Cintia M. Lapilli

Universality Away from Critical Points in Two-Dimensional Phase Transitions

The p-state clock model in two dimensions is a system of discrete rotors with a quasiliquid phase in a region T14. We show that, for p>4 and above a temperature T(eu), all macroscopic thermal averages become identical to those of the continuous rotor (p=infinity). 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> or =8, the collapse starts in the quasiliquid phase and makes the transition at T2 identical to the Berezinskii-Kosterlitz-Thouless (BKT) transition of the continuous rotor. For p< or =6, the transition at T2 is below T(eu) and no longer a BKT transition. The results generate a range of experimental predictions, such as the motion of magnetic domain walls, and limits on macroscopic distinguishability of different microscopic interactions.

HIGH-SURFACE-AREA BIOCARBONS FOR REVERSIBLE ON-BOARD STORAGE OF NATURAL GAS AND HYDROGEN

An overview is given of the development of advanced nanoporous carbons as storage materials for natural gas (methane) and molecular hydrogen in on-board fuel tanks for nextgeneration clean automobiles. The carbons are produced in a multi-step process from corncob, have surface areas of up to 3500 m 2 /g, porosities of up to 0.8, and reversibly store, by physisorption, record amounts of methane and hydrogen. Current best gravimetric and volumetric storage capacities are: 250 g CH4/kg carbon and 130 g CH4/liter carbon (199 V/V) at 35 bar and 293 K; and 80 g H2/kg carbon and 47 g H2/liter carbon at 47 bar and 77 K. This is the first time the DOE methane storage target of 180 V/V at 35 bar and ambient temperature has been reached and exceeded. The hydrogen values compare favorably with the 2010 DOE targets for hydrogen, excluding cryogenic components. A prototype adsorbed natural gas (ANG) tank, loaded with carbon monoliths produced accordingly and currently undergoing a road test in Kansas City, is described. A preliminary analysis of the surface and pore structure is given that may shed light on the mechanisms leading to the extraordinary storage capacities of these materials. The analysis includes pore-size distributions from nitrogen adsorption isotherms; spatial organization of pores across the entire solid from small-angle x-ray scattering (SAXS); pore entrances from scanning electron microscopy (SEM) and transmission electron microscopy (TEM); H2 binding energies from temperature-programmed desorption (TPD); and analysis of surface defects from Raman spectra. For future materials, expected to have higher H2 binding energies via appropriate surface functionalization, preliminary projections of H2 storage capacities based on molecular dynamics simulations of adsorption of H2 on graphite, are reported.

Complex pore spaces create record-breaking methane storage system for natural-gas vehicles

P. Pfeifer, L. Aston, M. Banks, S. Barker, J. Burress, S. Carter, J. Coleman, S. Crockett, C. Faulhaber, J. Flavin, M. Gordon, L. Hardcastle, Z. Kallenborn, M. Kemiki, C. Lapilli, J. Pobst, R. Schott, P. Shah, S. Spellerberg, G. Suppes, D. Taylor, A. Tekeei, C. Wexler, and M. Wood University of Missouri, Columbia, Missouri 65211, USA P. Buckley, T. Breier, J. Downing, S. Eastman, P. Freeze, S. Graham, S. Grinter, A. Howard, J. Martinez, D. Radke, and T. Vassalli Midwest Research Institute, Kansas City, Missouri 64110, USA J. Ilavsky Argonne National Laboratory, Argonne, Illinois 60439, USA Received 27 August 2007; published online 27 December 2007 DOI: 10.1063/1.2786007

Collective excitations in quantum Hall liquid crystals: Single-mode approximation calculations

A variety of recent experiments probing the low-temperature transport properties of quantum Hall systems have suggested an interpretation in terms of liquid crystalline mesophases dubbed quantum Hall liquid crystals. The single mode approximation (SMA) has been a useful tool for the determination of the excitation spectra of various systems such as phonons in {sup 4}He and in the fractional quantum Hall effect. In this paper we calculate (via the SMA) the spectrum of collective excitations in a quantum Hall liquid crystal by considering nematic, tetratic, and hexatic generalizations of Laughlins trial wave function having twofold, fourfold, and sixfold broken rotational symmetry, respectively. In the limit of zero wave vector q the dispersion of these modes is singular, with a gap that is dependent on the direction along which q=0 is approached for nematic and tetratic liquid crystalline states, but remains regular in the hexatic state, as permitted by the fourth order wave-vector dependence of the (projected) oscillator strength and static structure factor.

Universality away from critical points in a thermostatistical model

Nature uses phase transitions as powerful regulators of processes ranging from climate to the alteration of phase behavior of cell membranes to protect cells from cold, building on the fact that thermodynamic properties of a solid, liquid, or gas are sensitive fingerprints of intermolecular interactions. The only known exceptions from this sensitivity are critical points. At a critical point, two phases become indistinguishable and thermodynamic properties exhibit universal behavior: systems with widely different intermolecular interactions behave identically. Here we report a major counterexample. We show that different members of a family of two-dimensional systems —the discrete p-state clock model— with different Hamiltonians describing different microscopic interactions between molecules or spins, may exhibit identical thermodynamic behavior over a wide range of temperatures. The results generate a comprehensive map of the phase diagram of the model and, by virtue of the discrete rotors behaving like continuous rotors, an emergent symmetry, not present in the Hamiltonian. This symmetry, or many-to-one map of intermolecular interactions onto thermodynamic states, demonstrates previously unknown limits for macroscopic distinguishability of different microscopic interactions.

Single Mode Approximation Spectrum for a Quantum Hall Liquid Crystal

The single mode approximation (SMA) has been a useful tool for the determination of the collective excitation spectra of various systems. For example, Feynman used it successfully for the calculation of the spectrum of sound waves in superfluid 4He [1]; and Girvin, MacDonald and Platzman employed a projected SMA to determine that of the fractional quantum Hall (QH) effect [2] for Laughlin‐like states [3]. In this paper we calculate the excitation spectrum for density fluctuations in a QH liquid crystal. We consider nematic, tetratic, and hexatic generalizations of Laughlin’s trial wave function [3] having two‐, four‐ and six‐fold broken rotational symmetry (BRS) respectively. We perform extensive Monte Carlo (MC) simulations to compute the density‐coupled spectrum of these liquid crystalline states in the lowest Landau levels. We find significant angular dependence of the spectrum with singular behavior for long wavelengths and considerable deepening of the magnetoroton as the anisotropy is increased.