Fermi hypernetted-chain study of half-filled Landau levels with broken rotational symmetry

DOI: 10.1103/PhysRevB.65.205307 http://link.aps.org/doi/10.1103/PhysRevB.65.205307We investigate broken rotational symmetry (BRS) states at half-filling of the valence Landau level (LL). We generalize Rezayi and Read's (RR) trial wave function, a special case of Jain's composite fermion (CF) wave functions, to include anisotropic coupling of the flux quanta to electrons, thus generating a nematic order in the underlying CF liquid. Using the Fermi hypernetted-chain method, which readily gives results in the thermodynamic limit, we determine the properties of these states in detail. By using the anisotropic pair distribution and static structure functions we determine the correlation energy and find that, as expected, RR's state is stable in the lowest LL, whereas BRS states may occur at half- filling of higher LL's, with a possible connection to the recently discovered quantum Hall liquid crystals

Universality away from critical points in two-dimensional phase transitions

http://arxiv.org/ftp/cond-mat/papers/0511/0511559.pdfThe p-state clock model in two dimensions is a system of discrete rotors with a quasi-liquid phase in a region T1 4. We show that, for p > 4 and above a temperature Teu, all macroscopic thermal averages, such as energy or magnetization, become identical to those of the continuous rotor (p = \infty). This collapse of thermodynamic observables creates a regime of extended universality in the phase diagram and an emergent symmetry, not present in the Hamiltonian. For p \ge 8, the collapse starts in the quasi-liquid phase and makes the transition at T2 identical to the Berezinskii-Kosterlitz-Thouless (BKT) transition of the con-tinuous rotor. For p \le 6, the transition at T2 is below Teu and no longer BKT. The results generate a comprehensive map of the critical properties at T1 and T2, and a range of experimental predictions, such as motion of magnetic domain walls, fabrication of identical devices from different building blocks, and limits on macro-scopic distinguishability of different microscopic interactions.Acknowledgment is made to the University of Missouri Research Board and Council, the Donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation (Grant No. EEC-0438469), for support of this research

Cross-over mechanism of the melting transition in monolayers of alkanes adsorbed on graphite and the universality of energy scaling

http://arxiv.org/ftp/arxiv/papers/0902/0902.4422.pdfThe interplay between the torsional potential energy and the scaling of the 1-4 van der Waals and Coulomb interactions determines the stiffness of flexible molecules. In molecular simulations often ad-hoc values for the scaling factor (SF) are adopted without adequate justification. In this letter we demonstrate for the first time that the precise value of the SF has direct consequences on the critical properties and mechanisms of systems undergoing a phase transition. By analyzing the melting of n-alkanes (hexane C6, dodecane C12, tetracosane C24) on graphite, we show that the SF is not a universal feature, that it monotonically decreases with the molecular length, and that it drives a cross-over between two distinct mechanisms for melting in such systems.Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund (PRF43277-B5) for the support of this research. This material is based upon work supported in part by the Department of Energy under award number DE-FG02-07ER46411. Computational resources were provided by the University of Missouri Bioinformatics Consortium

Melting of hexane monolayers adsorbed on graphite: the role of domains and defect formation

http://arxiv.org/ftp/arxiv/papers/0903/0903.1065.pdfWe present the first large-scale molecular dynamics simulations of hexane on graphite that completely reproduces all experimental features of the melting transition. The canonical ensemble simulations required and used the most realistic model of the system: (i) fully atomistic representation of hexane; (ii) explicit site-by-site interaction with carbon atoms in graphite; (iii) CHARMM force field with carefully chosen adjustable parameters of non-bonded interaction; (iv) numerous $\ge$ 100 ns runs, requiring a total computation time of ca. 10 CPU-years. This has allowed us to determine correctly the mechanism of the transition: molecular reorientation within lamellae without perturbation of the overall adsorbed film structure. We observe that the melted phase has a dynamically reorienting domain-type structure whose orientations reflect that of graphite.This material is based upon work supported in part by the Department of Energy under Award Number DE-FG02-07ER46411. Acknowledgment is made to the Donors of The American Chemical Society Petroleum Research Fund (PRF43277-B5). Computational support was provided by the University of Missouri Bioinformatics Consortium

Quantum energy levels of hydrogen adsorbed on nanoporous carbons: an intrinsic probe for pore structure, and improving Monte Carlo simulations of adsorption [abstract]

Only abstract of poster available.Track IV: Materials for Energy ApplicationsHydrogen is the lightest molecule in nature, making both rotational and translational degrees of freedom eminently quantum mechanical (especially at low temperatures). For isolated molecules the first excited (degenerate) rotational states are at about 175 K above the (non-degenerate) ground state. When the hydrogen molecule is adsorbed, however, interaction with the substrate partially eliminates this degeneracy due to the different adsorption strengths of the different rotational states of the molecule. In this talk, we consider the adsorption of hydrogen in nanometer-size pores in carbon. We show that the rotation-vibration energy levels are strongly dependent on the pore structure (geometry and size). This dependence may be probed by inelastic neutron scattering as a local, non-destructive, probe intrinsic to the system, to characterize nanopores (in fact, using H2 as the probe makes sure that the pore structure probed is relevant for H2 adsorption). The rotation-vibration energy levels were also used as input for grand canonical Monte Carlo simulations of H2 adsorption, improving the accuracy of the simulations. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411."This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411.

Structural and energetic factors in designing a perfect nano-porous sorbent for hydrogen storage [abstract]

Only abstract of poster available.Track IV: Materials for Energy ApplicationsCarbons are one of several promising groups of materials for hydrogen storage by adsorption. However, the heat of hydrogen physisorption in such materials is low, in the range of about 4-8 kJ/mol which limits the total amount of hydrogen adsorbed at P = 100 bar to ~2 wt% at room temperature and about ~10 wt% at 77 K. To get better storage capacity, the adsorbing surfaces must be modified, either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. Here we analyze the variation of interaction energy between a molecule of hydrogen and graphene-based sorbents prepared as hypothetical modifications of the graphene layer. In particular, we show that partial substitution of carbons (for example, by boron) modifies both the symmetry of the energy landscape and strength of hydrogen physisorption. The effect of substituent extends over several sites of graphene lattice making the surface more heterogeneous. This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411."This material is based on work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46411.

Enhanced hydrogen adsorption in boron substituted carbon nanospaces

doi:10.1063/1.3251788Activated carbons are one of promising groups of materials for reversible storage of hydrogen by physisorption. However, the heat of hydrogen adsorption in such materials is relatively low, in the range of about 4-8 kJ/mol, which limits the total amount of hydrogen adsorbed at P = 100 bar to ∼ 2 wt % at room temperature and ∼ 8 wt % at 77 K. To improve the sorption characteristics the adsorbing surfaces must be modified either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. In this letter we present ab initio calculations and Monte Carlo simulations showing that substitution of 5%-10% of atoms in a nanoporous carbon by boron atoms results in significant increases in the adsorption energy (up to 10-13.5 kJ/mol) and storage capacity ( ∼ 5 wt % at 298 K, 100 bar) with a 97% delivery rateThis material is based on work supported in part by the Department of Energy under Award Nos. DE-FG02-07ER46411 and DE-FC36-08GO18142 (L.F., B.K., P.F., and C.W.). We acknowledge the Wroclaw and Poznan Supercomputing and Networking Centers and University of Missouri Bioinformatics Consortium for the use of their computational facilities

Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite. Explicit Hydrogen Molecular Dynamics study

http://arxiv.org/ftp/arxiv/papers/0805/0805.1435.pdfWe discuss Molecular Dynamics (MD) computer simulations of a tetracosane (C24H50) monolayer physisorbed onto the basal plane of graphite. The adlayer molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom structure having six graphene layers. The tetracosane dynamics modeled in the fully atomistic manner agree well with experiment. The low-temperature ordered solid organizes in rectangular centered structure, incommensurate with underlying graphite. Above T = 200 K, as the molecules start to lose their translational and orientational order via gauche defect formation, a weak smectic mesophase (observed experimentally but never reproduced in United Atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions are included in the model. If the electrostatic charges are set to zero (as it is in UA force field), the melting temperature increases by ~70 K with respect to the experimental value. When the non-bonded 1-4 interaction is not scaled, the melting temperature decreases by ~90 K. If the scaling factor is set to 0.5, the melting occurs at T = 350 K, in very good agreement with experimental data.Acknowledgment is made to the Donors of The American Chemical Society Petroleum Research Fund (PRF43277 - B5), and the University of Missouri Research Board, for the support of this research. This material is based upon work supported in part by the Department of Energy under Award Number DE-FG02-07ER46411

Hydrogen storage in engineered carbon nanospaces

doi: 10.1088/0957-4484/20/20/204026It is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities (~80 g H2/kg carbon, ~50 g H2/liter carbon, at 50 bar and 77 K). Nanopores generate high storage capacities (a) by having high surface area to volume ratios, and (b) by hosting deep potential wells through overlapping substrate potentials from opposite pore walls, giving rise to a binding energy nearly twice the binding energy in wide pores. Experimental case studies are presented with surface areas as high as 3100 m2 g−1, in which 40% of all surface sites reside in pores of width ~0.7 nm and binding energy ~9 kJ mol−1, and 60% of sites in pores of width>1.0 nm and binding energy ~5 kJ mol−1. The findings, including the prevalence of just two distinct binding energies, are in excellent agreement with results from molecular dynamics simulations. It is also shown, from statistical mechanical models, that one can experimentally distinguish between the situation in which molecules do (mobile adsorption) and do not (localized adsorption) move parallel to the surface, how such lateral dynamics affects the hydrogen storage capacity, and how the two situations are controlled by the vibrational frequencies of adsorbed hydrogen molecules parallel and perpendicular to the surface: in the samples presented, adsorption is mobile at 293 K, and localized at 77 K. These findings make a strong case for it being possible to significantly increase hydrogen storage capacities in nanoporous carbons by suitable engineering of the nanopore space.This material is based upon work supported in part by the Department of Energy under Award No. DE-FG02-07ER46411. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02-06CH11357. CW and RC gratefully acknowledge the University of Missouri Bioinformatics Consortium for the use of their computational facilities. The authors would like to thank M Frederick Hawthorne, Francisco Rodr´ıguez-Reinoso, Louis Schlapbach, Andreas Z¨uttel, Bogdan Kuchta, Lucyna Firlej, Michael Roth, and Michael Gordon for valuable contributions. Finally, the authors would like to acknowledge helpful contributions by Hiden Isochema Ltd,Warrington, UK