25 research outputs found
The spin anisotropy of the magnetic excitations in the normal and superconducting states of optimally doped YBa2Cu3O6.9 studied by polarized neutron spectroscopy
We use inelastic neutron scattering with spin polarization analysis to study
the magnetic excitations in the normal and superconducting states of
YBa2Cu3O6.9. Polarization analysis allows us to determine the spin polarization
of the magnetic excitations and to separate them from phonon scattering. In the
normal state, we find unambiguous evidence of magnetic excitations over the
10-60meV range of the experiment with little polarization dependence to the
excitations. In the superconducting state, the magnetic response is enhanced
near the "resonance energy" and above. At lower energies, 10<E<30meV, the local
susceptibility becomes anisotropic, with the excitations polarized along the
c-axis being suppressed. We find evidence for a new diffuse anisotropic
response polarized perpendicular to the c-axis which may carry significant
spectral weight.Comment: Various typos and errors (inc assignment of components) correcte
High-energy spin waves in the spin-1 square-lattice antiferromagnet LaNiO
Inelastic neutron scattering is used to study the magnetic excitations of the
square-lattice antiferromagnet LaNiO. We find that the spin waves
cannot be described by a simple classical (harmonic) Heisenberg model with only
nearest-neighbor interactions. The spin-wave dispersion measured along the
antiferromagnetic Brillouin-zone boundary shows a minimum energy at the
position as is observed in some square-lattice
antiferromagnets. Thus, our results suggest that the quantum dispersion
renormalization effects or longer-range exchange interactions observed in
cuprates and other square-lattice antiferromagnets are also present in
LaNiO. We also find that the overall intensity of the spin-wave
excitations is suppressed relative to linear spin-wave theory indicating that
covalency is important. Two-magnon scattering is also observed
Low-temperature ferroelectric phase and magnetoelectric coupling in the underdoped La_2CuO_(4+x)
We report the discovery of a ferroelectric ground state below 4.5 K in highly
underdoped La_2CuO_(4+x) accompanied by slow charge dynamics which develop
below T~40 K. An anisotropic magnetoelectric response has also been observed,
indicating considerable spin-charge coupling in this lightly doped "parent"
high temperature copper-oxide superconductor. The ferroelectric state is
proposed to develop from polar nanoregions, in which spatial inversion symmetry
is locally broken due to non-stoichiometric carrier doping.Comment: 7 Pages, 6 Figures, supplementary materia
Dynamical spin susceptibility in La2 CuO4 studied by resonant inelastic x-ray scattering
Resonant inelastic X-ray scattering (RIXS) is a powerful probe of elementary
excitations in solids. It is now widely applied to study magnetic excitations.
However, its complex cross-section means that RIXS has been more difficult to
interpret than inelastic neutron scattering (INS). Here we report
high-resolution RIXS measurements of magnetic excitations of La2CuO4, the
antiferromagnetic parent of one system of high-temperature superconductors. At
high energies (~2 eV), the RIXS spectra show angular-dependent dd orbital
excitations which are found to be in good agreement with single-site multiplet
calculations. At lower energies (<0.3 eV), we show that the
wavevector-dependent RIXS intensities are proportional to the product of the
single-ion spin-flip cross section and the dynamical susceptibility of the
spin-wave excitations. When the spin-flip crosssection is dividing out, the
RIXS magnon intensities show a remarkable resemblance to INS data. Our results
show that RIXS is a quantitative probe the dynamical spin susceptibility in
cuprate and therefore should be used for quantitative investigation of other
correlated electron materials.Comment: 12 page
Progress in Neutron Scattering Studies of Spin Excitations in High-Tc Cuprates
Neutron scattering experiments continue to improve our knowledge of spin
fluctuations in layered cuprates, excitations that are symptomatic of the
electronic correlations underlying high-temperature superconductivity.
Time-of-flight spectrometers, together with new and varied single crystal
samples, have provided a more complete characterization of the magnetic energy
spectrum and its variation with carrier concentration. While the spin
excitations appear anomalous in comparison with simple model systems, there is
clear consistency among a variety of cuprate families. Focusing initially on
hole-doped systems, we review the nature of the magnetic spectrum, and
variations in magnetic spectral weight with doping. We consider connections
with the phenomena of charge and spin stripe order, and the potential
generality of such correlations as suggested by studies of magnetic-field and
impurity induced order. We contrast the behavior of the hole-doped systems with
the trends found in the electron-doped superconductors. Returning to hole-doped
cuprates, studies of translation-symmetry-preserving magnetic order are
discussed, along with efforts to explore new systems. We conclude with a
discussion of future challenges.Comment: revised version, to be published in JPSJ, 20 pages, 21 figure
Neutron Scattering and Its Application to Strongly Correlated Systems
Neutron scattering is a powerful probe of strongly correlated systems. It can
directly detect common phenomena such as magnetic order, and can be used to
determine the coupling between magnetic moments through measurements of the
spin-wave dispersions. In the absence of magnetic order, one can detect diffuse
scattering and dynamic correlations. Neutrons are also sensitive to the
arrangement of atoms in a solid (crystal structure) and lattice dynamics
(phonons). In this chapter, we provide an introduction to neutrons and neutron
sources. The neutron scattering cross section is described and formulas are
given for nuclear diffraction, phonon scattering, magnetic diffraction, and
magnon scattering. As an experimental example, we describe measurements of
antiferromagnetic order, spin dynamics, and their evolution in the
La(2-x)Ba(x)CuO(4) family of high-temperature superconductors.Comment: 31 pages, chapter for "Strongly Correlated Systems: Experimental
Techniques", edited by A. Avella and F. Mancin
Influence of apical oxygen on the extent of in-plane exchange interaction in cuprate superconductors
In high Tc superconductors the magnetic and electronic properties are
determined by the probability that valence electrons virtually jump from site
to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion
and favored by hopping integrals. The spatial extent of the latter is related
to transport properties, including superconductivity, and to the dispersion
relation of spin excitations (magnons). Here, for three antiferromagnetic
parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer
Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical
atoms, we compare the magnetic spectra measured by resonant inelastic x-ray
scattering over a significant portion of the reciprocal space and with
unprecedented accuracy. We observe that the absence of apical oxygens increases
the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D
exchange-bond network. These results establish a corresponding relation between
the exchange interactions and the crystal structure, and provide fresh insight
into the materials dependence of the superconducting transition temperature.Comment: 9 pages, 4 figures, 1 Table, 42 reference
Magnetism and its microscopic origin in iron-based high-temperature superconductors
High-temperature superconductivity in the iron-based materials emerges from,
or sometimes coexists with, their metallic or insulating parent compound
states. This is surprising since these undoped states display dramatically
different antiferromagnetic (AF) spin arrangements and Nel
temperatures. Although there is general consensus that magnetic interactions
are important for superconductivity, much is still unknown concerning the
microscopic origin of the magnetic states. In this review, progress in this
area is summarized, focusing on recent experimental and theoretical results and
discussing their microscopic implications. It is concluded that the parent
compounds are in a state that is more complex than implied by a simple Fermi
surface nesting scenario, and a dual description including both itinerant and
localized degrees of freedom is needed to properly describe these fascinating
materials.Comment: 14 pages, 4 figures, Review article, accepted for publication in
Nature Physic
Genetic variation and evolution in the red cell carbonic anhydrase isozymes of macaque monkeys
The electrophoretic phenotypes of the two isozymes of red cell carbonic anhydrase, CA I and CA II, are described in nine species of macaque monkeys from southeast Asia and Japan. Twelve phenotypes of CA I, apparently under the control of seven alleles, and five phenotypes of CA II, under the control of three alleles, were found in the different macaque populations studied. Extensive electrophoretic polymorphisms of CA I were found in three species (Macaca nemestrina, Macaca speciosa , and Macaca fuscata) , and polymorphisms at the CA II locus were found in Macaca irus, Macaca mulatta , and M. nemestrina . In addition to the electrophoretic polymorphisms at the CA I locus in M. nemestrina , an inherited deficiency of CA I was also discovered in which approximately 30% of the individuals in all populations of M. nemestrina tested showed the deficient phenotype. Although the recessive gene controlling this deficiency appears to be an allele of the CA I locus, it is postulated that the CA I deficiency could also be under the control of a closely linked gene. The comparative data on the extent of genetic variation observed in the two isozymes of red cell carbonic anhydrase in macaques appear to support the concept that CA I has evolved more rapidly than CA II in mammals.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44171/1/10528_2004_Article_BF00485644.pd