3,422 research outputs found
Manifestation of geometric frustration on magnetic and thermodynamic properties of pyrochlores (X=Ti, Zr)
We present here magnetization, specific heat and Raman studies on
single-crystalline specimens of the first pyrochlore member of
the rare-earth titanate series. Its analogous compound in the
rare-earth zirconate series is also investigated in the polycrystalline form.
The Sm spins in remain unordered down to at least T = 0.5 K. The
absence of magnetic ordering is attributed to very small values of exchange
() and dipolar interaction ()
between the spins in this pyrochlore. In contrast, the pyrochlore
is characterized by a relatively large value of Sm-Sm spin
exchange (); however, long-range ordering of the
spins is not established at least down to T = 0.67 K, due to
frustration of the spins on the pyrochlore lattice. The ground state
of ions in both pyrochlores is a well-isolated Kramer's doublet. The
higher-lying crystal field excitations are observed in the low-frequency region
of the Raman spectra of the two compounds recorded at T = 10 K. At higher
temperatures, the magnetic susceptibility of shows a broad
maximum at T = 140 K while that of changes monotonically. Whereas
is a promising candidate for investigating spin-fluctuations on a
frustrated lattice as indicated by our data, the properties of
seem to conform to a conventional scenario where geometrical frustration of the
spin exclude their long-range ordering.Comment: 24 pages, 6 figures, Accepted for publication in Phys. Rev.
Correlated Prompt Fission Data in Transport Simulations
Detailed information on the fission process can be inferred from the
observation, modeling and theoretical understanding of prompt fission neutron
and -ray~observables. Beyond simple average quantities, the study of
distributions and correlations in prompt data, e.g., multiplicity-dependent
neutron and \gray~spectra, angular distributions of the emitted particles,
-, -, and -~correlations, can place stringent
constraints on fission models and parameters that would otherwise be free to be
tuned separately to represent individual fission observables. The FREYA~and
CGMF~codes have been developed to follow the sequential emissions of prompt
neutrons and -rays~from the initial excited fission fragments produced
right after scission. Both codes implement Monte Carlo techniques to sample
initial fission fragment configurations in mass, charge and kinetic energy and
sample probabilities of neutron and ~emission at each stage of the
decay. This approach naturally leads to using simple but powerful statistical
techniques to infer distributions and correlations among many observables and
model parameters. The comparison of model calculations with experimental data
provides a rich arena for testing various nuclear physics models such as those
related to the nuclear structure and level densities of neutron-rich nuclei,
the -ray~strength functions of dipole and quadrupole transitions, the
mechanism for dividing the excitation energy between the two nascent fragments
near scission, and the mechanisms behind the production of angular momentum in
the fragments, etc. Beyond the obvious interest from a fundamental physics
point of view, such studies are also important for addressing data needs in
various nuclear applications. (See text for full abstract.)Comment: 39 pages, 57 figure files, published in Eur. Phys. J. A, reference
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Are Steadily Moving Crystals Unstable?
We study the dynamics of small fluctuations about the uniform state of a
crystal moving through a dissipative medium, e.g. a sedimenting colloidal
crystal or a moving flux lattice, using a set of continuum equations for the
displacement fields, and a one-dimensional driven lattice-gas model for the
coupled concentration and tilt fields. For the colloidal crystal we predict a
continuous nonequilibrium phase transition to a clumped state above a critical
Peclet number.Comment: 4 pages, revtex, 2 .eps figures, uses epsf.sty; To be published in
Phys. Rev. Lett. This version is substantially rewritten but the essential
content is the same as befor
Temperature dependent Raman and x-ray studies of spin-ice pyrochlore and non-magnetic pyrochlore
We present here temperature-dependent Raman, x-ray diffraction and specific
heat studies between room temperature and 12 K on single crystals of spin-ice
pyrochlore compound and its non-magnetic analogue .
Raman data show a "new" band not predicted by factor group analysis of
Raman-active modes for the pyrochlore structure in , appearing
below a temperature of 110 K with a concomitant contraction of the cubic
unit cell volume as determined from the powder x-ray diffraction analysis. Low
temperature Raman experiments on O-isotope substituted
confirm the phonon origin of the "new" mode. These findings, absent in
, suggest that the room temperature cubic lattice of the
pyrochlore undergoes a "subtle" structural transformation near
. We find anomalous \textit{red-shift} of some of the phonon modes in both
the and the as the temperature decreases, which is
attributed to strong phonon-phonon anharmonic interactions.Comment: 28 pages, 9 figures (Accepted for publication in Physical Review B
Open and endovascular repair of the nontraumatic isolated aortic arch aneurysm
ObjectiveRepair of isolated aortic arch aneurysms (nontraumatic) by either open (OAR) or endovascular (TEVAR) methods is associated with need for hypothermic circulatory arrest, complex debranching procedures, or use of marginal proximal landing zones. This study evaluates outcomes for treatment of this cohort.MethodsOf 2153 patients undergoing arch repair (1993-2013), 137 (mean age, 60 years) were treated with isolated arch resection for nontraumatic aneurysms. Treatment was by open (n = 93), hybrid (n = 11), or TEVAR (n = 33) methods, with the last two approaches reserved for poor OAR candidates. Treatment was predominantly for saccular (n = 53) or fusiform (n = 30) aneurysms or dissection (n = 15). Rupture was present in 15%. Prior aortic repair was performed in the ascending (n = 30), arch (n = 40), descending (n = 24), or abdominal (n = 9) aorta. Propensity score adjustment was performed for multivariable analysis to account for baseline differences in patient groups as well as treatment selection bias.ResultsEarly mortality was seen in nine patients (7%). Morbidity included stroke (n = 9), paraplegia (n = 1), and need for dialysis (n = 5) or tracheostomy (n = 10). A composite outcome of death and stroke was independently predicted by advancing age (P = .055) and performance of a hybrid procedure (P = .012). The 15-year survival was 59%, with late mortality predicted by increasing age, presence of peripheral vascular disease, and perioperative stroke (all P < .05). The 10-year freedom from aortic rupture or reintervention was 75% and was higher after OAR (2-year OAR, 94% vs TEVAR or hybrid, 78%; P = .018). After propensity-adjusted Cox regression analysis, both prior abdominal aortic aneurysmectomy (P = .017) and an endovascular or hybrid procedure (P = .001) independently predicted late aortic rupture or need for reintervention.ConclusionsIsolated arch repair remains a high-risk procedure occurring frequently in the reoperative setting. Despite being performed in a higher risk group, endovascular strategies yielded similar outcomes but with an increased risk for aorta-related complications. These data support ongoing efforts to develop branched endografts specifically tailored for arch disease to potentially reduce morbidity related to currently available approaches
Granger Causality and Cross Recurrence Plots in Rheochaos
Our stress relaxation measurements on wormlike micelles using a Rheo-SALS
(rheology + small angle light scattering) apparatus allow simultaneous
measurements of the stress and the scattered depolarised intensity. The latter
is sensitive to orientational ordering of the micelles. To determine the
presence of causal influences between the stress and the depolarised intensity
time series, we have used the technique of linear and nonlinear Granger
causality. We find there exists a feedback mechanism between the two time
series and that the orientational order has a stronger causal effect on the
stress than vice versa. We have also studied the phase space dynamics of the
stress and the depolarised intensity time series using the recently developed
technique of cross recurrence plots (CRPs). The presence of diagonal line
structures in the CRPs unambiguously proves that the two time series share
similar phase space dynamics.Comment: 10 pages, 7 figure
Mean-field analysis of the q-voter model on networks
We present a detailed investigation of the behavior of the nonlinear q-voter
model for opinion dynamics. At the mean-field level we derive analytically, for
any value of the number q of agents involved in the elementary update, the
phase diagram, the exit probability and the consensus time at the transition
point. The mean-field formalism is extended to the case that the interaction
pattern is given by generic heterogeneous networks. We finally discuss the case
of random regular networks and compare analytical results with simulations.Comment: 20 pages, 10 figure
Non-resonant microwave absorption studies of superconducting MgB_2
Non-resonant microwave absorption(NRMA) studies of superconducting MgB_2 at a
frequency of 9.43 GHz in the field range -50 Gauss to 5000 Gauss are reported.
The NRMA results indicate near absence of intergranular weak links. A linear
temperature dependence of the lower critical field H_c1 is observed indicating
a non s-wave superconductivity. However, the phase reversal of the NRMA signal
which could suggest d-wave symmetry is also not observed.Comment: 8 pages, 2 figure
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