14 research outputs found
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
Evaluating the use of the Child and Adolescent Intellectual Disability Screening Questionnaire (CAIDS-Q) to estimate IQ in children with low intellectual ability
In situations where completing a full intellectual assessment is not possible or desirable the clinician or researcher may require an alternative means of accurately estimating intellectual functioning. There has been limited research in the use of proxy IQ measures in children with an intellectual disability or low IQ. The present study aimed to provide a means of converting total scores from a screening tool (the Child and Adolescent Intellectual Disability Screening Questionnaire: CAIDS-Q) to an estimated IQ. A series of linear regression analyses were conducted on data from 428 children and young people referred to clinical services, where FSIQ was predicted from CAIDS-Q total scores. Analyses were conducted for three age groups between ages 6 and 18 years. The study presents a conversion table for converting CAIDS-Q total scores to estimates of FSIQ, with corresponding 95% prediction intervals to allow the clinician or researcher to estimate FSIQ scores from CAIDS-Q total scores. It is emphasised that, while this conversion may offer a quick means of estimating intellectual functioning in children with a below average IQ, it should be used with caution, especially in children aged between 6 and 8 years old
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
Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2
Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors1–3, a central question has been whether magnetism in these materials arises from weakly correlated electrons4,5, as in the case of spin density wave in pure chromium6, requires strong electron correlations7, or can even be described in terms of localized electrons8,9 such as the AF insulating state of copper oxides10. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe1.9Ni0.1As2 (Tc = 20 K), which allows us to obtain the size of the fluctuating magnetic moment 〈m2〉, and its energy distribution11,12. We find that superconducting BaFe1.9Ni0.1As2 and AF BaFe2As2 (ref. 13) both have fluctuating magnetic moments 〈m2 〉 ≈ 3.2 µ2B pe
Quenched Magnon excitations by oxygen sublattice reconstruction in (SrCuO2)n/(SrTiO3)2 superlattices
Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides
In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors,
superconductivity occurs when electrons form coherent Cooper pairs below the
superconducting transition temperature Tc. Although the kinetic energy of
paired electrons increases in the superconducting state relative to the normal
state, the reduction in the ion lattice energy is sufficient to give the
superconducting condensation energy. For iron pnictide superconductors derived
from electron or hole doping of their antiferromagnetic (AF) parent compounds,
the microscopic origin for supercnductivity is unclear. Here we use neutron
scattering to show that high-Tc superconductivity only occurs for iron
pnictides with low-energy itinerant electron-spin excitation coupling and high
energy spin excitations. Since our absolute spin susceptibility measurements
for optimally hole-doped iron pnictide reveal that the change in magnetic
exchange energy below and above Tc can account for the superconducting
condensation energy, we conclude that the presence of both high-energy spin
excitations giving rise to a large magnetic exchange coupling J and low-energy
spin excitations coupled to the itinerant electrons is essential for high-Tc
superconductivity in iron pnictides.Comment: 9 pages, 4 figures in the main article; 11 pages, 13 figures in the
supplementary material