7,232 research outputs found
Refraction of shear zones in granular materials
We study strain localization in slow shear flow focusing on layered granular
materials. A heretofore unknown effect is presented here. We show that shear
zones are refracted at material interfaces in analogy with refraction of light
beams in optics. This phenomenon can be obtained as a consequence of a recent
variational model of shear zones. The predictions of the model are tested and
confirmed by 3D discrete element simulations. We found that shear zones follow
Snell's law of light refraction.Comment: 4 pages, 3 figures, minor changes, jounal ref. adde
Jet triggered Type Ia supernovae in radio-galaxies?
We report the serendipitous discovery of a supernova (SN) in the nearby
radio-galaxy 3C 78. Observations obtained with the STIS spectrograph on board
the Hubble Space Telescope show, at a distance of 0.54 arcsec (300 pc) from the
galaxy nucleus, a second bright source, not present in previous images. As this
source was fortuitously covered by the spectrograph slit its spectrum was
obtained and it is characteristic of a Type Ia SN. This SN is closely aligned
with the radio-jet of 3C 78. Analysis of historical records shows that such a
close association between jet and supernova occurred in 6 of the 14 reported
SNe in radio-galaxies. The probability that this results from a random
distribution of SN in the host galaxy is less than 0.05%. We then argue that
jets might trigger supernova explosions.Comment: 9 pages, 3 figures, 1 table, to appear in ApJL, 20 Jul 200
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
Molecular dynamics of folding of secondary structures in Go-type models of proteins
We consider six different secondary structures of proteins and construct two
types of Go-type off-lattice models: with the steric constraints and without.
The basic aminoacid-aminoacid potential is Lennard Jones for the native
contacts and a soft repulsion for the non-native contacts. The interactions are
chosen to make the target secondary structure be the native state of the
system. We provide a thorough equilibrium and kinetic characterization of the
sequences through the molecular dynamics simulations with the Langevin noise.
Models with the steric constraints are found to be better folders and to be
more stable, especially in the case of the -structures. Phononic spectra
for vibrations around the native states have low frequency gaps that correlate
with the thermodynamic stability. Folding of the secondary structures proceeds
through a well defined sequence of events. For instance, -helices fold
from the ends first. The closer to the native state, the faster establishment
of the contacts. Increasing the system size deteriorates the folding
characteristics. We study the folding times as a function of viscous friction
and find a regime of moderate friction with the linear dependence. We also
consider folding when one end of a structure is pinned which imitates
instantaneous conditions when a protein is being synthesized. We find that,
under such circumstances, folding of helices is faster and of the
-sequences slower.Comment: REVTeX, 14 pages, EPS figures included, JCP in pres
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