4,819 research outputs found
Force indeterminacy in the jammed state of hard disks
Granular packings of hard discs are investigated by means of contact dynamics
which is an appropriate technique to explore the allowed force-realizations in
the space of contact forces. Configurations are generated for given values of
the friction coefficient, and then an ensemble of equilibrium forces is found
for fixed contacts. We study the force fluctuations within this ensemble. In
the limit of zero friction the fluctuations vanish in accordance with the
isostaticity of the packing. The magnitude of the fluctuations has a
non-monotonous friction dependence. The increase for small friction can be
attributed to the opening of the angle of the Coulomb cone, while the decrease
as friction increases is due to the reduction of connectivity of the
contact-network, leading to local, independent clusters of indeterminacy. We
discuss the relevance of indeterminacy to packings of deformable particles and
to the mechanical response properties.Comment: 4 pages, 3 figures. Minor changes, journal reference adde
On the magnetic equation of state in (2+1)-flavor QCD
A first study of critical behavior in the vicinity of the chiral phase
transition of (2+1)-flavor QCD is presented. We analyze the quark mass and
volume dependence of the chiral condensate and chiral susceptibilities in QCD
with two degenerate light quark masses and a strange quark. The strange quark
mass (m_s) is chosen close to its physical value; the two degenerate light
quark masses (m_l) are varied in a wide range 1/80 \le m_l/m_s \le 2/5, where
the smallest light quark mass value corresponds to a pseudo-scalar Goldstone
mass of about 75 MeV. All calculations are performed with staggered fermions on
lattices with temporal extent Nt=4. We show that numerical results are
consistent with O(N) scaling in the chiral limit. We find that in the region of
physical light quark mass values, m_l/m_s \simeq 1/20, the temperature and
quark mass dependence of the chiral condensate is already dominated by
universal properties of QCD that are encoded in the scaling function for the
chiral order parameter, the magnetic equation of state. We also provide
evidence for the influence of thermal fluctuations of Goldstone modes on the
chiral condensate at finite temperature. At temperatures below, but close to
the chiral phase transition at vanishing quark mass, this leads to a
characteristic dependence of the light quark chiral condensate on the square
root of the light quark mass.Comment: 18 pages, 18 EPS-file
Frictional coupling between sliding and spinning motion
We show that the friction force and torque, acting at a dry contact of two
objects moving and rotating relative to each other, are inherently coupled. As
a simple test system, a sliding and spinning disk on a horizontal flat surface
is considered. We calculate, and also measure, how the disk is slowing down,
and find that it always stops its sliding and spinning motion at the same
moment. We discuss the impact of this coupling between friction force and
torque on the physics of granular materials.Comment: 4 pages, 5 figures; submitte
Resonant continuum in the Hartree-Fock+BCS approximation
A method for incorporating the effect of the resonant continuum into
Hartree-Fock+BCS equations is proposed. The method is applied for the case of
a neutron-rich nucleus calculated with a Skyrme-type force plus a zero-range
pairing interaction and the results are compared with
Hartree-Fock-Bogoliubov calculations. It is shown that the widths of resonant
states have an important effect on the pairing properties of nuclei close to
the drip line.Comment: 9 pages, 2 figures, comparison with HFB adde
- …