105,900 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
Quantum interference in nested d-wave superconductors: a real-space perspective
We study the local density of states around potential scatterers in d-wave
superconductors, and show that quantum interference between impurity states is
not negligible for experimentally relevant impurity concentrations. The two
impurity model is used as a paradigm to understand these effects analytically
and in interpreting numerical solutions of the Bogoliubov-de Gennes equations
on fully disordered systems. We focus primarily on the globally particle-hole
symmetric model which has been the subject of considerable controversy, and
give evidence that a zero-energy delta function exists in the DOS. The
anomalous spectral weight at zero energy is seen to arise from resonant
impurity states belonging to a particular sublattice, exactly as in the
2-impurity version of this model. We discuss the implications of these findings
for realistic models of the cuprates.Comment: 12 pages, 10 figs, submitted to Phys. Rev.
Two impurities in a d-wave superconductor:local density of states
We study the problem of two local potential scatterers in a d-wave
superconductor, and show how quasiparticle bound state wave functions
interfere. Each single-impurity electron and hole resonance energy is in
general split in the presence of a second impurity into two, corresponding to
one even parity and one odd parity state. We calculate the local density of
states (LDOS), and argue that scanning tunneling microscopy (STM) measurements
should be capable of extracting information about the Green's function in the
pure system by a systematic study of 2-impurity configurations. In some
configurations highly localized, long-lived states are predicted. We discuss
the effects of realistic band structures, and how 2-impurity STM measurements
could help distinguish between current explanations of LDOS impurity spectra in
the BSCCO-2212 system.Comment: 16 pages,21 figure,New Version to be Published on P.R.
Numerical simulations of negative-index refraction in wedge-shaped metamaterials
A wedge-shaped structure made of split-ring resonators (SRR) and wires is
numerically simulated to evaluate its refraction behavior. Four frequency
bands, namely, the stop band, left-handed band, ultralow-index band, and
positive-index band, are distinguished according to the refracted field
distributions. Negative phase velocity inside the wedge is demonstrated in the
left-handed band and the Snell's law is conformed in terms of its refraction
behaviors in different frequency bands. Our results confirmed that negative
index of refraction indeed exists in such a composite metamaterial and also
provided a convincing support to the results of previous Snell's law
experiments.Comment: 18 pages, 6 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
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