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We study the superfluid response, the energetic and structural properties of a one-dimensional ultracold Bose gas in an optical lattice of arbitrary strength. We use the Bose-Fermi mapping in the limit of infinitely large repulsive interaction and the diffusion Monte Carlo method in the case of finite interaction. For slightly incommensurate fillings we find a superfluid behavior which is discussed in terms of vacancies and interstitials. It is shown that both the excitation spectrum and static structure factor are different for the cases of microscopic and macroscopic fractions of defects. This system provides a extremely well-controlled model for studying defect-induced superfluidity., 14 pages, 13 figures, published version

By using the diffusion Monte Carlo method, we obtained the full phase diagram of $^{3}\mathrm{He}$ on top of graphite preplated with a solid layer of $^{4}\mathrm{He}$. All the $^{4}\mathrm{He}$ atoms of the substrate were explicitly considered and allowed to move during the simulation. We found that the ground state is a liquid of density $0.007\ifmmode\pm\else\textpm\fi{}0.001$ ${\AA{}}^{\ensuremath{-}2}$, in good agreement with available experimental data. This is significantly different from the case of $^{3}\mathrm{He}$ on clean graphite, in which both theory and experiment agree on the existence of a gas-liquid transition at low densities. Upon an increase in $^{3}\mathrm{He}$ density, we predict a first-order phase transition between a dense liquid and a registered 7/12 phase, the 4/7 phase being found metastable in our calculations. At larger second-layer densities, a final transition is produced to an incommensurate triangular phase.

Strongly interacting systems of dipolar bosons in three dimensions confined by harmonic traps are analyzed using the exact Path Integral Ground State Monte Carlo method. By adding a repulsive two-body potential, we find a narrow window of interaction parameters leading to stable ground- state configurations of droplets in a crystalline arrangement. We find that this effect is entirely due to the interaction present in the Hamiltonian without resorting to additional stabilizing mechanisms or specific three-body forces. We analyze the number of droplets formed in terms of the Hamiltonian parameters, relate them to the corresponding s-wave scattering length, and discuss a simple scaling model for the density profiles. Our results are in qualitative agreement with recent experiments showing a quantum Rosensweig instability in trapped Dy atoms., 5 pages, 4 figures

Recent heat-capacity experiments show quite unambiguously the existence of a liquid $^{3}\mathrm{He}$ phase adsorbed on graphite. This liquid is stable at an extremely low density, possibly one of the lowest found in nature. Previous theoretical calculations of the same system, and in strictly two dimensions, agree with the result that this liquid phase is not stable and the system is in the gas phase. We calculated the phase diagram of normal $^{3}\mathrm{He}$ adsorbed on graphite at $T=0$ using quantum Monte Carlo methods. Considering a fully corrugated substrate, we observe that at densities lower than $0.006\text{ }\text{ }{\AA{}}^{\ensuremath{-}2}$ the system is a very dilute gas that, at that density, is in equilibrium with a liquid of density $0.014\text{ }\text{ }{\AA{}}^{\ensuremath{-}2}$. Our prediction matches very well the recent experimental findings on the same system. On the contrary, when a flat substrate is considered, no gas-liquid coexistence is found, in agreement with previous calculations. We also report results on the different solid structures, and on the corresponding phase transitions that appear at higher densities.

The ground-state properties of spin polarized hydrogen H↓ in two dimensions (2D) are obtained by means of diffusion Monte Carlo calculations. Using the most accurate to date ab initio H↓–H↓ interatomic potential we have studied hydrogen gas phase, from the very dilute regime until densities above its freezing point. For very low densities, the equation of state of the gas can be described in terms of the gas parameter na2, where a is the s-wave scattering length in 2D. The solid phase in 2D has also been studied up to high pressures and the gas-solid phase transition determined using the double-tangent Maxwell construction.

The ground-state properties of spin-polarized deuterium with only one occupied nuclear spin state (D↓1) at zero temperature are studied by means of the variational Monte Carlo (VMC) method. Influence of the backflow correlations on the ground-state energy of this Fermi system under investigation is also explored. The calculations have been performed using several interatomic potentials and the quantum phase of the D↓1 system is discussed. Our results are compared with other previously reported variational results.

The behavior of pairs and clusters of vacancies in solid 4He crystals is studied with diffusion Monte Carlo method. We use a trial function suitable for describing solid 4He with long range order, arbitrary number of unoccupied sites and explicit Bose symmetry. It is found that vacancy clusters are unstable and collapse in all considered systems. We find no signature of stability of bound vacancy clusters of any size. Vacancies are found to exhibit weak attraction and a rough estimation of the binding energy is reported.

Extensive neutron scattering data on liquid helium 4 are available, and have been analyzed to give a number of physical quantities, e.g. static structure factor S(q), excitation energy, and roton linewidth. X-rays also give access to S(q) .H ow- ever, a comprehensive comparison between experimental data and theoretical results, including their dependence on pressure, is still lacking. The static response function χ(q) has been particularly overlooked, despite its fundamental role in theories of inhomogeneous helium. We present here a critical review about the strength of the main peaks of S(q) and χ(q). We include in the comparison the analysis of unpub- lished neutron data and new Monte-Carlo calculations of χ(q). We find a significant discrepancy between experiments, and suggest corrections which account for some of the differences. We give recommendations for the best values to use for S(q) and χ(q).

We have carried out a microscopic study of the dynamic structure factor of liquid $^4$He across the normal-superfluid transition temperature using the path integral Monte Carlo method. The ill-posed problem of the inverse Laplace transform, from the imaginary-time intermediate scattering function to the dynamic response, is tackled by stochastic optimization. Our results show a quasi-particle peak and a small and broad multiphonon contribution. In spite of the lack of strength in the collective peaks, we clearly identify the rapid dropping of the roton peak amplitude when crossing the transition temperature $T_\lambda$. Other properties such as the static structure factor, static response, and one-phonon contribution to the response are also calculated at different temperatures. The changes of the phonon-roton spectrum with the temperature are also studied. An overall agreement with available experimental data is achieved., Comment: 10 pages

Over the years many He-He interaction potentials have been developed, some very sophisticated, including various corrections beyond Born-Oppenheimer approximation. Most of them were used to predict properties of helium dimers and trimers, examples of exotic quantum states, whose experimental study proved to be very challenging. Recently, detailed structural properties of helium trimers were measured for the first time, allowing a comparison with theoretical predictions and possibly enabling the evaluation of different interaction potentials. The comparisons already made included adjusting the maxima of both theoretical and experimental correlation functions to one, so the overall agreement between theory and experiment appeared satisfactory. However, no attempt was made to evaluate the quality of the interaction potentials used in the calculations. In this work, we calculate the experimentally measured correlation functions using both new and old potentials, compare them with experimental data and rank the potentials. We use diffusion Monte Carlo simulations at $T=0\ \mathrm{K}$, which give within statistical noise exact results of the ground state properties. All models predict both trimers $^4\mathrm{He}_{3}$ and $^4\mathrm{He}_2{}^3\mathrm{He}$ to be in a quantum halo state., Comment: 21 pages, 7 figures, 4 tables

By doing quantum Monte Carlo ab initio simulations we show that dipolar excitons, which are now under experimental study, actually are strongly correlated systems. Strong correlations manifest in significant deviations of excitation spectra from the Bogoliubov one, large Bose condensate depletion, short-range order in the pair correlation function, and peak(s) in the structure factor.

We present recent results from a diffusion Monte Carlo study of small 3 He drops in the limit of zero temperature. Using the same methodology than in previous calculations carried out for bulk 3 He , we have obtained accurate results for their energy and structure properties. A relevant concern of this analysis has been the determination of the stability threshold for self binding. Our results show that the smallest self-bound drop is formed by N = 30 atoms, with preferred orbitals for open shells corresponding to maximum value of the spin. The quality of the upper bound energies provided by the fixed node approximation is analyzed by performing released-node estimations for short released times. Possible improvements of the nodal surface by including BCS-like correlation functions are also discussed.

We report the quantum phase diagram of a one-dimensional Coulomb wire obtained using the path integral Monte Carlo (PIMC) method. The exact knowledge of the nodal points of this system permits us to find the energy in an exact way, solving the sign problem which spoils fermionic calculations in higher dimensions. The results obtained allow for the determination of the stability domain, in terms of density and temperature, of the one-dimensional Wigner crystal. At low temperatures, the quantum wire reaches the quantum-degenerate regime, which is also described by the diffusion Monte Carlo method. Increasing the temperature the system transforms to a classical Boltzmann gas which we simulate using classical Monte Carlo. At large enough density, we identify a one-dimensional ideal Fermi gas which remains quantum up to higher temperatures than in two- and three-dimensional electron gases. The obtained phase diagram as well as the energetic and structural properties of this system are relevant to experiments with electrons in quantum wires and to Coulomb ions in one-dimensional confinement., Comment: 5 pages, 4 figures

We present a new set of three-body interaction models based on the Bruch-McGee (BM) potential that are suitable for the study of the energy, structural and elastic properties of solid 4He at high pressure. Our ab initio three-body potentials are obtained from the fit to total energies and atomic forces computed with the van der Waals density functional theory method due to Grimme, and represent an improvement with respect to previously reported three-body interaction models. In particular, we show that some of the introduced BM parametrizations reproduce closely the experimental equation of state and bulk modulus of solid helium up to a pressure of ~ 60 GPa, when used in combination with standard pairwise interaction models in diffusion Monte Carlo simulations. Importantly, we find that recent predictions reporting a surprisingly small variation of the kinetic energy and Lindeman ratio on quantum crystals under increasing pressure are likely to be artifacts produced by the use of incomplete interaction models. Also, we show that the experimental variation of the shear modulus, C44, at P < 25 GPa can be quantitatively described with the new set of three-body BM potentials. At higher pressures, however, the agreement between our C44 results and experiments deteriorates and thus we argue that higher order many-body terms in the expansion of the atomic interactions probably are necessary in order to better describe elasticity in very dense solid 4He., 11 pages, 7 figures

We present a diffusion Monte Carlo simulation of metastable superfluid 4 He at zero temperature and pressures beyond freezing (~ 25 bar) up to 275 bar. The equation of state of liquid 4 He is extended to the overpressurized regime, where the pressure dependence of the static structure factor and the condensate fraction is obtained. Along this large pressure range, excited-state energy corresponding to the roton has been determined using the release-node technique. Our results show that both the roton energies and the condensate fraction decrease with increasing pressure but do not become zero. We compare our calculations to recent experimental data in overpressurized regime.

The ground state of the ionic solids LiD and LiH is theoretically studied using the Variational Monte Carlo (VMC) method. Our main focus has been the calculation of relevant properties of the H− and D− ions using a fully quantum approach. In particular, we report results on their kinetic energies, mean-squared displacements, Einstein frequencies, radial distribution functions, and density profiles around the sites. The microscopic results obtained for both isotopes show corrections beyond trivial isotopic effects due to their quantum behavior. Finally, the VMC results are compared with predictions from self consistent average phonon (SCAP) theory at T = 0 K.

We present a variational Monte Carlo calculation of a small 3He cluster within the Jastrow-Feenberg-Slater wave function approach. The trial wave function incorporates, in a progressive way, higher order correlations to elucidate at a quantitative level the relevance and possible interference of the different correlation mechanisms. As proved previously in bulk, the present results show the importance of backflow correlations to achieve a realistic description of this Fermi system. On the other hand, the introduction of three-body dynamical correlations also improves the ground-state energy, reducing appreciably the difference between this variational approach and more exact results obtained using the diffusion Monte Carlo method.

In recent experiments, Balibar and collaborators have been able to produce metastable superfluid $^4$He up to approximately 200 bar, a pressure much higher than the freezing point (25 bar). Interpretation of some of their experimental data requires the knowledge of the equation of state of bulk $^4$He in this high-pressure regime. As this data is not experimentally accessible, Maris and collaborators have proposed to use the data extrapolated from analytical fits to the well known equation of state in the stable regime. Moreover, combining these extrapolations with density functional estimations of the dependence of the roton energy on the pressure, Maris and co. predicted the existence of a maximum liquid pressure ($\sim$ 200 bar) due to the achievement of a zero-energy roton. Using the same theoretical analysis that allowed in the past to reproduce accurately the equation of state of liquid $^4$He in the stable domain, we present now extended results for this equation of state up to $\sim$ 280 bar. Our calculations, based on the diffusion Monte Carlo method, show some significant differences with the proposed extrapolations, which could be relevant for future experiments. Also, preliminary data show a rather slow decrease of the roton energy with increasing pressure, which locates the possible instability much higher in pressure than Maris's predictions.

An exhaustive analysis of 4He clusters at T=0 using the diffusion Monte Carlo method has been carried out searching for possible shell structures. Contrary to previous microscopic calculations, we have focused our attention on a one-by-one number of atoms calculation around the experimental magic numbers Nm. The energy per particle and density profiles are therefore obtained with high resolution. The resugts obtained for the chemical potential as a function of the number of atoms do not show any signature of shell effects around the selected Nm values. Influence on the density profiles is not observed either.

We utilize structural information from microscopic variational and diffusion Monte Carlo calculations for two- and three-dimensional 3He to calculate the dynamic structure function and the effective mass of 3He. Static effective interactions are constructed from the density- and spin structure functions using sumrules. It is shown that mugtiparticle excitations are important for understanding the dynamic structure function. We formulate a theory that contains such excitations and maintains the mo and ml sumrules. Our implementation of the theory describes, at a semi-quantitative level, the salient experimental features of S(k, ω): A lowering of the collective mode energy, a reduction of the strength of the particle-hole band, and mugtiparticle excitations above the particle-hole continuum. The contribution of both spinand density fluctuations to the effective mass are of comparable size, spinfluctuations are somewhat more important in 2D than in 3D. Our resugts show, in agreement with experiments, a flattening of the single particle selfenergy with increasing density, which eventually leads to a divergent effective mass.

A quantum Monte Carlo study at zero temperature of twodimensional H2 on top of a solid Rb surface is presented. The lattice constituted by the alkali atoms frustrates the formation of solid H2, which is the natural phase of pure two-dimensional H2. Therefore, H2 with the Rb atoms acting like fixed impurities remains in a liquid phase even at zero temperature. We present resugts for the H2 equation of state, structural properties, superfluid density and condensate fraction for a fixed Rb lattice.

We present a first-principles computational study of solid $^{4}\mathrm{He}$ at $T=0$ K and pressures up to $\ensuremath{\sim}160$ GPa. Our computational strategy consists in using van der Waals density functional theory (DFT-vdW) to describe the electronic degrees of freedom in this material, and the diffusion Monte Carlo (DMC) method to solve the Schr\"odinger equation describing the behavior of the quantum nuclei. For this, we construct an analytical interaction function based on the pairwise Aziz potential that closely matches the volume variation of the cohesive energy calculated with DFT-vdW in dense helium. Interestingly, we find that the kinetic energy of solid $^{4}\mathrm{He}$ does not increase appreciably with compression for $P\ensuremath{\ge}85$ GPa. Also, we show that the Lindemann ratio in dense solid $^{4}\mathrm{He}$ amounts to $0.10$ almost independently of pressure. The reliability of customary quasiharmonic DFT (QH DFT) approaches in describing quantum nuclear effects in solids is also studied. We find that QH DFT simulations, although provide a reasonable equation of state in agreement with experiments, are not able to reproduce correctly these critical effects in compressed $^{4}\mathrm{He}$. In particular, we disclose huge discrepancies of at least $\ensuremath{\sim}50%$ in the calculated $^{4}\mathrm{He}$ kinetic energies using both the QH DFT and present DFT-DMC methods.

We present results of a path integral Monte Carlo (PIMC) calculation of the momentum distribution of Ne and normal 4 He at low temperatures. In the range of temperatures analyzed, consequently exchanges can be disregarded and both systems are considered Boltzmann quantum liquids. Their quantum character is well reflected in their momentum distributions which show clear departures from the classical limit. The PIMC momentum distributions are sampled using a new and more efficient method that generalises the standard McMillan approach. Kinetic energies of both systems as a function of temperature and at a fixed density are also reported.

We present the results of a diffusion Monte Carlo calculation on the groundstate energy of 4He adsorbed in the interstitial channels of a bundle of (10, 10) nanotubes. The one-dimensionality of that system is analyzed by comparing the bundle results with the equation of state of purely 1D 4He. In spite of the narrowness of the intersites, significant differences in both the binding energy and the equilibrium density of the two systems appear. Moreover, additional attraction provided by neighboring interchannels filled with helium modifies in a sizeable amount the equation of state of a single intersite.

The ground-state properties of one-dimensional 3He are studied using quantum Monte Carlo methods. The equation of state is calculated in a wide range of physically relevant densities and is well reproduced by a power-series fit. The Luttinger liquid theory is found to describe the long-range properties of the correlation function. The density dependence of the Luttinger parameter is explicitly found and interestingly it shows a non-monotonic behavior. Depending on the density, the static structure factor can be a smooth function of the momentum or might contain a peak of a finite or infinite height. Although no phase transitions are present in the system, we identify a number of physically different regimes, including an ideal Fermi gas, a "Bose-gas", a "super-Tonks-Girardeau" regime, and a "quasi-crystal"., 4 figures, published version

We analyze the ground state of a system of dipolar bosons moving in the $XY$ plane and such that their dipolar moments are all aligned in a fixed direction in space. We focus on the general case where the polarization field forms a generic angle $\ensuremath{\alpha}$ with respect to the $Z$ axis. We use the path-integral ground-state method to analyze the static properties of the system as both $\ensuremath{\alpha}$ and the density $n$ vary over a wide range where the system is stable. We use the maximum of the static structure function as an order parameter to characterize the different phases and the transition lines among them. We find that, in addition to a superfluid gas and a solid phase, the system reaches a stripe phase at large tilting angles that is entirely induced by the anisotropic character of the interaction. We also show that the quantum phase transition from the gas to the stripe phase is of second order and report approximate values for the critical exponents.

We study a system of two bosons of one species and a third boson of a second species in a one-dimensional parabolic trap at zero temperature. We assume contact repulsive inter- and intra-species interactions. By means of an exact diagonalization method we calculate the ground and excited states for the whole range of interactions. We use discrete group theory to classify the eigenstates according to the symmetry of the interaction potential. We also propose and validate analytical ansatzs gaining physical insight over the numerically obtained wavefunctions. We show that, for both approaches, it is crucial to take into account that the distinguishability of the third atom implies the absence of any restriction over the wavefunction when interchanging this boson with any of the other two. We find that there are degeneracies in the spectra in some limiting regimes, that is, when the inter-species and/or the intra-species interactions tend to infinity. This is in contrast with the three-identical boson system, where no degeneracy occurs in these limits. We show that, when tuning both types of interactions through a protocol that keeps them equal while they are increased towards infinity, the systems's ground state resembles that of three indistinguishable bosons. Contrarily, the systems's ground state is different from that of three-identical bosons when both types of interactions are increased towards infinity through protocols that do not restrict them to be equal. We study the coherence and correlations of the system as the interactions are tuned through different protocols, which permit to built up different correlations in the system and lead to different spatial distributions of the three atoms., 14 pages, 7 figures

We consider the ground state of a bilayer system of dipolar bosons, where dipoles are oriented by an external field in the direction perpendicular to the parallel planes. Quantum Monte Carlo methods are used to calculate the ground-state energy, the one-body and two-body density matrix, and the superfluid response as a function of the separation between layers. We find that by decreasing the interlayer distance for fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from a single-particle to a pair superfluid. The single-particle superfluid is characterized by a finite value of both the atomic condensate and the super-counterfluid density. The pair superfluid phase is found to be stable against formation of many-body cluster states and features a gap in the spectrum of elementary excitations., Comment: 4 figures

We study the zero-temperature quantum phase transition between liquid and hcp solid $^{4}\mathrm{He}$. We use the variational method with a simple yet exchange-symmetric and fully explicit wave function. It is found that the optimized wave function undergoes spontaneous symmetry breaking and describes the quantum solidification of helium at 22 atm. The explicit form of the wave function allows us to consider various contributions to the phase transition. We find that the employed wave function is an excellent candidate for describing both a first-order quantum phase transition and the ground state of a Bose solid.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We study monolayers of noble gas atoms (Xe, Kr, Ar, and Ne) deposited on individual ultraclean suspended nanotubes. For this, we record the resonance frequency of the mechanical motion of the nanotube, since it provides a direct measure of the coverage. The latter is the number of adsorbed atoms divided by the number of the carbon atoms of the suspended nanotube. Monolayers form when the temperature is lowered in a constant pressure of noble gas atoms. The coverage of Xe monolayers remains constant at 1/6 over a large temperature range. This finding reveals that Xe monolayers are solid phases with a triangular atomic arrangement, and are commensurate with the underlying carbon nanotube. By comparing our measurements to theoretical calculations, we identify the phases of Ar and Ne monolayers as fluids, and we tentatively describe Kr monolayers as solid phases. These results underscore that mechanical resonators made from single nanotubes are excellent probes for surface science. © 2014 American Physical Society., We acknowledge support from the European Union through the Graphene Flagship (Grant No. 604391), the ERCcarbonNEMS project, and a Marie Curie Grant (No. 271938), the Spanish state (Grant No. MAT2012-31338), and the Catalan government (AGAUR, SGR). C.G. and J. B. acknowledge partial financial support from the Junta de de Andalucía Group PAI-205, Grant No. FQM-5987, MICINN (Spain) Grants No. FIS2010-18356 and No. FIS2011-25275, and Generalitat de Catalunya Grant No. 2009SGR-1003.

We have studied molecular hydrogen in a pure 1D geometry and inside a narrow carbon nanotube by means of the diffusion Monte Carlo method. The one dimensionality of ${\mathrm{H}}_{2}$ in the nanotube is well maintained in a large density range, this system being closer to an ideal 1D fluid than liquid ${}^{4}\mathrm{He}$ in the same setup. ${\mathrm{H}}_{2}$ shares with ${}^{4}\mathrm{He}$ the existence of a stable liquid phase and a quasicontinuous liquid-solid transition at very high linear densities.

The application of the diffusion Monte Carlo method to a strongly interacting Fermi system as normal liquid 3He is explored. We show that the fixed-node method together with the released-node technique and a systematic method to analytically improve the nodal surface constitute an efficient strategy to improve the calculation up to a desired accuracy. This methodology shows unambiguously that backflow correlations are enough to generate an equation of state of liquid 3He in excellent agreement with experimental data from equilibrium up to freezing.

We have investigated the feasibility of the hot-filament chemical vapour deposition (HFCVD) technique to synthesise carbon nanostructures with attractive electron emission properties on nickel and cobalt thin films and dots. For this purpose, a systematic study of the influence of deposition conditions on both growth morphology and emission characteristics was undertaken and complemented by micro-Raman characterisations. Depending on the substrate preparation and synthesis conditions, carbon nanostructures with completely different growth morphologies were prepared. In particular, we have highlighted the peculiarity of the HFCVD technique to favour an oriented and coral-like growth mode of a complex material composed essentially of carbon nanotubes and nanoparticles. By this means, a threshold electric field as low as 4 V μm −1 has been reached for a deposition temperature compatible with glass panels.

Apart form their possible technological applications, the adsorption of quantum liquids inside carbon nanotubes allows for an experimental realization of quasi-one dimensional systems. We present a comparison between energetic and structural properties of 4 He and H 2 inside a (5,5) nanotube and in a pure 1D arrangement based in diffusion Monte Carlo calculations.

We report results of diffusion Monte Carlo calculations for both ${}^{4}\mathrm{He}$ absorbed in a narrow single walled carbon nanotube $(R=3.42\AA{})$ and strictly one-dimensional ${}^{4}\mathrm{He}.$ Inside the tube, the binding energy of liquid ${}^{4}\mathrm{He}$ is approximately three times larger than on planar graphite. At low linear densities, ${}^{4}\mathrm{He}$ in a nanotube is an experimental realization of a one-dimensional quantum fluid. However, when the density increases the structural and energetic properties of both systems differ. At high density, a quasicontinuous liquid-solid phase transition is observed in both cases.

We have applied the fixed-node and the released-node Monte Carlo techniques to calculate the phonon-roton excitation spectrum (e(q)) of superfluid4He. An excellent agreement with the experimental spectrum is achieved both at the equilibrium and near the freezing densities. The strength of the single excitation peak Z(q) in the dynamic structure function S(q, ω) shows an overall agreement with experimental data. New results for e(q) and Z(q) at a negative pressure close to the spinodal point are also reported.

The static properties of a single hydrogen atom and an hydrogen molecule in bulk superfluid4He are studied by means of the diffusion Monte Carlo method. The analysis includes atomic hydrogen, deuterium and tritium, and molecular hydrogen and deuterium. Our results, which show some differences with previous variational calculations, are in agreement with the only available experimental data corresponding to atomic deuterium. In contrast to the unbinding presented by the atomic impurities, the molecular ones do bind with energies higher than the chemical potential of pure4He.

Ground state of weakly bound dimers and trimers with a radius extending well into the classically forbidden region is explored, with the goal to test the predicted universality of quantum halo states. The focus of the study are molecules consisting of T$\downarrow$, D$\downarrow$, $^3$He, $^4$He and alkali atoms, where interaction between particles is much better known than in the case of nuclei, which are traditional examples of quantum halos. The study of realistic systems is supplemented by model calculations in order to analyze how low-energy properties depend on the interaction potential. The use of variational and diffusion Monte Carlo methods enabled very precise calculation of both size and binding energy of the trimers. In the quantum halo regime, and for large values of scaled binding energies, all clusters follow almost the same universal line. As the scaled binding energy decreases, Borromean states separate from tango trimers., Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Lett

We present variational calculations of the one-body density matrices and momentum distributions for 3 He- 4 He mixtures in the zero-temperature limit, in the framework of the correlated basis functions theory. The ground-state wave function contains two- and three-body correlations and the matrix elements are computed by ~Fermi! hypernetted chain techniques. The dependence on the 3 He concentration (x 3) of the 4 He condensate fraction ( n04) ) and of the 3 He pole strength (ZF) is studied along the P50 isobar. At low 3 He concentrations, the computed 4 He condensate fraction is not significantly affected by the 3 He statistics. Despite the low x 3 values, ZF is found to be quite smaller than that of the corresponding pure 3 He because of the strong 3 He- 4 He correlations and of the overall, large total density r. A small increase of n04) along x 3 is found, which is mainly due to the decrease of r with respect to the pure- 4 He phase. @S0163-1829~97!03242-6#

The experimental realization of a thin layer of spin-polarized hydrogen adsorbed on top of the surface of superfluid 4He provides one of the best examples of a stable nearly two-dimensional quantum Bose gas. We report a theoretical study of this system using quantum Monte Carlo methods in the limit of zero temperature. Using the full Hamiltonian of the system, composed of a superfluid 4He slab and the adsorbed hydrogen layer, we calculate the main properties of its ground state using accurate models for the pair interatomic potentials. Comparing the results for the layer with the ones obtained for a strictly two-dimensional (2D) setup, we analyze the departure from the 2D character when the density increases. Only when the coverage is rather small the use of a purely 2D model is justified. The condensate fraction of the layer is significantly larger than in 2D at the same surface density, being as large as $60\%$ at the largest coverage studied., 6 pages, to appear in J. Chem. Phys

We study the elastic properties of incommensurate solid ${}^{4}$He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants (${C}_{11}$, ${C}_{12}$, ${C}_{13}$, ${C}_{33}$, and ${C}_{44}$), longitudinal and transversal speeds of sound, and the $T=0$ Debye temperature of incommensurate and commensurate hcp ${}^{4}$He using the diffusion Monte Carlo method. Our results show that under compression, the commensurate crystal is globally stiffer than the incommensurate, however at pressures close to melting (i.e., $P\ensuremath{\sim}25$ bar) some of the elastic constants accounting for strain deformations of the hcp basal plane (${C}_{12}$ and ${C}_{13}$) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of ${}^{4}$He (${C}_{44}$) undergoes a small reduction.

We study the zero-temperature phase diagrams of H2 adsorbed on the three structures predicted for graphane (chair, boat and washboard graphane), using a dif- fusion Monte Carlo technique. Graphane is the hydrogenated version of graphene, in which each carbon atom changes its hybridization to sp 3 and forms a covalent bond with a hydrogen atom. Our results show that the ground state of H2 adsorbed on all three types of graphane is a √ 3 × √ 3 solid, similar to the structures found both for H2 and D2 on graphene. When the H2 density increases, the system undergoes a first order phase transition to a triangular incommensurate solid. This change is direct in the case of washboard graphane, but indirect via different commensurate structures in the other cases. The total hydrogen weight percentage on the three graphane types in their ground states is in the range 10 % to 12 %, depending on if one or both graphane surfaces are covered with H2.

By means of diffusion Monte Carlo calculations, we obtained the phase diagrams of a first and second layer of 4He on graphene and on the outside of different isolated armchair carbon nanotubes with radii in the range 3.42 to 10.85 Å. That corresponds to tubes between the (5, 5) and (16, 16) in standard nomenclature. In both cases, the ground state is either a liquid (second layer on graphene and on nanotubes whose radii is greater than ∼7 Å) or an incommensurate solid (for thinner tubes). In the former case, upon a density increase, the system undergoes a first-order phase transition to another incommensurate solid. A study of the influence of the C–He potential (isotropic or anisotropic) on the phase diagrams is also presented.

We report diffusion Monte Carlo calculations on the phase diagrams of para-H2 and ortho-D2 adsorbed on top of a first layer of the same substances on graphene. We found that the ground state of the second layer is a triangular incommensurate solid for both isotopes. The densities for promotion to a second layer and for the onset of a two-dimensional solid on that second layer compare favorably to available experimental data in both cases.

We present a diffusion Monte Carlo study of a single vortex in two-dimensional superfluid liquid {sup 4}He within the fixed-node approximation. We use both the Feynman phase and an improved phase which includes backflow correlations to model the nodal surface of the vortex wave function. Results for the particle density, core radius, and excitation energies are presented. {copyright} {ital 1996 The American Physical Society.}

The ground-state properties of two-dimensional liquid {sup 4}He at zero temperature are studied by means of a quadratic diffusion Monte Carlo method. As interatomic potential we use a revised version of the HFDHE2 Aziz potential which is expected to give a better description of the interaction between helium atoms. The equation of state is determined with great accuracy over a wide range of densities in the liquid phase from the spinodal point up to the freezing density. The spinodal decomposition density is estimated and other properties of the liquid, such as the radial distribution function, static form factor, and density dependence of the condensate fraction, are all presented. {copyright} {ital 1996 The American Physical Society.}

A study of final-state effects (FSE) on the dynamic structure function of superfluid {sup 4}He in the Gersch-Rodriguez formalism is presented. The main ingredients needed in the calculation are the momentum distribution and the semidiagonal two-body density matrix. The influence of these ground-state quantities on the FSE is analyzed. A variational form of {rho}{sub 2} is used, even though simpler forms turn out to give accurate results if properly chosen. Comparison to the experimental response at high momentum transfer is performed. The predicted response is quite sensitive to slight variations on the value of the condensate fraction, the best agreement with experiment being obtained with {ital n}{sub 0}=0.082. Sum rules of the FSE broadening function are also derived and commented. Finally, it is shown that Gersch-Rodriguez theory produces results as accurate as those coming from other more recent FSE theories. {copyright} {ital 1996 The American Physical Society.}

We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H2) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H2 molecules, we employ the diffusion Monte Carlo method and realistic semi-empirical pairwise potentials describing the interactions between H2-H2 and Na-H2 species. In particular, we calculate the energetic, structural and superfluid properties of two-dimensional Na-H2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H2 ground-state is a liquid with a significantly large superfluid fraction of ~30%. The appearance of p-H2 superfluid response is due to the fact that the interactions between Na atoms and H2 molecules are less attractive than between H2 molecules. This induces a considerable reduction of the hydrogen density which favours the stabilization of the liquid phase., Comment: 7 pages, 6 figures, submitted

We show that a two-component mixture of a few repulsively interacting ultracold atoms in a one-dimensional trap possesses very different quantum regimes and that the crossover between them can be induced by tuning the interactions in one of the species. In the composite fermionization regime, where the interactions between both components are large, none of the species show large occupation of any natural orbital. Our results show that by increasing the interaction in one of the species, one can reach the phase-separated regime. In this regime, the weakly interacting component stays at the center of the trap and becomes almost fully phase coherent, while the strongly interacting component is displaced to the edges of the trap. The crossover is sharp, as observed in the in the energy and the in the largest occupation of a natural orbital of the weakly interacting species. Such a transition is a purely mesoscopic effect which disappears for large atom numbers., 5 pages, 3 figures