Axial morphology along the Southern Chile Rise

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Geology 315-318 (2012): 58-63, doi:10.1016/j.margeo.2012.06.001.Morphology of four spreading segments on the southern Chile Rise is described based on multi-beam bathymetric data collected along the axial zones. The distribution of axial volcanoes, the character of rift valley scarps, and the average depths vary between Segment 1 in the south, terminating at the Chile Triple Junction, and Segment 4 in the north, which are separated by three intervening transform faults. Despite this general variability, there is a consistent pattern of clockwise rotation of the southern-most axial volcanic ridge within each of Segments 2, 3, and 4, relative to the overall trend of the rift valley. A combination of local ridge-transform intersection stresses and regional tectonics may influence spreading axis evolution in this sense.This work was supported by NOAA/OE grant NA08OAR4600757 and University of California Ship Funds

17O NMR study of q=0 spin excitations in a nearly ideal S=1/2 1D Heisenberg antiferromagnet, Sr2CuO3, up to 800 K

We used 17O NMR to probe the uniform (wavevector q=0) electron spin excitations up to 800 K in Sr2CuO3 and separate the q=0 from the q=\pm\pi/a staggered components. Our results support the logarithmic decrease of the uniform spin susceptibility below T ~ 0.015J, where J=2200 K. From measurement of the dynamical spin susceptibility for q=0 by the spin-lattice relaxation rate 1/T_{1}, we demonstrate that the q=0 mode of spin transport is ballistic at the T=0 limit, but has a diffusion-like contribution at finite temperatures even for T << J.Comment: Submitted to Phys. Rev. Lett. 4 pages, 4 figure

Recruitment, growth and mortality of an Antarctic hexactinellid sponge, Anoxycalyx joubini.

Polar ecosystems are sensitive to climate forcing, and we often lack baselines to evaluate changes. Here we report a nearly 50-year study in which a sudden shift in the population dynamics of an ecologically important, structure-forming hexactinellid sponge, Anoxycalyx joubini was observed. This is the largest Antarctic sponge, with individuals growing over two meters tall. In order to investigate life history characteristics of Antarctic marine invertebrates, artificial substrata were deployed at a number of sites in the southern portion of the Ross Sea between 1967 and 1975. Over a 22-year period, no growth or settlement was recorded for A. joubini on these substrata; however, in 2004 and 2010, A. joubini was observed to have settled and grown to large sizes on some but not all artificial substrata. This single settlement and growth event correlates with a region-wide shift in phytoplankton productivity driven by the calving of a massive iceberg. We also report almost complete mortality of large sponges followed over 40 years. Given our warming global climate, similar system-wide changes are expected in the future

Critical dynamics of a spin-5/2 2D isotropic antiferromagnet

We report a neutron scattering study of the dynamic spin correlations in Rb$_2$MnF$_4$, a two-dimensional spin-5/2 antiferromagnet. By tuning an external magnetic field to the value for the spin-flop line, we reduce the effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal two-dimensional isotropic antiferromagnet. From the shape of the quasielastic peak as a function of temperature, we demonstrate dynamic scaling for this system and find a value for the dynamical exponent $z$. We compare these results to theoretical predictions for the dynamic behavior of the two-dimensional Heisenberg model, in which deviations from $z=1$ provide a measure of the corrections to scaling.Comment: 5 pages, 4 figures. Submitted to Physical Review B, Rapid Communication

Nuclear spin relaxation rates in two-leg spin ladders

Using the transfer-matrix DMRG method, we study the nuclear spin relaxation rate 1/T_1 in the two-leg s=1/2 ladder as function of the inter-chain (J_{\perp}) and intra-chain (J_{|}) couplings. In particular, we separate the q_y=0 and \pi contributions and show that the later contribute significantly to the copper relaxation rate ^{63}(1/T_1) in the experimentally relevant coupling and temperature range. We compare our results to both theoretical predictions and experimental measures on ladder materials.Comment: Few modifications from the previous version 4 pages, 5 figures, accepted for publication in PR

63Cu NQR evidence of dimensional crossover to anisotropic 2d regime in S= 1/2 three-leg ladder Sr2Cu3O5

We probed spin-spin correlations up to 725 K with 63Cu NQR in the S= 1/2 three-leg ladder Sr2Cu3O5. We present experimental evidence that below 300 K, weak inter-ladder coupling causes dimensional crossover of the spin-spin correlation length \xi from quasi-1d (\xi ~ 1/T) to anisotropic 2d regime (\xi \~ exp[2\pi\rho_{s}/T], where 2\pi\rho_{s} = 290 +/- 30 K is the effective spin stiffness). This is the first experimental verification of the renormalized classical behavior of the anisotropic non-linear sigma model in 2d, which has been recently proposed for the striped phase in high T_{c} cuprates.Comment: 4 pages, 3 figure

Measuring Positive Childhood Experiences: Testing the structural and predictive validity of the Health Outcomes from Positive Experiences (HOPE) framework

OBJECTIVE: Positive childhood experiences (PCEs), that occur within secure and nurturing social environments, are fundamental to healthy physical, socio-emotional, and cognitive development. However, reliable measures of these experiences are not yet widely available. We used data from the Longitudinal Study of Australian Children (LSAC) to empirically represent and psychometrically evaluate three primary domains of PCEs defined within the Health Outcomes from Positive Experiences (HOPE) framework, specifically: (1) nurturing and supportive relationships; (2) safe and protective environments and; (3) constructive social engagement and connectedness. METHODS: LSAC is a nationally representative cohort that has followed young Australians from birth since 2004. LSAC data were used to represent the three primary HOPE-PCEs domains (0-11 years) across four interrelated PCEs constructs: (1) positive parenting, (2) trusting and supportive relationships, (3) supportive neighbourhood and home learning environments, and (4) social engagement and enjoyment. Confirmatory factor analysis was used to test the proposed four-factor structure. Predictive validity was examined through associations with mental health problems and academic difficulties at 14-15 years. RESULTS: The four-factor structure was supported by empirical data at each time point. Higher exposure to PCEs across each domain was associated with lower reporting of mental health problems (β=-0.20 to -2.05) and academic difficulties (β=-0.01 to -0.13) in adolescence. CONCLUSIONS: The four LSAC-based HOPE-PCEs have sufficient internal coherence and predictive validity to offer a potentially useful way of conceptualizing and measuring PCEs in future cohort studies and intervention trials aiming to enhance understanding of, and mitigate the negative impacts of, adverse childhood experiences

Evidence for Ballistic Thermal Conduction in the One-Dimensional S=1/2 Heisenberg Antiferromagnetic Spin System Sr2CuO3

We have measured the thermal conductivity of the one-dimensional (1D) S=1/2 Heisenberg antiferromagnetic spin system of Sr2Cu1-xPdxO3 single crystals including nonmagnetic impurities of Pd2+. It has been found that the mean free path of spinons along the 1D spin chain at low temperatures is very close to the average length of finite spin chains between spin defects estimated from the magnetic susceptibility measurements. This proves that the thermal conduction due to spinons at low temperatures in Sr2CuO3 is ballistic as theoretically expected [Zotos et al.: Phys. Rev. Lett. 55 (1997) 11029]

Thermal conductivity via magnetic excitations in spin-chain materials

We discuss the recent progress and the current status of experimental investigations of spin-mediated energy transport in spin-chain and spin-ladder materials with antiferromagnetic coupling. We briefly outline the central results of theoretical studies on the subject but focus mainly on recent experimental results that were obtained on materials which may be regarded as adequate physical realizations of the idealized theoretical model systems. Some open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure