1,618 research outputs found
Rheological aging and rejuvenation in solid friction contacts
We study the low-velocity (0.1--100 m.s) frictional properties of
interfaces between a rough glassy polymers and smooth silanized glass, a
configuration which gives direct access to the rheology of the adhesive joints
in which shear localizes. We show that these joints exhibit the full
phenomenology expected for confined quasi 2D soft glasses: they strengthen
logarithmically when aging at rest, and weaken (rejuvenate) when sliding.
Rejuvenation is found to saturate at large velocities. Moreover, aging at rest
is shown to be strongly accelerated when waiting under finite stress below the
static threshold
Time-dependent quantum transport: an exact formulation based on TDDFT
An exact theoretical framework based on Time Dependent Density Functional
Theory (TDDFT) is proposed in order to deal with the time-dependent quantum
transport in fully interacting systems. We use a \textit{partition-free}
approach by Cini in which the whole system is in equilibrium before an external
electric field is switched on. Our theory includes the interactions between the
leads and between the leads and the device. It is well suited for calculating
measurable transient phenomena as well as a.c. and other time-dependent
responses. We show that the steady-state current results from a
\textit{dephasing mechanism} provided the leads are macroscopic and the device
is finite. In the d.c. case, we obtain a Landauer-like formula when the
effective potential of TDDFT is uniform deep inside the electrodes.Comment: final version, 7 pages, 1 figur
Perturbation of Tunneling Processes by Mechanical Degrees of Freedom in Mesoscopic Junctions
We investigate the perturbation in the tunneling current caused by
non-adiabatic mechanical motion in a mesoscopic tunnel junction. A theory
introduced by Caroli et al. \cite{bi1,bi2,bi3} is used to evaluate second order
self-energy corrections for this non-equilibrium situation lacking
translational invariance. Inelastic signatures of the mechanical degrees of
freedom are found in the current-voltage characteristics. These give
rise to sharp features in the derivative spectrum, .Comment: 22 pages LaTeX + 3 uuencoded PS picture
Interplay between shear loading and structural aging in a physical gel
We show that the aging of the mechanical relaxation of a gelatin gel exhibits
the same scaling phenomenology as polymer and colloidal glasses. Besides,
gelatin is known to exhibit logarithmic structural aging (stiffening). We find
that stress accelerates this process. However, this effect is definitely
irreducible to a mere age shift with respect to natural aging. We suggest that
it is interpretable in terms of elastically-aided elementary (coilhelix)
local events whose dynamics gradually slows down as aging increases geometric
frustration
The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction across a tunneling junction out of equilibrium
The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic
- spin impurities across a tunneling junction is studied when the system
is driven out of equilibrium through biasing the junction. The nonequilibrium
situation is handled with the Keldysh time-loop perturbation formalism in
conjunction with appropriate coupling methods for tunneling systems due to
Caroli and Feuchtwang. We find that the presence of a nonequilibrium bias
across the junction leads to an interference of several fundamental
oscillations, such that in this tunneling geometry, it is possible to tune the
interaction between ferromagnetic and antiferromagnetic coupling at a fixed
impurity configuration, simply by changing the bias across the junction.
Furthermore, it is shown that the range of the RKKY interaction is altered out
of equilibrium, such that in particular the interaction energy between two
slabs of spins scales extensively with the thickness of the slabs in the
presence of an applied bias.Comment: 38 pages revtex preprint; 5 postscript figures; submitted to Phys.
Rev.
Correlated Nanoscopic Josephson Junctions
We discuss correlated lattice models with a time-dependent potential across a
barrier and show how to implement a Josephson-junction-like behavior. The
pairing occurs by a correlation effect enhanced by the symmetry of the system.
In order to produce the effect we need a mild distortion which causes avoided
crossings in the many-body spectrum. The Josephson-like response involves a
quasi-adiabatic evolution in the time-dependent field. Besides, we observe an
inverse-Josephson (Shapiro) current by applying an AC bias; a supercurrent in
the absence of electromotive force can also be excited. The qualitative
arguments are supported by explicit exact solutions in prototype 5-atom
clusters with on-site repulsion. These basic units are then combined in
ring-shaped systems, where one of the units sits at a higher potential and
works as a barrier. In this case the solution is found by mapping the
low-energy Hamiltonian into an effective anisotropic Heisenberg chain. Once
again, we present evidence for a superconducting flux quantization, i.e. a
Josephson-junction-like behavior suggesting the build-up of an effective order
parameter already in few-electron systems. Some general implications for the
quantum theory of transport are also briefly discussed, stressing the
nontrivial occurrence of asymptotic current oscillations for long times in the
presence of bound states.Comment: 12 pages, 2 figures, to appear in J. Phys. - Cond. Ma
Driven activation versus thermal activation
Activated dynamics in a glassy system undergoing steady shear deformation is
studied by numerical simulations. Our results show that the external driving
force has a strong influence on the barrier crossing rate, even though the
reaction coordinate is only weakly coupled to the nonequilibrium system. This
"driven activation" can be quantified by introducing in the Arrhenius
expression an effective temperature, which is close to the one determined from
the fluctuation-dissipation relation. This conclusion is supported by
analytical results for a simplified model system.Comment: 5 pages, 3 figure
Dry Friction due to Adsorbed Molecules
Using an adiabatic approximation method, which searches for Tomlinson
model-like instabilities for a simple but still realistic model for two
crystalline surfaces in the extremely light contact limit, with mobile
molecules present at the interface, sliding relative to each other, we are able
to account for the virtually universal occurrence of "dry friction." The model
makes important predictions for the dependence of friction on the strength of
the interaction of each surface with the mobile molecules.Comment: four pages of latex, figure provide
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