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The Bose polaron has attracted theoretical and experimental interest because the mobile impurity is surrounded by a bath that undergoes a superfluid-to-normal phase transition. Although many theoretical works have studied this system in its ground state, only few analyze its behavior at finite temperature. We have studied the effect of temperature on a Bose polaron system performing ab-initio Path Integral Monte Carlo simulations. This method is able to approach the critical temperature without losing accuracy, in contrast with perturbative approximations. We have calculated the polaron energy for the repulsive and attractive branches and we have observed an asymmetric behavior between the two branches. When the potential is repulsive, the polaron energy decreases when the temperature increases, and contrariwise for the attractive branch. Our results for the effective mass and the dynamical structure factor of the polaron show unambiguously that its quasiparticle nature disappears close to the critical temperature, in agreement with recent experimental findings. Finally, we have also estimated the fraction of bosons in the condensate as well as the superfluid fraction, and we have concluded that the impurity hinders the condensation of the rest of bosons., 9 pages, 6 figures

We reveal an intriguing anomaly in the temperature dependence of the specific heat of a one-dimensional Bose gas. The observed peak holds for arbitrary interaction and remembers a superfluid-to-normal phase transition in higher dimensions, but phase transitions are not allowed in one dimension. The presence of the anomaly signals a region of unpopulated states which behaves as an energy gap and is located below the hole branch in the excitation spectrum. The anomaly temperature is found to be of the same order of the energy of the maximum of the hole branch. We rely on the Bethe Ansatz to obtain the specific heat exactly and provide interpretations of the analytically tractable limits. The dynamic structure factor is computed with the Path Integral Monte Carlo method for the first time. We notice that at temperatures similar to the anomaly threshold, the energy of the thermal fluctuations become comparable with the maximal hole energy, leading to a qualitative change in the structure of excitations. This excitation pattern experiences the breakdown of the quasi-particle description for any value of the interaction strength at the anomaly, similarly to any superfluid phase transition at the critical temperature. We provide indications for future observations and how the hole anomaly can be employed for in-situ thermometry, identifying different collisional regimes and understanding other anomalies in atomic, solid-state, electronic, spin-chain and ladder systems., Comment: Main Text: 16 pages, 6 figures. Appendices: 11 pages, 4 figures. Open-access datasets can be found at 10.5821/data-2117-353128-1

Some discrepancies between experimental results on quantum droplets made of a mixture of $^{39}\mathrm{K}$ atoms in different hyperfine states and their analysis within extended Gross-Pitaevskii theory (which incorporates beyond mean-field corrections) have been recently solved by introducing finite-range effects into the theory. Here we study the influence of these effects on the monopole and quadrupole excitation spectrum of extremely dilute quantum droplets using a density functional built from first-principles quantum Monte Carlo calculations, which can be easily introduced in the existing Gross-Pitaevskii numerical solvers. Our results show differences of up to $20%$ with those obtained within the extended Gross-Pitaevskii theory, likely providing another way to observe finite-range effects in mixed quantum droplets by measuring their lowest excitation frequencies.

We revisited the phase diagram of the second layer of $^{4}\mathrm{He}$ on top of graphite using quantum Monte Carlo methods. Our aim was to explore the existence of the novel phases suggested recently in experimental works, and determine their properties and stability limits. We found evidence of a superfluid quantum phase with hexatic correlations, induced by the corrugation of the first Helium layer, and a quasi-two-dimensional supersolid corresponding to a $7/12$ registered phase. The $4/7$ commensurate solid was found to be unstable, while the triangular incommensurate crystals, stable at large densities, were normal.

We studied the miscibility of two dipolar quantum gases in the limit of zero temperature. The system under study is composed by a mixture of two Bose gases with dominant dipolar interaction in a two-dimensional harmonic confinement. The dipolar moments are considered all to be perpendicular to the plane, turning the dipolar potential in a purely repulsive and isotropic model. Our analysis is carried out by using the diffusion Monte Carlo method which allows for an exact solution to the many-body problem within some statistical noise. Our results show that the miscibility between the two species is rather constrained as a function of the relative dipolar moments and masses of the two components. A narrow regime is predicted where both species mix and we introduce an adimensional parameter whose value predicts quite accurately the miscibility of the two dipolar gases., Comment: 14 pages, 7 figures

Ground state properties of a fermionic Coulomb gas are calculated using the fixed-node diffusion Monte Carlo method. The validity of the composite boson description is tested for different densities. We extract the exciton-exciton $s$-wave scattering length by solving the four-body problem in a harmonic trap and mapping the energy to that of two trapped bosons. The equation of state is consistent with the Bogoliubov theory for composite bosons interacting with the obtained $s$-wave scattering length. The perturbative expansion at low density has contributions physically coming from (a) exciton binding energy, (b) mean-field Gross-Pitaevskii interaction between excitons, (c) quantum depletion of the excitonic condensate (Lee-Huang-Yang terms for composite bosons). In addition, for low densities we find a good agreement with the Bogoliubov bosonic theory for the condensate fraction of excitons. The equation of state in the opposite limit of large density is found to be well described by the perturbative theory including (a) mixture of two ideal Fermi gases (b) exchange energy. We find that for low densities both energetic and coherent properties are correctly described by the picture of composite bosons (excitons)., 12 pages, 4 figures

We study thermal properties of a trapped Bose-Bose mixture in a dilute regime using quantum Monte Carlo methods. Our main aim is to investigate the dependence of the superfluid density and the condensate fraction on temperature, for the mixed and separated phases. To this end, we use the diffusion Monte Carlo method, in the zero-temperature limit, and the path-integral Monte Carlo method for finite temperatures. The results obtained are compared with solutions of the coupled Gross-Pitaevskii equations for the mixture at zero temperature. We notice the existence of an anisotropic superfluid density in some phase-separated mixtures. Our results also show that the temperature evolution of the superfluid density and condensate fraction is slightly different, showing noteworthy situations where the superfluid fraction is smaller than the condensate fraction., Comment: accepted for publication in Phys. Rev. A

We analyze the role played by quantum fluctuations on a Raman Spin-Orbit Coupled system in the stripe phase. We show that beyond mean-field effects stabilize the collapse predicted by mean-field theory and induce the emergence of two phases: a gas and a liquid, which also show spatial periodicity along a privileged direction. We show that the energetically favored phase is determined by the Raman coupling and the spin-dependent scattering lengths. We obtain the ground-state solution of the finite system by solving the extended Gross-Pitaevskii equation and find self-bound, droplet-like solutions that feature internal structure through a striped pattern. We estimate the critical number for binding associated to these droplets and show that their value is experimentally accessible. We report an approximate energy functional in order to ease the evaluation of the Lee-Huang-Yang correction in practical terms., Comment: 10 pages, 6 Figures

We study a two-component mixture of fermionic dipoles in two dimensions at zero temperature, interacting via a purely repulsive $1/r^3$ potential. This model can be realized with ultracold atoms or molecules, when their dipole moments are aligned in the confinement direction orthogonal to the plane. We characterize the unpolarized mixture by means of the Diffusion Monte Carlo technique. Computing the equation of state, we identify the regime of validity for a mean-field theory based on a low-density expansion and compare our results with the hard-disk model of repulsive fermions. At high density, we address the possibility of itinerant ferromagnetism, namely whether the ground state can be fully polarized in the fluid phase. Within the fixed-node approximation, we show that the accuracy of Jastrow-Slater trial wave functions, even with the typical two-body backflow correction, is not sufficient to resolve the relevant energy differences. By making use of the iterative-backflow improved trial wave functions, we observe no signature of a fully-polarized ground state up to the freezing density., Comment: 12 pages, 6 figures

We have carried out a study of the momentum distribution and of the spectrum of elementary excitations of liquid $$^4$$ He across the normal–superfluid transition temperature, using the path integral Monte Carlo method. Our results for the momentum distribution in the superfluid regime show that a kink is present in the range of momenta corresponding to the roton excitation. This effect disappears when crossing the transition temperature to the normal fluid, in a behavior currently unexplained by theory.

We study a harmonically confined Bose-Bose mixture using quantum Monte Carlo methods. Our results for the density profiles are systematically compared with mean-field predictions derived through the Gross-Pitaevskii equation in the same conditions. The phase space as a function of the interaction strengths and the relation between masses is quite rich. The miscibility criterion for the homogeneous system applies rather well to the system, with some discrepancies close to the critical line for separation. We observe significant differences between the mean-field results and the Monte Carlo ones, that magnify when the asymmetry between masses increases. In the analyzed interaction regime, we observe universality of our results which extend beyond the applicability regime for the Gross-Pitaevskii equation., Comment: 9 pages

We study the superfluid properties of a system of fully polarized dipolar bosons moving in the $XY$ plane. We focus on the general case where the polarization field forms an arbitrary angle $\ensuremath{\alpha}$ with respect to the $Z$ axis, while the system is still stable. We use the diffusion Monte Carlo and the path integral ground state methods to evaluate the one-body density matrix and the superfluid fractions in the region of the phase diagram where the system forms stripes. Despite its oscillatory behavior, the presence of a finite large-distance asymptotic value in the $s$-wave component of the one-body density matrix indicates the existence of a Bose condensate. The superfluid fraction along the stripes direction is always close to 1, while in the $Y$ direction decreases to a small value that is nevertheless different from zero. These two facts confirm that the stripe phase of the dipolar Bose system is a clear candidate for an intrinsic supersolid without the presence of defects as described by the Andreev-Lifshitz mechanism.

We present a first-principles study of the energy and elastic properties of solid helium at pressures below the range in which it is energetically stable. We find that the limit of mechanical stability in hcp \(^{4}\)He is \(P_{s} = -33.8(1)\) bar, which lies significantly below the spinodal pressure found in the liquid phase (i.e., \(-9.6\) bar). Furthermore, we show that the pressure variation of the transverse and longitudinal sound velocities close to \(P_{s}\) does not follow a power law of the form \(\propto \left( P - P_{s} \right) ^{\gamma }\), in contrast to what is observed in the fluid.

We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degeneracy has also been observed in thin films of hydrogen adsorbed on the 4He surface allowing for the possibility of observing the Berezinskii–Kosterlitz–Thouless superfluid transition.

We study the ground state of a bilayer system of dipolar bosons with dipoles oriented by an external field perpendicularly to the two parallel planes. By decreasing the interlayer distance, for a fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from an atomic to a pair superfluid. We investigate the excitation spectrum across this transition by using microscopic approaches. Quantum Monte Carlo methods are employed to obtain the static structure factors and intermediate scattering functions in imaginary time. The dynamic response is calculated using both the correlated basis functions (CBF) method and the approximate inversion of the Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic phase, both density and spin excitations are gapless. However, in the pair-superfluid phase a gap opens in the excitation energy of the spin mode. For small separation between layers, the minimal spin excitation energy equals the binding energy of a dimer and is twice the gap value., Comment: 8 pages, 5 figures

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

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 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.

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 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

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.

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#