Ground State Phase Diagram of Parahydrogen in One Dimension

The low-temperature phase diagram of parahydrogen in one dimension is studied by quantum Monte Carlo simulations, whose results are interpreted within the framework of Luttinger liquid theory. We show that, contrary to what was claimed in a previous study [Phys. Rev. Lett. 85, 2348 (2000)], the equilibrium phase is a crystal. The phase diagram mimics that of parahydrogen in two dimensions, with a single quasicrystaline phase and no quantum phase transition; i.e., it is qualitatively different from that of Helium-four in one dimension.Comment: Eq. (1) corrected (published version has pi at the denominator of the first term of the right hand side, instead of pi squared

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.Comment: Replaced with published versio

Mesoscopic dipolar quantum crystals

The ground state of a two-dimensional, harmonically confined mesoscopic assembly of up to thirty polar molecules is studied by computer simulations. As the strength of the confining trap is increased, clusters evolve from superfluid, to supersolid, to insulating crystals. For strong confinement, the crystalline structure can be predicted based on classical energetics. However, clusters of specific numbers of particles (i.e., N=12 and N=19) display a {\it non-classical crystalline structure}, stabilized by quantum effects, in an intermediate range of confinement strength. In these cases, coexistence of quantum and classical crystalline configurations is observed at finite temperature.Comment: 5 pages, 4 figure

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.Comment: 7 pages, 6 color figure

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.Comment: Replaced with published versio

Quantum statistics and the momentum distribution of liquid para-hydrogen

Extensive Monte Carlo simulations of bulk liquid para-hydrogen at a temperature T=16.5 K have been carried out using the continuous-space Worm Algorithm. Results for the momentum distribution, as well as for the kinetic energy per particle and the pair correlation function are reported. The static equilibrium thermodynamic properties of this system can be generally computed by assuming that molecules are distinguishable. However, the one-body density matrix (and the associated momentum distribution) are affected by particle indistinguishability and quantum statistics, to an extent that lends itself to experimental observation. Comparisons with available experimental data and other theoretical and numerical calculations are offered.Comment: Replaced with published versio

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.Comment: Replaced with published versio

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

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 discussedComment: 6 pages, 5 figures, 1 tabl

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.Comment: 7 pages, 4 figures in colo