32 research outputs found
Structural and Magnetic Properties of Pyrochlore Solid Solutions (Y,Lu)2Ti2-x(Nb,Ta)xO7+/-y
The synthesis and characterization of the pyrochlore solid solutions,
Y2Ti2-xNbxO7-y, Lu2Ti2-xNbxO7-y, Y2Ti2-xTaxO7-y and Lu2TiTaO7-y (-0.4<y<0.5),
is described. Synthesis at 1600 C, and 10-5 Torr yields oxygen deficiency in
all systems. All compounds are found to be paramagnetic and semiconducting,
with the size of the local moments being less, in some cases substantially
less, than the expected value for the number of nominally unpaired electrons
present. Thermogravimetric analysis (TGA) shows that all compounds can be fully
oxidized while retaining the pyrochlore structure, yielding oxygen rich
pyrochlores as white powders. Powder neutron diffraction of Y2TiNbO7-based
samples was done. Refinement of the data for oxygen deficient Y2TiNbO6.76
indicates the presence of a distribution of oxygen over the 8b and 48f sites.
Refinement of the data for oxygen rich Y2TiNbO7.5 shows these sites to be
completely filled, with an additional half filling of the 8a site. The magnetic
and TGA data strongly suggest a preference for a Ti3+/(Nb,Ta)5+ combination, as
opposed to Ti4+/(Nb,Ta)4+, in this pyrochlore family. In addition, the evidence
clearly points to Ti3+ as the source of the localized moments, with no evidence
for localized Nb4+ moments.Comment: Accepted to Journal of Solid State Chemistr
Freezing and large time scales induced by geometrical frustration
We investigate the properties of an effective Hamiltonian with competing
interactions involving spin and chirality variables, relevant for the
description of the {\it trimerized} version of the spin-1/2 {\it kagome}
antiferromagnet. Using classical Monte Carlo simulations, we show that
remarkable behaviors develop at very low temperatures. Through an {\it order by
disorder} mechanism, the low-energy states are characterized by a dynamical
freezing of the chiralities, which decouples the lattice into ``dimers'' and
``triangles'' of antiferromagnetically coupled spins. Under the presence of an
external magnetic field, the particular topology of the chiralities induces a
very slow spin dynamics, reminiscent of what happens in ordinary spin glasses.Comment: 12 pages, 13 figure
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Age at first birth in women is genetically associated with increased risk of schizophrenia
Prof. Paunio on PGC:n jäsenPrevious studies have shown an increased risk for mental health problems in children born to both younger and older parents compared to children of average-aged parents. We previously used a novel design to reveal a latent mechanism of genetic association between schizophrenia and age at first birth in women (AFB). Here, we use independent data from the UK Biobank (N = 38,892) to replicate the finding of an association between predicted genetic risk of schizophrenia and AFB in women, and to estimate the genetic correlation between schizophrenia and AFB in women stratified into younger and older groups. We find evidence for an association between predicted genetic risk of schizophrenia and AFB in women (P-value = 1.12E-05), and we show genetic heterogeneity between younger and older AFB groups (P-value = 3.45E-03). The genetic correlation between schizophrenia and AFB in the younger AFB group is -0.16 (SE = 0.04) while that between schizophrenia and AFB in the older AFB group is 0.14 (SE = 0.08). Our results suggest that early, and perhaps also late, age at first birth in women is associated with increased genetic risk for schizophrenia in the UK Biobank sample. These findings contribute new insights into factors contributing to the complex bio-social risk architecture underpinning the association between parental age and offspring mental health.Peer reviewe