46 research outputs found
The Challenge of Unconventional Superconductivity
During the past few decades, several new classes of superconductors have been
discovered. Most of these do not appear to be related to traditional
superconductors. As a consequence, it is felt by many that for these materials,
superconductivity arises from a different source than the electron-ion
interactions that are at the heart of conventional superconductivity.
Developing a rigorous theory for any of these classes of materials has proven
to be a difficult challenge, and will continue to be one of the major problems
in physics in the decades to come.Comment: 14 pages, 4 figure
On the Bilayer Coupling in the Yttrium-Barium Family of High Temperature Superconductors
We present and solve a model for the susceptibility of two CuO2 planes
coupled by an interplane coupling J_perp and use the results to analyze a
recent "cross-relaxation" NMR experiment on Y2Ba4Cu7O15. We deduce that in this
material the product of J_perp and the maximum value of the in-plane
susceptibility chi_max varies from approximately 0.2 at T = 200 K to 0.4 at T =
120 K and that this implies the existence of a temperature dependent in-plane
spin correlation length. Using estimates of chi_max from the literature we find
5 meV < J_perp < 20 meV. We discuss the relation of the NMR results to neutron
scattering results which have been claimed to imply that in YBa2Cu3O_{6+x} the
two planes of a bilayer are perfectly anticorrelated. We also propose that the
recently observed 41 meV excitation in YBa2Cu3O7 is an exciton pulled down
below the superconducting gap by J_perp.Comment: 11 pages, 3 postscript figures (uuencoded and compressed
On the multi-orbital band structure and itinerant magnetism of iron-based superconductors
This paper explains the multi-orbital band structures and itinerant magnetism
of the iron-pnictide and chalcogenides. We first describe the generic band
structure of an isolated FeAs layer. Use of its Abelian glide-mirror group
allows us to reduce the primitive cell to one FeAs unit. From
density-functional theory, we generate the set of eight Fe and As
localized Wannier functions for LaOFeAs and their tight-binding (TB)
Hamiltonian, . We discuss the topology of the bands, i.e. allowed and
avoided crossings, the origin of the d6 pseudogap, as well as the role of the
As orbitals and the elongation of the FeAs tetrahedron. We then
couple the layers, mainly via interlayer hopping between As orbitals,
and give the formalism for simple and body-centered tetragonal stackings. This
allows us to explain the material-specific 3D band structures. Due to the high
symmetry, several level inversions take place as functions of or
pressure, resulting in linear band dispersions (Dirac cones). The underlying
symmetry elements are, however, easily broken, so that the Dirac points are not
protected, nor pinned to the Fermi level. From the paramagnetic TB Hamiltonian,
we form the band structures for spin spirals with wavevector by coupling
and . The band structure for stripe order is studied as a
function of the exchange potential, , using Stoner theory. Gapping of
the Fermi surface (FS) for small requires matching of FS dimensions
(nesting) and -orbital characters. The origin of the propeller-shaped FS is
explained. Finally, we express the magnetic energy as the sum over
band-structure energies, which enables us to understand to what extent the
magnetic energies might be described by a Heisenberg Hamiltonian, and the
interplay between the magnetic moment and the elongation of the FeAs4
tetrahedron
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
Evolution of the resonance and incommensurate spin fluctuations in superconducting YBaCuO
Polarized and unpolarized neutron triple-axis spectrometry was used to study
the dynamical magnetic susceptibility as
a function of energy () and wave vector () in a wide
temperature range for the bilayer superconductor YBaCuO with
oxygen concentrations, , of 0.45, 0.5, 0.6, 0.7, 0.8, 0.93, and 0.95. The
most prominent features in the magnetic spectra include a spin gap in the
superconducting state, a pseudogap in the normal state, the much-discussed
resonance, and incommensurate spin fluctuations below the resonance. We
establish the doping dependence of the spin gap in the superconducting state,
the resonance energy, and the incommensurability of the spin fluctuations. We
discuss in detail the procedure used for separating the magnetic scattering
from phonon and other spurious effects. In the comparison of our experimental
results with various microscopic theoretical models, particular emphasis was
made to address the similarities and differences in the spin fluctuations of
YBaCuO and LaSrCuO.Comment: 23 pages with 30 figures, Phys. Rev. B (in press). If necessary,
contact me for higher resolution figure
Preliminary Evidence for Cell Membrane Amelioration in Children with Cystic Fibrosis by 5-MTHF and Vitamin B12 Supplementation: A Single Arm Trial
Cystic fibrosis (CF) is one of the most common fatal autosomal recessive disorders in the Caucasian population caused by mutations of gene for the cystic fibrosis transmembrane conductance regulator (CFTR). New experimental therapeutic strategies for CF propose a diet supplementation to affect the plasma membrane fluidity and to modulate amplified inflammatory response. The objective of this study was to evaluate the efficacy of 5-methyltetrahydrofolate (5-MTHF) and vitamin B12 supplementation for ameliorating cell plasma membrane features in pediatric patients with cystic fibrosis.A single arm trial was conducted from April 2004 to March 2006 in an Italian CF care centre. 31 children with CF aged from 3 to 8 years old were enrolled. Exclusion criteria were diabetes, chronic infections of the airways and regular antibiotics intake. Children with CF were supplemented for 24 weeks with 5-methyltetrahydrofolate (5-MTHF, 7.5 mg /day) and vitamin B12 (0.5 mg/day). Red blood cells (RBCs) were used to investigate plasma membrane, since RBCs share lipid, protein composition and organization with other cell types. We evaluated RBCs membrane lipid composition, membrane protein oxidative damage, cation content, cation transport pathways, plasma and RBCs folate levels and plasma homocysteine levels at baseline and after 24 weeks of 5-MTHF and vitamin B12 supplementation. In CF children, 5-MTHF and vitamin B12 supplementation (i) increased plasma and RBC folate levels; (ii) decreased plasma homocysteine levels; (iii) modified RBC membrane phospholipid fatty acid composition; (iv) increased RBC K(+) content; (v) reduced RBC membrane oxidative damage and HSP70 membrane association.5-MTHF and vitamin B12 supplementation might ameliorate RBC membrane features of children with CF.ClinicalTrials.gov NCT00730509