Volcanic forcing improves Atmosphere-Ocean Coupled General Circulation Model scaling performance

Recent Atmosphere-Ocean Coupled General Circulation Model (AOGCM) simulations of the twentieth century climate, which account for anthropogenic and natural forcings, make it possible to study the origin of long-term temperature correlations found in the observed records. We study ensemble experiments performed with the NCAR PCM for 10 different historical scenarios, including no forcings, greenhouse gas, sulfate aerosol, ozone, solar, volcanic forcing and various combinations, such as it natural, anthropogenic and all forcings. We compare the scaling exponents characterizing the long-term correlations of the observed and simulated model data for 16 representative land stations and 16 sites in the Atlantic Ocean for these scenarios. We find that inclusion of volcanic forcing in the AOGCM considerably improves the PCM scaling behavior. The scenarios containing volcanic forcing are able to reproduce quite well the observed scaling exponents for the land with exponents around 0.65 independent of the station distance from the ocean. For the Atlantic Ocean, scenarios with the volcanic forcing slightly underestimate the observed persistence exhibiting an average exponent 0.74 instead of 0.85 for reconstructed data.Comment: 4 figure

Thermal conductivity of Mg-doped CuGeO_3 at very low temperatures: Heat conduction by antiferromagnetic magnons

Thermal conductivity \kappa is measured at very low temperatures down to 0.28 K for pure and Mg-doped CuGeO_3 single crystals. The doped samples carry larger amount of heat than the pure sample at the lowest temperature. This is because antiferromagnetic magnons appear in the doped samples and are responsible for the additional heat conductivity, while \kappa of the pure sample represents phonon conductivity at such low temperatures. The maximum energy of the magnon is estimated to be much lower than the spin-Peierls-gap energy. The result presents the first example that \kappa at very low temperatures probes the magnon transport in disorder-induced antiferromagnetic phase of spin-gap systems

The first-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in Cu_(1-x)M_xGeO_3

We have performed detailed magnetic susceptibility measurements as well as synchrotron x-ray diffraction studies to determine the temperature vs concentration ($T$ - $x$) phase diagram of Cu${}_{1-x}$Mg${}_x$GeO${}_3$. We observe clear double peaks in the magnetic susceptibility implying two antiferromagnetic (AF) transition temperatures in samples with Mg concentrations in the range 0.0237 $\le x \le$ 0.0271. We also observe a drastic change in the inverse correlation length in this concentration range by x-ray diffraction. The drastic change of the AF transition temperature as well as the disappearance of the spin-Peierls (SP) phase have been clarified; these results are consistent with a first-order phase transition between dimerized AF (D-AF) and uniform AF (U-AF) phases as reported by T. Masuda {\it et al.} \lbrack Phys. Rev. Lett. {\bf 80}, 4566 (1998)\rbrack. The $T$ - $x$ phase diagram of Cu${}_{1-x}$Zn${}_x$GeO${}_3$ is similar to that of Cu${}_{1-x}$Mg${}_x$GeO${}_3$, which suggests that the present phase transition is universal for Cu${}_{1-x}M_{x}$GeO${}_3$.Comment: 7 pages, 5 figures. submitted to PR

Angular momentum spatial distribution symmetry breaking in Rb by an external magnetic field

Excited state angular momentum alignment -- orientation conversion for atoms with hyperfine structure in presence of an external magnetic field is investigated. Transversal orientation in these conditions is reported for the first time. This phenomenon occurs under Paschen Back conditions at intermediate magnetic field strength. Weak radiation from a linearly polarized diode laser is used to excite Rb atoms in a cell. The laser beam is polarized at an angle of pi/4 with respect to the external magnetic field direction. Ground state hyperfine levels of the 5S_1/2 state are resolved using laser-induced fluorescence spectroscopy under conditions for which all excited 5P_3/2 state hyperfine components are excited simultaneously. Circularly polarized fluorescence is observed to be emitted in the direction perpendicular to both to the direction of the magnetic field B and direction of the light polarization E. The obtained circularity is shown to be in quantitative agreement with theoretical predictions.Comment: Accepted for publication in Phys. Rev.

Structural Critical Scattering Study of Mg-Doped CuGeO3

We report a synchrotron x-ray scattering study of the diluted spin-Peierls (SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at which the spin gap is established is significantly higher than the temperature T_s at which the SP dimerization attains long-range order. The latter is observed only for xx_c the SP correlation length quickly decreases with increasing x. We argue that impurity-induced competing interactions play a central role in these phenomena.Comment: 5 pages, 4 embedded eps figures, to appear in PR

Using the past to constrain the future: how the palaeorecord can improve estimates of global warming

Climate sensitivity is defined as the change in global mean equilibrium temperature after a doubling of atmospheric CO2 concentration and provides a simple measure of global warming. An early estimate of climate sensitivity, 1.5-4.5{\deg}C, has changed little subsequently, including the latest assessment by the Intergovernmental Panel on Climate Change. The persistence of such large uncertainties in this simple measure casts doubt on our understanding of the mechanisms of climate change and our ability to predict the response of the climate system to future perturbations. This has motivated continued attempts to constrain the range with climate data, alone or in conjunction with models. The majority of studies use data from the instrumental period (post-1850) but recent work has made use of information about the large climate changes experienced in the geological past. In this review, we first outline approaches that estimate climate sensitivity using instrumental climate observations and then summarise attempts to use the record of climate change on geological timescales. We examine the limitations of these studies and suggest ways in which the power of the palaeoclimate record could be better used to reduce uncertainties in our predictions of climate sensitivity.Comment: The final, definitive version of this paper has been published in Progress in Physical Geography, 31(5), 2007 by SAGE Publications Ltd, All rights reserved. \c{opyright} 2007 Edwards, Crucifix and Harriso

Bond-Dilution-Induced Quantum Phase Transitions in Heisenberg Antiferromagnets

Bond-dilution effects on the ground state of the square-lattice antiferromagnetic Heisenberg model, consisting of coupled bond-alternating chains, are investigated by means of the quantum Monte Carlo simulation. It is found that, when the ground state of the non-diluted system is a non-magnetic state with a finite spin gap, a sufficiently weak bond dilution induces a disordered state with a mid gap in the original spin gap, and under a further stronger bond dilution an antiferromagnetic long-range order emerges. While the site-dilution-induced long-range order is induced by an infinitesimal concentration of dilution, there exists a finite critical concentration in the case of bond dilution. We argue that this essential difference is due to the occurrence of two types of effective interactions between induced magnetic moments in the case of bond dilution, and that the antiferromagnetic long-range-ordered phase does not appear until the magnitudes of the two interactions become comparable.Comment: 7 pages, 13 figure

Developing independent investigators for clinical research relevant for Africa

Sustainable research capacity building requires training individuals at multiple levels within a supportive institutional infrastructure to develop a critical mass of independent researchers. At many African medical institutions, a PhD is important for academic promotion and is, therefore, an important focal area for capacity building programs. We examine the training at the Infectious Diseases Institute (IDI) as a model for in-country training based on systems capacity building and attention to the academic environment. PhD training in Africa should provide a strong research foundation for individuals to perform independent, original research and to mentor others. Training the next generation of researchers within excellent indigenous academic centers of excellence with strong institutional infrastructure will empower trainees to ask regionally relevant research questions that will benefit Africans