438 research outputs found
Large magnetic field-induced spectral weight enhancement of high-energy spin excitations in
We report electronic Raman scattering experiments on a superconducting single crystal in a magnetic field. At low
temperatures, the spectral weight of the high-energy two-magnon peak increases
linearly with field and is amplified by a factor of more than two at 14 T. The
effect disappears at elevated temperatures and is not present in undoped . This observation is discussed in terms of an electronically
inhomogeneous state in which the field enhances the volume fraction of a phase
with local antiferromagnetic order at the expense of the superconducting phase.Comment: to appear in PR
Phonon anomalies in pure and underdoped R{1-x}K{x}Fe{2}As{2} (R = Ba, Sr) investigated by Raman light scattering
We present a detailed temperature dependent Raman light scattering study of
optical phonons in Ba{1-x}K{x}Fe{2}As{2} (x ~ 0.28, superconducting Tc ~ 29 K),
Sr{1-x}K{x}Fe{2}As{2} (x ~ 0.15, Tc ~ 29 K) and non-superconducting
BaFe{2}As{2} single crystals. In all samples we observe a strong continuous
narrowing of the Raman-active Fe and As vibrations upon cooling below the
spin-density-wave transition Ts. We attribute this effect to the opening of the
spin-density-wave gap. The electron-phonon linewidths inferred from these data
greatly exceed the predictions of ab-initio density functional calculations
without spin polarization, which may imply that local magnetic moments survive
well above Ts. A first-order structural transition accompanying the
spin-density-wave transition induces discontinuous jumps in the phonon
frequencies. These anomalies are increasingly suppressed for higher potassium
concentrations. We also observe subtle phonon anomalies at the superconducting
transition temperature Tc, with a behavior qualitatively similar to that in the
cuprate superconductors.Comment: 5 pages, 6 figures, accepted versio
Magnetic Resonant excitations in High-{} superconductors
The observation of an unusual spin resonant excitation in the superconducting
state of various High-Tc ~copper oxides by inelastic neutron scattering
measurements is reviewed. This magnetic mode % (that does not exist in
conventional superconductors) is discussed in light of a few theoretical models
and likely corresponds to a spin-1 collective mode.Comment: 4 figures, Proceedings conference MSM'03 (september 2003) in Monastir
(Tunisia) to be published in Phys. Stat. Solid
Incommensurate spin density modulation in a copper-oxide chain compound with commensurate charge order
Neutron diffraction has been used to determine the magnetic structure of
NaCuO, a stoichiometric compound containing chains based on
edge-sharing CuO plaquettes. The chains are doped with 2/5 hole per Cu site
and exhibit long-range commensurate charge order with an onset well above room
temperature. Below K, the neutron data indicate long-range collinear
magnetic order with a spin density modulation whose propagation vector is
commensurate along and incommensurate perpendicular to the chains. Competing
interchain exchange interactions are discussed as a possible origin of the
incommensurate magnetic order
Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2
Single-crystal neutron diffraction has been used to determine the
incommensurate magnetic structure of NaCu2O2, a compound built up of chains of
edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice
symmetry were identified by a group-theoretical analysis, and their magnetic
structure factors were compared to the experimentally observed Bragg
intensities. In conjunction with other experimental data, this analysis yields
an elliptical helix structure in which both the helicity and the polarization
plane alternate among copper-oxide chains. This magnetic ground state is
discussed in the context of the recently reported multiferroic properties of
other copper-oxide chain compounds
Superconductivity in epitaxial thin films of NaxCoO2 y D2O
The observation of superconductivity in the layered transition metal oxide
NaxCoO2 y H2O (K. Takada et al., Nature 422, 53 (2003)) has caused a tremendous
upsurge of scientific interest due to its similarities and its differences to
the copper based high-temperature superconductors. Two years after the
discovery, we report the fabrication of single-phase superconducting epitaxial
thin films of Na0.3CoO2 x 1.3 D2O grown by pulsed laser deposition technique.
This opens additional roads for experimental research exploring the
superconducting state and the phase diagram of this unconventional material.Comment: 3 pages, 5 figure
Energy Gaps and Kohn Anomalies in Elemental Superconductors
The momentum and temperature dependence of the lifetimes of acoustic phonons
in the elemental superconductors Pb and Nb was determined by resonant spin-echo
spectroscopy with neutrons. In both elements, the superconducting energy gap
extracted from these measurements was found to converge with sharp anomalies
originating from Fermi-surface nesting (Kohn anomalies) at low temperatures.
The results indicate electron many-body correlations beyond the standard
theoretical framework for conventional superconductivity. A possible mechanism
is the interplay between superconductivity and spin- or charge-density-wave
fluctuations, which may induce dynamical nesting of the Fermi surface
Magnon Heat Transport in doped
We present results of the thermal conductivity of and single-crystals which represent model systems for the
two-dimensional spin-1/2 Heisenberg antiferromagnet on a square lattice. We
find large anisotropies of the thermal conductivity, which are explained in
terms of two-dimensional heat conduction by magnons within the CuO planes.
Non-magnetic Zn substituted for Cu gradually suppresses this magnon thermal
conductivity . A semiclassical analysis of
is shown to yield a magnon mean free path which scales
linearly with the reciprocal concentration of Zn-ions.Comment: 4 pages, 3 figure
Resonant magnetic excitations at high energy in superconducting
A detailed inelastic neutron scattering study of the high temperature
superconductor provides evidence of new resonant
magnetic features, in addition to the well known resonant mode at 41 meV: (i) a
commensurate magnetic resonance peak at 53 meV with an even symmetry under
exchange of two adjacent layers; and (ii) high energy
incommensurate resonant spin excitations whose spectral weight is around 54
meV. The locus and the spectral weight of these modes can be understood by
considering the momentum shape of the electron-hole spin-flip continuum of
d-wave superconductors. This provides new insight into the interplay between
collective spin excitations and the continuum of electron-hole excitations.Comment: 5 figure
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