of the phases of He-4 on GF and hBN. We have verified that also in the case of graphene such electric field does not modify the stability of the commensurate sqrt{3}*sqrt{3}R30 phase.

cond-mat.other↗

arXiv2021

Vortex properties in the extended supersolid phase of dipolar Bose-Einstein condensates

We study the properties of singly-quantized linear vortices in the supersolid phase of a dipolar Bose-Einstein condensate at zero temperature modeling $^{164}$Dy atoms. The system is extended in the $x-y$ plane and confined by a harmonic trap in the the polarization direction $z$. Our study is based on a generalized Gross-Pitaevskii equation. We characterize the ground state of the system in terms of spatial order and superfluid fraction and compare the properties of a single vortex and of a vortex dipole in the superfluid phase (SFP) and in the supersolid phase (SSP). At variance with a vortex in the SFP, which is free to move in the superfluid, a vortex in the SSP is localized at the interstitial sites and does not move freely. We have computed the energy barrier for motion from an equilibrium site to another. The fact that the vortex is submitted to a periodic potential has a dramatic effect on the dynamics of a vortex dipole made of two counter rotating parallel vortices; instead of rigidly translating as in the SFP, the vortex and anti-vortex approach each other by a series of jumps from one site to another until they annihilate in a very short time and their energy is transferred to bulk excitations.

cond-mat.quant-gas↗

arXiv2019

Prediction for two spatially modulated superfluids: $^4$He on fluorographene and on hexagonal BN

We have derived the adsorption potential of $^4$He atoms on fluorographene (GF), on graphane and on hexagonal boron nitride (hBN) by a recently developed ab initio method that incorporates the van der Waals interaction. The $^4$He monolayer on GF and on hBN is studied by state-of-the-art quantum simulations at T=0 K. With our adsorption potentials we find that in both cases the ground state of $^4$He monolayer is a fluid and not an ordered state with localized atoms as on graphite and on graphene. In the case of GF the present result is in qualitative agreement with the superfluid phase that was obtained using an empirical adsorption potential [M. Nava et al., Phys. Rev. B 86, 174509 (2012)]. This fluid state of $^4$He on GF and on hBN is characterized by a very large density modulation and at the equilibrium density the ratio $Γ$ between the largest and the smallest local density along the direction of two neighboring adsorption sites and averaged over the perpendicular direction is $Γ$ = 1.91 for GF and $Γ$ = 1.65 for hBN. Recent experiments [J. Nyeki et al., Nature Physics 13, 455 (2017)] have discovered a superfluid phase in the second layer $^4$He. This is a spatially modulated superfluid that turns out to have anomalous thermal properties. This gives a strong motivation for an experimental study of monolayer $^4$He on GF and on hBN that we predict to be a superfluid with a much stronger spatial modulation.

cond-mat.mes-hall↗

arXiv2018

Formation of cluster crystals in an ultra-soft potential model on a spherical surface

We investigate the formation of cluster crystals with multiply occupied lattice sites on a spherical surface in systems of ultra-soft particles interacting via repulsive, bounded pair potentials. Not all interactions of this kind lead to clustering: we generalize the criterion devised in C.N. Likos et al., Phys. Rev. E, 2001, 63, 031206 to spherical systems in order to distinguish between cluster forming systems and fluids which display reentrant melting. We use both DFT and Monte Carlo simulations to characterize the behavior of the system, and obtain semi-quantitative agreement between the two. Furthermore, we study the effect of topological frustration on the system due to the sphere curvature by comparing the properties of disclinations, i.e., clusters with fewer than six neighbors, and non-defective clusters. Disclinations are shown to be less stable, contain fewer particles, and be closer to their neighbors than other lattice points: these properties are explained on the basis of geometric and energetic considerations.

cond-mat.soft↗

arXiv2018

Sliding states of a soft-colloid cluster crystal: Cluster versus single-particle hopping

We study a two-dimensional model for interacting colloidal particles which displays spontaneous clustering. Within this model we investigate the competition between the pinning to a periodic corrugation potential, and a sideways constant pulling force which would promote a sliding state. For a few sample particle densities and amplitudes of the periodic corrugation potential we investigate the depinning from the statically pinned to the dynamically sliding regime. This sliding state exhibits the competition between a dynamics where entire clusters are pulled from a minimum to the next and a dynamics where single colloids or smaller groups leave a cluster and move across the corrugation energy barrier to join the next cluster downstream in the force direction. Both kinds of sliding states can occur either coherently across the entire sample, or asynchronously: the two regimes result in different average mobilities. Finite temperature tends to destroy separate sliding regimes, generating a smoother dependence of the mobility on the driving force.

cond-mat.soft↗

arXiv2018

Probing quantum turbulence in $^4$He by quantum evaporation measurements

Theory of superfluid $^4$He shows that, due to strong correlations and backflow effects, the density profile of a vortex line has the character of a density modulation and it is not a simple rarefaction region as found in clouds of cold bosonic atoms. We find that the basic features of this density modulation are represented by a wave--packet of cylindrical symmetry in which rotons with positive group velocity have a dominant role: The vortex density modulation can be viewed as a cloud of virtual excitations, mainly rotons, sustained by the phase of the vortex wave function. This suggests that in a vortex reconnection some of these rotons become real so that a vortex tangle is predicted to be a source of non-thermal rotons. The presence of such vorticity induced rotons can be verified by measurements at low temperature of quantum evaporation of $^4$He atoms. We estimate the rate of evaporation and this turns out to be detectable by current instrumentation. Additional information on the microscopic processes in the decay of quantum turbulence will be obtained if quantum evaporation by high energy phonons should be detected.

cond-mat.other↗

arXiv2015

Depletion interaction between spheres of unequal size and demixing in binary mixtures of colloids

The possibility to induce demixing in a colloidal mixture by adding small polymers, or other equivalent depletant agents, is theoretically investigated. By use of Mean Field Theory, suitably generalized to deal with short range effective interactions, the phase diagram of a binary mixture ofcolloidal particles (modelled as hard spheres) in a solvent is determined as a function of the polymer concentration on the basis of the Asakura-Oosawa model.The topology of the phase diagram changes when the relative size of the colloidal particles is reduced: the critical line connecting the liquid-vapour critical points of the two pure fluids breaks and the branch starting from the critical point of the bigger particles bends to higher volume fractions, where concentration fluctuations drive the transition. The effects of a softer colloid-polymer interaction is also investigated: Even the presence of a small repulsive tail in the potential gives rise to a significant lowering of the stability threshold. In this case, phase transitions may take place by adding just a few percent of depletant in volume fraction. These results may be relevant for the interpretation of recent experiments of solidification kinetics in colloidal mixtures.

cond-mat.soft↗

arXiv2014

Quantum Monte Carlo study of a vortex in superfluid $^4$He and search for a vortex state in the solid

We have performed a microscopic study of a straight quantized vortex line in three dimensions in condensed $^4$He at zero temperature using the Shadow Path Integral Ground State method and the fixed-phase approximation. We have characterized the energy and the local density profile around the vortex axis in superfluid $^4$He at several densities, ranging from below the equilibrium density up to the overpressurized regime. For the Onsager-Feynman (OF) phase our results are exact and represent a benchmark for other theories. The inclusion of backflow correlations in the phase improves the description of the vortex with respect to the OF phase by a large reduction of the core energy of the topological excitation. At all densities the phase with backflow induces a partial filling of the vortex core and this filling slightly increases with density. The core size slightly decreases for increasing density and the density profile has well defined density dependent oscillations whose wave vector is closer to the wave vector of the main peak in the static density response function rather than to the roton wave vector. Our results can be applied to vortex rings of large radius $R$ and we find good agreement with the experimental value of the energy as function of $R$ without any free parameter. We have studied also $^4$He above the melting density in the solid phase using the same functional form for the phase as in the liquid. We found that off-diagonal properties of the solid are not qualitatively affected by the velocity field induced by the vortex phase, both with and without backflow correlations. Therefore we find evidence that a perfect $^4$He crystal is not a marginally stable quantum solid in which rotation would be able to induce off-diagonal long-range coherence.

cond-mat.other↗

arXiv2013

Novel behaviors of monolayer quantum gases on Graphene, Graphane and Fluorographene

This article discusses the behavior of submonolayer quantum films (He and H2) on graphene and newly discovered surfaces that are derived from graphene. Among these substrates are graphane (abbreviated GH), which has an H atom bonded to each C atom, and fluorographene (GF). For the graphite case, extensive experimental and theoretical investigations have revealed that the phase diagrams of the bose gases 4He and para-H2 are qualitatively similar, differing primarily in a higher characteristic temperature scale for H2 than for He. The phase behaviors of these films on one side of pristine graphene, or both sides of free-standing graphene, are expected to be similar to those on graphite. We point out the possibility of novel phenomena in adsorption on graphene related to the large flexibility of the graphene sheet, to the non--negligible interaction between atoms adsorbed on opposite sides of the sheet and to the perturbation effect of the adsorbed layer on the Dirac electrons. In contrast, the behaviors predicted on GF and GH surfaces are very different from those on graphite, a result of the different corrugation, i.e., the lateral variation of the potential experienced by these gases. As a result of this novel geometry and potential, distinct properties are observed. For example, the 4He film's ground-state on graphite is a two-dimensional (2D) crystal commensurate with the substrate; on GF and GH, instead, it is predicted to be an anisotropic superfluid. On GF the anisotropy is so extreme that the roton excitations are very anisotropic, as if the bosons are moving in a multiconnected space along the bonds of a honeycomb lattice. Such a novel phase has not been predicted or observed previously on any substrate. Also, in the case of 3He the film's ground-state is a fluid, thus offering the possibility of studying an anisotropic Fermi fluid with a tunable density.

cond-mat.mes-hall↗

arXiv2012

Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group

The Hierarchical Reference Theory (HRT) of fluids is a general framework for the description of phase transitions in microscopic models of classical and quantum statistical physics. The foundations of HRT are briefly reviewed in a self-consistent formulation which includes both the original sharp cut-off procedure and the smooth cut-off implementation, which has been recently investigated. The critical properties of HRT are summarized, together with the behavior of the theory at first order phase transitions. However, the emphasis of this presentation is on the close relationship between HRT and non perturbative renormalization group methods, as well as on recent generalizations of HRT to microscopic models of interest in soft matter and quantum many body physics.

cond-mat.stat-mech↗

arXiv2011

An investigation of the SCOZA for narrow square-well potentials and in the sticky limit

We present a study of the self consistent Ornstein-Zernike approximation (SCOZA) for square-well (SW) potentials of narrow width delta. The main purpose of this investigation is to elucidate whether in the limit delta --> 0, the SCOZA predicts a finite value for the second virial coefficient at the critical temperature B2(Tc), and whether this theory can lead to an improvement of the approximate Percus-Yevick solution of the sticky hard-sphere (SHS) model due to Baxter [R. J. Baxter, J. Chem. Phys. 49, 2770 (1968)]. For SW of non vanishing delta, the difficulties due to the influence of the boundary condition at high density already encountered in an earlier investigation [E. Schoell-Paschinger, A. L. Benavides, and R. Castaneda-Priego, J. Chem. Phys. 123, 234513 (2005)] prevented us from obtaining reliable results for delta < 0.1. In the sticky limit this difficulty can be circumvented, but then the SCOZA fails to predict a liquid-vapor transition. The picture that emerges from this study is that for delta --> 0, the SCOZA does not fulfill the expected prediction of a constant B2(Tc) [M. G. Noro and D. Frenkel, J. Chem. Phys. 113, 2941 (2000)], and that for thermodynamic consistency to be usefully exploited in this regime, one should probably go beyond the Ornstein-Zernike ansatz.

cond-mat.soft↗

arXiv2009

The smooth cut-off Hierarchical Reference Theory of fluids

We provide a comprehensive presentation of the Hierarchical Reference Theory (HRT) in the smooth cut-off formulation. A simple and self-consistent derivation of the hierarchy of differential equations is supplemented by a comparison with the known sharp cut-off HRT. Then, the theory is applied to a hard core Yukawa fluid (HCYF): a closure, based on a mean spherical approximation ansatz, is studied in detail and its intriguing relationship to the self consistent Ornstein-Zernike approximation is discussed. The asymptotic properties, close to the critical point are investigated and compared to the renormalization group results both above and below the critical temperature. The HRT free energy is always a convex function of the density, leading to flat isotherms in the two-phase region with a finite compressibility at coexistence. This makes HRT the sole liquid-state theory able to obtain directly fluid-fluid phase equilibrium without resorting to the Maxwell construction. The way the mean field free energy is modified due to the inclusion of density fluctuations suggests how to identify the spinodal curve. Thermodynamic properties and correlation functions of the HCYF are investigated for three values of the inverse Yukawa range: z=1.8, z=4 and z=7 where Monte Carlo simulations are available. The stability of the liquid-vapor critical point with respect to freezing is also studied.

cond-mat.soft↗

arXiv2008

Liquid-vapor transition from a microscopic theory: Beyond the Maxwell construction

A smooth cut-off formulation of the Hierarchical Reference Theory (HRT) is developed and applied to a Yukawa fluid. The HRT equations are derived and numerically solved leading to: the expected renormalization group structure in the critical region, non classical critical exponents and scaling laws, a convex free energy in the whole phase diagram (including the two-phase region), finite compressibility at coexistence, together with a fully satisfactory comparison with available numerical simulations. This theory, which also guarantees the correct short range behavior of two body correlations, represents a major improvement over the existing liquid state theories.

cond-mat.stat-mech↗

arXiv2007

Smooth cutoff formulation of hierarchical reference theory for a scalar phi4 field theory

The phi4 scalar field theory in three dimensions, prototype for the study of phase transitions, is investigated by means of the hierarchical reference theory (HRT) in its smooth cutoff formulation. The critical behavior is described by scaling laws and critical exponents which compare favorably with the known values of the Ising universality class. The inverse susceptibility vanishes identically inside the coexistence curve, providing a first principle implementation of the Maxwell construction, and shows the expected discontinuity across the phase boundary, at variance with the usual sharp cutoff implementation of HRT. The correct description of first and second order phase transitions within a microscopic, nonperturbative approach is thus achieved in the smooth cutoff HRT.

cond-mat.stat-mech↗