1,905 research outputs found
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
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
Control of multiferroic domains by external electric fields in TbMnO3
The control of multiferroic domains through external electric fields has been
studied by dielectric measurements and by polarized neutron diffraction on
single-crystalline TbMnO. Full hysteresis cycles were recorded by varying
an external field of the order of several kV/mm and by recording the chiral
magnetic scattering as well as the charge in a sample capacitor. Both methods
yield comparable coercive fields that increase upon cooling.Comment: 12 pages, 6 figure
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
Evidence for a temperature-induced spin-state transition of Co3+ in La2-xSrxCoO4
We study the magnetic susceptibility of mixed-valent La2-xSrxCoO4 single
crystals in the doping range of 0.5<= x <= 0.8 for temperatures up to 1000 K.
The magnetism below room temperature is described by paramagnetic Co2+ in the
high-spin state and by Co3+ in the non-magnetic low-spin state. Above room
temperature, an increase in susceptibility compared to the behavior expected
from Co2+ is seen, which we attribute to a spin-state transition of Co3+. The
susceptibility is analyzed by comparison to full-multiplet calculations for the
thermal population of the high- and intermediate-spin states of Co3+
Renormalization group trajectories from resonance factorized S-matrices
We propose and investigate a large class of models possessing resonance
factorized S-matrices. The associated Casimir energy describes a rich pattern
of renormalization group trajectories related to flows in the coset models
based on the simply laced Lie Algebras. From a simplest resonance S-matrix,
satisfying the ``-property'', we predict new flows in non-unitary
minimal models.Comment: (7 pages) (no figures included
Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling
High-resolution and high-flux neutron as well as X-ray powder-diffraction
experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with
0<x<0.15 in order to study the crystal and magnetic structure. The magnetic
symmetry of the undoped compound corresponds to those reported for ReOFeAs
(with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered
magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger
than the values previously reported for this compound. A sizable ordered
magnetic moment is observed up to a F-doping of 4.5% whereas there is no
magnetic order for a sample with a F concentration of x=0.06. In the undoped
sample, several interatomic distances and FeAs4 tetrahedra angles exhibit
pronounced anomalies connected with the broad structural transition and with
the onset of magnetism supporting the idea of strong magneto-elastic coupling
in this material.Comment: 8 pages, 7 figures, regular articl
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
Antiperiodic solitons of the Goldstone model on S^1
Our purpose is to present all static solutions of the Goldstone model on a
circle in 1+1 dimensions with an antiperiodicity condition imposed on the
scalar fields. Jacobi elliptic and standard trigonometric functions are used to
express the solutions found and stability analysis of the latter is what
follows. Classically stable quasi-topological solitons are identified.Comment: 12 pages, 3 figures, 3 references added, some corrections made.
Accepted for publication in Nonlinearit
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
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