Field-induced local moments around nonmagnetic impurities in metallic cuprates

We consider a defect in a strongly correlated host metal and discuss, within a slave boson mean field formalism for the $t-t'-J$ model, the formation of an induced paramagnetic moment which is extended over nearby sites. We study in particular an impurity in a metallic band, suitable for modelling the optimally doped cuprates, in a regime where the impurity moment is paramagnetic. The form of the local susceptibility as a function of temperature and doping is found to agree well with recent NMR experiments, without including screening processes leading to the Kondo effect.Comment: 7 pages, submitted to Phys Rev

Normal State Magnetic Properties of Ni and Zn Substituted in YBa_{2}Cu_{3} O_{6+x}: Hole-Doping Dependence

We present SQUID susceptibility data on Zn and Ni substituted YBa_{2}Cu_{3}O_{6+x}. Cross-checks with NMR yield an unprecedented accuracy in the estimate of the magnetic susceptibility associated with the substituants, from the underdoped to the lightly overdoped case. This allows us to determine the Weiss temperature \theta for YBCO: its value is very small for all hole dopings n_h. Since in conventional metals, the Kondo temperature, $T_{K}<\theta$, magnetic screening effects would not be expected for $T\gg \theta$; in contrast, increasing n_h produces a reduction of the small moment induced by Zn^{2+} and a nearly constant effective moment for Ni^{2+} corresponding to a spin 1/2 rather than to a spin 1.Comment: 4 pages, 5 figures, to be published in Europhysics Letter

Helium condensation in aerogel: avalanches and disorder-induced phase transition

We present a detailed numerical study of the elementary condensation events (avalanches) associated to the adsorption of $^4$He in silica aerogels. We use a coarse-grained lattice-gas description and determine the nonequilibrium behavior of the adsorbed gas within a local mean-field analysis, neglecting thermal fluctuations and activated processes. We investigate the statistical properties of the avalanches, such as their number, size and shape along the adsorption isotherms as a function of gel porosity, temperature, and chemical potential. Our calculations predict the existence of a line of critical points in the temperature-porosity diagram where the avalanche size distribution displays a power-law behavior and the adsorption isotherms have a universal scaling form. The estimated critical exponents seem compatible with those of the field-driven Random Field Ising Model at zero temperature.Comment: 16 pages, 14 figure

Spitz and wingless, emanating from distinct borders, cooperate to establish cell fate across the engrailed domain in the drosophila epidermis

A key step in development is the establishment of cell type diversity across a cellular field. Segmental patterning within the Drosophila embryonic epidermis is one paradigm for this process. At each parasegment boundary, cells expressing the Wnt family member Wingless confront cells expressing the homeoprotein Engrailed. The Engrailed-expressing cells normally differentiate as one of two alternative cell types. In investigating the generation of this cell type diversity among the 2-cell-wide Engrailed stripe, we previously showed that Wingless, expressed just anterior to the Engrailed cells, is essential for the specification of anterior Engrailed cell fate. In a screen for additional mutations affecting Engrailed cell fate, we identified anterior open/yan, a gene encoding an inhibitory ETS-domain transcription factor that is negatively regulated by the Rasl-MAP kinase signaling cascade. We find that Anterior Open must be inactivated for posterior Engrailed cells to adopt their correct fate. This is achieved by the EGF receptor (DER), which is required autonomously in the Engrailed cells to trigger the Ras1-MAP kinase pathway. Localized activation of DER is accomplished by restricted processing of the activating ligand, Spitz. Processing is confined to the cell row posterior to the Engrailed domain by the restricted expression of Rhomboid. These cells also express the inhibitory ligand Argos, which attenuates the activation of DER in cell rows more distant from the ligand source. Thus, distinct signals flank each border of the Engrailed domain, as Wingless is produced anteriorly and Spitz posteriorly. Since we also show that En cells have the capacity to respond to either Wingless or Spitz, these cells must choose their fate depending on the relative level of activation of the two pathways.Louise O’Keefe, Scott T. Dougan, Limor Gabay, Erez Raz, Ben-Zion Shilo and Stephen DiNard

A temperature-controlled device for volumetric measurements of Helium adsorption in porous media

We describe a set-up for studying adsorption of helium in silica aerogels, where the adsorbed amount is easily and precisely controlled by varying the temperature of a gas reservoir between 80 K and 180 K. We present validation experiments and a first application to aerogels. This device is well adapted to study hysteresis, relaxation, and metastable states in the adsorption and desorption of fluids in porous media

Trapping of Metal Atoms and Metal Clusters by Chabazite under Severe Redox Stress

[EN] The remarkable ability of Al-containing CHA zeolite to trap and stabilize noble single-metal atoms and metal clusters has facilitated the design of sinter-resistant materials for catalytic applications that require severe reaction conditions. At high temperatures in O-2, volatile MOx species appear to be fixated by the zeolite Al centers to prevent Ostwald-ripening sintering mechanisms, whereas small metal clusters (<100 atoms) are stabilized in H-2 without further aggregation as coalescence by Brownian motion is inhibited because of an encapsulation effect. Evidences of the possibility to trap the metal released from a second adjacent surface (e.g., SiO2 and Al2O3), upon metal migration over micrometer distances, are provided. These properties have opened the possibility to prepare several noble-metal atoms and clusters inside small-pore zeolites, including bimetallic formulation, by simple wetness impregnations or solid-to-solid transformations followed by standard calcination procedures, resulting in improved catalytic performances compared to other nonreducible supports in reactions that subject the catalysts to severe redox stress, such as the water-gas-shift reaction.This work has been supported by the Spanish Government-MINECO through "Severo Ochoa" (SEV 2012-0267) and MAT2015-71261-R, by the European Union through ERC-AdG-2014-671093 (SynCatMatch) and by the Fundacion Ramon Areces through a research contract of the "Life and Materials Science" program. The Electron Microscopy Service of the UPV is acknowledged for their help in sample characterization. This research used beamline 9-BM and 20-ID of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank Isabel Millet, Elisa Garcia, and Paul Stevens for technical assistance, and Aaron Sattler, Randall Meyer, Rob Carr, and Gary Casty for review of the manuscript and interesting scientific discussions. We appreciate the support of ExxonMobil Research and Engineering in this fundamental research area.Moliner Marin, M.; Gabay, JE.; Kliewer, CE.; Serna Merino, PM.; Corma Canós, A. (2018). Trapping of Metal Atoms and Metal Clusters by Chabazite under Severe Redox Stress. ACS Catalysis. 8(10):9520-9528. https://doi.org/10.1021/acscatal.8b01717S9520952881