137 research outputs found
Superexchange coupling and spin susceptibility spectral weight in undoped monolayer cuprates
A systematic inelastic neutron scattering study of the superexchange
interaction in three different undoped monolayer cuprates (La_2CuO_4, Nd_2CuO_4
and Pr_2CuO_4) has been performed using conventional triple axis technique. We
deduce the in-plane antiferromagnetic (AF) superexchange coupling which
actually presents no simple relation versus crystallographic parameters. The
absolute spectral weight of the spin susceptibility has been obtained and it is
found to be smaller than expected even when quantum corrections of the AF
ground state are taken into account.Comment: 13 pages, 1 table, 3 figure
Model for the low-temperature magnetic phases observed in doped YBa_2Cu_3O_{6+x}
A classical statistical model for the antiferromagnetic (AFM) ordering of the
Cu-spins in the CuO_2 planes of reduced YBa_2Cu_3O_{6+x} type materials is
presented. The magnetic phases considered are the experimentally observed
high-temperature AFI phase with ordering vector Q_I=(1/2,1/2,0), and the
low-temperature phases: AFII with Q_II=(1/2,1/2,1/2) and intermediate TA (Turn
Angle) phases TAI, TAII and TAIII with components of both ordering vectors. It
is shown that the AFII and TA phases result from an effective ferromagnetic
(FM) type coupling mediated by free spins in the CuO_x basal plane. Good
agreement with experimental data is obtained for realistic model parameters.Comment: 11 pages, 2 Postscript figures, Submitted to Phys.Rev.Let
Low energy magnetic excitations of the Mn_{12}-acetate spin cluster observed by neutron scattering
We performed high resolution diffraction and inelastic neutron scattering
measurements of Mn_{12}-acetate. Using a very high energy resolution, we could
separate the energy levels corresponding to the splitting of the lowest S
multiplet. Data were analyzed within a single spin model (S=10 ground state),
using a spin Hamiltonian with parameters up to 4^{th} order.
The non regular spacing of the transition energies unambiguously shows the
presence of high order terms in the anisotropy (D= -0.457(2) cm^{-1}, B_4^0 =
-2.33(4) 10^{-5}cm^{-1}).
The relative intensity of the lowest energy peaks is very sensitive to the
small transverse term, supposed to be mainly responsible for quantum tunneling.
This allows an accurate determination of this term in zero magnetic field
(B_4^4 = \pm 3.0(5) 10^{-5} cm^{-1}). The neutron results are discussed in view
of recent experiments and theories.Comment: 4 pages ? 3 figures, submitted to Physical Review Lette
The Transcriptional Programme of Human Heart Valves Reveals the Natural History of Infective Endocarditis
Infective endocarditis (IE) is an infectious disease that is mainly caused by Staphylococcus aureus and Streptococcus sp. It usually leads to valvular destruction and vegetation formation. Its pathophysiology is badly understood and likely involves immune and coagulation systems with close interactions with the microorganism. Our objective was to evaluate host response by comparing transcriptional profiles of cardiac valves from IE patients with controls. Hierarchical clustering revealed a signature of IE consisting of 146 genes. Among the 89 up-regulated genes, we identified two genes strongly associated with IE: metalloproteinase 12 (MMP-12) and aquaporin-9, a member of the aquaglyceroporin membrane channel family. The up-regulation of MMP-12 gene is strengthened by the down-modulation of the gene encoding its inhibitor TIMP3. In addition, MMP-12 was expressed in macrophages infiltrating EI valves. We also found that aquaporin-9 was expressed in endothelial cells lining neo-vessel lumen, suggesting that aquaporin-9 might be associated with neovascularization of infected valves leading to tissue oedema secondary to the inflammatory process. The Gene Ontology annotation and the resulting functional classification showed that most up-regulated genes account for recruitment of inflammatory cells in vegetations, angiogenesis and remodelling of endocardium tissue. A network analysis confirmed the involvement of molecules related to the remodelling of endocardium tissue and angiogenesis in IE. It also evidenced the role of caspases, especially that of caspase-9 and intrinsic apoptotic pathway in IE. Based on this study we propose a scenario for the natural history of IE in humans. Some parameters identified in this work could become tools for measuring the disease activity and should be tested as biomarkers for diagnosis or prognosis assessment in future studies
Antiferromagnetic ordering in a 90 K copper oxide superconductor
Using elastic neutron scattering, we evidence a commensurate
antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-
cuprate (y=0.013, =93 K). As
in the Co-free system, the spin excitation spectrum is dominated by a magnetic
resonance peak at 41 meV but with a reduced spectral weight. The substitution
of Co thus leads to a state where AF and SC cohabit showing that the CuO
plane is a highly antiferromagnetically polarizable medium even for a sample
where T remains optimum.Comment: 3 figure
Neutron scattering search for static magnetism in oxygen ordered YBa2Cu3O6.5
We present elastic and inelastic neutron scattering results on highly oxygen
ordered YBa2Cu3O6.5 ortho-II. We find no evidence for the presence of ordered
magnetic moments to a sensitivity of 0.003 Bohr magnetons, an order of
magnitude smaller than has been suggested in theories of orbital or
d-density-wave (DDW) currents. The absence of sharp elastic peaks, shows that
the d-density-wave phase is not present, at least for the superconductor with
the doping of 6.5 and the ordered ortho-II structure. We cannot exclude the
possibility that a broad peak may exist with extremely short-range DDW
correlations. For less ordered or more doped crystals it is possible that
disorder may lead to static magnetism. We have also searched for the large
normal state spin gap that is predicted to exist in an ordered DDW phase.
Instead of a gap we find that the Q-correlated spin susceptibility persists to
the lowest energies studied, 6 meV. Our results are compatible with the
coexistence of superconductivity with orbital currents, but only if they are
dynamic, and exclude a sharp phase transition to an ordered d-density-wave
phase.Comment: 6 pages 4 figures RevTex Submitted to Phys Rev B January 23, 200
4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)
We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.Comment: 8 pages, 10 figures included, to appear in Phys. Rev.
Systematics of two-component superconductivity in from microwave measurements of high quality single crystals
Systematic microwave surface impedance measurements of YBCO single crystals
grown in crucibles reveal new properties that are not directly seen
in similar measurements of other YBCO samples. Two key observations obtained
from complex conductivity are: a new normal conductivity peak at around 80K and
additional pairing below 65K. High pressure oxygenation of one of the crystals
still yields the same results ruling out any effect of macroscopic segregation
of O-deficient regions. A single complex order parameter cannot describe these
data, and the results suggest at least two superconducting components.
Comparisons with model calculations done for various decoupled two-component
scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals
show that the 80K quasiparticle peak is correlated with the normal state
inelastic scattering rate. Close to Tc, the data follow a mean-field behavior.
Overall, our results strongly suggest the presence of multiple pairing
temperature and energy scales in .Comment: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses
graphicx. Postscript version also available at
http://sagar.physics.neu.edu/preprints.htm
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