Characterizing PSPACE with Shallow Non-Confluent P Systems

In P systems with active membranes, the question of understanding the power of non-confluence within a polynomial time bound is still an open problem. It is known that, for shallow P systems, that is, with only one level of nesting, non-con uence allows them to solve conjecturally harder problems than con uent P systems, thus reaching PSPACE. Here we show that PSPACE is not only a bound, but actually an exact characterization. Therefore, the power endowed by non-con uence to shallow P systems is equal to the power gained by con uent P systems when non-elementary membrane division and polynomial depth are allowed, thus suggesting a connection between the roles of non-confluence and nesting depth

Rates and Equilibria for a Photoisomerizable Antagonist at the Acetylcholine Receptor of Electrophorus Electroplaques

Voltage-jump and light-flash experiments have been performed on isolated Electrophorus electroplaques exposed simultaneously to nicotinic agonists and to the photoisomerizable compound 2,2'-bis-[α-(trimethylammonium)methyl]-azobenzene (2BQ). Dose-response curves are shifted to the right in a nearly parallel fashion by 2BQ, which suggests competitive antagonism; dose-ratio analyses show apparent dissociation constants of 0.3 and 1 µM for the cis and trans isomers, respectively. Flash-induced trans → cis concentration jumps produce the expected decrease in agonist-induced conductance; the time constant is several tens of milliseconds. From the concentration dependence of these rates, we conclude that the association and dissociation rate constants for the cis-2BQ-receptor binding are approximately ~ 10^8 M^(-1) s^(-1) and 60 s^(-1) at 20ºC; the Q_(10) is 3. Flash-induced cis → trans photoisomerizations produce molecular rearrangements of the ligand-receptor complex, but the resulting relaxations probably reflect the kinetics of buffered diffusion rather than of the interaction between trans-2BQ and the receptor. Antagonists seem to bind about an order of magnitude more slowly than agonists at nicotinic receptors

Effect of dimensionality on the charge-density-wave in few-layers 2H-NbSe2_2

We investigate the charge density wave (CDW) instability in single and double layers, as well as in the bulk 2H-NbSe$_{2}$. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk 2H-NbSe$_{2}$. We predict that both mono- and bilayer NbSe$_{2}$ undergo a CDW instability. However, while in the bulk the instability occurs at a momentum $\mathbf{q}_{CDW}\approx{2/3}\mathbf{\Gamma M}$, in free-standing layers it occurs at $\mathbf{q}_{CDW}\approx{1/2}\mathbf{\Gamma M}$. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at $\mathbf{q}\approx\mathbf{q}_{CDW}$ plays in forming the CDW ground state.Comment: 4 pages 5 figure

Hygrothermal damage mechanisms in graphite-epoxy composites

T300/5209 and T300/5208 graphite epoxy laminates were studied experimentally and analytically in order to: (1) determine the coupling between applied stress, internal residual stress, and moisture sorption kinetics; (2) examine the microscopic damage mechanisms due to hygrothermal cycling; (3) evaluate the effect of absorbed moisture and hygrothermal cycling on inplane shear response; (4) determine the permanent loss of interfacial bond strength after moisture absorption and drying; and (5) evaluate the three dimensional stress state in laminates under a combination of hygroscopic, thermal, and mechanical loads. Specimens were conditioned to equilibrium moisture content under steady exposure to 55% or 95% RH at 70 C or 93 C. Some specimens were tested subsequent to moisture conditioning and 100 cycles between -54 C and either 70 C or 93 C

Simulating counting oracles with cooperation

We prove that monodirectional shallow chargeless P systems with active membranes and minimal cooperation working in polynomial time precisely characterise P#P k , the complexity class of problems solved in polynomial time by deterministic Turing machines with a polynomial number of parallel queries to an oracle for a counting problem

Structure, Stability, Edge States and Aromaticity of Graphene Ribbons

We determine the stability, the geometry, the electronic and magnetic structure of hydrogen-terminated graphene-nanoribbons edges as a function of the hydrogen content of the environment by means of density functional theory. Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum pressures. Under more standard conditions, the most stable structures are the mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with one di- and two mono-hydrogenated sites. At high hydrogen-concentration ``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in analogy to the spontaneous breaking of graphene into small-width nanoribbons observed experimentally in solution. The stability and the existence of exotic edge electronic-states and/or magnetism is rationalized in terms of simple concepts from organic chemistry (Clar's rule)Comment: 4 pages, 3 figures, accepted for publication by Physical Review Letter

Structure and stability of graphene nanoribbons in oxygen, carbon dioxide, water, and ammonia

We determine, by means of density functional theory, the stability and the structure of graphene nanoribbon (GNR) edges in presence of molecules such as oxygen, water, ammonia, and carbon dioxide. As in the case of hydrogen-terminated nanoribbons, we find that the most stable armchair and zigzag configurations are characterized by a non-metallic/non-magnetic nature, and are compatible with Clar's sextet rules, well known in organic chemistry. In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in oxygen-rich atmosphere should preferentially be along the armchair direction, while water-saturated GNRs should present zigzag edges. Our results promise to be particularly useful to GNRs synthesis, since the most recent and advanced experimental routes are most effective in water and/or ammonia-containing solutions.Comment: accepted for publication in PR

Characterizing PSPACE with Shallow Non-Confluent P Systems

In P systems with active membranes, the question of understanding the power of non-confluence within a polynomial time bound is still an open problem. It is known that, for shallow P systems, that is, with only one level of nesting, non-con uence allows them to solve conjecturally harder problems than con uent P systems, thus reaching PSPACE. Here we show that PSPACE is not only a bound, but actually an exact characterization. Therefore, the power endowed by non-con uence to shallow P systems is equal to the power gained by con uent P systems when non-elementary membrane division and polynomial depth are allowed, thus suggesting a connection between the roles of non-confluence and nesting depth

First-Principles Wannier Functions of Silicon and Gallium Arsenide

We present a self-consistent, real-space calculation of the Wannier functions of Si and GaAs within density functional theory. We minimize the total energy functional with respect to orbitals which behave as Wannier functions under crystal translations and, at the minimum, are orthogonal. The Wannier functions are used to calculate the total energy, lattice constant, bulk modulus, and the frequency of the zone-center TO phonon of the two semiconductors with the accuracy required nowadays in ab-initio calculations. Furthermore, the centers of the Wannier functions are used to compute the macroscopic polarization of Si and GaAs in zero electric field. The effective charges of GaAs, obtained by finite differentiation of the polarization, agree with the results of linear response theory.Comment: 12 pages, 2 PostScript figures, RevTeX, to appear in Physical Review

Introducing a Space Complexity Measure for P Systems

We define space complexity classes in the framework of membrane computing, giving some initial results about their mutual relations and their connection with time complexity classes, and identifying some potentially interesting problems which require further research