42,377 research outputs found
Generation and recovery of strain in (28)Si-implanted pseudomorphic GeSi films on Si(100)
Effects of ion implantation of 320 keV Si-28 at room temperature in pseudomorphic metastable GexSi1-x (x almost-equal-to 0.04, 0.09, 0.13) layers approximately 170 nm thick grown on Si(100) wafers were characterized by x-ray double-crystal diffractometry and MeV He-4 channeling spectrometry. The damage induced by implantation produces additional compressive strain in the GexSi1-x layers, superimposed on the intrinsic compressive strain of the heterostructures. This strain rises with the dose proportionally for doses below several times 10(14) Si-28/cm2. Furthermore, for a given dose, the strain increases with the Ge content in the layer. Upon thermal processing, the damage anneals out and the strain recovers to the value before implantation. Amorphized samples (doses of greater than 2 x 10(15) Si-28/cm2) regrow poorly
Cellular buckling in I-section struts
An analytical model that describes the interactive buckling of a thin-walled
I-section strut under pure compression based on variational principles is
presented. A formulation combining the Rayleigh--Ritz method and continuous
displacement functions is used to derive a system of differential and integral
equilibrium equations for the structural component. Numerical continuation
reveals progressive cellular buckling (or snaking) arising from the nonlinear
interaction between the weakly stable global buckling mode and the strongly
stable local buckling mode. The resulting behaviour is highly unstable and when
the model is extended to include geometric imperfections it compares
excellently with some recently published experiments.Comment: 23 pages, 12 figures. Submitted for special issue of Thin-Walled
Structure
Defect production in Si(100) by 19F, 28Si, 40Ar, and 131Xe implantation at room temperature
We used x-ray double-crystal diffractometry and MeV 4He channeling spectrometry to study quantitatively the damage produced in Si(100) at room temperature by 230-keV 19F, 230-keV 28Si, 250-keV 40Ar, or 570-keV 131Xe implantation. The measured defect concentration and the perpendicular strain have the same depth profile, and both are depleted near the surface compared to the Frenkel pair concentration calculated from computer simulation. The perpendicular strain is proportional to the defect concentration with a coefficient of B~0.01 common to all implanted species. The maximum value of the perpendicular strain and of the defect concentration rises nonlinearly with the dose for all species. The damage produced by different implanted species depends on the dose in approximately the same way save for a scaling factor of the dose. In the regime of low damage, the strain and the defect concentration rise linearly with increasing dose. The slope of this rise with dose increases with the square of the Frenkel pairs produced per unit dose of incident ions, as calculated from computer simulations. This fact means that stable defects produced by room-temperature implantation in Si(100) cannot be predicted by a linear cascade model
Defects production and annealing in self-implanted Si
230-keV 28Si ions were implantated into Si(100) at room temperature with doses from 1014 to 1015/cm2. The samples were analyzed by x-ray double crystal diffractometry and 2-MeV 4He ion channeling spectrometry. The implanted layer has a parallel lattice spacing equal to that of the unimplanted substrate. The perpendicular lattice spacing is larger than that of the unimplanted substrate and is proportional to the defect concentration extracted from the channeling measurement. Both the perpendicular lattice spacing and the defect concentration increase nonlinearly with ion dose. The defect concentration initially increases slowly with dose until a critical value (~15%, at 4×1014/cm2), then rises rapidly, and finally a continuous amorphous layer forms. The initial sluggish increase of the damage is due to the considerable recombination of point defects at room temperature. The rapid growth of the defect concentration is attributed to the reduction of the threshold energy for atomic displacement in a predamaged crystal. The amorphization is envisioned as a cooperative process initiated by a spontaneous collapse of heavily damaged crystalline regions. The annealing behavior of the damaged layer reveals various stages of defect recovery, indicating that the damage consists of a hierarchy of various defect structures of vacancy and interstitial aggregates
Damage production and annealing in 28Si-implanted CoSi2/Sim(111) heterostructures
The damage in epitaxial CoSi2 films 500 nm thick grown on Si(111) produced by room-temperature implantation of 150 keV 28Si were investigated by 2-MeV 4He channeling spectrometry, double-crystal x-ray diffractometry, and electrical resistivity measurements. The damage in the films can be categorized into two types. In lightly (heavily) damaged CoSi2 the damage is in the form of point-like (extended) defects. The resistivity of lightly damaged CoSi2 films rises with the dose of implantation. Electrical defects correlate well with structural ones in lightly damaged films. The resistivity of heavily damaged films flattens off while the structural defects continue to rise with the dose, so that resistivity no longer correlates with structural defects. Upon thermal annealing, lightly damaged films can fully recover structurally and electrically, whereas heavily damaged films do so only electrically. A residual structural damage remains even after annealing at 800 °C for 60 min
Wind-driven Accretion in Protoplanetary Disks. I: Suppression of the Magnetorotational Instability and Launching of the Magnetocentrifugal Wind
We perform local, vertically stratified shearing-box MHD simulations of
protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account
the effects of both Ohmic resistivity and ambipolar diffusion (AD). The
magnetic diffusion coefficients are evaluated self-consistently from a look-up
table based on equilibrium chemistry. We first show that the inclusion of AD
dramatically changes the conventional picture of layered accretion. Without net
vertical magnetic field, the system evolves into a toroidal field dominated
configuration with extremely weak turbulence in the far-UV ionization layer
that is far too inefficient to drive rapid accretion. In the presence of a weak
net vertical field (plasma beta~10^5 at midplane), we find that the MRI is
completely suppressed, resulting in a fully laminar flow throughout the
vertical extent of the disk. A strong magnetocentrifugal wind is launched that
efficiently carries away disk angular momentum and easily accounts for the
observed accretion rate in PPDs. Moreover, under a physical disk wind geometry,
all the accretion flow proceeds through a strong current layer with thickness
of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4
times the sound speed. Both Ohmic resistivity and AD are essential for the
suppression of the MRI and wind launching. The efficiency of wind transport
increases with increasing net vertical magnetic flux and the penetration depth
of the FUV ionization. Our laminar wind solution has important implications on
planet formation and global evolution of PPDs.Comment: 23 pages, 13 figures, accepted to Ap
Dynamics of Solids in the Midplane of Protoplanetary Disks: Implications for Planetesimal Formation
(Abridged) We present local 2D and 3D hybrid numerical simulations of
particles and gas in the midplane of protoplanetary disks (PPDs) using the
Athena code. The particles are coupled to gas aerodynamically, with
particle-to-gas feedback included. Magnetorotational turbulence is ignored as
an approximation for the dead zone of PPDs, and we ignore particle self-gravity
to study the precursor of planetesimal formation. Our simulations include a
wide size distribution of particles, ranging from strongly coupled particles
with dimensionless stopping time tau_s=Omega t_stop=1e-4 to marginally coupled
ones with tau_s=1 (where Omega is the orbital frequency, t_stop is the particle
friction time), and a wide range of solid abundances. Our main results are: 1.
Particles with tau_s>=0.01 actively participate in the streaming instability,
generate turbulence and maintain the height of the particle layer before
Kelvin-Helmholtz instability is triggered. 2. Strong particle clumping as a
consequence of the streaming instability occurs when a substantial fraction of
the solids are large (tau_s>=0.01) and when height-integrated solid to gas mass
ratio Z is super-solar. 3. The radial drift velocity is reduced relative to the
conventional Nakagawa-Sekiya-Hayashi (NSH) model, especially at high Z. We
derive a generalized NSH equilibrium solution for multiple particle species
which fits our results very well. 4. Collision velocity between particles with
tau_s>=0.01 is dominated by differential radial drift, and is strongly reduced
at larger Z. 5. There exist two positive feedback loops with respect to the
enrichment of local disk solid abundance and grain growth. All these effects
promote planetesimal formation.Comment: 25 pages (emulate apj), accepted to Ap
Parametrization of the Driven Betatron Oscillation
An AC dipole is a magnet which produces a sinusoidally oscillating dipole
field and excites coherent transverse beam motion in a synchrotron. By
observing this coherent motion, the optical parameters can be directly measured
at the beam position monitor locations. The driven oscillation induced by an AC
dipole will generate a phase space ellipse which differs from that of the free
oscillation. If not properly accounted for, this difference can lead to a
misinterpretation of the actual optical parameters, for instance, of 6% or more
in the cases of the Tevatron, RHIC, or LHC. The effect of an AC dipole on the
linear optics parameters is identical to that of a thin lens quadrupole. By
introducing a new amplitude function to describe this new phase space ellipse,
the motion produced by an AC dipole becomes easier to interpret. Beam position
data taken under the influence of an AC dipole, with this new interpretation in
mind, can lead to more precise measurements of the normal Courant-Snyder
parameters. This new parameterization of the driven motion is presented and is
used to interpret data taken in the FNAL Tevatron using an AC dipole.Comment: 8 pages, 8 figures, and 1 tabl
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