89,879 research outputs found
The Spin Stiffness and the Transverse Susceptibility of the Half-filled Hubbard Model
The spin stiffness and the transverse susceptibility of the square lattice half-filled Hubbard model are calculated as a
function of the Hubbard parameter ratio by series expansions around the
Ising limit. We find that the calculated spin-stiffness, transverse
susceptibility, and sublattice magnetization for the Hubbard model smoothly
approach the Heisenberg values for large . The results are compared for
different with RPA and other numerical studies.Comment: 9 Revtex pages, 3 Postscript figures, Europhys. Lett. in pres
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Lateral shearing interferometry for high-NA EUV wavefront metrology
We present a lateral shearing interferometer suitable for high-NA EUV wavefront metrology. In this interferometer, a geometric model is used to accurately characterize and predict systematic errors that come from performing interferometry at high NA. This interferometer is compatible with various optical geometries, including systems where the image plane is tilted with respect to the optical axis, as in the Berkeley MET5. Simulation results show that the systematic errors in tilted geometries can be reduced by aligning the shearing interferometer grating and detector parallel to the image plane. Subsequent residual errors can be removed by linear fitting
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
Pairing of 1-hexyl-3-methylimidazolium and tetrafluoroborate ions in n-pentanol
Molecular dynamics simulations are obtained and analyzed to study pairing of
1-hexyl-3-methylimidazolium and tetrafluoroborate ions in n-pentanol, in
particular by evaluating the potential-of-mean-force between counter ions. The
present molecular model and simulation accurately predicts the dissociation
constant Kd in comparison to experiment, and thus the behavior and magnitudes
for the ion-pair pmf at molecular distances, even though the dielectric
constant of the simulated solvent differs from the experimental value by about
30%. A naive dielectric model does not capture molecule structural effects such
as multiple conformations and binding geometries of the Hmim+ and BF4-
ion-pairs. Mobilities identify multiple time-scale effects in the
autocorrelation of the random forces on the ions, and specifically a slow,
exponential time-decay of those long-ranged forces associated here with
dielectric friction effects.Comment: 5 pages, 7 figures. V2: Figs. 4 & 7 redrawn for better visual clarity
with log-scales. No change in results. In press J. Chem. Phys. 201
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
Topography of Spin Liquids on a Triangular Lattice
Spin systems with frustrated anisotropic interactions are of significant
interest due to possible exotic ground states. We have explored their phase
diagram on a nearest-neighbor triangular lattice using the density-matrix
renormalization group and mapped out the topography of the region that can
harbor a spin liquid. We find that this spin-liquid phase is continuously
connected to a previously discovered spin-liquid phase of the isotropic
model. The two limits show nearly identical spin correlations,
making the case that their respective spin liquids are isomorphic to each
other.Comment: Accepted to PRL; 5 p., 11+ p. supplemental; main text is longer than
the accepted versio
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