6,555 research outputs found
(In)commensurability, scaling and multiplicity of friction in nanocrystals and application to gold nanocrystals on graphite
The scaling of friction with the contact size and (in)commensurabilty of
nanoscopic and mesoscopic crystals on a regular substrate are investigated
analytically for triangular nanocrystals on hexagonal substrates. The crystals
are assumed to be stiff, but not completely rigid. Commensurate and
incommensurate configurations are identified systematically. It is shown that
three distinct friction branches coexist, an incommensurate one that does not
scale with the contact size () and two commensurate ones which scale
differently (with and ) and are associated with various
combinations of commensurate and incommensurate lattice parameters and
orientations. This coexistence is a direct consequence of the two-dimensional
nature of the contact layer, and such multiplicity exists in all geometries
consisting of regular lattices. To demonstrate this, the procedure is repeated
for rectangular geometry. The scaling of irregularly shaped crystals is also
considered, and again three branches are found (). Based
on the scaling properties, a quantity is defined which can be used to classify
commensurability in infinite as well as finite contacts. Finally, the
consequences for friction experiments on gold nanocrystals on graphite are
discussed
The 3-graviton vertex function in thermal quantum gravity
The high temperature limit of the 3-graviton vertex function is studied in
thermal quantum gravity, to one loop order. The leading () contributions
arising from internal gravitons are calculated and shown to be twice the ones
associated with internal scalar particles, in correspondence with the two
helicity states of the graviton. The gauge invariance of this result follows in
consequence of the Ward and Weyl identities obeyed by the thermal loops, which
are verified explicitly.Comment: 19 pages, plain TeX, IFUSP/P-100
Predicting crystal structures: the Parrinello-Rahman method revisited
By suitably adapting a recent approach [A. Laio and M. Parrinello, PNAS, 99,
12562 (2002)] we develop a powerful molecular dynamics method for the study of
pressure-induced structural transformations. We use the edges of the simulation
cell as collective variables. In the space of these variables we define a
metadynamics that drives the system away from the local minimum towards a new
crystal structure. In contrast to the Parrinello-Rahman method our approach
shows no hysteresis and crystal structure transformations can occur at the
equilibrium pressure. We illustrate the power of the method by studying the
pressure-induced diamond to simple hexagonal phase transition in a model of
silicon.Comment: 5 pages, 2 Postscript figures, submitte
On the Infrared Behavior of the Pressure in Thermal Field Theories
We study non-perturbatively, via the Schwinger-Dyson equations, the leading
infrared behavior of the pressure in the ladder approximation. This problem is
discussed firstly in the context of a thermal scalar field theory, and the
analysis is then extended to the Yang-Mills theory at high temperatures. Using
the Feynman gauge, we find a system of two coupled integral equations for the
gluon and ghost self-energies, which is solved analytically. The solutions of
these equations show that the contributions to the pressure, when calculated in
the ladder approximation, are finite in the infrared domain.Comment: 20 pages plus 4 figures available by request, IFUSP/P-100
The graviton self-energy in thermal quantum gravity
We show generally that in thermal gravity, the one-particle irreducible
2-point function depends on the choice of the basic graviton fields. We derive
the relevant properties of a physical graviton self-energy, which is
independent of the parametrization of the graviton field. An explicit
expression for the graviton self-energy at high-temperature is given to
one-loop order.Comment: 13 pages, 2 figure
Dynamical transitions in incommensurate systems
In the dynamics of the undamped Frenkel-Kontorova model with kinetic terms,
we find a transition between two regimes, a floating incommensurate and a
pinned incommensurate phase. This behavior is compared to the static version of
the model. A remarkable difference is that, while in the static case the two
regimes are separated by a single transition (the Aubry transition), in the
dynamical case the transition is characterized by a critical region, in which
different phenomena take place at different times. In this paper, the
generalized angular momentum we have previously introduced, and the dynamical
modulation function are used to begin a characterization of this critical
region. We further elucidate the relation between these two quantities, and
present preliminary results about the order of the dynamical transition.Comment: 7 pages, 6 figures, file 'epl.cls' necessary for compilation
provided; subm. to Europhysics Letter
Energy flow of moving dissipative topological solitons
We study the energy flow due to the motion of topological solitons in
nonlinear extended systems in the presence of damping and driving. The total
field momentum contribution to the energy flux, which reduces the soliton
motion to that of a point particle, is insufficient. We identify an additional
exchange energy flux channel mediated by the spatial and temporal inhomogeneity
of the system state. In the well-known case of a DC external force the
corresponding exchange current is shown to be small but non-zero. For the case
of AC driving forces, which lead to a soliton ratchet, the exchange energy flux
mediates the complete energy flow of the system. We also consider the case of
combination of AC and DC external forces, as well as spatial discretization
effects.Comment: 24 pages, 5 figures, submitted to Chao
Forward Flux Sampling-type schemes for simulating rare events: Efficiency analysis
We analyse the efficiency of several simulation methods which we have
recently proposed for calculating rate constants for rare events in stochastic
dynamical systems, in or out of equilibrium. We derive analytical expressions
for the computational cost of using these methods, and for the statistical
error in the final estimate of the rate constant, for a given computational
cost. These expressions can be used to determine which method to use for a
given problem, to optimize the choice of parameters, and to evaluate the
significance of the results obtained. We apply the expressions to the
two-dimensional non-equilibrium rare event problem proposed by Maier and Stein.
For this problem, our analysis gives accurate quantitative predictions for the
computational efficiency of the three methods.Comment: 19 pages, 13 figure
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Faculty responses to business school branding: a discursive approach
It is increasingly recognized that the branding of universities presents a different set of challenges from corporate, for-profit sectors. However, much remains unknown about how faculty in particular interpret and make sense of branding in this complex environment. This paper investigates faculty responses to branding through a qualitative interview-based study of four business schools. Our discursive approach to understanding faculty responses highlights the fluid and reflexive nature of brand engagement, in which faculty adopt a number of stances towards their schoolâs branding efforts. In particular, the study identifies three main faculty responses to branding: endorsement, ambivalence and cynicism. The study highlights the ambiguities created from higher education brand management efforts, and the multiple ways that faculty exploit, frame and resist the branding of their business schools. We conclude by discussing the implications of these findings for branding in university contexts
Thermal matter and radiation in a gravitational field
We study the one-loop contributions of matter and radiation to the
gravitational polarization tensor at finite temperatures. Using the
analytically continued imaginary-time formalism, the contribution of matter is
explicitly given to next-to-leading () order. We obtain an exact form for
the contribution of radiation fields, expressed in terms of generalized Riemann
zeta functions. A general expression is derived for the physical polarization
tensor, which is independent of the parametrization of graviton fields. We
investigate the effective thermal masses associated with the normal modes of
the corresponding graviton self-energy.Comment: 32 pages, IFUSP/P-107
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