621 research outputs found
Dynamical Spin Response Functions for Heisenberg Ladders
We present the results of a numerical study of the 2 by L spin 1/2 Heisenberg
ladder. Ground state energies and the singlet-triplet energy gaps for L =
(4-14) and equal rung and leg interaction strengths were obtained in a Lanczos
calculation and checked against earlier calculations by Barnes et al. (even L
up to 12). A related moments technique is then employed to evaluate the
dynamical spin response for L=12 and a range of rung to leg interaction
strength ratios (0 - 5). We comment on two issues, the need for
reorthogonalization and the rate of convergence, that affect the numerical
utility of the moments treatment of response functions.Comment: Revtex, 3 figure
Area evolution, bulk viscosity and entropy principles for dynamical horizons
We derive from Einstein equation an evolution law for the area of a trapping
or dynamical horizon. The solutions to this differential equation show a causal
behavior. Moreover, in a viscous fluid analogy, the equation can be interpreted
as an energy balance law, yielding to a positive bulk viscosity. These two
features contrast with the event horizon case, where the non-causal evolution
of the area and the negative bulk viscosity require teleological boundary
conditions. This reflects the local character of trapping horizons as opposed
to event horizons. Interpreting the area as the entropy, we propose to use an
area/entropy evolution principle to select a unique dynamical horizon and time
slicing in the Cauchy evolution of an initial marginally trapped surface.Comment: Some references added, 5 pages, Phys. Rev. D, in pres
Topological spin excitations of Heisenberg antiferromagnets in two dimensions
In this paper we discuss the construction and the dynamics of vortex-like
topological spin excitations in the Schwinger-boson description of Heisenberg
antiferromagnets in two dimensions. The topological spin excitations are Dirac
fermions (with gap) when spin value is a half-integer. Experimental and
theoretical implications of these excitations are being investigated.Comment: Latex file, no figur
Doping-Dependent Raman Resonance in the Model High-Temperature Superconductor HgBa2CuO4+d
We study the model high-temperature superconductor HgBa2CuO4+d with
electronic Raman scattering and optical ellipsometry over a wide doping range.
The resonant Raman condition which enhances the scattering cross section of
"two-magnon" excitations is found to change strongly with doping, and it
corresponds to a rearrangement of inter-band optical transitions in the 1-3 eV
range seen by ellipsometry. This unexpected change of the resonance condition
allows us to reconcile the apparent discrepancy between Raman and x-ray
detection of magnetic fluctuations in superconducting cuprates. Intriguingly,
the strongest variation occurs across the doping level where the antinodal
superconducting gap reaches its maximum.Comment: 4 pages, 4 figures, contact authors for Supplemental Materia
Charge-Transfer Excitations in the Model Superconductor HgBaCuO
We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of
charge-transfer excitations in the 2-8 eV range in the structurally simple
compound HgBaCuO at optimal doping ( K).
The spectra exhibit a significant dependence on the incident photon energy
which we carefully utilize to resolve a multiplet of weakly-dispersive ( eV) electron-hole excitations, including a mode at 2 eV. The observation
of this 2 eV excitation suggests the existence of a charge-transfer pseudogap
deep in the superconducting phase. Quite generally, our data demonstrate the
importance of exploring the incident photon energy dependence of the RIXS cross
section.Comment: 5 pages, 3 figure
Unraveling the Nature of Charge Excitations in LaCuO with Momentum-Resolved Cu -edge Resonant Inelastic X-ray Scattering
Results of model calculations using exact diagonalization reveal the orbital
character of states associated with different Raman loss peaks in Cu -edge
resonant inelastic X-ray scattering (RIXS) from LaCuO. The model
includes electronic orbitals necessary to highlight non-local Zhang-Rice
singlet, charge transfer and - excitations, as well as states with apical
oxygen 2 character. The dispersion of these excitations is discussed with
prospects for resonant final state wave-function mapping. A good agreement with
experiments emphasizes the substantial multi-orbital character of RIXS profiles
in the energy transfer range 1-6 eV.Comment: Original: 4.5 pages. Replaced: 4 pages and 4 figures with updated
content and reference
Angle-resolved photoemission spectroscopy study of HgBaCuO
HgBaCuO (Hg1201) has been shown to be a model cuprate for
scattering, optical, and transport experiments, but angle-resolved
photoemission spectroscopy (ARPES) data are still lacking owing to the absence
of a charge-neutral cleavage plane. We report on progress in achieving the
experimental conditions for which quasiparticles can be observed in the
near-nodal region of the Fermi surface. The d-wave superconducting gap is
measured and found to have a maximum of 39 meV. At low temperature, a kink is
detected in the nodal dispersion at approximately 51 meV below the Fermi level,
an energy that is different from other cuprates with comparable T. The
superconducting gap, Fermi surface, and nodal band renormalization measured
here provide a crucial momentum-space complement to other experimental probes
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