23,845 research outputs found
O uso do lodo de esgoto na agricultura: precauções com os contaminantes orgânicos.
bitstream/CNPMA/7108/1/documentos_64.pd
From Quantum Dynamics to the Canonical Distribution: General Picture and a Rigorous Example
Derivation of the canonical (or Boltzmann) distribution based only on quantum
dynamics is discussed. Consider a closed system which consists of mutually
interacting subsystem and heat bath, and assume that the whole system is
initially in a pure state (which can be far from equilibrium) with small energy
fluctuation. Under the "hypothesis of equal weights for eigenstates", we derive
the canonical distribution in the sense that, at sufficiently large and typical
time, the (instantaneous) quantum mechanical expectation value of an arbitrary
operator of the subsystem is almost equal to the desired canonical expectation
value. We present a class of examples in which the above derivation can be
rigorously established without any unproven hypotheses.Comment: LaTeX, 8 pages, no figures. The title, abstract and some discussions
are modified to stress physical motivation of the work. References are added
to [2]. This version will appear in Phys. Rev. Lett. There is an accompanying
unpublished note (cond-mat/9707255
Chiral Crystal Growth under Grinding
To study the establishment of homochirality observed in the crystal growth
experiment of chiral molecules from a solution under grinding, we extend the
lattice gas model of crystal growth as follows. A lattice site can be occupied
by a chiral molecule in R or S form, or can be empty. Molecules form
homoclusters by nearest neighbor bonds. They change their chirality if they are
isolated monomers in the solution. Grinding is incorporated by cutting and
shafling the system randomly. It is shown that Ostwald ripening without
grinding is extremely slow to select chirality, if possible. Grinding alone
also cannot achieve chirality selection. For the accomplishment of
homochirality, we need an enhanced chirality change on crystalline surface.
With this "autocatalytic effect" and the recycling of monomers due to rinding,
an exponential increase of crystal enantiomeric excess to homochiral state is
realized.Comment: 10 pages, 5 figure
Ultrafast photocurrent measurement of the escape time of electrons and holes from carbon nanotube PN junction photodiodes
Ultrafast photocurrent measurements are performed on individual carbon
nanotube PN junction photodiodes. The photocurrent response to sub-picosecond
pulses separated by a variable time delay {\Delta}t shows strong photocurrent
suppression when two pulses overlap ({\Delta}t = 0). The picosecond-scale decay
time of photocurrent suppression scales inversely with the applied bias VSD,
and is twice as long for photon energy above the second subband E22 as compared
to lower energy. The observed photocurrent behavior is well described by an
escape time model that accounts for carrier effective mass.Comment: 8 pages Main text, 4 Figure
Distribuição ambiental de poluentes orgânicos encontrados em lodos de esgoto.
O objetivo deste trabalho foi apresentar uma modelagem da distribuição ambiental de vinte e nove poluentes orgânicos encontrados em amostras de lodos de esgoto das estações de tratamento de esgoto de Barueri e Suzano, SP. Lodos de esgoto são misturas de produtos orgânicos e inorgânicos gerados nos processos primários e secundários de tratamento que, mediante adequado controle de qualidade, podem ser reutilizados como fertilizantes agrÃcolas. O modelo de fugacidade nÃvel I foi aplicado a um sistema compartimental hipotético constituÃdo de ar, água, solo, sedimento, biota aquática e plantas. Foi utilizada a massa molecular, a pressão de vapor, a solubilidade em água, a constante de Henry, o coeficiente de partição octanol-água e a meia-vida no solo de cada um dos poluentes. O coeficiente de sorção no solo, os fatores de bioconcentração em organismos aquáticos, raÃzes e na seiva do xilema de plantas foram calculados por meio de expressões que correlacionam cada um desses parâmetros com o coeficiente de partição octanol-água do poluente. Foram estimados e usados os coeficientes de partição folha-ar e ar-água e o Ãndice GUS de cada poluente. A modelagem e os cálculos dos fatores de bioconcentração e dos coeficientes de partição revelam os compartimentos preferenciais dos poluentes
Energy Anomaly and Polarizability of Carbon Nanotubes
The energy of electron Fermi sea perturbed by external potential, represented
as energy anomaly which accounts for the contribution of the deep-lying states,
is analyzed for massive d = 1+1 Dirac fermions on a circle. The anomaly is a
universal function of the applied field, and is related to known
field-theoretic anomalies. We express transverse polarizability of Carbon
nanotubes via the anomaly, in a way which exhibits the universality and
scale-invariance of the response dominated by pi-electrons and qualitatively
different from that of dielectric and conducting shells. Electron band
transformation in a strong-field effect regime is predicted.Comment: 4 pg
Ground-state energy of the electron liquid in ultrathin wires
The ground-state energy and the density correlation function of the electron
liquid in a thin one-dimensional wire are computed. The calculation is based on
an approximate mapping of the problem with a realistic Coulomb interaction law
onto exactly solvable models of mathematical physics. This approach becomes
asymptotically exact in the limit of small wire radius but remains numerically
accurate even for modestly thin wires.Comment: (v3) Replaced with the published version. 4 pages, 1 figur
Narrow gap Luttinger liquid in Carbon nanotubes
Electron interactions reinforce minigaps induced in metallic nanotubes by an
external field and turn the gap field dependence into a universal power law. An
exactly solvable Gross-Neveau model with an SU(4) symmetry is derived for
neutral excitations near half-filling. Charge excitations, described by a
sin-Gordon perturbation of Luttinger liquid theory, are composite solitons
formed by the charged and neutral fields with two separate length scales.
Charge compressibility at finite density, evaluated in terms of inter-soliton
interaction, exhibits a crossover from overlapping to non-overlapping soliton
state. Implications for the Coulomb blockade measurements are discussed.Comment: 4 pages, 2 figure
Distribution of the spacing between two adjacent avoided crossings
We consider the frequency at which avoided crossings appear in an energy
level structure when an external field is applied to a quantum chaotic system.
The distribution of the spacing in the parameter between two adjacent avoided
crossings is investigated. Using a random matrix model, we find that the
distribution of these spacings is well fitted by a power-law distribution for
small spacings. The powers are 2 and 3 for the Gaussian orthogonal ensemble and
Gaussian unitary ensemble, respectively. We also find that the distributions
decay exponentially for large spacings. The distributions in concrete quantum
chaotic systems agree with those of the random matrix model.Comment: 11 page
Gluon propagators and center vortices in gluon plasma
We study electric and magnetic components of the gluon propagators in
quark-gluon plasma in terms of center vortices by using a quenched simulation
of SU(2) lattice theory. In the Landau gauge, the magnetic components of the
propagators are strongly affected in the infrared region by removal of the
center vortices, while the electric components are almost unchanged by this
procedure. In the Coulomb gauge, the time-time correlators, including an
instantaneous interaction, also have an essential contribution from the center
vortices. As a result, one finds that magnetic degrees of freedom in the
infrared region couple strongly to the center vortices in the deconfinement
phase.Comment: 12 pages, 11 figure
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