3,052 research outputs found
Quenching of Cross Sections in Nucleon Transfer Reactions
Cross sections for proton knockout observed in (e,e'p) reactions are
apparently quenched by a factor of ~0.5, an effect attributed to short-range
correlations between nucleons. Here we demonstrate that such quenching is not
restricted to proton knockout, but a more general phenomenon associated with
any nucleon transfer. Measurements of absolute cross sections on a number of
targets between 16O and 208Pb were analyzed in a consistent way, with the cross
sections reduced to spectroscopic factors through the distorted-wave Born
approximation with global optical potentials. Across the 124 cases analyzed
here, induced by various proton- and neutron-transfer reactions and with
angular momentum transfer l=0-7, the results are consistent with a quenching
factor of 0.55. This is an apparently uniform quenching of single-particle
motion in the nuclear medium. The effect is seen not only in (d,p) reactions
but also in reactions with A=3 and 4 projectiles, when realistic wave functions
are used for the projectiles.Comment: 5 pages, 3 figures, accepted to Physical Review Letter
Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)
We have studied the low temperature a.c. magnetic susceptibility of the
diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the
frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y
or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that
the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges
for x > 0.4 and persists to x = 1.98. This behavior can be understood as a
non-monotonic dependence of the quantum spin relaxation time with dilution. The
results suggest that the observed spin freezing is fundamentally a single spin
process which is affected by the local environment, rather than the development
of spin-spin correlations as earlier data suggested.Comment: 26 pages, 9 figure
Slow spin relaxation in a highly polarized cooperative paramagnet
We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.Comment: Accepted for publication in Physical Review Letter
Time Dependent Effects and Transport Evidence for Phase Separation in La_{0.5}Ca_{0.5}MnO_{3}
The ground state of La_{1-x}Ca_{x}MnO_{3} changes from a ferromagnetic
metallic to an antiferromagnetic charge-ordered state as a function of Ca
concentration at x ~ 0.50. We present evidence from transport measurements on a
sample with x = 0.50 that the two phases can coexist, in agreement with other
observations of phase separation in these materials. We also observe that, by
applying and then removing a magnetic field to the mainly charge-ordered state
at some temperatures, we can "magnetically anneal" the charge order, resulting
in a higher zero-field resistivity. We also observe logarithmic time dependence
in both resistivity and magnetization after a field sweep at low temperatures.Comment: 9 pages, LATEX, 3 postscript figure
Spin Freezing in Geometrically Frustrated Antiferromagnets with Weak Disorder
We investigate the consequences for geometrically frustrated antiferromagnets
of weak disorder in the strength of exchange interactions. Taking as a model
the classical Heisenberg antiferromagnet with nearest neighbour exchange on the
pyrochlore lattice, we examine low-temperature behaviour. We show that random
exchange generates long-range effective interactions within the extensively
degenerate ground states of the clean system. Using Monte Carlo simulations, we
find a spin glass transition at a temperature set by the disorder strength.
Disorder of this type, which is generated by random strains in the presence of
magnetoelastic coupling, may account for the spin freezing observed in many
geometrically frustrated magnets.Comment: 4 pages, 5 figure
Low Velocity Granular Drag in Reduced Gravity
We probe the dependence of the low velocity drag force in granular materials
on the effective gravitational acceleration (geff) through studies of spherical
granular materials saturated within fluids of varying density. We vary geff by
a factor of 20, and we find that the granular drag is proportional to geff,
i.e., that the granular drag follows the expected relation Fprobe = {\eta}
{\rho}grain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical
cylinder with depth of insertion, hprobe, diameter dprobe, moving through
grains of density {\rho}grain, and where {\eta} is a dimensionless constant.
This dimensionless constant shows no systematic variation over four orders of
magnitude in effective grain weight, demonstrating that the relation holds over
that entire range to within the precision of our data
Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice
We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through
measurements of the a.c. magnetic susceptibility. While the characteristic spin
relaxation time is thermally activated at high temperatures, it becomes almost
temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling
dominates the relaxation process below that temperature. As the low-entropy
spin ice state develops below Tice ~ 4 K, the spin relaxation time increases
sharply with decreasing temperature, suggesting the emergence of a collective
degree of freedom for which thermal relaxation processes again become important
as the spins become highly correlated
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