3,265 research outputs found
Algebraic, geometric, and stochastic aspects of genetic operators
Genetic algorithms for function optimization employ genetic operators patterned after those observed in search strategies employed in natural adaptation. Two of these operators, crossover and inversion, are interpreted in terms of their algebraic and geometric properties. Stochastic models of the operators are developed which are employed in Monte Carlo simulations of their behavior
Charge-ordering, commensurability and metallicity in the phase diagram of layered Na(x)CoO(2)
The phase diagram of non-hydrated Na(x)CoO(2) has been determined by changing
the Na content x using a series of chemical reactions. As x increases from 0.3,
the ground state goes from a paramagnetic metal to a charge-ordered insulator
(at x=1/2) to a `Curie-Weiss metal' (around 0.70), and finally to a weak-moment
magnetically ordered state (x>0.75). The unusual properties of the state at 1/2
(including particle-hole symmetry at low T and enhanced thermal conductivity)
are described. The strong coupling between the Na ions and the holes is
emphasized.Comment: 4 pages with 3 figures, changed conten
Pressure effects in the triangular layered cobaltites NaxCoO2
We have measured transport properties as a function of temperature and
pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition
temperature at 53K increases with pressure, while paradoxically the sample
passes from an insulating to a metallic ground state. A similar transition is
observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample
transforms the sample from a metallic to an insulating state. We discuss our
results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure
A Novel Taxonomy of Opportunities and Risks in Massively Multiplayer Online Role Playing Games
The explosion and rapid embrace of Massively Multiplayer Online Role Playing Games (MMORPG) has provided players with unique, fully immersive three dimensional environments in which they can express themselves in a myriad of ways. Players can develop unique skill sets, share knowledge, explore and experiment with various identities and network with like-minded individuals. Evidence suggests however, that these opportunities are accompanied with a number of risks including addiction, desensitisation and threats to the privacy of personal information. This paper presents a novel Taxonomy of opportunities and risks in the specific context of Massively Multiplayer Online Role Playing Games
Large enhancement of the thermopower in NaCoO at high Na doping
Research on the oxide perovskites has uncovered electronic properties that
are strikingly enhanced compared with those in conventional metals. Examples
are the high critical temperatures of the cuprate superconductors and the
colossal magnetoresistance in the manganites. The conducting layered cobaltate
displays several interesting electronic phases as is varied
including water-induced superconductivity and an insulating state that is
destroyed by field. Initial measurements showed that, in the as-grown
composition, displays moderately large thermopower and
conductivity . However, the prospects for thermoelectric cooling
applications faded when the figure of merit was found to be small at this
composition (0.60.7). Here we report that, in the poorly-explored
high-doping region 0.75, undergoes an even steeper enhancement. At the
critical doping 0.85, (at 80 K) reaches values 40 times
larger than in the as-grown crystals. We discuss prospects for low-temperature
thermoelectric applications.Comment: 6 pages, 7 figure
NMR studies of Successive Phase Transitions in Na0.5CoO2 and K0.5CoO2
59Co- and 23Na-NMR measurements have been carried out on polycrystalline and
c-axis aligned samples of Na0.5CoO2, which exhibits successive transitions at
temperatures T = 87 K (= Tc1) and T = 53 K (= Tc2). 59Co-NMR has also been
carried out on c-axis aligned crystallites of K0.5CoO2 with similar successive
transitions at Tc1 ~ 60 K and Tc2 ~ 20 K. For Na0.5CoO2, two sets of three NMR
lines of 23Na nuclei explained by considering the quadrupolar frequencies nuQ
~1.32 and 1.40 MHz have been observed above Tc1, as is expected from the
crystalline structure. Rather complicated but characteristic variation of the
23Na-NMR spectra has been observed with varying T through the transition
temperatures, and the internal fields at two crystallographically distinct Na
sites are discussed on the basis of the magnetic structures reported
previously. The internal fields at two distinct Co sites observed below Tc1 and
the 591/T1-T curves of Na0.5CoO2 and K0.5CoO2 are also discussed in a
comparative way.Comment: 7 pages, 10 figures, submitted to J. Phys. Soc. Jpn, correction is
made in right colum of p6 (35th line) as K0.5CoO2-->Na0.5CoO
Evidence of many-body localization in 2D from quantum Monte Carlo simulation
We use the stochastic series expansion quantum Monte Carlo method, together
with the eigenstate-to-Hamiltonian mapping approach, to map the localized
ground states of the disordered two-dimensional Heisenberg model, to excited
states of a target Hamiltonian. The localized nature of the ground state is
established by studying the spin stiffness, local entanglement entropy, and
local magnetization. This construction allows us to define many body localized
states in an energy resolved phase diagram thereby providing concrete numerical
evidence for the existence of a many-body localized phase in two dimensions.Comment: 8 pages, 6 figure
- …