This paper demonstrates that nanospace engineering of KOH activated carbon is possible by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. High specific surface areas, porosities, sub-nanometer (<1 nm) and supra-nanometer (1-5 nm) pore volumes are quantitatively controlled by a combination of KOH concentration and activation temperature. The process typically leads to a bimodal pore size distribution, with a large, approximately constant number of sub-nanometer pores and a variable number of supra-nanometer pores. We show how to control the number of supra-nanometer pores in a manner not achieved previously by chemical activation. The chemical mechanism underlying this control is studied by following the evolution of elemental composition, specific surface area, porosity, and pore size distribution during KOH activation and preceding H(3)PO(4) activation. The oxygen, nitrogen, and hydrogen contents decrease during successive activation steps, creating a nanoporous carbon network with a porosity and surface area controllable for various applications, including gas storage. The formation of tunable sub-nanometer and supra-nanometer pores is validated by sub-critical nitrogen adsorption. Surface functional groups of KOH activated carbon are studied by microscopic infrared spectroscopy.

We derive the long-wavelength elastic theory for the quantum Hall smectic state starting from the HartreeFock approximation. Dislocations in this state lead to an effective nematic model for TϾ0, which undergoes a disclination unbinding transition from a phase with algebraic orientational order into an isotropic phase. We obtain transition temperatures that are in qualitative agreement with recent experiments that have observed large anisotropies of the longitudinal resistivities in half-filled Landau levels, lending credence to the liquid crystal interpretation of experiments. DOI: 10.1103/PhysRevB.64.115312 PACS number͑s͒: 73.43.Ϫf, 64.70.Md, 73.20.Mf, 73.50.Jt Recent experiments 1-3 in high-mobility two-dimensional electron systems ͑2DES͒ have revealed remarkable phenomena in the transitional regions between the different plateau of the Hall conductance. In particular, striking anisotropies and nonlinearities in the magnetotransport were observed for Landau level ͑LL͒ filling factors near ϭnϩ1/2, for nу4, corresponding to partially filled LL indices Lу2. This anisotropy tends to align with the crystalline axes of the sample, but can be reoriented by the application of in-plane magnetic fields, 4,5 and resistance ratios as high as R xx /R yy ϳ3500 have been observed. 6 This anisotropic behavior has been attributed to the formation of a striped phase. A unidirectional charge density wave ͑UCDW͒ had been predicted several years ago 7 for nearly-half-filled high LL's; exact diagonalizations for systems of up to 12 electrons 8 corroborate this picture for Lу2, and many experimental results can be qualitatively understood under the assumption of a UCDW. The presence of stripes has already been directly observed in a large class of low-dimensional, strongly correlated electronic systems, 9 and the present experimental evidence in quantum Hall devices is compelling, even if still somewhat circumstantial. 10 Due to the similarities of the UCDW state with a classical smectic liquid crystal, these states have been dubbed quantum Hall smectics by Fradkin and Kivelson. 11,12 In two dimensions thermal fluctuations destroy the positional order, 13 but the system should still exhibit anisotropic transport as long as there is some remnant of orientational order ͑alge-braic order in the quantum Hall nematic͒. 14 As the temperature is increased, the algebraic orientational order will disappear in a Kosterlitz-Thouless ͑KT͒ disclination-unbinding transition. 15 To study this process we have mapped the interacting electron system ͑in the Hartree-Fock approximation͒ onto a classical smectic ͑the UCDW͒. We then consider the role of thermal fluctuations ͑phonons and dislocations͒ in reducing the order from smectic to nematic at larger distances. Without the use of any fitting parameters, and using only experimentally accessible values for the electron density and the width of the 2DES, we are able to estimate values for the disclination-unbinding transition temperature, which are in qualitative agreement with the transport measu...

We report the results of molecular dynamics simulations of a complete monolayer of hexane physisorbed onto the basal plane of graphite. At low temperatures the system forms a herringbone solid. With increasing temperature, a solid-to-nematic liquid-crystal transition takes place at T 1 = 138± 2 K followed by another transition at T 2 = 176± 3 K into an isotropic fluid. We characterize the different phases by calculating various order parameters, coordinate distributions, energetics, spreading pressure, and correlation functions, most of which are in reasonable agreement with available experimental evidence. In addition, we perform simulations where the Lennard-Jones interaction strength, corrugation potential strength, and dihedral rigidity are varied in order to better characterize the nature of the two transitions. We find that both phase transitions are facilitated by a "footprint reduction" of the molecules via tilting and to a lesser degree via creation of gauche defects in the molecules.

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.

We discuss an alternative accurate Monte Carlo method to calculate the ground-state energy and related quantities for Laughlin states of the fractional quantum Hall effect in a disk geometry. This alternative approach allows us to obtain accurate bulk regime ͑thermodynamic limit͒ values for various quantities from Monte Carlo simulations with a small number of particles ͑much smaller than that needed with standard Monte Carlo approaches͒.

It is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities (∼80 g H 2 /kg carbon, ∼50 g H 2 /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 m 2 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.

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Breakdown of the Quantum Hall Effect at high values of injected current is explained as a consequence of an abrupt formation of a metallic "river" percolating from one edge of the sample to the other. Such river is formed when lakes of compressible liquid, where the long-range disorder potential is screened, get connected with each other due to the strong electric field. Our theory predicts critical currents consistent with experiment values and explains various features of the breakdown.Since the discovery of the Quantum Hall Effect (QHE) [1] great variety of phenomena associated with the behavior of the two-dimensional electron gas (2DEG) in the strong magnetic field has been studied both experimentally and theoretically. In this paper we discuss the breakdown of the QHE at high injected currents. First observed by Ebert [2] as a sudden onset of the dissipation when the injected current exceeds some critical value, the breakdown of the integer QHE (IQHE) has been intensively investigated since then [3]. While the absence of the dissipation (σ xx = 0) and the quantization of the transverse conductivity σ xy in the IQHE regime at low temperatures are well understood, the breakdown of the dissipationless regime has not been given a clear explanation. Existing theories of this effect [4] include production of hot electrons [5], inter-Landau level transitions [6] in the high local electric field (due to tunnelling or emission of phonons), increase in the number of the delocalized states in the Landau level [7]. None of these produced results consistent with experiments: except for the hot-electron picture, all others predict values of the critical current density that are two orders of magnitude higher than those observed experimentally. Also, neither of them can explain hysteresis and localized nature of the breakdown phenomenon, nor the existence of the transient switching and broadband noise before the breakdown, as well as peculiar steps in the magnetic field dependence in the regime of critical current. Besides, all these theories need to use some artificial assumptions and are too complicated to be correct.Experiments on the breakdown of the QHE are performed on GaAs heterostructures or on high-quality MOSFET devices both characterized by the presence of long-range fluctuations of the disorder potential. In the GaAs systems the disorder potential due to the remote dopants has predominantly long wave-length fluctuations (λ > d > l H , where λ is the wavelength of the fluctuations, d is the spacer thickness, and l H is the magnetic length). Long-range potential fluctuations are also present in high-mobility MOSFET devices though their nature is not that evident [8]. When the magnetic field and the density of the 2DEG correspond to a filling factor close to an integer number, these fluctuations are not screened by the electrons in most parts of the system [9]. The region with completely filled Landau level percolates through the sample, leading to the QHE (at the end of this paper we discuss what happens when th...