ALBERT

Description: Article Superconductivity in the iron pnictides is believed to be related to quantum critical fluctuations. Putzke et al. observe unexpected anomalies in the critical fields of BaFe 2 (As 1−x P x ) 2 that emerge close to its magnetic critical point, which they argue is a generic feature of quantum critical superconductivity. Nature Communications doi: 10.1038/ncomms6679 Authors: C. Putzke, P. Walmsley, J. D. Fletcher, L. Malone, D. Vignolles, C. Proust, S. Badoux, P. See, H. E. Beere, D. A. Ritchie, S. Kasahara, Y. Mizukami, T. Shibauchi, Y. Matsuda, A. Carrington

Description: The threshold of incipient bedload motion, expressed either as a critical force or as a critical water discharge, is a key parameter in bedload transport prediction. Measuring the threshold of motion is difficult, and reliable data from natural streams are rare. By recording the vibrations triggered by bedload particles when moving over a steel plate mounted in the channel bed, we determined the time at start and end of bedload transport in four streams, where discharge is continuously monitored. The threshold discharge scatters over approximately one order of magnitude for each stream, reinforcing previous observations that critical discharge is characterized by a distribution of values. We interpret a strong correlation between the discharge at the start of transport and the discharge at the end of transport of the previous event to reflect temporal changes in bed structure and consequent effects on the driving and resisting forces acting on the bed.

Description: ABSTRACT Depending on their behaviour during extreme floods, streams can be divided into two distinct classes, which derive from a simple threshold model of transport dynamics. ‘Flood-cleaning’ streams erode during high flows and deposit during small and medium flows. ‘Flood-depositing’ streams deposit during high flows and erode during small and medium flows. Using published descriptions of the geomorphic effects of large floods, rivers with a wide range of drainage areas and other characteristics are classified as either ‘flood-cleaning’ or ‘flood-depositing’. In bedrock channels, this behaviour can lead to a feedback effect, the ‘overprint effect’, between sediment transport processes and bedrock erosion, which can modulate long-term bedrock erosion rates. The ‘overprint effect’ arises when alluvium covers the bedrock and typical alluvial channel forms (e.g., meandering or braiding patterns, armour layers or bedforms) develop, which influence sediment transport rates. This effect may accelerate or decelerate sediment export from a reach, causing increased or decreased long-term bedrock erosion rates. The ‘overprint effect’ is illustrated using field data from the Erlenbach, Switzerland, and its implications for channel dynamics and bedrock erosion are discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Description: ABSTRACT Topographic measurements are essential for the study of earth surface processes. Three-dimensional data have been conventionally obtained through terrestrial laser scanning or photogrammetric methods. However, particularly in steep and rough terrain, high-resolution field measurements remain challenging and often require new creative approaches. In this paper, range imaging is evaluated as an alternative method for obtaining surface data in such complex environments. Range imaging is an emerging time-of-flight technology, using phase shift measurements on a multi-pixel sensor to generate a distance image of a surface. Its suitability for field measurements has yet not been tested. We found ambient light and surface reflectivity to be the main factors affecting error in distance measurements. Low-reflectivity surfaces and strong illumination contrasts under direct exposure to sunlight lead to noisy distance measurements. However, regardless of lighting conditions, the accuracy of range imaging was markedly improved by averaging multiple images of the same scene. For medium ambient lighting (shade) and a light-coloured surface the measurement uncertainty was approximately 9 mm. To further test the suitability of range imaging for field applications we measured a reach of a steep mountain stream with a horizontal resolution of circa 1 cm (in the focal plane of the camera), allowing for the interpolation of a digital elevation model on a 2 cm grid. Comparison with an elevation model obtained from terrestrial laser scanning for the same site revealed that both models show similar degrees of topographic detail. Despite limitations in measurement range and accuracy, particularly at bright ambient lighting, range imaging offers three dimensional data in real time and video mode without the need of post-processing. Therefore, range imaging is a useful complement or alternative to existing methods for high-resolution measurements in small- to medium-scale field sites. Copyright © 2012 John Wiley & Sons, Ltd.

Description: Nature Physics 9, 761 (2013). doi:10.1038/nphys2792 Authors: Neven Barišić, Sven Badoux, Mun K. Chan, Chelsey Dorow, Wojciech Tabis, Baptiste Vignolle, Guichuan Yu, Jérôme Béard, Xudong Zhao, Cyril Proust & Martin Greven The metallic state of the underdoped high- Tc cuprates has remained an enigma: how may seemingly disconnected Fermi-surface segments, observed in zero magnetic field as a result of the opening of a partial gap (the pseudogap), possess conventional quasiparticle properties? How do the small Fermi-surface pockets evidenced by the observation of quantum oscillations emerge as superconductivity is suppressed in high magnetic fields? Such quantum oscillations, discovered in underdoped YBa2Cu3O6.5 (Y123; ref.  ) and YBa2Cu4O8 (Y124; ref. ), signify the existence of a conventional Fermi surface. However, owing to the complexity of the crystal structures of Y123 and Y124 (CuO2 double layers, CuO chains, low structural symmetry), it has remained unclear whether the quantum oscillations are specific to this particular family of cuprates. Numerous theoretical proposals have been put forward to explain the source of quantum oscillations, including materials-specific scenarios involving CuO chains and scenarios involving the quintessential CuO2 planes. Here we report the observation of quantum oscillations in underdoped HgBa2CuO4+δ (Hg1201), a model cuprate superconductor with individual CuO2 layers, high tetragonal symmetry and no CuO chains. This observation proves that quantum oscillations are a universal property of the underdoped CuO2 planes, and it opens the door to quantitative future studies of the metallic state and of the Fermi-surface reconstruction phenomenon in this structurally simplest cuprate.

Description: Journal of the American Chemical Society DOI: 10.1021/ja506646s

Description: Steep mountain streams typically feature macro-roughness elements like boulders, step-pool sequences, and a varying channel width. Flow resistance because of such roughness elements appears to be an important control on bedload transport rates. Many commonly used bedload transport equations overestimate the transport in steep streams by orders of magnitude. Few approaches take into account the typical macro-roughness elements, and systematic tests of these models with field observations are lacking. In the present study several approaches were considered that allow calculating the contribution of macro-roughness elements to flow resistance. These approaches were combined with bedload transport equations and the predictions were compared to field measurements of discharge, transported bedload volumes, and channel characteristics in 13 Swiss mountain streams. The streams have channel slopes ranging from 2% to 19%, and catchment areas of 0.5 to 170 km2. For six streams there were time series of sediment yields, mostly measured annually, and for the other seven streams sediment volume estimates were available for large flood events in 2000 and 2005. All tested equation combinations achieved an improvement in bedload prediction compared to a reference equation that was uncorrected for macro-roughness. The prediction accuracy mainly depended on the size and density of the macro-roughness and on flow conditions. The best performance overall was achieved by an empirical approach accounting for macro-roughness, on the basis of an independent data set of flow resistance measurements.

Description: Steep mountain streams typically feature macroroughness elements like large immobile boulders or channel-spanning bedforms such as step-pool sequences. The effects of macroroughness on resistance and flow velocity are not well understood and appropriate field parameters for representing macroroughness in flow velocity equations have not been identified. The prediction of flow velocity in rough and steep streams therefore remains challenging. We measured flow velocity and several macroroughness parameters, i.e., boulder concentration, boulder diameter and protrusion, and roughness of longitudinal channel profiles in six reaches of steep mountain streams with plane bed/riffle, step-pool, and cascade channel morphologies. The between-site variations in flow resistance can be explained to a large degree by nondimensionalization of discharge and flow velocity using channel slope and a characteristic roughness length. Using any of our roughness parameters as the characteristic roughness length, this nondimensionalization leads to a similarity collapse of the entire data set. The remaining differences in flow resistance among the streams are related to dimensionless measures of macroroughness that describe the concentration of boulders or step density in a reach. Boulder concentration represents the measure best describing the data and is used in a simple regression equation for flow velocity. The predictions were better than predictions by the variable power law equation proposed by Ferguson. Although the regression might not be statistically significant, the observed trends suggest that boulder concentration partly explains the residual variance of between-site variation of flow resistance.

Description: [1]  Steep mountain streams typically feature macroroughness elements like large immobile boulders or channel-spanning bedforms such as step-pool sequences. The effects of macroroughness on resistance and flow velocity are not well understood and appropriate field parameters for representing macroroughness in flow velocity equations have not been identified. The prediction of flow velocity in rough and steep streams therefore remains challenging. We measured flow velocity and several macroroughness parameters, i.e., boulder concentration, boulder diameter and protrusion, and roughness of longitudinal channel profiles in six reaches of steep mountain streams with plane bed/riffle, step-pool, and cascade channel morphologies. The between-site variations in flow resistance can be explained to a large degree by nondimensionalization of discharge and flow velocity using channel slope and a characteristic roughness length. Using any of our roughness parameters as the characteristic roughness length, this nondimensionalization leads to a similarity collapse of the entire data set. The remaining differences in flow resistance among the streams are related to dimensionless measures of macroroughness that describe the concentration of boulders or step density in a reach. Boulder concentration represents the measure best describing the data and is used in a simple regression equation for flow velocity. The predictions were better than predictions by the variable power law equation proposed by Ferguson. Although the regression might not be statistically significant, the observed trends suggest that boulder concentration partly explains the residual variance of between-site variation of flow resistance.