Small-Angle Neutron Scattering and Magnetization Study of HoNi2B2C

The superconducting and magnetic properties of HoNi2B2C single crystals are investigated through transport, magnetometry and small-angle neutron scattering measurements. In the magnetic phases that enter below the superconducting critical temperature, the small-angle neutron scattering data uncover networks of magnetic surfaces. These likely originate from uncompensated moments e.g. at domain walls pinned to crystallographic grain boundaries. The field and temperature dependent behaviour appears consistent with the metamagnetic transitions reported in earlier works.Comment: 11 pages , 4 figures, submitted to Low Temperature Physic

Elastic domains in antiferromagnets

We consider periodic domain structures which appear due to the magnetoelastic interaction if the antiferromagnetic crystal is attached to an elastic substrate. The peculiar behavior of such structures in an external magnetic field is discussed. In particular, we find the magnetic field dependence of the equilibrium period and the concentrations of different domains

18-Year Land-Surface Hydrology Model Simulations for a Midlatitude Grassland Catchment in Valdai, Russia

Off-line simulations of improved bucket hydrology and Simplified Simple Biosphere (SSiB) models are performed for a grassland vegetation catchment region, located at the Valdai water-balance research station in Russia, forced by observed meteorological and simulated actinometric data for 1966-83. Evaluation of the model simulations is performed using observations of total soil moisture in the top 1 m, runoff, evaporation, snow depth, and water-table depth made within the catchment. The Valdai study demonstrates that using only routine meteorological measurements, long-term simulations of land-surface schemes suitable for model evaluation can be made. The Valdai dataset is available for use in the evaluation of other land-surface schemes. Both the SSiB and the bucket models reproduce the observed hydrology averaged over the simulation period (1967-83) and its interannual variability reasonably well. However, the models' soil moisture interannual variability is too low during the fall and winter when compared to observations. In addition, some discrepancies in the models' seasonal behavior with respect to observations are seen. The models are able to reproduce extreme hydrological events to some degree, but some inconsistencies in the model mechanisms are seen. The bucket model's soil-moisture variability is limited by its inability to rise above its prescribed field capacity for the case where the observed water table rises into the top 1-m layer of soil, which can lead to erroneous simulations of evaporation and runoff. SSiB's snow depth simulations are generally too low due to high evaporation from the snow surface. SSiB typically produces drainage out of its bottom layer during the summer, which appears inconsistent to the runoff observations of the catchment

Vortex studies in superconducting Ba(Fe0.93Co0.07)2As2

We present small-angle neutron scattering (SANS) and Bitter decoration studies of the superconducting vortices in Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$}. A highly disordered vortex configuration is observed at all measured fields, and is attributed to strong pinning. This conclusion is supported by the absence of a Meissner rim in decoration images obtained close to the sample edge. The field dependence of the magnitude of the SANS scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry, consistent with the decoration images which show primarily 6-fold coordinated vortex domains. An analysis of the scattered intensity shows that this decreases much more rapidly than expected from estimates of the upper critical field, consistent with the large degree of disorder.Comment: 5 pages, 4 figure

Self-organized criticality induced by quenched disorder: experiments on flux avalanches in NbHx_x films

We present an experimental study of the influence of quenched disorder on the distribution of flux avalanches in type-II superconductors. In the presence of much quenched disorder, the avalanche sizes are power-law distributed and show finite size scaling, as expected from self-organized criticality (SOC). Furthermore, the shape of the avalanches is observed to be fractal. In the absence of quenched disorder, a preferred size of avalanches is observed and avalanches are smooth. These observations indicate that a certain minimum amount of disorder is necessary for SOC behavior. We relate these findings to the appearance or non-appearance of SOC in other experimental systems, particularly piles of sand.Comment: 4 pages, 4 figure

Observation of the magnetic domain structures in Cu0,47_{0,47}Ni0,53_{0,53} thin films at low temperatures

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu$_{0,47}$Ni$_{0,53}$ alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the top of the films well below the Curie temperature T$_{Curie}$ ($\sim$ 60 K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm.Comment: 4 pages, 5 figures, will be published in JETP Let

Impurity Effect on the In-plane Penetration Depth of the Organic Superconductors κ\kappa-(BEDT-TTF)2X_2X (XX = Cu(NCS)2_2 and Cu[N(CN)2_2]Br)

We report the in-plane penetration depth $\lambda_{\parallel}$ of single crystals $\kappa$-(BEDT-TTF)$_2X$ ($X=$ Cu(NCS)$_2$ and Cu[N(CN)$_2$]Br) by means of the reversible magnetization measurements under the control of cooling-rate. In $X$ = Cu(NCS)$_2$, $\lambda_{\parallel}(0)$ as an extrapolation toward $T$ = 0 K does not change by the cooling-rate within the experimental accuracy, while $T_{\textrm{c}}$ is slightly reduced. On the other hand, in $X$ = Cu[N(CN)$_2$]Br, $\lambda_{\parallel}(0)$ indicates a distinct increase by cooling faster. The different behavior of $\lambda_{\parallel}(0)$ on cooling-rate between the two salts is quantitatively explained in terms of the local-clean approximation (London model), considering that the former salt belongs to the very clean system and the later the moderate clean one. The good agreement with this model demonstrates that disorders of ethylene-group in BEDT-TTF introduced by cooling faster increase the electron(quasiparticle)-scattering, resulting in shorter mean free path.Comment: 8 pages, 9 figure

MICRODISC GEL ELECTROPHORESIS IN SODIUM DODECYL SULFATE OF ORGANIC MATERIAL FROM RAT OTOCONIAL COMPLEXES *

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74849/1/j.1749-6632.1981.tb30921.x.pd

Vortex Chains in Anisotropic Superconductors

High-T_c superconductors in small magnetic fields directed away from the crystal symmetry axes have been found to exhibit inhomogeneous chains of flux lines (vortices), in contrast to the usual regular triangular flux-line lattice. We review the experimental observations of these chains, and summarize the theoretical background that explains their appearance. We treat separately two classes of chains: those that appear in superconductors with moderate anisotropy due to an attractive part of the interaction between tilted flux lines, and those with high anisotropy where the tilted magnetic flux is created by two independent and perpendicular crossing lattices. In the second case it is the indirect attraction between a flux line along the layers (Josephson vortex) and a flux line perpendicular to the layers (pancake vortex stack) that leads to the formation of chains of the pancake vortex stacks. This complex system contains a rich variety of phenomena, with several different equilibrium phases, and an extraordinary dynamic interplay between the two sets of crossing vortices. We compare the theoretical predictions of these phenomena with the experimental observations made to date. We also contrast the different techniques used to make these observations. While it is clear that this system forms a wonderful playground for probing the formation of structures with competing interactions, we conclude that there are important practical implications of the vortex chains that appear in highly anisotropic superconductors.Comment: Topical review for Journal of Physics: Condensed Matter; large pdf file 1.9M

Northern Eurasia Future Initiative (NEFI): facing the challenges and pathways of global change in the 21st century

During the past several decades, the Earth system has changed significantly, especially across Northern Eurasia. Changes in the socio-economic conditions of the larger countries in the region have also resulted in a variety of regional environmental changes that can have global consequences. The Northern Eurasia Future Initiative (NEFI) has been designed as an essential continuation of the Northern Eurasia Earth Science Partnership Initiative (NEESPI), which was launched in 2004. NEESPI sought to elucidate all aspects of ongoing environmental change, to inform societies and, thus, to better prepare societies for future developments. A key principle of NEFI is that these developments must now be secured through science-based strategies co-designed with regional decision makers to lead their societies to prosperity in the face of environmental and institutional challenges. NEESPI scientific research, data, and models have created a solid knowledge base to support the NEFI program. This paper presents the NEFI research vision consensus based on that knowledge. It provides the reader with samples of recent accomplishments in regional studies and formulates new NEFI science questions. To address these questions, nine research foci are identified and their selections are briefly justified. These foci include: warming of the Arctic; changing frequency, pattern, and intensity of extreme and inclement environmental conditions; retreat of the cryosphere; changes in terrestrial water cycles; changes in the biosphere; pressures on land-use; changes in infrastructure; societal actions in response to environmental change; and quantification of Northern Eurasia's role in the global Earth system. Powerful feedbacks between the Earth and human systems in Northern Eurasia (e.g., mega-fires, droughts, depletion of the cryosphere essential for water supply, retreat of sea ice) result from past and current human activities (e.g., large scale water withdrawals, land use and governance change) and potentially restrict or provide new opportunities for future human activities. Therefore, we propose that Integrated Assessment Models are needed as the final stage of global change assessment. The overarching goal of this NEFI modeling effort will enable evaluation of economic decisions in response to changing environmental conditions and justification of mitigation and adaptation efforts