67 research outputs found
Finite-temperature phase diagram and critical point of the Aubry pinned-sliding transition in a 2D monolayer
The Aubry unpinned--pinned transition in the sliding of two incommensurate
lattices occurs for increasing mutual interaction strength in one dimension
() and is of second order at , turning into a crossover at nonzero
temperatures. Yet, real incommensurate lattices come into contact in two
dimensions (), at finite temperature, generally developing a mutual
Novaco-McTague misalignment, conditions in which the existence of a sharp
transition is not clear. Using a model inspired by colloid monolayers in an
optical lattice as a test case, simulations show a sharp Aubry transition
between an unpinned and a pinned phase as a function of corrugation. Unlike
, the transition is now of first order, and, importantly, remains well
defined at . It is heavily structural, with a local rotation of moir\'e
pattern domains from the nonzero initial Novaco-McTague equilibrium angle to
nearly zero. In the temperature () -- corrugation strength () plane,
the thermodynamical coexistence line between the unpinned and the pinned phases
is strongly oblique, showing that the former has the largest entropy. This
first-order Aubry line terminates with a novel critical point , marked
by a susceptibility peak. The expected static sliding friction upswing between
the unpinned and the pinned phase decreases and disappears upon heating from
to . The experimental pursuit of this novel scenario is proposed.Comment: 9 pages, 9 figure
Friction Boosted by Equilibrium Misalignment of Incommensurate Two-Dimensional Colloid Monolayers
Colloidal 2D monolayers sliding in an optical lattice are of recent
importance as a frictional system. In the general case when the monolayer and
optical lattices are incommensurate, we predict two important novelties, one in
the static equilibrium structure, the other in the frictional behavior under
sliding. Structurally, realistic simulations show that the colloid layer should
possess in full equilibrium a small misalignment rotation angle relative to the
optical lattice, an effect so far unnoticed but visible in some published
experimental moir\'e patterns. Under sliding, this misalignment has the effect
of boosting the colloid monolayer friction by a considerable factor over the
hypothetical aligned case discussed so far. A frictional increase of similar
origin must generally affect other incommensurate adsorbed monolayers and
contacts, to be sought out case by case.Comment: 9 pages, 11 figures (including Supplemental Material
Modeling friction: From nanoscale to mesoscale
The physics of sliding friction is gaining impulse from nanoscale and
mesoscale experiments, simulations, and theoretical modeling. This Colloquium
reviews some recent developments in modeling and in atomistic simulation of
friction, covering open-ended directions, unconventional nanofrictional
systems, and unsolved problems.Comment: 26 pages, 14 figures, Rev. Mod. Phys. Colloquiu
Hysteresis from dynamically pinned sliding states
We report a surprising hysteretic behavior in the dynamics of a simple
one-dimensional nonlinear model inspired by the tribological problem of two
sliding surfaces with a thin solid lubricant layer in between. In particular,
we consider the frictional dynamics of a harmonic chain confined between two
rigid incommensurate substrates which slide with a fixed relative velocity.
This system was previously found, by explicit solution of the equations of
motion, to possess plateaus in parameter space exhibiting a remarkable
quantization of the chain center-of-mass velocity (dynamic pinning) solely
determined by the interface incommensurability. Starting now from this
quantized sliding state, in the underdamped regime of motion and in analogy to
what ordinarily happens for static friction, the dynamics exhibits a large
hysteresis under the action of an additional external driving force F_ext. A
critical threshold value F_c of the adiabatically applied force F_ext is
required in order to alter the robust dynamics of the plateau attractor. When
the applied force is decreased and removed, the system can jump to intermediate
sliding regimes (a sort of ``dynamic'' stick-slip motion) and eventually
returns to the quantized sliding state at a much lower value of F_ext. On the
contrary no hysteretic behavior is observed as a function of the external
driving velocity.Comment: 12 pages, 5 figures, ECOSS 200
AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers
In the atomic force microscope, the nanoscale force topography of even
complex surface superstructures is extracted by the changing vibration
frequency of a scanning tip. An alternative dissipation topography with similar
or even better contrast has been demonstrated recently by mapping the
(x,y)-dependent tip damping but the detailed damping mechanism is still
unknown. Here we identify two different tip dissipation mechanisms: local
mechanical softness and hysteresis. Motivated by recent data, we describe both
of them in a onedimensional model of Moire' superstructures of incommensurate
overlayers. Local softness at "soliton" defects yields a dissipation contrast
that can be much larger than the corresponding density or corrugation contrast.
At realistically low vibration frequencies, however, a much stronger and more
effective dissipation is caused by the tip-induced nonlinear jumping of the
soliton, naturally developing bistability and hysteresis. Signatures of this
mechanism are proposed for experimental identification.Comment: 5 pages, 5 figures, Phys Rev B 81, 045417 (2010
Graphene on h-BN: to align or not to align?
The contact strength, adhesion and friction, between graphene and an
incommensurate crystalline substrate such as {\it h}-BN depends on their
relative alignment angle . The well established Novaco-McTague (NM)
theory predicts for a monolayer graphene on a hard bulk {\it h}-BN crystal face
a small spontaneous misalignment, here \,\,0.45 degrees
which if realized would be relevant to a host of electronic properties besides
the mechanical ones. Because experimental equilibrium is hard to achieve, we
inquire theoretically about alignment or misalignment by simulations based on
dependable state-of-the-art interatomic force fields. Surprisingly at first, we
find compelling evidence for , i.e., full energy-driven alignment
in the equilibrium state of graphene on {\it h}-BN. Two factors drive this
deviation from NM theory. First, graphene is not flat, developing on {\it h}-BN
a long-wavelength out-of-plane corrugation. Second, {\it h}-BN is not hard,
releasing its contact stress by planar contractions/expansions that accompany
the interface moir\'e structure. Repeated simulations by artificially forcing
graphene to keep flat, and {\it h}-BN to keep rigid, indeed yield an
equilibrium misalignment similar to as expected. Subsequent
sliding simulations show that friction of graphene on {\it h}-BN, small and
essentially independent of misalignments in the artificial frozen state,
strongly increases in the more realistic corrugated, strain-modulated, aligned
state
Sliding Over a Phase Transition
The effects of a displacive structural phase transition on sliding friction
are in principle accessible to nanoscale tools such as the Atomic Force
Microscopy, yet they are still surprisingly unexplored. We present model
simulations demonstrating and clarifying the mechanism and potential impact of
these effects. A structural order parameter inside the material will yield a
contribution to stick-slip friction that is nonmonotonic as temperature crosses
the phase transition, peaking at the critical Tc where critical fluctuations
are strongest, and the sliding-induced order parameter local flips from one
value to another more numerous. Accordingly, the friction below Tc is larger
when the order parameter orientation is such that flips are more effectively
triggered by the slider. The observability of these effects and their use for
friction control are discussed, for future application to sliding on the
surface of and ferro- or antiferro-distortive materials.Comment: Accepted on PR
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