1,929 research outputs found
Potential Energy Landscape Equation of State
Depth, number, and shape of the basins of the potential energy landscape are
the key ingredients of the inherent structure thermodynamic formalism
introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys.
Rev. A 25, 978 (1982)]. Within this formalism, an equation of state based only
on the volume dependence of these landscape properties is derived. Vibrational
and configurational contributions to pressure are sorted out in a transparent
way. Predictions are successfully compared with data from extensive molecular
dynamics simulations of a simple model for the fragile liquid orthoterphenyl.Comment: RevTeX4, 4 pages, 5 figure
Evaluation of configurational entropy of a model liquid from computer simulations
Computer simulations have been employed in recent years to evaluate the
configurational entropy changes in model glass-forming liquids. We consider two
methods, both of which involve the calculation of the `intra-basin' entropy as
a means for obtaining the configurational entropy. The first method involves
the evaluation of the intra-basin entropy from the vibrational frequencies of
inherent structures, by making a harmonic approximation of the local potential
energy topography. The second method employs simulations that confine the
liquid within a localized region of configuration space by the imposition of
constraints; apart from the choice of the constraints, no further assumptions
are made. We compare the configurational entropies estimated for a model liquid
(binary mixture of particles interacting {\it via} the Lennard-Jones potential)
for a range of temperatures, at fixed density.Comment: 10 pages, 5 figures, Proceedings of "Unifying Concepts in Glass
Physics" Trieste 1999 (to appear in J. Phys. Cond. Mat.
Stabilization of trajectories for systems with nonholonomic constraints
A technique for stabilizing nonholonomic systems to trajectories is presented. It is well known that such systems cannot be stabilized to a point using smooth static-state feedback. The authors suggest the use of control laws for stabilizing a system about a trajectory, instead of a point. Given a nonlinear system and a desired nominal feasible trajectory, an explicit control law which will locally exponentially stabilize the system to the desired trajectory is given. The theory is applied to several examples, including a car-like robot
Comment on "Quantum mechanics of smeared particles"
In a recent article, Sastry has proposed a quantum mechanics of smeared
particles. We show that the effects induced by the modification of the
Heisenberg algebra, proposed to take into account the delocalization of a
particle defined via its Compton wavelength, are important enough to be
excluded experimentally.Comment: 2 page
Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids
The liquid-gas spinodal and the glass transition define ultimate boundaries
beyond which substances cannot exist as (stable or metastable) liquids. The
relation between these limits is analyzed {\it via} computer simulations of a
model liquid. The results obtained indicate that the liquid - gas spinodal and
the glass transition lines intersect at a finite temperature, implying a glass
- gas mechanical instability locus at low temperatures. The glass transition
lines obtained by thermodynamic and dynamic criteria agree very well with each
other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids
A liquid can exist under conditions of thermodynamic stability or
metastability within boundaries defined by the liquid-gas spinodal and the
glass transition line. The relationship between these boundaries has been
investigated previously using computer simulations, the energy landscape
formalism, and simplified model calculations. We calculate these stability
boundaries semi-analytically for a model glass forming liquid, employing
accurate liquid state theory and a first-principles approach to the glass
transition. These boundaries intersect at a finite temperature, consistent with
previous simulation-based studies.Comment: Minor text revisions. Fig.s 4, 5 update
Stability, Gain, and Robustness in Quantum Feedback Networks
This paper concerns the problem of stability for quantum feedback networks.
We demonstrate in the context of quantum optics how stability of quantum
feedback networks can be guaranteed using only simple gain inequalities for
network components and algebraic relationships determined by the network.
Quantum feedback networks are shown to be stable if the loop gain is less than
one-this is an extension of the famous small gain theorem of classical control
theory. We illustrate the simplicity and power of the small gain approach with
applications to important problems of robust stability and robust
stabilization.Comment: 16 page
Potential Energy Landscape and Long Time Dynamics in a Simple Model Glass
We analyze the properties of a Lennard-Jones system at the level of the
potential energy landscape. After an exhaustive investigation of the
topological features of the landscape of the systems, obtained studying small
size sample, we describe the dynamics of the systems in the multi-dimensional
configurational space by a simple model. This consider the configurational
space as a connected network of minima where the dynamics proceeds by jumps
described by an appropriate master equation. Using this model we are able to
reproduce the long time dynamics and the low temperature regime. We investigate
both the equilibrium regime and the off-equilibrium one, finding those typical
glassy behavior usually observed in the experiments such as: {\it i)} stretched
exponential relaxation, {\it ii)} temperature-dependent stretching parameter,
{\it iii)} breakdown of the Stokes-Einstein relation, and {\it iv)} appearance
of a critical temperature below which one observes deviation from the
fluctuation-dissipation relation as consequence of the lack of equilibrium in
the system.Comment: 11 pages (Latex), 9 ps figure
Origin for the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-T_c cuprates
We report measurements of the anisotropy of the spin echo decay for the inner
layer Cu site of the triple layer cuprate, Hg_0.8Re_0.2Ba_2Ca_2Cu_3O_8 (T_c=126
K) in the pseudogap T regime below T_pg ~ 170 K and the corresponding analysis
for their interpretation. As the field alignment is varied, the shape of the
decay curve changes from Gaussian (H_0 \parallel c) to single exponential (H_0
\perp c). The latter characterizes the decay caused by the fluctuations of
adjacent Cu nuclear spins caused by their interactions with electron spins. The
angular dependence of the second moment (T_{2M}^{-2} \equiv )
deduced from the decay curves indicates that T_{2M}^{-2} for H_0 \parallel c,
which is identical to T_{2G}^{-2} (T_{2G} is the Gaussian component), is
substantially enhanced, as seen in the pseudogap regime of the bilayer systems.
Comparison of T_{2M}^{-2} between H_0 \parallel c and H_0 \perp c indicates
that this enhancement is caused by electron spin correlations between the inner
and the outer CuO_2 layers. These results provide the answer to the
long-standing controversy regarding the opposite T dependences of (T_1T)^{-1}
and T_{2G}^{-2} in the pseudogap regime of bi- and trilayer systems.Comment: 4 pages, 4 figure
Growth of carbon nanotubes on quasicrystalline alloys
We report on the synthesis of carbon nanotubes on quasicrystalline alloys.
Aligned multiwalled carbon nanotubes (MWNTs) on the conducting faces of
decagonal quasicrystals were synthesized using floating catalyst chemical vapor
deposition. The alignment of the nanotubes was found perpendicular to the
decagonal faces of the quasicrystals. A comparison between the growth and tube
quality has also been made between tubes grown on various quasicrystalline and
SiO2 substrates. While a significant MWNT growth was observed on decagonal
quasicrystalline substrate, there was no significant growth observed on
icosahedral quasicrystalline substrate. Raman spectroscopy and high resolution
transmission electron microscopy (HRTEM) results show high crystalline nature
of the nanotubes. Presence of continuous iron filled core in the nanotubes
grown on these substrates was also observed, which is typically not seen in
MWNTs grown using similar process on silicon and/or silicon dioxide substrates.
The study has important implications for understanding the growth mechanism of
MWNTs on conducting substrates which have potential applications as heat sinks
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