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The equation of state of H2 adsorbed in the interstitial channels of a carbon nanotube bundle has been calculated using the diffusion Monte Carlo method. The possibility of a lattice dilation, induced by H2 adsorption, has been analyzed by modeling the cohesion energy of the bundle. The influence of factors like the interatomic potentials, the nanotube radius and the geometry of the channel on the bundle swelling is systematically analyzed. The most critical input is proved to be the C-H2 potential. Using the same model than in planar graphite, which is expected to be also accurate in nanotubes, the dilation is observed to be smaller than in previous estimations or even inexistent. H2 is highly unidimensional near the equilibrium density, the radial degree of freedom appearing progressively at higher densities., Comment: Accepted for publication in PRB

The implementation and reliability of a quadratic diffusion Monte Carlo method for the study of ground-state properties of atoms are discussed. We show in the simple yet non-trivial calculation of the binding energy of the Li atom that the method presented is effectively second-order in the time step. The fulfilment of the expected quadratic behavior relies on some basic requirements of the trial wave function used for importance sampling, in the context of the fixed-node approximation. Expectation values of radial operators are calculated by means of a pure estimation based on the forward walking methodology. It is shown that accurate results without extrapolation errors can be obtained with a pure algorithm that can be easily implemented in any previous diffusion Monte Carlo program., Comment: RevTex, 20 pages, 3 figures, accepted in J. Chem. Phys

We investigate the phenomenon of Bose-Einstein condensation and superfluidity in a Bose gas at zero temperature with disorder. By using the Diffusion Monte-Carlo method we calculate the superfluid and the condensate fraction of the system as a function of density and strength of disorder. In the regime of weak disorder we find agreement with the analytical results obtained within the Bogoliubov model. For strong disorder the system enters an unusual regime where the superfluid fraction is smaller than the condensate fraction., Comment: 4 pages, 4 Postscript figures

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 H2 in the nanotube is well maintained in a large density range, this system being closer to an ideal 1D fluid than liquid 4He in the same setup. H2 shares with 4He the existence of a stable liquid phase and a quasi-continuous liquid-solid transition at very high linear densities., Comment: 5 pages, 4 figures, 2 Tables

We use a diffusion Monte Carlo method to calculate the lowest energy state of a uniform gas of bosons interacting through different model potentials, both strictly repulsive and with an attractive well. We explicitly verify that at low density the energy per particle follows a universal behavior fixed by the gas parameter na^3. In the regime of densities typical for experiments in trapped Bose-condensed gases, the corrections to the mean-field energies greatly exceed the differences due to the details of the potential., Comment: 4 pages Latex, 4 postscript figures

Diffusion Monte Carlo results for the phonon-roton excitation branch in bulk liquid $^4$He at zero temperature are presented. The sign problem associated to the excited wave function has been dealt both with the fixed-node approximation and the released-node technique. The upper bounds provided by the fixed-node approximation are shown to become exact when using the released-node method. An excellent agreement with experimental data is achieved both at the equilibrium and near the freezing densities., Comment: 12 pages, RevTex, 3 ps figures included

We present a diffusion Monte Carlo study of a single vortex in two-dimensional superfluid liquid $^4$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 wavefunction. Results for the particle density, core radius and excitation energies are presented., Comment: 11 pages, 3 postscript figures, Revtex

A study of Final State Effects (FSE) on the dynamic structure function of superfluid $^4$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 $\rho_2$ 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 $n_0=0.082$. 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., Comment: 20 pages, RevTex 3.0, 11 figures available upon request, to be appear in Phys. Rev. B

The ground-state properties of two-dimensional liquid $^4$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 radial distribution function, static form factor, momentum distribution and density dependence of the condensate fraction are all presented., Comment: 19 pages, RevTex 3.0, 7 figures available upon request

The sum rule approach is used to derive upper bounds for the dispersion law $\omega_0(q)$ of the elementary excitations of a Bose superfluid. Bounds are explicitly calculated for the phonon-roton dispersion in superfluid $^4$He, both at equilibrium ($\rho=0.02186$ \AA$^{-3}$) and close to freezing ($\rho=0.02622$ \AA$^{-3}$). The bound $\omega_0(q) \le 2S(q)\mid\chi(q)\mid^{-1}$, where $S(q)$ and $\chi(q)$ are the static structure factor and density response respectively, is calculated microscopically for several values of the wavevector $q$. The results provide a significant improvement with respect to the Feynman approximation $\omega_F(q)= q^2(2mS(q))^{-1}$. A further, stronger bound, requiring the additional knowledge of the current correlation function is also investigated. New results for the current correlation function are presented., Comment: 13 pages, REVTEX, 7 figures available upon request or at http://anubis.science.unitn.it/~dalfovo/papers/papers.html

A Monte Carlo algorithm for computing quantum mechanical expectation values of coordinate operators in many body problems is presented. The algorithm, that relies on the forward walking method, fits naturally in a Green's Function Monte Carlo calculation, i.e., it does not require side walks or a bilinear sampling method. Our method evidences stability regions large enough to accurately sample unbiased pure expectation values. The proposed algorithm yields accurate results when it is applied to test problems as the hydrogen atom and the hydrogen molecule. An excellent description of several properties of a fully many body problem as liquid $^4$He at zero temperature is achieved., Comment: 20 pages, RevTex 3.0, 7 figures available upon request to the Authors (FAX or mail)

A microscopic calculation of the effective mass of one $^4$He impurity in homogeneous liquid $^3$He at zero temperature is performed for an extended Jastrow--Slater wave function, including two-- and three--body dynamical correlations and also backflow correlations between the $^4$He atom and the particles in the medium. The effective mass at saturation density, $m_4^*/m_4=1.21$, is in very good agreement with the recent experimental determination by Edwards {\it et al}. The three-- particle correlations appear to give a small contribution to the effective mass and different approximations for the three--particle distribution function give almost identical results for $m_4^*/m_4$., Comment: 12 pages, RevTex 3.0, one figure available upon request, FENUPC02-94

A Quadratic Diffusion Monte Carlo method has been used to obtain the equation of state of liquid $^4$He including the negative pressure region down to the spinodal point. The atomic interaction used is a renewed version (HFD-B(HE)) of the Aziz potential, which reproduces quite accurately the features of the experimental equation of state. The spinodal pressure has been calculated and the behavior of the sound velociy around the spinodal density has been analyzed., Comment: 10 pages, RevTex 3.0, with 4 PostScript figures included

By means of a Quadratic Diffusion Monte Carlo method we have performed a comparative analysis between the Aziz potential and a revised version of it. The results demonstrate that the new potential produces a better description of the equation of state for liquid $^4$He. In spite of the improvement in the description of derivative magnitudes of the energy, as the pressure or the compressibility, the energy per particle which comes from this new potential is lower than the experimental one. The inclusion of three-body interactions, which give a repulsive contribution to the potential energy, makes it feasible that the calculated energy comes close to the experimental result., Comment: 36 pages, LaTex, 11 PostScript figures included

Relevant features of the dynamic structure function $S(q,\omega)$ in $^3$He-$^4$He mixtures at zero temperature are investigated starting from known properties of the ground state. Sum rules are used to fix rigorous constraints to the different contributions to $S(q,\omega)$, coming from $^3$He and $^4$He elementary excitations, as well as to explore the role of the cross term $S^{(3,4)}(q,\omega)$. Both the low-$q$ (phonon-roton $^4$He excitations and 1p-1h $^3$He excitations) and high-$q$ (deep inelastic scattering) ranges are discussed., Comment: 29 pages, Plain TeX, 11 figures available by request from dalfovo@itnvax.cineca.it