2,252 research outputs found
Growth of epitaxial nanowires by controlled coarsening of strained islands
We show that elongated nanowires can be grown on crystal surfaces by allowing
large strained two-dimensional islands to desorb by varying the adatom
supersaturation or chemical potential. The width of the wires formed in this
process is determined by a competition between the repulsive elastic
interactions of the long edges of the wires and the thermodynamic driving force
which tends to decrease the distance between these edges. The proposed
mechanism allows for control of the wire sizes by changing the growth
conditions, in particular, the vapor pressure of the material that is being
deposited
On the energetic origin of self-limiting trenches formed around Ge/Si quantum dots
At high growth temperatures, the misfit strain at the boundary of Ge quantum
dots on Si(001) is relieved by formation of trenches around the base of the
islands. The depth of the trenches has been observed to saturate at a level
that depends on the base-width of the islands. Using finite element
simulations, we show that the self-limiting nature of trench depth is due to a
competition between the elastic relaxation energy gained by the formation of
the trench and the surface energy cost for creating the trench. Our simulations
predict a linear increase of the trench depth with the island radius, in
quantitative agreement with the experimental observations of Drucker and
coworkers
The atomistic structure and energy of nascent dislocation loops
An harmonic lattice theory is used, in conjunction with Mura's theory of eigendistorsions, to study the structure and energetics of nascent dislocation loops in face-centred-cubic (FCC) crystals. An analytical expression for the activation energies of such loops is derived. The results obtained herein indicate that thermal activation of small dislocation loops is possible at high stress levels such as those found in the vicinity of a crack tip. The implications of these results in understanding phenomena such as the brittle-ductile transition are discussed
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
Frequency shifts and depth dependence of premotor beta band activity during perceptual decision-making
Neural activity in the premotor and motor cortices shows prominent structure in the beta frequency range (13–30 Hz). Currently, the behavioral relevance of this beta band activity (BBA) is debated. The underlying source of motor BBA and how it changes as a function of cortical depth are also not completely understood. Here, we addressed these unresolved questions by investigating BBA recorded using laminar electrodes in the dorsal premotor cortex of 2 male rhesus macaques performing a visual reaction time (RT) reach discrimination task. We observed robust BBA before and after the onset of the visual stimulus but not during the arm movement. While poststimulus BBA was positively correlated with RT throughout the beta frequency range, prestimulus correlation varied by frequency. Low beta frequencies (∼12–20 Hz) were positively correlated with RT, and high beta frequencies (∼22–30 Hz) were negatively correlated with RT. Analysis and simulations suggested that these frequency-dependent correlations could emerge due to a shift in the component frequencies of the prestimulus BBA as a function of RT, such that faster RTs are accompanied by greater power in high beta frequencies. We also observed a laminar dependence of BBA, with deeper electrodes demonstrating stronger power in low beta frequencies both prestimulus and poststimulus. The heterogeneous nature of BBA and the changing relationship between BBA and RT in different task epochs may be a sign of the differential network dynamics involved in cue expectation, decision-making, motor preparation, and movement execution.Published versio
Possible ferro-spin nematic order in NiGa2S4
We explore the possibility that the spin-1 triangular lattice magnet NiGa2 S4
may have a ferro-nematic ground state with no frozen magnetic moment but a
uniform quadrupole moment. Such a state may be stabilized by biquadratic spin
interactions. We describe the physical properties of this state and suggest
experiments to help verify this proposal. We also contrast this state with a
`non-collinear' nematic state proposed earlier by Tsunetsugu and Arikawa for
NiGa2S4
(k, n)-fractonic Maxwell theory
Fractons emerge as charges with reduced mobility in a class of gauge theories. Here, we generalize fractonic theories of U(1) type to what we call (k, n)-fractonic Maxwell theory, which employs symmetric rank-n tensors of k forms (rank-k antisymmetric tensors) as "vector potentials." The generalization, valid in any spatial dimension d, has two key manifestations. First, the objects with mobility restrictions extend beyond simple charges to higher-order multipoles (dipoles, quadrupoles, etc.) all the way to (n - 1)th-order multipoles, which we call the order-n fracton condition. Second, these fractonic charges themselves are characterized by tensorial densities of (k - 1)-dimensional extended objects. For any (k, n), the theory can be constructed to have a gapless "photon modes" with dispersion omega similar to vertical bar q vertical bar(z), where the integer z can range from 1 to n
Quasicontinuum Models of Interfacial Structure and Deformation
Microscopic models of the interaction between grain boundaries (GBs) and both
dislocations and cracks are of importance in understanding the role of
microstructure in altering the mechanical properties of a material. A recently
developed mixed atomistic and continuum method is extended to examine the
interaction between GBs, dislocations and cracks. These calculations elucidate
plausible microscopic mechanisms for these defect interactions and allow for
the quantitative evaluation of critical parameters such as the stress to
nucleate a dislocation at a step on a GB and the force needed to induce GB
migration.Comment: RevTex, 4 pages, 4 figure
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