1,656 research outputs found
Quantum Dynamics of the Taub Universe in a Generalized Uncertainty Principle framework
The implications of a Generalized Uncertainty Principle on the Taub
cosmological model are investigated. The model is studied in the ADM reduction
of the dynamics and therefore a time variable is ruled out. Such a variable is
quantized in a canonical way and the only physical degree of freedom of the
system (related to the Universe anisotropy) is quantized by means of a modified
Heisenberg algebra. The analysis is performed at both classical and quantum
level. In particular, at quantum level, the motion of wave packets is
investigated. The two main results obtained are as follows. i) The classical
singularity is probabilistically suppressed. The Universe exhibits a stationary
behavior and the probability amplitude is peaked in a determinate region. ii)
The GUP wave packets provide the right behavior in the establishment of a
quasi-isotropic configuration for the Universe.Comment: 10 pages, 4 figures; v2: section added, to appear on PR
Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy
The effective Mott gap measured by scanning tunneling microscopy (STM) in the
lightly doped Mott insulator differs
greatly from values reported by photoemission and optical experiments. Here, we
show that this is a consequence of the poor electronic screening of the
tip-induced electric field in this material. Such effects are well known from
STM experiments on semiconductors, and go under the name of tip-induced band
bending (TIBB). We show that this phenomenon also exists in the lightly doped
Mott insulator and that, at doping
concentrations of , it causes the measured energy gap in the sample
density of states to be bigger than the one measured with other techniques. We
develop a model able to retrieve the intrinsic energy gap leading to a value
which is in rough agreement with other experiments, bridging the apparent
contradiction. At doping we further observe circular features
in the conductance layers that point to the emergence of a significant density
of free carriers in this doping range, and to the presence of a small
concentration of donor atoms. We illustrate the importance of considering the
presence of TIBB when doing STM experiments on correlated-electron systems and
discuss the similarities and differences between STM measurements on
semiconductors and lightly doped Mott insulators.Comment: 9 pages, 5 figure
40. The gas-phase ammoxidation of n-hexane to unsaturated C6 dinitriles, intermediates for hexamethylenediamine synthesis
This paper reports about an investigation on the catalytic gas-phase ammoxidation of n-hexane aimed at the production of 1,6-C6 dinitriles, precursors for the synthesis of hexamethylenediamine. Catalysts tested were those also active and selective in the ammoxidation of propane to acrylonitrile: rutile-type V/Sb and Sn/V/Nb/Sb mixed oxides. Several N-containing compounds formed; however, the selectivity to cyano-containing aliphatic linear C6 compounds was low, due to the relevant contribution of side reactions such as combustion, cracking and formation of heavy compounds.INGLES
Minimal Length and the Quantum Bouncer: A Nonperturbative Study
We present the energy eigenvalues of a quantum bouncer in the framework of
the Generalized (Gravitational) Uncertainty Principle (GUP) via quantum
mechanical and semiclassical schemes. In this paper, we use two equivalent
nonperturbative representations of a deformed commutation relation in the form
[X,P]=i\hbar(1+\beta P^2) where \beta is the GUP parameter. The new
representation is formally self-adjoint and preserves the ordinary nature of
the position operator. We show that both representations result in the same
modified semiclassical energy spectrum and agrees well with the quantum
mechanical description.Comment: 14 pages, 2 figures, to appear in Int. J. Theor. Phy
The oxygen-assisted transformation of propane to COx/H2 through combined oxidation and WGS reactions catalyzed by vanadium oxide-based catalysts
This paper reports about the gas-phase oxidation of propane catalyzed by bulk vanadium oxide and by alumina- and silica-supported vanadium oxide. The reaction was studied with the aim of finding conditions at which the formation of H2 and CO2 is preferred over that of CO, H2O and of products of alkane partial oxidation. It was found that with bulk V2O5 considerable amounts of H2 are produced above 400 8C, the temperature at which the limiting reactant, oxygen, is totally consumed. The formation of H2 derived from the combination of: (i) oxidation reactions, with generation of CO, CO2, oxygenates (mainly acetic acid), propylene and H2O, all occurring in the fraction of catalytic bed that operated in the presence of gas-phase oxygen, and (ii) WGS reaction, propane dehydrogenation and coke formation, that instead occurred in the fraction of bed operating under anaerobic conditions. This combination of different reactions in a single catalytic bed was possible because of the reduction of V2O5 to V2O3 at high temperature, in the absence of gas-phase oxygen. In fact, vanadium sesquioxide was found to be an effective catalyst for the WGS, while V2O5 was inactive in this reaction. The same combination of reactions was not possible when vanadium oxide was supported over high-surface area silica or alumina; this was attributed to the fact that in these catalysts vanadium was not reduced below the oxidation state V4+, even under reaction conditions leading to total oxygen conversion. In consequence, these catalysts produced less H2 than bulk vanadium oxide
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