979 research outputs found
Step bunching of vicinal 6H-SiC{0001} surfaces
We use kinetic Monte Carlo simulations to understand growth- and
etching-induced step bunching of 6H-SiC{0001} vicinal surfaces oriented towards
[1-100] and [11-20]. By taking account of the different rates of surface
diffusion on three inequivalent terraces, we reproduce the experimentally
observed tendency for single bilayer height steps to bunch into half unit cell
height steps. By taking account of the different mobilities of steps with
different structures, we reproduce the experimentally observed tendency for
adjacent pairs of half unit cell height steps to bunch into full unit cell
height steps. A prediction of our simulations is that growth-induced and
etching-induced step bunching lead to different surface terminations for the
exposed terraces when full unit cell height steps are present.Comment: 10 pages, 12 figure
Frictional shear cracks
We discuss crack propagation along the interface between two dissimilar
materials. The crack edge separates two states of the interface, ``stick'' and
``slip''. In the slip region we assume that the shear stress is proportional to
the sliding velocity, i.e. the linear viscous friction law. In this picture the
static friction appears as the Griffith threshold for crack propagation. We
calculate the crack velocity as a function of the applied shear stress and find
that the main dissipation comes from the macroscopic region and is mainly due
to the friction at the interface. The relevance of our results to recent
experiments,
Baumberger et al, Phys. Rev. Lett. 88, 075509 (2002), is discussed
Transfer matrix solution of the Wako-Sait\^o-Mu\~noz-Eaton model augmented by arbitrary short range interactions
The Wako-Sait{\^o}-Mu\~noz-Eaton (WSME) model, initially introduced in the
theory of protein folding, has also been used in modeling the RNA folding and
some epitaxial phenomena. The advantage of this model is that it admits exact
solution in the general inhomogeneous case (Bruscolini and Pelizzola, 2002)
which facilitates the study of realistic systems. However, a shortcoming of the
model is that it accounts only for interactions within continuous stretches of
native bonds or atomic chains while neglecting interstretch (interchain)
interactions. But due to the biopolymer (atomic chain) flexibility, the
monomers (atoms) separated by several non-native bonds along the sequence can
become closely spaced. This produces their strong interaction. The inclusion of
non-WSME interactions into the model makes the model more realistic and
improves its performance. In this study we add arbitrary interactions of finite
range and solve the new model by means of the transfer matrix technique. We can
therefore exactly account for the interactions which in proteomics are
classified as medium- and moderately long-range ones.Comment: 15 pages, 2 figure
Fracture and Friction: Stick-Slip Motion
We discuss the stick-slip motion of an elastic block sliding along a rigid
substrate. We argue that for a given external shear stress this system shows a
discontinuous nonequilibrium transition from a uniform stick state to uniform
sliding at some critical stress which is nothing but the Griffith threshold for
crack propagation. An inhomogeneous mode of sliding occurs, when the driving
velocity is prescribed instead of the external stress. A transition to
homogeneous sliding occurs at a critical velocity, which is related to the
critical stress. We solve the elastic problem for a steady-state motion of a
periodic stick-slip pattern and derive equations of motion for the tip and
resticking end of the slip pulses. In the slip regions we use the linear
viscous friction law and do not assume any intrinsic instabilities even at
small sliding velocities. We find that, as in many other pattern forming
system, the steady-state analysis itself does not select uniquely all the
internal parameters of the pattern, especially the primary wavelength. Using
some plausible analogy to first order phase transitions we discuss a ``soft''
selection mechanism. This allows to estimate internal parameters such as crack
velocities, primary wavelength and relative fraction of the slip phase as
function of the driving velocity. The relevance of our results to recent
experiments is discussed.Comment: 12 pages, 7 figure
Strain induced stabilization of stepped Si and Ge surfaces near (001)
We report on calculations of the formation energies of several [100] and
[110] oriented step structures on biaxially stressed Si and Ge (001) surfaces.
It is shown that a novel rebonded [100] oriented single-height step is strongly
stabilized by compressive strain compared to most well-known step structures.
We propose that the side walls of ``hut''-shaped quantum dots observed in
recent experiments on SiGe/Si films are made up of these steps. Our
calculations provide an explanation for the nucleationless growth of shallow
mounds, with steps along the [100] and [110] directions in low- and high-misfit
films, respectively, and for the stability of the (105) facets under
compressive strain.Comment: to appear in Appl. Phys. Lett.; v2=minor corrections,figs resize
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