Orbital-selective Mott-insulator transition in Ca 2 - x Sr x RuO 4

The electronic structures of the metallic and insulating phases of the alloy series Ca_2-xSr_xRuO₄ ( 0 ? x ? 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. In the end members the groundstate respectively is an orbitally non-degenerate antiferromagnetic insulator (x = 0) and a good metal (x = 2). For x > 0.5 the observed Curie-Weiss paramagnetic metallic state which possesses a local moment with the unexpected spin S = 1/2, is explained by the coexistence of localized and itinerant Ru-4d-orbitals. For 0.2 < x < 0.5 we propose a state with partial orbital and spin ordering. An effective model for the localized orbital and spin degrees of freedom is discussed. The metal-insulator transition at x = 0.2 is attributed to a switch in the orbital occupation associated with a structural change of the crystal. Received 27 July 2001     

Progress in Parametric Pumping

Parametric pumping is the name given to a novel separation device developed by Richard Wilhelm and co-workers^(1,2,3). The principle of operation depends on an immobile phase (e.g. adsorbent) to alternately retard and release selected species. This action coupled with a synchronous periodic fluid flow causes the selectively adsorbed species to be literally pumped from one region to another. In the successful experiments^(2,3) a single packed column was used with reservoirs attached to each end. A water jacket surrounding the bed was heated or cooled synchronously with periodic fluid motion. Figure 1 outlines the principle of operation. Essentially, the process operates in a “bucket-brigade” fashion. If one follows the travels of a single solute (see Figure 1) in a non-adsorbable solvent we see that during downflow the column is cooled thus retarding (adsorption step) the solute, allowing the solvent to move unimpeded. When the flow direction changes to an upward motion, the column is heated, releasing (desorption step) the solute and sweeping it upward. Repeating this cycle, one sees that the solute hops in bucket-brigade fashion toward the upper reservoir until it is eventually captured. Separation factors (i.e., ratio of rich to lean reservoir composition) as large as 10⁵:1 have been reported^(2,3). The fundamental driving force for separation is the adsorbed phase concentration difference at the two operating temperatures. This difference depends on the shape of the isotherms as shown in Figure 2, The method of operation thus outlined has been called the direct thermal-mode⁽³⁾. The principle of operation is not limited to a thermally induced retardation-release step, and in fact Thompson and Bass^(4,5) recently used electrode potential to induce retardation. Furthermore, in a short note Lee and Kirwan⁽⁶⁾ outlined results whereby a porous carbon electrode sustained markedly different adsorption levels of biosubstances (glucoamylase) with Increases in applied voltage. In one of his earlier papers⁽³⁾, Wilhelm suggests a variety of driving potentials may be used, including electrical, pressure, magnetic, and chemical (e.g., pH). Obviously, any potential which can be effectively tuned so as to induce a retard-re lease mechanism could be used in designing a parametric pump. The trick is to discover a retardation-release mechanism which is selective, that is, only the desired species is pumped, while other species remain stagnant or move in the opposite direction (e.g., abnormal isotherm). By way of introduction, it should be noted that the parametric pumping technique is not limited to closed (batch) systems, but can be operated in a continuous and semi-continuous mode. Recent work on such open systems will also be treated in another section.     

Genetic variants in novel pathways influence blood pressure and cardiovascular disease risk

Blood pressure is a heritable trait influenced by several biological pathways and responsive to environmental stimuli. Over one billion people worldwide have hypertension (≥140?mm?Hg systolic blood pressure or ≥90?mm?Hg diastolic blood pressure). Even small increments in blood pressure are associated with an increased risk of cardiovascular events. This genome-wide association study of systolic and diastolic blood pressure, which used a multi-stage design in 200,000 individuals of European descent, identified sixteen novel loci: six of these loci contain genes previously known or suspected to regulate blood pressure (GUCY1A3-GUCY1B3, NPR3-C5orf23, ADM, FURIN-FES, GOSR2, GNAS-EDN3); the other ten provide new clues to blood pressure physiology. A genetic risk score based on 29 genome-wide significant variants was associated with hypertension, left ventricular wall thickness, stroke and coronary artery disease, but not kidney disease or kidney function. We also observed associations with blood pressure in East Asian, South Asian and African ancestry individuals. Our findings provide new insights into the genetics and biology of blood pressure, and suggest potential novel therapeutic pathways for cardiovascular disease prevention.     

Doped spin liquid: Luttinger sum rule and low temperature order

We analyze a model of two-leg Hubbard ladders weakly coupled by interladder tunneling. At half filling a semimetallic state with small Fermi pockets is induced beyond a threshold tunneling strength. The sign changes in the single electron Green's function relevant for the Luttinger sum rule now take place at surfaces with both zeros and infinities with important consequences for the interpretation of angle-resolved photoemission spectroscopy experiments. Residual interactions between electron and holelike quasiparticles cause a transition to long range order at low temperatures. The theory can be extended to small doping leading to superconducting order.     

Systematic improvement of variational Monte Carlo using Lanczos iterations

We present a new numerical technique which combines the variational Monte Carlo and the Lanczos methods without suffering from the fermion sign problem. Lanczos iterations allow systematic improvement of trial wavefunctions while Monte Carlo sampling permits treatment of large lattices. As in the usual Lanczos method we find it useful to symmetrize the starting wavefunction in order to accelerate convergence. We apply our method to the 2D AFM Heisenberg model in the fermionic electron representation, which allows us to compare with results from the equivalent bosonic spin representation. Using d-wave RVB states as starting wavefunctions shows that after only one iteration between 70 and 80% of the difference between the variational energy and the ground state energy (as determined by GFMC) is recovered, and a similar improvement is observed in the second iteration. Leaving the spin-singlet sector by introducing antiferromagnetic correlations reduces the symmetry and the relative improvement in energy drops below 50% for one iteration. Our method allows us also to see trends in observables. Relative to the d-wave RVB states we find an enhancement in the spinspin correlations, consistent with the expectation that the true ground state has long-range order.