1,514 research outputs found
Degrees of Freedom: Expanding College Opportunities - for Currently and Formerly Incarcerated Californians
This report begins with a background on the higher education and criminal justice systems in California. This background section highlights the vocabulary and common pathways for each system, and provides a primer on California community colleges. Part II explains why California needs this initiative. Part III presents the landscape of existing college programs dedicated to criminal justice-involved populations in the community and in jails and prisons. This landscape identifies promising strategies and sites of innovation across the state, as well as current challenges to sustaining and expanding these programs. Part IV lays out concrete recommendations California should take to realize the vision of expanding high-quality college opportunities for currently and formerly incarcerated individuals. It includes guidelines for developing high-quality, sustainable programs, building and strengthening partnerships, and shaping the policy landscape, both by using existing opportunities and by advocating for specific legislative and policy changes. Profiles of current college students and graduates with criminal records divide the sections and offer first-hand accounts of the joys and challenges of a college experience
Long wavelength structural anomalies in jammed systems
The structural properties of static, jammed packings of monodisperse spheres
in the vicinity of the jamming transition are investigated using large-scale
computer simulations. At small wavenumber , we argue that the anomalous
behavior in the static structure factor, , is consequential of an
excess of low-frequency, collective excitations seen in the vibrational
spectrum. This anomalous feature becomes more pronounced closest to the jamming
transition, such that at the transition point. We introduce an
appropriate dispersion relation that accounts for these phenomena that leads us
to relate these structural features to characteristic length scales associated
with the low-frequency vibrational modes of these systems. When the particles
are frictional, this anomalous behavior is suppressed providing yet more
evidence that jamming transitions of frictional spheres lie at lower packing
fractions that that for frictionless spheres. These results suggest that the
mechanical properties of jammed and glassy media may therefore be inferred from
measurements of both the static and dynamical structure factors.Comment: 8 pages, 6 figure captions. Completely revised version to appear in
Phys. Rev.
Breakdown of Kinetic Compensation Effect in Physical Desorption
The kinetic compensation effect (KCE), observed in many fields of science, is
the systematic variation in the apparent magnitudes of the Arrhenius parameters
, the energy of activation, and , the preexponential factor, as a
response to perturbations. If, in a series of closely related activated
processes, these parameters exhibit a strong linear correlation, it is expected
that an isokinetic relation will occur, then the rates become the same at a
common compensation temperature . The reality of these two phenomena
continues to be debated as they have not been explicitly demonstrated and their
physical origins remain poorly understood. Using kinetic Monte Carlo
simulations on a model interface, we explore how site and adsorbate
interactions influence the Arrhenius parameters during a typical desorption
process. We find that their transient variations result in a net partial
compensation, due to the variations in the prefactor not being large enough to
completely offset those in , both in plots that exhibit a high degree of
linearity and in curved non-Arrhenius plots. In addition, the observed
isokinetic relation arises due to a transition to a non-interacting regime, and
not due to compensation between and . We expect our results to
provide a deeper insight into the microscopic events that originate
compensation effects and isokinetic relations in our system, and in other
fields where these effects have been reported.Comment: 11 pages, 17 figures, 3 table
Granular flow down a rough inclined plane: transition between thin and thick piles
The rheology of granular particles in an inclined plane geometry is studied
using molecular dynamics simulations. The flow--no-flow boundary is determined
for piles of varying heights over a range of inclination angles . Three
angles determine the phase diagram: , the angle of repose, is the
angle at which a flowing system comes to rest; , the maximum angle
of stability, is the inclination required to induce flow in a static system;
and is the maximum angle for which stable, steady state flow is
observed. In the stable flow region , three
flow regimes can be distinguished that depend on how close is to
: i) : Bagnold rheology, characterized by a
mean particle velocity in the direction of flow that scales as
, for a pile of height , ii)
: the slow flow regime, characterized by a linear
velocity profile with depth, and iii) : avalanche flow
characterized by a slow underlying creep motion combined with occasional free
surface events and large energy fluctuations. We also probe the physics of the
initiation and cessation of flow. The results are compared to several recent
experimental studies on chute flows and suggest that differences between
measured velocity profiles in these experiments may simply be a consequence of
how far the system is from jamming.Comment: 19 pages, 14 figs, submitted to Physics of Fluid
Fragility and hysteretic creep in frictional granular jamming
The granular jamming transition is experimentally investigated in a
two-dimensional system of frictional, bi-dispersed disks subject to
quasi-static, uniaxial compression at zero granular temperature. Currently
accepted results show the jamming transition occurs at a critical packing
fraction . In contrast, we observe the first compression cycle exhibits
{\it fragility} - metastable configuration with simultaneous jammed and
un-jammed clusters - over a small interval in packing fraction (). The fragile state separates the two conditions that define
with an exponential rise in pressure starting at and an exponential
fall in disk displacements ending at . The results are explained
through a percolation mechanism of stressed contacts where cluster growth
exhibits strong spatial correlation with disk displacements. Measurements with
several disk materials of varying elastic moduli and friction coefficients
, show friction directly controls the start of the fragile state, but
indirectly controls the exponential slope. Additionally, we experimentally
confirm recent predictions relating the dependence of on . Under
repetitive loading (compression), the system exhibits hysteresis in pressure,
and the onset increases slowly with repetition number. This friction
induced hysteretic creep is interpreted as the granular pack's evolution from a
metastable to an eventual structurally stable configuration. It is shown to
depend upon the quasi-static step size which provides the only
perturbative mechanism in the experimental protocol, and the friction
coefficient which acts to stabilize the pack.Comment: 12 pages, 10 figure
Geometric origin of excess low-frequency vibrational modes in amorphous solids
Glasses have a large excess of low-frequency vibrational modes in comparison
with crystalline solids. We show that such a feature is a necessary consequence
of the geometry generic to weakly connected solids. In particular, we analyze
the density of states of a recently simulated system, comprised of weakly
compressed spheres at zero temperature. We account for the observed a)
constancy of the density of modes with frequency, b) appearance of a
low-frequency cutoff, and c) power-law increase of this cutoff with
compression. We predict a length scale below which vibrations are very
different from those of a continuous elastic body.Comment: 4 pages, 2 figures. Argument rewritten, identical result
Density of states in random lattices with translational invariance
We propose a random matrix approach to describe vibrational excitations in
disordered systems. The dynamical matrix M is taken in the form M=AA^T where A
is some real (not generally symmetric) random matrix. It guaranties that M is a
positive definite matrix which is necessary for mechanical stability of the
system. We built matrix A on a simple cubic lattice with translational
invariance and interaction between nearest neighbors. We found that for certain
type of disorder phonons cannot propagate through the lattice and the density
of states g(w) is a constant at small w. The reason is a breakdown of affine
assumptions and inapplicability of the elasticity theory. Young modulus goes to
zero in the thermodynamic limit. It strongly reminds of the properties of a
granular matter at the jamming transition point. Most of the vibrations are
delocalized and similar to diffusons introduced by Allen, Feldman et al., Phil.
Mag. B v.79, 1715 (1999).Comment: 4 pages, 5 figure
Normal Modes in Model Jammed Systems in Three Dimensions
Vibrational spectra and normal modes of mechanically stable particle packings
in three dimensions are analyzed over a range of compressions, from near the
jamming transition, where the packings lose their rigidity, to far above it. At
high frequency, the normal modes are localized at all compressions. At low
frequency, the nature of the modes depends somewhat on compression. At large
compressions, far from the transition, the lowest-frequency normal modes have
some plane-wave character, though less than one would expect for a crystalline
or isotropic solid. At low compressions near the jamming transition, the
lowest-frequency modes are neither plane-wave-like nor localized. We
characterize these differences, highlighting the unusual dispersion behavior
that emerges for marginally jammed solids.Comment: Under review at Phys. Rev. E. Lower resolution figures her
Fractal dimensions of jammed packings with power-law particle size distributions in two and three dimensions
Static structure factors are computed for large-scale, mechanically stable,
jammed packings of frictionless spheres (three dimensions) and disks (two
dimensions) with broad, power-law size dispersity characterized by the exponent
. The static structure factor exhibits diverging power-law behavior for
small wavenumbers, allowing us to identify a structural fractal dimension,
. In three dimensions, for ,
such that each of the structure factors can be collapsed onto a universal
curve. In two dimensions, we instead find for
. Furthermore, we show that the fractal behavior
persists when rattler particles are removed, indicating that the long
wavelength structural properties of the packings are controlled by the large
particle backbone conferring mechanical rigidity to the system. A numerical
scheme for computing structure factors for triclinic unit cells is presented
and employed to analyze the jammed packings.Comment: 5 figures, 1 tabl
Gravity-driven Dense Granular Flows
We report and analyze the results of numerical studies of dense granular
flows in two and three dimensions, using both linear damped springs and
Hertzian force laws between particles. Chute flow generically produces a
constant density profile that satisfies scaling relations suggestive of a
Bagnold grain inertia regime. The type of force law has little impact on the
behavior of the system. Bulk and surface flows differ in their failure criteria
and flow rheology, as evidenced by the change in principal stress directions
near the surface. Surface-only flows are not observed in this geometry.Comment: 4 pages, RevTeX 3.0, 4 PostScript figures (5 files) embedded with
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