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Massimo Boninsegni

Publications and source records attributed to Massimo Boninsegni.

At least 37 records · Page 2Linked to original sources
arXiv2022

Bound state of a He-3 atom at the interface of crystal and superfluid He-4

The bound state of a He-3 atom at the interface between coexisting crystalline and superfluid phases of He-4 is studied theoretically by means of first principle Quantum Monte Carlo simulations. We consider both the case of a solid/superfluid He-4 interface, as well as that of a few solid layers of He-4 forming on an attractive substrate. It is observed that the He-3 bound state is sharply localized in a well-defined, quasi-2D layer of He-4 in the region intermediate between the solid and the liquid. The layer in which the He-3 atom resides displays an intriguing interplay of atomic localization and quantum-mechanical exchanges; is not necessarily the first "non-solid" layer, as suggested in previous studies, and it is affected by the attractive strength of the substrate.

cond-mat.other↗
arXiv2022

Phase diagram of hard core bosons with anisotropic interactions

The phase diagram of lattice hard core bosons with nearest-neighbor interactions allowed to vary independently, from repulsive to attractive, along different crystallographic directions, is studied by Quantum Monte Carlo simulations. We observe a superfluid phase, as well as two crystalline phases at half filling, either checkerboard or striped. Just like in the case of isotropic interactions, no supersolid phase is observed.

cond-mat.stat-mech↗
arXiv2022

Quasi-one-dimensional $^4$He in nanopores

Low temperature structural and superfluid properties of $^4$He confined in cylindrical nanopores are theoretically investigated by means of first principle Quantum Monte Carlo (QMC) simulations. We vary the density of $^4$He inside the pore, as well as the pore diameter and the potential describing the interaction of each $^4$He atom with the pore surface. Accordingly, the $^4$He fluid inside the pore forms either a single channel along the axis, or a series of concentric cylindrical shells, with varying degrees of shell overlap. In the limit of pore length greatly exceeding its radius, the $^4$He fluid always displays markedly one-dimensional behavior, with no "dimensional crossover" above some specific pore radius and/or as multiple concentric shells form, in contrast to what recently claimed by other authors [Phys. Rev. B 101, 104505 (2020)]. Indeed, the predicted robustness of one-dimensional physics suggests that this system may offer a broadly viable pathway to the experimental observation of exotic behavior of, e.g., junctions of interacting Tomonaga-Luttinger liquids, in an appropriately designed network of nanopores.

cond-mat.mes-hall↗
arXiv2021

Morphology of dipolar Bose droplets

The ground state of a free standing, self-bound droplet comprising four hundred dipolar Bose particles with aligned dipole moments, with an additional purely repulsive two-body interaction, is investigated by Quantum Monte Carlo simulations. The focus here is on the evolution of the cluster as the effective range of the repulsive interaction is varied. We identify a ``classical'' regime, in which binding arises exclusively from the dipolar potential energy and the cluster is a quasi-one-dimensional filament, and a ``quantum'' regime of prolate droplets, held together to a significant degree by quantum-mechanical exchanges. The transition between the two regimes occurs abruptly.

cond-mat.quant-gas↗
arXiv2021

Superfluid transition and specific heat of the 2D x-y model: Monte Carlo simulation

We present results of large-scale Monte Carlo simulations of the 2D classical x-y model on the square lattice. We obtain high accuracy results for the superfluid fraction and for the specific heat as a function of temperature, for systems of linear size L up to 4096. Our estimate for the superfluid transition temperature is consistent with those furnished in all previous studies. The specific heat displays a well-defined peak, whose shape and position are independent of the size of the lattice for L > 256, within the statistical uncertainties of our calculations. The implications of these results on the interpretation of experiments on adsorbed thin films of He-4 are discussed.

cond-mat.stat-mech↗
arXiv2021

Muonium hydride: the lowest density crystal

A muonium hydride molecule is a bound state of muonium and hydrogen atoms. It has half the mass of a parahydrogen molecule and very similar electronic properties in its ground state. The phase diagram of an assembly of such particles is investigated by first principle quantum simulations. In the bulk limit, the low-temperature equilibrium phase is a crystal of extraordinarily low density, lower than that of any other known atomic or molecular crystal. Despite the low density and particle mass, the melting temperature is surprisingly high (close to 9 K). No (metastable) supersolid phase is observed. We investigated the physical properties of nanoscale clusters (up to 200 particles) of muonium hydride and found the superfluid response to be greatly enhanced compared to that of parahydrogen clusters. The possible experimental realization of these systems is discussed.

cond-mat.stat-mech↗
arXiv2021

Quasi-2D parahydrogen: On the verge of turning superfluid?

First principle computer simulations of a thin parahydrogen film adsorbed on a silica substrate at low temperature (below 6 K) yield no evidence that the top layer is liquid and/or in the proximity of a superfluid transition, as claimed in recent experimental work [T. Makiuchi et al., Phys. Rev. Lett. 123, 245301 (2019)]. Computed values of first and second layer completion densities are in quantitative agreement with experiment, but as observed also on other substrates, the top layer is an insulating crystal, quantum-mechanical exchanges of molecules are non-existent, and the overall physical behavior of the system can be understood largely along classical lines.

cond-mat.stat-mech↗
arXiv2020

Specific heat of thin $^4$He films on graphite

The specific heat of a two-layer He-4 film adsorbed on a graphite substrate is estimated as a function of temperature by Quantum Monte Carlo simulations. The results are consistent with recent experimental observations, in that they broadly reproduce their most important features. However, neither the "supersolid" nor the "superfluid hexatic" phases, of which experimental data are claimed to be evidence, are observed. It is contended that heat capacity measurements alone may not be a good predictor of structural and superfluid transitions in this system, as their interpretation is often ambiguous.

cond-mat.stat-mech↗
arXiv2020

Tuning the quantumness of simple Bose systems: A universal phase diagram

We present a comprehensive theoretical study of the phase diagram of a system of many Bose particles interacting with a two-body central potential of the so-called Lennard-Jones form. First-principles path-integral computations are carried out, providing essentially exact numerical results on the thermodynamic properties. The theoretical model used here provides a realistic and remarkably general framework for describing simple Bose systems ranging from crystals to normal fluids to superfluids and gases. The interplay between particle interactions on the one hand, quantum indistinguishability and delocalization on the other, is characterized by a single quantumness parameter, which can be tuned to engineer and explore different regimes. Taking advantage of the rare combination of the versatility of the many-body Hamiltonian and the possibility for exact computations, we systematically investigate the phases of the systems as a function of pressure (P) and temperature (T), as well as the quantumness parameter. We show how the topology of the phase diagram evolves from the known case of He-4, as the system is made more (and less) quantum, and compare our predictions with available results from mean-field theory. Possible realization and observation of the phases and physical regimes predicted here are discussed in various experimental systems, including hypothetical muonic matter.

cond-mat.stat-mech↗
arXiv2020

Roton excitation in overpressurized superfluid He-4

We carry out a theoretical investigation of overpressurized superfluid phases of He-4 by means of quantum Monte Carlo (QMC) simulations. As a function of density, we study structural and superfluid properties, and estimate the energy of the roton excitation by inverting imaginary-time density correlation functions computed by QMC, using Maximum Entropy. We estimate the pressure at which the roton energy vanishes to be about 100 bars, which we identify with the spinodal density, i.e., the upper limit for the existence of a metastable superfluid phase.

cond-mat.stat-mech↗
arXiv2020

Comment on "Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipolar stripes"

In a recent article [R. Bombin, F. Mazzanti and J. Boronat, Phys. Rev. A100, 063614 (2019)], it is contended that a two-dimensional system of dipolar bosons, with dipole moments aligned at particular angles with respect to the direction perpendicular to the plane of motion, featuring a "striped" crystalline ground state, in turn undergoes a Berezinskii-Kosterlitz-Thouless superfluid transition at low temperature, making it a two-dimensional supersolid. We show here that the results provided therein, obtained by means of Quantum Monte Carlo simulations, do not actually support such a conclusion. Rather, they are consistent with that expounded in our work [J. Low Temp. Phys. 196, 413 (2019)], namely that the striped ground state is insulating (i.e., non-superfluid in the conventional sense), essentially behaving like a system of quasi-one-dimensional, parallel independent chains. We attribute the incorrectness of the conclusion reached by Bombin et al. to the very small sizes of their simulated system, which do not allow for a reliable extrapolation to the thermodynamic limit.

cond-mat.quant-gas↗
arXiv2020

Superfluid response of parahydrogen clusters in superfluid He-4

First principle computer simulations yield evidence of a finite superfluid response at low temperature of a parahydrogen cluster of 15 molecules in bulk superfluid He-4. The superfluid fraction is worth ~44% at T=0.25 K, growing to about 66% at T=0.15 K, i.e., it is substantially reduced compared to that of the same cluster in vacuo, due to higher molecular localization. The implications of these findings on the interpretation of experiments with linear molecules embedded in parahydrogen clusters immersed in superfluid helium are discussed.

cond-mat.other↗
arXiv2019

Dipolar bosons in one dimension: the case of longitudinal dipole alignment

We study by quantum Monte Carlo simulations the low-temperature phase diagram of dipolar bosons confined to one dimension, with dipole moments aligned along the direction of particle motion. A hard core repulsive potential of varying range ($σ$) is added to the dipolar interaction, in order to ensure stability of the system against collapse. In the $σ\to 0$ limit the physics of the system is dominated by the potential energy and the ground state is quasi-crystalline; as $σ$ is increased the attractive part of the interaction weakens and the equilibrium phase evolves from quasi-crystalline to a non-superfluid liquid. At a critical value $σ_c$, the kinetic energy becomes dominant and the system undergoes a quantum phase transition from a self-bound liquid to a gas. In the gaseous phase with $σ\toσ_c$, at low density attractive interactions bring the system into a "weak" superfluid regime. However, gas-liquid coexistence also occurs, as a result of which the topologically protected superfluid regime is not approached.

cond-mat.stat-mech↗
arXiv2019

Patterned Supersolids in Dipolar Bose Systems

We study by means of first principle Quantum Monte Carlo simulations the ground state phase diagram of a system of dipolar bosons with aligned dipole moments, and with the inclusion of a two-body repulsive potential of varying range. The system is shown to display a supersolid phase in a relatively broad region of the phase diagram, featuring different crystalline patterns depending on the density and on the range of the repulsive part of the interaction (scattering length). The supersolid phase is sandwiched between a classical crystal of parallel filaments and a homogeneous superfluid phase. We show that a "roton" minimum appears in the elementary excitation spectrum of the superfluid as the system approaches crystallization.

cond-mat.quant-gas↗
arXiv2019

Absence of superfluidity in 2D dipolar Bose striped crystals

We present results of computer simulations at low temperature of a two-dimensional system of dipolar bosons, with dipole moments aligned at an arbitrary angle with respect to the direction perpendicular to the plane. The phase diagram includes a homogeneous superfluid phase, as well as triangular and striped crystalline phases, as the particle density and the tilt angle are varied. In the striped solid, no phase coherence among stripes and consequently no ``supersolid" phase is found, in disagreement with recent theoretical predictions.

cond-mat.stat-mech↗
arXiv2019

Second layer crystalline phase of helium films on graphite

We investigate theoretically the existence at low temperature of a commensurate (4/7) crystalline phase of a layer of either He isotope on top of a He-4 layer adsorbed on graphite. We make use of a recently developed, systematically improvable variational approach which allows us to treat both isotopes on an equal footing. We confirm that no commensurate crystalline second layer of He-4 forms, in agreement with all recent calculations. Interestingly and more significantly, we find that even for He-3 there is no evidence of such a phase, as the system freezes into an {\it incommensurate} crystal at a coverage lower than that (4/7) at which a commensurate one has been predicted, and for which experimental claims have been made. Implications on the interpretation of recent experiments with helium on graphite are discussed.

cond-mat.other↗
arXiv2018

Dynamic structure factor of superfluid He-4 from Quantum Monte Carlo: Maximum Entropy revisited

We use the Maximum Entropy Method (MaxEnt) to estimate the dynamic structure factor of superfluid He-4 at T=1 K, by inverting imaginary-time density correlation functions computed by a Quantum Monte Carlo (QMC) simulation. Our procedure consists of a Metropolis random walk in the space of all possible spectral images, sampled from a probability density which includes the entropic prior, in the context of the so-called "classic" MaxEnt. Comparison with recent work by other authors shows that, contrary to what is often stated, sharp features in the reconstructed image are not "washed out" by the entropic prior if the underlying QMC data have sufficient accuracy. Only spurious features that tend to appear in a straightforward chi-square minimization are suppressed.

cond-mat.stat-mech↗