1,456 research outputs found
On Charge-3 Cyclic Monopoles
We determine the spectral curve of charge 3 BPS su(2) monopoles with C_3
cyclic symmetry. The symmetry means that the genus 4 spectral curve covers a
(Toda) spectral curve of genus 2. A well adapted homology basis is presented
enabling the theta functions and monopole data of the genus 4 curve to be given
in terms of genus 2 data. The Richelot correspondence, a generalization of the
arithmetic mean, is used to solve for this genus 2 curve. Results of other
approaches are compared.Comment: 34 pages, 16 figures. Revision: Abstract added and a few small
change
Origin and roles of a strong electron-phonon interaction in cuprate oxide superconductors
A strong electron-phonon interaction arises from the modulation of the
superexchange interaction by phonons. As is studied in Phys. Rev. B 70, 184514
(2004), Cu-O bond stretching modes can be soft around (pm pi/a, 0) and (0, pm
pi/a), with a the lattice constant of CuO_2 planes. In the critical region of
SDW, where antiferromagnetic spin fluctuations are developed around nesting
wave numbers Q of the Fermi surface, the stretching modes can also be soft
around 2Q. Almost symmetric energy dependences of the 2Q component of the
density of states, which are observed in the so called stripe and checker-board
states, cannot be explained by CDW with 2Q following the complete softening of
the 2Q modes, but they can be explained by a second-harmonic effect of SDW with
Q. The strong electron-phonon interaction can play no or only a minor role in
the occurrence of superconductivity.Comment: 5 pages, 1 fugur
Structural and magnetic aspects of the metal insulator transition in CaSrRuO
The phase diagram of CaSrRuO has been studied by neutron
diffraction on powder and single-crystalline samples. The experiments reveal
antiferromagnetic order and structural distortions characterized by tilts and
rotations of the RuO-octahedra. There is strong evidence that the
structural details of the isovalent samples tune the magnetic as well as the
electronic behavior. In particular we observe for low Sr-concentration a metal
insulator transition associated with a structural change and magnetic ordering
Magnetoelastic coupling in RETiO3 (RE = La, Nd, Sm, Gd, Y)
A detailed analysis of the crystal structure in RETiO3 with RE = La, Nd, Sm,
Gd, and Y reveals an intrinsic coupling between orbital degrees of freedom and
the lattice which cannot be fully attributed to the structural deformation
arising from bond-length mismatch. The TiO6 octahedra in this series are all
irregular with the shape of the distortion depending on the RE ionic radius.
These octahedron distortions vary more strongly with temperature than the tilt
and rotation angles. Around the Ti magnetic ordering all compounds exhibit
strong anomalies in the thermal-expansion coefficients, these anomalies exhibit
opposite signs for the antiferromagnetic and ferromagnetic compounds.
Furthermore the strongest effects are observed in the materials close to the
magnetic cross-over from antiferromagnetic to ferromagnetic order
Non-crystallographic reduction of generalized Calogero-Moser models
We apply a recently introduced reduction procedure based on the embedding of non-crystallographic Coxeter groups into crystallographic ones to Calogero–Moser systems. For rational potentials the familiar generalized Calogero Hamiltonian is recovered. For the Hamiltonians of trigonometric, hyperbolic and elliptic types, we obtain novel integrable dynamical systems with a second potential term which is rescaled by the golden ratio. We explicitly show for the simplest of these non-crystallographic models, how the corresponding classical equations of motion can be derived from a Lie algebraic Lax pair based on the larger, crystallographic Coxeter group
Orbital order in La0.5Sr1.5MnO4: beyond a common local Jahn-Teller picture
The standard way to find the orbital occupation of Jahn-Teller (JT) ions is
to use structural data, with the assumption of a one-to-one correspondence
between the orbital occupation and the associated JT distortion, e.g. in O6
octahedron. We show, however, that this approach in principle does not work for
layered systems. Specifically, using the layered manganite La0.5Sr1.5MnO4 as an
example, we found from our x-ray absorption measurements and theoretical
calculations, that the type of orbital ordering strongly contradicts the
standard local distortion approach for the Mn3+O6 octahedra, and that the
generally ignored long-range crystal field effect and anisotropic hopping
integrals are actually crucial to determine the orbital occupation. Our
findings may open a pathway to control of the orbital state in multilayer
systems and thus of their physical properties.Comment: 4+ pages, 4 figure
The oxygen isotope effect in the ab-plane reflectance of underdoped YBa_2Cu_3O_{7-delta}
We have measured the effect of oxygen isotope substitution on the ab-plane
reflectance of underdoped YBCO. The frequency shift of the transverse optic
phonons due to the substitution of O-16 by O-18 yields an isotope effect of the
expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1.
The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of
alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the
superconducting state. These observations suggest that the shoulder is of
electronic origin and not due to a phonon mode as has been suggested recently.Comment: 4 pages 2 figure
High resolution X-ray scattering studies of structural phase transitions in underdoped LaBaCuO
We have studied structural phase transitions in high quality underdoped
LaBaCuO single crystals using high resolution x-ray scattering
techniques. Critical properties associated with the continuous High Temperature
Tetragonal (HTT, ) to Middle Temperature Orthorhombic (MTO, )
phase transition were investigated in single crystal samples with x=0.125,
0.095, and 0.08 and we find that all behavior is consistent with three
dimensional XY criticality, as expected from theory. Power law behavior in the
orthorhombic strain, 2(a-b)/(a+b), is observed over a remarkably wide
temperature range, spanning most of the MTO regime in the phase diagram. Low
temperature measurements investigating the Low Temperature Tetragonal (LTT,
) phase, below the strongly discontinuous MTOLTT phase
transition, in x=0.125 and x=0.095 samples show that the LTT phase is
characterized by relatively broad Bragg scattering, compared with that observed
at related wavevectors in the HTT phase. This shows that the LTT phase is
either an admixture of tetragonal and orthorhombic phases, or that it is
orthorhombic with very small orthorhombic strain, consistent with the ``less
orthorhombic" low temperature structure previously reported in mixed
LaSrBaCuO single crystals. We compare the complex
temperature-composition phase diagram for the location of structural and
superconducting phase transitions in underdoped LaBaCuO and
find good agreement with results obtained on polycrystalline samples.Comment: 8 pages, 7 figures, 1 tabl
Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction
Strong resonant enhancements of the charge-order and spin-order
superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when
x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The
pronounced photon-energy and polarization dependences of these diffraction
intensities allow for a critical determination of the local symmetry of the
ordered spin and charge carriers. We found that not only the antiferromagnetic
order but also the charge-order superstructure resides within the NiO2 layers;
the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with
the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the
cuprates.Comment: 4 pages, 3 figure
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