69 research outputs found
Spectral Properties of Magnetic Excitations in Cuprate Two-Leg Ladder Systems
This article summarizes and extends the recent developments in the
microscopic modeling of the magnetic excitations in cuprate two-leg ladder
systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange
terms plus an additional four-spin interaction which is about five times
smaller. We give an overview over the relevant energies like the one-triplon
dispersion, the energies of two-triplon bound states and the positions of
multi-triplon continua and over relevant spectral properties like spectral
weights and spectral densities in the parameter regime appropriate for cuprate
systems. It is concluded that an almost complete understanding of the magnetic
excitations in undoped cuprate ladders has been obtained as measured by
inelastic neutron scattering, inelastic light (Raman) scattering and infrared
absorption.Comment: 26 pages, 10 figures, review for Mod. Phys. Lett.
Orthorhombic versus monoclinic symmetry of the charge-ordered state of NaV2O5
High-resolution X-ray diffraction data show that the low-temperature
superstructure of alpha-NaV2O5 has an F-centered orthorhombic 2a x 2b x 4c
superlattice. A structure model is proposed, that is characterized by layers
with zigzag charge order on all ladders and stacking disorder, such that the
averaged structure has space group Fmm2. This model is in accordance with both
X-ray scattering and NMR data. Variations in the stacking order and disorder
offer an explanation for the recently observed devils staircase of the
superlattice period along c.Comment: REVTEX, 4 pages including 2 figures, shortened, submitted to PR
Discussion of a spin-cluster model for the low temperature phase of NaV_2O_5
We discuss magnetic excitations of a spin-cluster model which has been
suggested to describe the low temperature phase of alpha'-NaV_2O_5. This model
fulfills all symmetry criteria proposed by recent x-ray investigations. We find
that this model is not able to describe the occurence of two well separated
magnon lines perpendicular to the ladder direction as observed in INS
experiments. We suggest further experimental analysis to generally distinguish
between models with double reflection or inversion symmetry.Comment: 4 pages, 4 figures, added a calculation of level repulsio
Magnetic properties of a spin-1/2 quadrumer chain
We study a novel S=1/2 cluster chain Hamiltonian which has recently been
proposed in the context of the charge ordered low-temperature phase of
alpha'-NaV2O5. We perform a detailed investigation of this model within a large
range of parameters using perturbation theory and Lanczos diagonalization.
Using model-specific local conservation laws and parameter-dependent mappings
to various effective low-energy Hamiltonians we uncover a rich phase diagram
and several regimes of gapful spin-excitations. We find that the overall
features of recent neutron scattering data on alpha'-NaV2O5 can be fitted
within this model, however using a set of parameters which seems unlikely.Comment: 9 pages REVTeX, 11 PostScript figures included using psfig.sty; final
version to appear in Phys. Rev. B: New appendix, modified Figs. 1 & 10 and
other small change
Dzyaloshinskii-Moriya interaction in NaVO: a microscopic study
We present a unified account of magnetic exchange and Raman scattering in the
quasi-one-dimensional transition-metal oxide NaVO. Based on a
cluster-model approach explicit expressions for the exchange integral and the
Raman-operator are given. It is demonstrated that a combination of the
electronic-structure and the Dzyaloshinskii-Moriya interaction, allowed by
symmetry in this material, are responsible for the finite Raman cross-section
giving rise to both, one- and two-magnon scattering amplitudes.Comment: 7 pages, 1 figur
Low-temperature structural model of hcp solid C
We report intermolecular potential-energy calculations for solid C_ and
determine the optimum static orientations of the molecules at low temperature;
we find them to be consistent with the monoclinic structural model proposed by
us in an earlier report [Solid State Commun. {\bf 105), 247 (1998)]. This model
indicates that the C_5 axis of the molecule is tilted by an angle 18^o
from the monoclinic b axis in contrast with the molecular orientation proposed
by Verheijen {\it et al.} [J. Chem. Phys. {\bf 166}, 287 (1992)] where the C_5
axis is parallel to the monoclinic b axis. In this calculation we have
incorporated the effective bond charge Coulomb potential together with the
Lennard-Jones potential between the molecule at the origin of the monoclinic
unit cell and its six nearest neighbours, three above and three below. The
minimum energy configuration for the molecular orientations turns out to be at
=18^o, =8^o, and =5^o, where , , and
define the molecular orientations.Comment: ReVTeX (4 pages) + 2 PostScript figure
Simple Metals at High Pressure
In this lecture we review high-pressure phase transition sequences exhibited
by simple elements, looking at the examples of the main group I, II, IV, V, and
VI elements. General trends are established by analyzing the changes in
coordination number on compression. Experimentally found phase transitions and
crystal structures are discussed with a brief description of the present
theoretical picture.Comment: 22 pages, 4 figures, lecture notes for the lecture given at the Erice
course on High-Pressure Crystallography in June 2009, Sicily, Ital
Can a frustrated spin-cluster model describe the low-temperature physics of NaV_2O_5 ?
Recent experimental evidence suggest the existence of three distinct
V-valence states (V^{+4}, V^{+4.5} and V^{+5}) in the low-temperature phase of
NaV_2O_5 in apparent discrepancy with the observed spin-gap. We investigate a
novel spin cluster model, consisting of weakly coupled, frustrated four-spin
clusters aligned along the crystallographic b-axis that was recently proposed
to reconcile these experimental observations. We have studied the phase diagram
and the magnon dispersion relation of this model using DMRG, exact
diagonalization and a novel cluster-operator theory. We find a spin-gap for all
parameter values and two distinct phases, a cluster phase and a Haldane phase.
We evaluate the size of the gap and the magnon dispersion and find no parameter
regime which would reproduce the experimental results. We conclude that this
model is inappropriate for the low-temperature regime of NaV_2O_5
Phase diagram of the quarter-filled extended Hubbard model on a two-leg ladder
We investigate the ground-state phase diagram of the quarter-filled Hubbard
ladder with nearest-neighbor Coulomb repulsion V using the Density Matrix
Renormalization Group technique. The ground-state is homogeneous at small V, a
``checkerboard'' charge--ordered insulator at large V and not too small on-site
Coulomb repulsion U, and is phase-separated for moderate or large V and small
U. The zero-temperature transition between the homogeneous and the
charge-ordered phase is found to be second order. In both the homogeneous and
the charge-ordered phases the existence of a spin gap mainly depends on the
ratio of interchain to intrachain hopping. In the second part of the paper, we
construct an effective Hamiltonian for the spin degrees of freedom in the
strong-coupling charge-ordered regime which maps the system onto a frustrated
spin chain. The opening of a spin gap is thus connected with spontaneous
dimerization.Comment: 12 pages, 13 figures, submitted to PRB, presentation revised, new
results added (metallic phase at small U and V
Hole crystallization in the spin ladder of Sr14Cu24O41
One of the deepest questions in condensed matter physics concerns what other
phases compete with superconductivity in high-transition-temperature (high-Tc)
superconductors. One candidate is the "stripe" phase, in which the carriers
(holes) condense into rivers of charge separating regions of
antiferromagnetism. A related but lesser known system is the "spin ladder",
which consists of two coupled chains of magnetic ions forming an array of
rungs. A doped ladder can be thought of as a high-Tc material with lower
dimensionality, and has been predicted to exhibit both superconductivity and an
insulating "hole crystal" phase in which the carriers are localised through
many-body interactions. The competition between the two resembles that between
static stripes and superconductivity in high-Tc materials. Here we report
evidence, from resonant x-ray scattering, for the existence of a hole crystal
in the doped spin ladder of Sr14Cu24O41. This phase exists without a detectable
distortion in the structural lattice, indicating it arises from many-body
effects. Our measurements confirm theoretical predictions and support the
picture that proximity to charge ordered states is a general property of
superconductivity in copper-oxides.Comment: 10 pages, 4 figure
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