99,636 research outputs found
Scaled frequency-dependent transport in the mesoscopically phase-separated colossal magnetoresistive manganite La_{0.625-y}Pr_yCa_{0.375}MnO_3
We address the issue of massive phase separation (PS) in manganite family of
doped Mott insulators through ac conductivity measurements on
LaPrCaMnO (0.375 y 0.275), and
establish applicability of the scaling theory of percolation in the critical
regime of the PS. Measurements of dc resistivity, magnetization (M(T)) and
electron diffraction show incomplete growth of a ferromagnetic (FM) metallic
component on cooling the high temperature charge ordered (CO) phase well below
Curie temperature. The impedance Z(T,f) measured over a frequency
(f) range of 10 Hz to 10 MHz in the critical regime follows a universal scaling
of the form R(T,0)g(f) with 0.86
and the normalized correlation length varying from 1 to 4, suggesting anomalous
diffusion of holes in percolating FM clusters.Comment: 12 pages and 5 figure
Formation of Compressed Flat Electron Beams with High Transverse-Emittance Ratios
Flat beams -- beams with asymmetric transverse emittances -- have important
applications in novel light-source concepts, advanced-acceleration schemes and
could possibly alleviate the need for damping rings in lepton colliders. Over
the last decade, a flat-beam-generation technique based on the conversion of an
angular-momentum-dominated beam was proposed and experimentally tested. In this
paper we explore the production of compressed flat beams. We especially
investigate and optimize the flat-beam transformation for beams with
substantial fractional energy spread. We use as a simulation example the
photoinjector of the Fermilab's Advanced Superconducting Test Accelerator
(ASTA). The optimizations of the flat beam generation and compression at ASTA
were done via start-to-end numerical simulations for bunch charges of 3.2 nC,
1.0 nC and 20 pC at ~37 MeV. The optimized emittances of flat beams with
different bunch charges were found to be 0.25 {\mu}m (emittance ratio is ~400),
0.13 {\mu}m, 15 nm before compression, and 0.41 {\mu}m, 0.20 {\mu}m, 16 nm
after full compression, respectively with peak currents as high as 5.5 kA for a
3.2-nC flat beam. These parameters are consistent with requirements needed to
excite wakefields in asymmetric dielectric-lined waveguides or produce
significant photon flux using small-gap micro-undulators.Comment: 17
Continuous quantum phase transition in a Kondo lattice model
We study the magnetic quantum phase transition in an anisotropic Kondo
lattice model. The dynamical competition between the RKKY and Kondo
interactions is treated using an extended dynamic mean field theory (EDMFT)
appropriate for both the antiferromagnetic and paramagnetic phases. A quantum
Monte Carlo approach is used, which is able to reach very low temperatures, of
the order of 1% of the bare Kondo scale. We find that the finite-temperature
magnetic transition, which occurs for sufficiently large RKKY interactions, is
first order. The extrapolated zero-temperature magnetic transition, on the
other hand, is continuous and locally critical.Comment: 4 pages, 4 figures; updated, to appear in PR
GW quasiparticle calculations with spin-orbit coupling for the light actinides
We report on the importance of GW self-energy corrections for the electronic
structure of light actinides in the weak-to-intermediate coupling regime. Our
study is based on calculations of the band structure and total density of
states of Np, U, and Pu using a one-shot GW approximation that includes
spin-orbit coupling within a full potential LAPW framework. We also present RPA
screened effective Coulomb interactions for the f-electron orbitals for
different lattice constants, and show that there is an increased contribution
from electron-electron correlation in these systems for expanded lattices. We
find a significant amount of electronic correlation in these highly localized
electronic systems.Comment: Accepted and to appear in Phys. Rev.
Transitions to improved confinement regimes induced by changes in heating in zero-dimensional models for tokamak plasmas
It is shown that rapid substantial changes in heating rate can induce
transitions to improved energy confinement regimes in zero-dimensional models
for tokamak plasma phenomenology. We examine for the first time the effect of
step changes in heating rate in the models of E-J.Kim and P.H.Diamond,
Phys.Rev.Lett. 90, 185006 (2003) and M.A.Malkov and P.H.Diamond, Phys.Plasmas
16, 012504 (2009) which nonlinearly couple the evolving temperature gradient,
micro-turbulence and a mesoscale flow; and in the extension of H.Zhu,
S.C.Chapman and R.O.Dendy, Phys.Plasmas 20, 042302 (2013), which couples to a
second mesoscale flow component. The temperature gradient rises, as does the
confinement time defined by analogy with the fusion context, while
micro-turbulence is suppressed. This outcome is robust against variation of
heating rise time and against introduction of an additional variable into the
model. It is also demonstrated that oscillating changes in heating rate can
drive the level of micro-turbulence through a period-doubling path to chaos,
where the amplitude of the oscillatory component of the heating rate is the
control parameter.Comment: 8 pages, 14 figure
Relationship Between the Azimuthal Dependencies of Nuclear Modification Factor and Ridge Yield
The azimuthal angular dependence of the nuclear modification factor
R_{AA}(p_T, phi,N_{part}) recently obtained by PHENIX is related at low p_T to
the trigger phi dependence of the ridge yield as measured by STAR in a
framework in which the azimuthal anisotropy is driven by semihard scattering
near the surface. Careful consideration of the initial geometry leads to the
determination of a surface segment in which the production of semihard partons
are responsible for the phi dependence of the inclusive distribution on the one
hand, and for the angular correlation in ridge phenomenology on the other. With
v_2 also being well reproduced along with R_{AA} and ridge yield, all relevant
phi dependencies in heavy-ion collisions can now be understood in a unified
description that emphasizes the ridge production whether or not a trigger is
used.Comment: This expanded version has additional discussions that render the
paper more readable without change of substance. It is to be published in
Phys. Rev.
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