arXiv subjects

Massimo Boninsegni

Publications and source records attributed to Massimo Boninsegni.

At least 55 records · Page 3Linked to original sources

arXiv2018

Computer simulation study of nanoscale size parahydrogen clusters

We present results of computer simulations of a parahydrogen cluster of a thousand molecules, corresponding to approximately 4 nm in diameter, at temperatures between 1 K and 10 K. Examination of structural properties suggests that the local environment experienced by molecules is very similar to that in solid bulk parahydrogen, especially near the center of the cluster, where crystallization originates. Albeit strongly suppressed compared to helium, quantum-mechanical exchanges are not entirely negligible at the lowest temperature, resulting in a small but significant molecular mobility enhancement with respect to the bulk crystalline phase. Although the overall superfluid response at the lowest temperature is only a few percents, there is evidence of a surprising "supersolid" core, as well as of a superfluid outer shell. Much like in fluid parahydrogen at the melting temperature, quantum-mechanical signatures can be detected in the momentum distribution.

cond-mat.mes-hall↗

arXiv2018

Adsorption of parahydrogen on graphene

We study the low temperature properties of a single layer of parahydrogen adsorbed on graphene, by means of Quantum Monte Carlo simulations. The computed phase diagram is very similar to that of helium on the same substrate, featuring commensurate solid phases with fillings 1/3 and 7/16, as well as domain wall phases at intermediate coverages. At higher coverage the system transitions to an incommensurate, compressible phase. Evidence of promotion of molecules to the second layer is observed at a coverage close 0.112 inverse square Angstroms, significantly above existing theoretical estimates.

cond-mat.mes-hall↗

arXiv2018

Search for superfluidity in supercooled liquid parahydrogen

The possible superfluid transition of supercooled liquid parahydrogen is investigated by quantum Monte Carlo simulations. The cooling protocol adopted here allows for the investigation of a fluid phase down to a temperature T=0.25 K. No evidence of superfluidity is found, as exchanges of identical particles are strongly suppressed even at the lowest temperature. Is shown that, contrary to a commonly held belief, it is not the well depth of the pair-wise interaction but rather its relatively large hard core diameter that physically hinders superfluidity in parahydrogen.

cond-mat.stat-mech↗

arXiv2017

Classical and quantum filaments in the ground state of trapped dipolar Bose gases

We study by quantum Monte Carlo simulations the ground state of a harmonically confined dipolar Bose gas with aligned dipole moments, and with the inclusion of a repulsive two-body potential of varying range. Two different limits can be clearly identified, namely a classical one in which the attractive part of the dipolar interaction dominates and the system forms an ordered array of parallel filaments, and a quantum-mechanical one, wherein filaments are destabilized by zero-point motion, and eventually the ground state becomes a uniform cloud. The physical character of the system smoothly evolves from classical to quantum mechanical as the range of the repulsive two-body potential increases. An intermediate regime is observed, in which ordered filaments are still present, albeit forming different structures from the ones predicted classically; quantum-mechanical exchanges of indistinguishable particles across different filaments allow phase coherence to be established, underlying a global superfluid response.

cond-mat.stat-mech↗

arXiv2017

Atomic displacements in quantum crystals

Displacements of atoms and molecules away from lattice sites in helium and parahydrogen solids at low temperature have been studied by means of Quantum Monte Carlo simulations. In the bcc phases of He-3 and He-4, atomic displacements are largely quantum-mechanical in character, even at melting. The computed Lindemann ratio at melting is found to be in good agreement with experimental results for He-4. Unlike the case of helium, in solid parahydrogen there exists near melting a significant thermal contribution to molecular vibrations, accounting for roughly half of the total effect. Although the Lindemann ratio at melting is in quantitative agreement with experiment, computed molecular mean square fluctuations feature a clear temperature dependence, in disagreement with recent experimental observations.

cond-mat.other↗

arXiv2017

The Momentum Distribution of Liquid $^4$He

We report high-resolution neutron Compton scattering measurements of liquid $^4$He under saturated vapor pressure. There is excellent agreement between the observed scattering and ab initio predictions of its lineshape. Quantum Monte Carlo calculations predict that the Bose condensate fraction is zero in the normal fluid, builds up rapidly just below the superfluid transition temperature, and reaches a value of approximately $7.5\%$ below 1 K. We also used model fit functions to obtain from the scattering data empirical estimates for the average atomic kinetic energy and Bose condensate fraction. These quantities are also in excellent agreement with ab initio calculations. The convergence between the scattering data and Quantum Monte Carlo calculations is strong evidence for a Bose broken symmetry in superfluid $^4$He.

cond-mat.other↗

arXiv2017

Absence of superfluidity in a quasi one-dimensional parahydrogen fluid adsorbed inside carbon nanotubes

A recent claim of superfluid behaviour of parahydrogen adsorbed inside armchair carbon nanotubes [M. Rossi and F. Ancilotto, Phys. Rev. B 94, 100502 (2016)] is disproven by means of first principle computer simulations. Conclusive numerical evidence shows that the low temperature equilibrium thermodynamic phase of parahydrogen adsorbed inside a (10,10) armchair nanotube features a crystalline shell adsorbed on the inner surface of the tube, and a well separated quasi one-dimensional central column showing no evidence of possible fluid or superfluid behaviour. Rather, the system is quasi-crystalline and non-superfluid; its physical character is qualitatively identical to that observed in similar confined settings extensively studied in the past. The incorrect prediction of superfluidity stems from the erroneous use of an effective one-dimensional interaction potential that does not describe parahydrogen at the center of the nanotube.

cond-mat.mes-hall↗

arXiv2017

Phases of dipolar bosons in a bilayer geometry

We study by first principle computer simulations the low temperature phase diagram of bosonic dipolar gases in a bilayer geometry, as a function of the two control parameters, i.e., the in-plane density and the interlayer distance. We observe four distinct phases, namely paired and decoupled superfluids, as well as a crystal of dimers and one consisting of two aligned crystalline layers. A direct quantum phase transition from a dimer crystal to two independent superfluids is observed in a relatively wide range of parameters. No supersolid phase is predicted for this system.

cond-mat.quant-gas↗

arXiv2016

Hard core repulsion and supersolid cluster crystals

We study the effect of a short-ranged hard-core repulsion on the stability and superfluid properties of the cluster crystal phase of two-dimensional (2D) soft core bosons. Results of Quantum Monte Carlo simulations on a cogent test case suggest that the main physical properties of the phase remain unaltered if the range d of the inner repulsive core is sufficiently short, even if the strength of the repulsion is several orders of magnitude greater than the outer soft core barrier. Only if d is an appreciable fraction of the size of the clusters (> 5%) does a sufficiently strong hard core repulsion cause the crystal to break down into a homogeneous superfluid; a moderate inner core repulsion enhances the superfluid response of the crystalline phase.

cond-mat.stat-mech↗

arXiv2016

Ground state phase diagram of Gaussian-Core bosons in two dimensions

The ground state of a two-dimensional (2D) system of Bose particles of spin zero, interacting via a repulsive Gaussian-Core potential, has been investigated by means of Quantum Monte Carlo simulations. The quantum phase diagram is qualitatively identical to that of 2D Yukawa bosons. While the system is a fluid at all densities for weak coupling, in the strong coupling regime it transitions upon compression from a low density superfluid to a crystal, and then into a reentrant superfluid phase. No evidence of a (supersolid) cluster crystal phase is seen.

cond-mat.stat-mech↗

arXiv2016

Quasi-1D parahydrogen in nanopores

The low temperature physics of parahydrogen (ph2) confined in cylindrical channels of diameter of the order of 1 nm is studied theoretically by Quantum Monte Carlo simulations. On varying the attractive strength of the wall of the cylindrical pore, as well as its diameter, the equilibrium phase evolves from a single quasi-1D channel along the axis, to a concentric cylindrical shell. It is found that the quasi-1D system retains a strong propensity to crystallization, even though on weakly attractive substrates quantum fluctuations reduce somewhat such a tendency compared to the purely 1D system. No evidence of a topologically protected superfluid phase (in the Luttinger sense) is observed. Implications on the possible existence of a bulk superfluid phase of parahydrogen are discussed

physics.chem-ph↗

arXiv2016

Low-density phases of $^3$He monolayers adsorbed on graphite

Quantum Monte Carlo simulations at zero temperature of a $^3$He monolayer adsorbed on graphite, either clean or preplated with $^4$He, unexpectedly point to a gas-liquid phase transition at a very low areal density of the order of 0.01Å$^{-2}$. This result stems from an essentially unbiased calculation of the ground state energy for an infinite, defect-free substrate which interacts with He atoms via a realistic potential, whereas the interaction between two He atoms includes two- and three-body terms. The sensitivity of the gas-liquid coexistence region on the model Hamiltonian employed is discussed.

cond-mat.stat-mech↗

arXiv2016

Absence of superfluidity in a parahydrogen film intercalated within a crystal of Na atoms

A recent claim of possible superfluid behaviour of parahydrogen films intercalated within a crystalline matrix of Na atoms is examined. Quantum Monte Carlo simulations at finite temperature yield strong numerical evidence that the system forms at low temperature a non-superfluid crystalline phase, commensurate with the underlying impurity lattice. The physics of this system is therefore qualitatively identical to that observed in similar settings, extensively studied in precedence. Comparison of numerical results obtained here, with those of the reference in which the prediction of superfluidity (disproven here) was made, points to likely bias in the computational methodology adopted therein.

cond-mat.other↗

arXiv2015

Superfluidity of 4He nanoclusters in confinement

Structure and superfluid response of nanoscale size helium-four clusters enclosed in spherical cavities are studied by computer simulations. The curved surface causes the formation of well-defined concentric shells, thus imparting to the system a very different structure from that of free standing clusters. On a strongly attractive substrate, superfluidity is only observed at low density, in the single layer coating the inner surface of the cavity. If the substrate is very weak (e.g., Li), on the other hand, a superfluid two-shell structure can form, whose physical properties interpolate between two and three dimensions. It is shown how experimental signatures of this physical behavior can be detected through measurements of the momentum distribution.

cond-mat.mes-hall↗

arXiv2015

Superfluid response of two-dimensional parahydrogen clusters in confinement

We study by computer simulations the effect of confinement on the superfluid properties of small two-dimensional (2D) parahydrogen clusters. For clusters of fewer than twenty molecules, the superfluid response in the low temperature limit is found to remain comparable in magnitude to that of free clusters, within a rather wide range of depth and size of the confining well. The resilience of the superfluid response is attributable to the "supersolid" character of these clusters. We investigate the possibility of establishing a bulk 2D superfluid "cluster crystal" phase of \paraH2, in which a global superfluid response would arise from tunnelling of molecules across adjacent unit cells. The computed energetics suggests that for clusters of about ten molecules, such a phase may be thermodynamically stable against the formation of the equilibrium insulating crystal, for values of the cluster crystal lattice constant possibly allowing tunnelling across adjacent unit cells

cond-mat.mes-hall↗

arXiv2014

Coexistence, interfacial energy, and the fate of microemulsions in 2D dipolar bosons

The superfluid-crystal quantum phase transition of a system of purely repulsive dipolar bosons in two dimensions is studied by Quantum Monte Carlo simulations at zero temperature. We determine freezing and melting densities, and estimate the energy per unit length of a macroscopic interface separating the two phases. The results rule out the microemulsion scenario for any physical realization of this system, given the exceedingly large predicted size of the bubbles.

cond-mat.stat-mech↗

arXiv2014

Enhanced superfluid response of parahydrogen in nanoscale confinement

Confinement has generally the effect of suppressing order in condensed matter. Indeed, phase transitions such as freezing, or the superfluid transition in liquid helium, occur at lower temperatures in confinement than they do in the bulk. We provide here an illustration of a physical setting in which the opposite takes place. Specifically, the enhancement of the superfluid response of parahydrogen confined to nanoscale size cavities is demonstrated by means of first principle computer simulations. Prospects to stabilize and observe the long sought but yet elusive bulk superfluid phase of parahydrogen in purposefully designed porous media are discussed.

cond-mat.mes-hall↗

arXiv2014

Systematics of small parahydrogen clusters in two dimensions

We studied by means of computer simulations the low temperature properties of two-dimensional parahydrogen clusters comprising between 7 and 30 molecules. Computed energetics is in quantitative agreement with that reported in the only previous study [Phys. Rev. B 65, 174527 (2002)], but a generally stronger superfluid response is obtained here for clusters with more than ten molecules. Moreover, all the clusters, including the smallest one, display a well-defined, clearly identifiable solidlike structure; with only one possible exception, those with fewer than 25 molecules are (almost) entirely superfluid at the lowest temperature considered here (i.e., 0.25 K), and can thus be regarded as nanoscale "supersolids". The implications of these results on a possible bulk two-dimensional superfluid phase of parahydrogen are discussed.

cond-mat.stat-mech↗