We calculate the electronic structure and magnetic properties of hydrogenated graphite surfaces using van der Waals density functional theory (DFT) and model Hamiltonians. We find, as previously reported, that the interaction between hydrogen atoms on graphene favors adsorption on different sublattices along with an antiferromagnetic coupling of the induced magnetic moments. On the contrary, when hydrogenation takes place on the surface of graphene multilayers or graphite (Bernal stacking), the interaction between hydrogen atoms competes with the different adsorption energies of the two sublattices. This competition may result in all hydrogen atoms adsorbed on the same sublattice and, thereby, in a ferromagnetic state for low concentrations. Based on the exchange couplings obtained from the DFT calculations, we have also evaluated the Curie temperature by mapping this system onto an Ising-like model with randomly located spins. Remarkably, the long-range nature of the magnetic coupling in these systems makes the Curie temperature size dependent and larger than room temperature for typical concentrations and sizes.Comment: 11 pages, 18 figure

We present a theoretical study of the dynamics of H atoms adsorbed on graphene bilayers with Bernal stacking. First, through extensive density functional theory calculations, including van der Waals interactions, we obtain the activation barriers involved in the desorption and migration processes of a single H atom. These barriers, along with attempt rates and the energetics of H pairs, are used as input parameters in kinetic Monte Carlo simulations to study the time evolution of an initial random distribution of adsorbed H atoms. The simulations reveal that, at room temperature, H atoms occupy only one sublattice before they completely desorb or form clusters. This sublattice selectivity in the distribution of H atoms may last for sufficiently long periods of time upon lowering the temperature down to 0• C. The final fate of the H atoms, namely, desorption or cluster formation, depends on the actual relative values of the activation barriers which can be tuned by doping. In some cases, a sublattice selectivity can be obtained for periods of time experimentally relevant even at room temperature. This result shows the possibility for observation and applications of the ferromagnetic state associated with such distribution.

We present a study of the adsorption and diffusion of CH4, CO2 and H2 molecules in clathrate hydrates using ab initio van der Waals density functional formalism [Dion et al. Phys. Rev. Lett. 92, 246401 (2004)]. We find that the adsorption energy is dominated by van der Waals interactions and that, without them, gas hydrates would not be stable. We calculate the maximum adsorption capacity as well as the maximum hydrocarbon size that can be adsorbed.The relaxation of the host lattice is essential for a good description of the diffusion activation energies, which are estimated to be of the order of 0.2, 0.4, and 1.0 eV for H2, CO2, and CH4, respectively.PACS numbers: 91.50He, 64.70kt, 84.60VeThe existence of complex crystalline structures, made of water molecules hosting in their cavities hydrocarbons and other molecules, that stabilize the otherwise unstable network, has been known for many years [1][2][3]. These gas hydrates, or clathrates, are stable at high pressures and low temperatures. They are very abundant in the Earth's permafrost and marine sediments [4][5][6], and they have been detected in other planetary bodies like Mars and some moons of Saturn [7,8]. They can be prepared in the laboratory under appropriate conditions, and different structural and spectroscopic measurements, like Xray and neutron diffraction, nuclear magnetic resonance (NMR), Raman and infrared spectroscopy, and others, have been performed to characterize their composition and crystal structure (see [2] and references therein).These compounds are important for many practical reasons. Historically, it was soon realized that their formation, in extraction and transportation pipes of hydrocarbons, had to be controlled to avoid their clogging. On a more global perspective, they could potentially be a huge source of hydrocarbons, with reserves much larger than those of oil and natural gas together [1]. However, their destabilization and release to the atmosphere, due to the temperature increase associated to global warming, constitutes a very serious environmental threat. Another source of interest is in their potential use for H 2 storage and also for CO 2 sequestration. On this respect, the extraction of CH 4 from natural hydrates, and its simultaneous substitution by CO 2 , preserving their structure and stability, would be an ideal operation. The viability of such an operation requires, however, a precise knowledge of several magnitudes, like the relative stability of CH 4 and CO 2 hydrates, and the diffusion barriers of these molecules in the networks. We address these magnitudes in this work.The crystalline structure of the clathrates is made up of H-bonded water molecules forming a network with cages of different shapes and sizes. Out of the various crystalline structures, we will address here the so called Structure I and Structure H (see Figure 1). For more details see for instance the Sloan's reviews [1][2][3] From a theoretical point of view, the study of these compounds represents a challenging task: it is necessary to deal...