3,627 research outputs found
Raman Response in Antiferromagnetic Two-Leg S=1/2 Heisenberg Ladders
The Raman response in the antiferromagnetic 2-leg S=1/2 Heisenberg ladder is
calculated for various couplings by continuous unitary transformations. For leg
couplings above 80% of the rung coupling a characteristic 2-peak structure
occurs with a point of zero intensity within the continuum. Experimental data
for CaV_2O_5 and La_yCa_(14-y)Cu_24O_41 are analyzed and the coupling constants
are determined. Evidence is found that the Heisenberg model is not sufficient
to describe cuprate ladders. We argue that a cyclic exchange term is the
appropriate extension.Comment: 4 pages with 4 figures include
The Structure of Operators in Effective Particle-Conserving Models
For many-particle systems defined on lattices we investigate the global
structure of effective Hamiltonians and observables obtained by means of a
suitable basis transformation. We study transformations which lead to effective
Hamiltonians conserving the number of excitations. The same transformation must
be used to obtain effective observables. The analysis of the structure shows
that effective operators give rise to a simple and intuitive perspective on the
initial problem. The systematic calculation of n-particle irreducible
quantities becomes possible constituting a significant progress. Details how to
implement the approach perturbatively for a large class of systems are
presented.Comment: 12 pages, 1 figure, accepted by J. Phys. A: Math. Ge
SrCu_2(BO_3)_2 - a Two Dimensional Spin Liquid
We study an extended Shastry-Sutherland model for SrCu_2(BO_3)_2 and analyze
the low lying parts of the energy spectrum by means of a perturbative unitary
transformation based on flow equations. The derivation of the 1-magnon
dispersion (elementary triplets) is discussed. Additionally, we give a
quantitative description (symmetries and energies) of bound states made from
two elementary triplets. Our high order results allow to fix the model
parameters for SrCu_2(BO_3)_2 precisely: J_1=6.16(10)meV, x:=J_2/J_1=0.603(3),
J_\perp=1.3(2)meV. To our knowledge this is the first quantitative treatment of
bound states in a true 2d model.Comment: 4 pages, 3 figures, Proceeding paper of the HFM2000 conference in
Waterloo, Canada, Jun 200
Symmetries and Triplet Dispersion in a Modified Shastry-Sutherland Model for SrCu_2(BO_3)_2
We investigate the one-triplet dispersion in a modified Shastry-Sutherland
Model for SrCu_2(BO_3)_2 by means of a series expansion about the limit of
strong dimerization. Our perturbative method is based on a continuous unitary
transformation that maps the original Hamiltonian to an effective, energy
quanta conserving block diagonal Hamiltonian H_{eff}. The dispersion splits
into two branches which are nearly degenerated. We analyse the symmetries of
the model and show that space group operations are necessary to explain the
degeneracy of the dispersion at k=0 and at the border of the magnetic Brillouin
zone. Moreover, we investigate the behaviour of the dispersion for small |k|
and compare our results to INS data.Comment: 9 pages, 8 figures accepted by J. Phys.: Condens. Matte
Quasiparticle Dynamics in the Kondo Lattice Model at Half Filling
We study spectral properties of quasiparticles in the Kondo lattice model in
one and two dimensions including the coherent quasiparticle dispersions, their
spectral weights and the full two-quasiparticle spectrum using a cluster
expansion scheme. We investigate the evolution of the quasiparticle band as
antiferromagnetic correlations are enhanced towards the RKKY limit of the
model. In both the 1D and the 2D model we find that a repulsive interaction
between quasiparticles results in a distinct antibound state above the
two-quasiparticle continuum. The repulsive interaction is correlated with the
emerging antiferromagnetic correlations and can therefore be associated with
spin fluctuations. On the square lattice, the antibound state has an extended
s-wave symmetry.Comment: 8 pages, 11 figure
Raman Response of Magnetic Excitations in Cuprate Ladders and Planes
An unified picture for the Raman response of magnetic excitations in cuprate
spin-ladder compounds is obtained by comparing calculated two-triplon Raman
line-shapes with those of the prototypical compounds SrCu2O3 (Sr123),
Sr14Cu24O41 (Sr14), and La6Ca8Cu24O41 (La6Ca8). The theoretical model for the
two-leg ladder contains Heisenberg exchange couplings J_parallel and J_perp
plus an additional four-spin interaction J_cyc. Within this model Sr123 and
Sr14 can be described by x:=J_parallel/J_perp=1.5, x_cyc:=J_cyc/J_perp=0.2,
J_perp^Sr123=1130 cm^-1 and J_perp^Sr14=1080 cm^-1. The couplings found for
La6Ca8 are x=1.2, x_cyc=0.2, and J_perp^La6Ca8=1130 cm^-1. The unexpected sharp
two-triplon peak in the ladder materials compared to the undoped
two-dimensional cuprates can be traced back to the anisotropy of the magnetic
exchange in rung and leg direction. With the results obtained for the isotropic
ladder we calculate the Raman line-shape of a two-dimensional square lattice
using a toy model consisting of a vertical and a horizontal ladder. A direct
comparison of these results with Raman experiments for the two-dimensional
cuprates R2CuO4 (R=La,Nd), Sr2CuO2Cl2, and YBa2Cu3O(6+delta) yields a good
agreement for the dominating two-triplon peak. We conclude that short range
quantum fluctuations are dominating the magnetic Raman response in both,
ladders and planes. We discuss possible scenarios responsible for the
high-energy spectral weight of the Raman line-shape, i.e. phonons, the
triple-resonance and multi-particle contributions.Comment: 10 pages, 6 figure
Optical spectroscopy of (La,Ca)14Cu24O41 spin ladders: comparison of experiment and theory
Transmission and reflectivity of La_x Ca_14-x Cu_24 O_41 two-leg spin-1/2
ladders were measured in the mid-infrared regime between 500 and 12000 1/cm.
This allows us to determine the optical conductivity sigma_1 directly and with
high sensitivity. Here we show data for x=4 and 5 with the electrical field
polarized parallel to the rungs (E||a) and to the legs (E||c). Three
characteristic peaks are identified as magnetic excitations by comparison with
two different theoretical calculations.Comment: 4 pages, 2 figures, submitted to SCES 200
Dispersion and Symmetry of Bound States in the Shastry-Sutherland Model
Bound states made from two triplet excitations on the Shastry-Sutherland
(ShaSu) lattice are investigated. Based on the perturbative unitary
transformation by flow equations quantitative properties like dispersions and
qualitative properties like symmetries are determined. The high order results
(up to (J_2/J_1)^{14}) permit to fix the parameters of SrCu_2(BO_3)_2
precisely: J_1=6.16(10)meV, x:=J_2/J_1=0.603(3), J_\perp=1.3(2)meV. At the
border of the magnetic Brillouin zone (MBZ) a general double degeneracy is
derived. An unexpected instability in the triplet channel at x=0.63 indicates a
first order transition towards a triplet condensate, related to classical
helical order.Comment: 4 pages, submitted to Phys. Rev. Let
The Equivalence Theorem and Effective Lagrangians
We point out that the equivalence theorem, which relates the amplitude for a
process with external longitudinally polarized vector bosons to the amplitude
in which the longitudinal vector bosons are replaced by the corresponding
pseudo-Goldstone bosons, is not valid for effective Lagrangians. However, a
more general formulation of this theorem also holds for effective interactions.
The generalized theorem can be utilized to determine the high-energy behaviour
of scattering processes just by power counting and to simplify the calculation
of the corresponding amplitudes. We apply this method to the phenomenologically
most interesting terms describing effective interactions of the electroweak
vector and Higgs bosons in order to examine their effects on vector-boson
scattering and on vector-boson-pair production in annihilation. The
use of the equivalence theorem in the literature is examined.Comment: 20 pages LaTeX, BI-TP 94/1
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