462 research outputs found
Plasma deposited diamondlike carbon on GaAs and InP
The properties of diamond like carbon films grown by RF flow discharge 30 kHz plasma using methane are reported. The Cls XPS line shape of films showed localized hybrid carbon bonds as low as 40 to as high as 95 percent. Infrared spectroscopy and N(15) nuclear reaction profiling data indicated 35 to 42 percent hydrogen, depending inversely on deposition temperature. The deposition rate of films on Si falls off exponentially with substrate temperature, and nucleation does not occur above 200 C on GaAs and InP. Optical data of the films showed bandgap values of 2.0 to 2.4 eV increasing monotonically with CH4 flow rate
Optical properties of hydrogenated amorphous carbon films grown from methane plasma
A 30 kHz ac glow discharge formed from methane gas was used to grow carbon films on InP substrates. Both the growth rate, and the realitive Ar ion sputtering rate at 3 keV varied monotonically with deposition power. Results from the N-15 nuclear reaction profile experiments indicated a slight drop in the hydrogen concentration as more energy was dissipated in the ac discharge. Values for the index of refraction and extinction coefficient ranged from 1.721 to 1.910 and 0 to -0.188, respectively. Optical bandgaps as high as 2.34 eV were determined
A dynamical classification of the range of pair interactions
We formalize a classification of pair interactions based on the convergence
properties of the {\it forces} acting on particles as a function of system
size. We do so by considering the behavior of the probability distribution
function (PDF) P(F) of the force field F in a particle distribution in the
limit that the size of the system is taken to infinity at constant particle
density, i.e., in the "usual" thermodynamic limit. For a pair interaction
potential V(r) with V(r) \rightarrow \infty) \sim 1/r^a defining a {\it
bounded} pair force, we show that P(F) converges continuously to a well-defined
and rapidly decreasing PDF if and only if the {\it pair force} is absolutely
integrable, i.e., for a > d-1, where d is the spatial dimension. We refer to
this case as {\it dynamically short-range}, because the dominant contribution
to the force on a typical particle in this limit arises from particles in a
finite neighborhood around it. For the {\it dynamically long-range} case, i.e.,
a \leq d-1, on the other hand, the dominant contribution to the force comes
from the mean field due to the bulk, which becomes undefined in this limit. We
discuss also how, for a \leq d-1 (and notably, for the case of gravity, a=d-2)
P(F) may, in some cases, be defined in a weaker sense. This involves a
regularization of the force summation which is generalization of the procedure
employed to define gravitational forces in an infinite static homogeneous
universe. We explain that the relevant classification in this context is,
however, that which divides pair forces with a > d-2 (or a < d-2), for which
the PDF of the {\it difference in forces} is defined (or not defined) in the
infinite system limit, without any regularization. In the former case dynamics
can, as for the (marginal) case of gravity, be defined consistently in an
infinite uniform system.Comment: 12 pages, 1 figure; significantly shortened and focussed, additional
references, version to appear in J. Stat. Phy
Probability distributions consistent with a mixed state
A density matrix may be represented in many different ways as a
mixture of pure states, \rho = \sum_i p_i |\psi_i\ra \la \psi_i|. This paper
characterizes the class of probability distributions that may appear in
such a decomposition, for a fixed density matrix . Several illustrative
applications of this result to quantum mechanics and quantum information theory
are given.Comment: 6 pages, submitted to Physical Review
Characterizing mixing and measurement in quantum mechanics
What fundamental constraints characterize the relationship between a mixture
of quantum states, the states being mixed,
and the probabilities ? What fundamental constraints characterize the
relationship between prior and posterior states in a quantum measurement? In
this paper we show that there are many surprisingly strong constraints on these
mixing and measurement processes that can be expressed simply in terms of the
eigenvalues of the quantum states involved. These constraints capture in a
succinct fashion what it means to say that a quantum measurement acquires
information about the system being measured, and considerably simplify the
proofs of many results about entanglement transformation.Comment: 12 page
Classical Helium Atom with Radiation Reaction
We study a classical model of Helium atom in which, in addition to the
Coulomb forces, the radiation reaction forces are taken into account. This
modification brings in the model a new qualitative feature of a global
character. Indeed, as pointed out by Dirac, in any model of classical
electrodynamics of point particles involving radiation reaction one has to
eliminate, from the a priori conceivable solutions of the problem, those
corresponding to the emission of an infinite amount of energy. We show that the
Dirac prescription solves a problem of inconsistency plaguing all available
models which neglect radiation reaction, namely, the fact that in all such
models most initial data lead to a spontaneous breakdown of the atom. A further
modification is that the system thus acquires a peculiar form of dissipation.
In particular, this makes attractive an invariant manifold of special physical
interest, the zero--dipole manifold, that corresponds to motions in which no
energy is radiated away (in the dipole approximation). We finally study
numerically the invariant measure naturally induced by the time--evolution on
such a manifold, and this corresponds to studying the formation process of the
atom. Indications are given that such a measure may be singular with respect to
that of Lebesgue.Comment: 16 pages, 3 figure
Fluctuations in Stationary non Equilibrium States
In this paper we formulate a dynamical fluctuation theory for stationary non
equilibrium states (SNS) which covers situations in a nonlinear hydrodynamic
regime and is verified explicitly in stochastic models of interacting
particles. In our theory a crucial role is played by the time reversed
dynamics. Our results include the modification of the Onsager-Machlup theory in
the SNS, a general Hamilton-Jacobi equation for the macroscopic entropy and a
non equilibrium, non linear fluctuation dissipation relation valid for a wide
class of systems
Global validity of the Master kinetic equation for hard-sphere systems
Following the recent establishment of an exact kinetic theory realized by the Master kinetic equation
which describes the statistical behavior of the Boltzmann-Sinai Classical Dynamical System (CDS), in
this paper the problem is posed of the construction of the related global existence and regularity theorems.
For this purpose, based on the global prescription of the same CDS for arbitrary single- and multiplecollision
events, first global existence is extablished for the N-body Liouville equation which is written
in Lagrangian differential and integral forms. This permits to reach the proof of global existence both
of generic N-body probability density functions (PDF) as well as of particular solutions which maximize
the statistical Boltzmann-Shannon entropy and are factorized in terms of the corresponding 1-body PDF.
The latter PDF is shown to be uniquely defined and to satisfy the Master kinetic equation globally in the
extended 1-body phase space. Implications concerning the global validity of the asymptotic Boltzmann
equation and Boltzmann H-theorem are discussed
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