139 research outputs found
Nucleation of Ge clusters at high temperatures on Ge/Si(001) wetting layer
Difference in nucleation of Ge quantum dots during Ge deposition at low (<
600C) and high (> 600C) temperatures on the Si(001) surface is studied by high
resolution scanning tunneling microscopy. Two process resulting in appearance
of {105}-faceted clusters on the Ge wetting layer have been observed at high
temperatures: Pyramids have been observed to nucleate via the previously
described formation of strictly determined structures, resembling blossoms,
composed by 16 dimers grouped in pairs and chains of 4 dimes on tops of the
wetting layer M x N patches, each on top of a separate single patch, just like
it goes on at low temperatures; an alternative process consists in faceting of
shapeless heaps of excess Ge atoms which arise in the vicinity of strong sinks
of adatoms, such as pits or steps. The latter process has never been observed
at low temperatures; it is typical only for the high-temperature deposition
mode.Comment: 13 pages, 4 figures; a revised versio
Propagation of thermal excitations in a cluster of vortices in superfluid 3He-B
We describe the first measurement on Andreev scattering of thermal
excitations from a vortex configuration with known density, spatial extent, and
orientations in 3He-B superfluid. The heat flow from a blackbody radiator in
equilibrium rotation at constant angular velocity is measured with two quartz
tuning fork oscillators. One oscillator creates a controllable density of
excitations at 0.2Tc base temperature and the other records the thermal
response. The results are compared to numerical calculations of ballistic
propagation of thermal quasiparticles through a cluster of rectilinear
vortices.Comment: 6 pages, 4 figure
Vortex lines or sheets - what is formed in dynamic drives?
In isotropic macroscopic quantum systems vortex lines can be formed while in
anisotropic systems also vortex sheets are possible. Based on measurements of
superfluid 3He-A, we present the principles which select between these two
competing forms of quantized vorticity: sheets displace lines if the frequency
of the external field exceeds a critical limit. The resulting topologically
stable state consists of multiple vortex sheets and has much faster dynamics
than the state with vortex lines.Comment: RevTex, 5 pages, sumbitted to Phys. Rev. Let
Energy Spectra of Quantum Turbulence: Large-scale Simulation and Modeling
In simulation of quantum turbulence within the Gross-Pitaevskii
equation we demonstrate that the large scale motions have a classical
Kolmogorov-1941 energy spectrum E(k) ~ k^{-5/3}, followed by an energy
accumulation with E(k) ~ const at k about the reciprocal mean intervortex
distance. This behavior was predicted by the L'vov-Nazarenko-Rudenko bottleneck
model of gradual eddy-wave crossover [J. Low Temp. Phys. 153, 140-161 (2008)],
further developed in the paper.Comment: (re)submitted to PRB: 5.5 pages, 4 figure
Quasiparticle scattering measurements of laminar and turbulent vortex flow in the spin-down of superfluid 3He-B
The dynamics of quantized vortices is studied in superfluid 3He-B after a
rapid stop of rotation. We use Andreev reflection of thermal excitations to
monitor vortex motion with quartz tuning fork oscillators in two different
experimental setups at temperatures below 0.2Tc. Deviations from ideal
cylindrical symmetry in the flow environment cause the early decay to become
turbulent. This is identified from a rapid initial overshoot in the vortex
density above the value before the spin-down and its subsequent decay with a
t^(-3/2) time dependence. The high polarization of the vortices along the
rotation axis significantly suppresses the effective turbulent kinematic
viscosity below the values reported for more homogeneous turbulence and leads
to a laminar late-time response. The vortex dissipation down to T < 0.15Tc is
determined from the precession frequency of the polarized vortex configuration.
In the limit of vanishing normal component density, the laminar dissipation is
found to approach a temperature-independent value, whose origin is currently
under discussion.Comment: 8 pages, 5 figure
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