Zero-temperature phase diagram of the second layer of 4^{\bf 4}He adsorbed on graphene

The phase diagram at zero temperature of $^4$He adsorbed on an helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of $^4$He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163 $\pm$ 0.005 \AA$^{-2}$ and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186 $\pm$ 0.003 \AA$^{-2}$. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density.Comment: 7 pages, accepted for publication in Phys. Rev.

4He adsorbed outside a single carbon nanotube

The phase diagrams of $^4$He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42 -- 10.85 \AA \ are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H$_2$ on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved $\sqrt 3 \times \sqrt 3$ registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered.Comment: 6 pages, accepted for publication in Phys. Rev.

Liquid and solid phases of 3He on graphite

Recent heat-capacity experiments show quite unambiguously the existence of a liquid $^3$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 $^3$He adsorbed on graphite at $T=0$ using quantum Monte Carlo methods. Considering a fully corrugated substrate we observe that at densities lower that 0.006 \AA$^{-2}$ the system is a very dilute gas, that at that density is in equilibrium with a liquid of density 0.014 \AA$^{-2}$. 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 the corresponding phase transitions that appear at higher densities.Comment: 5 page

Isotopic effects of hydrogen adsorption in carbon nanotubes

We present diffusion Monte Carlo calculations of D$_2$ adsorbed inside a narrow carbon nanotube. The 1D D$_2$ equation of state is reported, and the one-dimensional character of the adsorbed D$_2$ is analyzed. The isotopic dependence of the constitutive properties of the quantum fluid are studied by comparing D$_2$ and H$_2$. Quantum effects due to their different masses are observed both in the energetic and the structural properties. The influence of the interatomic potential in one-dimensional systems is also studied by comparing the properties of D$_2$ and $^4$He which have nearly the same mass but a sizeably different potential. The physics of molecular hydrogen adsorbed in the interstitial channels of a bundle of nanotubes is analyzed by means of both a diffusion Monte Carlo calculation and an approximate mean field method.Comment: 17 pages, revtex, 9 ps figures, to be appear in Phys. Rev.

Phase diagram of H2 adsorbed on graphene

The phase diagram of the first layer of H$_2$ adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo (DMC) simulations. We have found that, as in the case of $^4$He, the ground state of molecular hydrogen is a $\sqrt3 \times \sqrt3$ commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered, and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids.Comment: 5 pages, 3 figure

4He adsorbed inside (10,10) single walled carbon nanotubes

Diffusion Monte Carlo calculations on the adsorption of $^4$He in open-ended single walled (10,10) nanotubes are presented. We have found a first order phase transition separating a low density liquid phase in which all $^4$He atoms are adsorbed close to the tube wall and a high density arrangement characterized by two helium concentric layers. The energy correction due to the presence of neighboring tubes in a bundle has also been calculated, finding it negligible in the density range considered.Comment: 5 pages, accepted for publication in Phys. Rev.

Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows no superfluid signal whereas $^4$He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in $^4$He without destroying the spatial solid order.Comment: 5 pages, accepted for publication in PR

Phases of 4^4He and H2_2 adsorbed on a single carbon nanotube

Using a diffusion Monte Carlo (DMC) technique, we calculated the phase diagrams of $^4$He and H$_2$ adsorbed on a single (5,5) carbon nanotube, one of the narrowest that can be obtained experimentally. For a single monolayer, when the adsorbate density increases, both species undergo a series of first order solid-solid phase transitions between incommensurate arrangements. Remarkably, the $^4$He lowest-density solid phase shows supersolid behavior in contrast with the normal solid that we found for H$_2$. The nature of the second-layer is also different for both adsorbates. Contrarily to what happens on graphite, the second-layer of $^4$He on that tube is a liquid, at least up to the density corresponding to a third-layer promotion on a flat substrate. However, the second-layer of H$_2$ is a solid that, at its lowest stable density, has a small but observable superfluid fraction