Flow to strong coupling in the two-dimensional Hubbard model

We extend the analysis of the renormalization group flow in the two-dimensional Hubbard model close to half-filling using the recently developed temperature flow formalism. We investigate the interplay of d-density wave and Fermi surface deformation tendencies with those towards d-wave pairing and antiferromagnetism. For a ratio of next nearest to nearest neighbor hoppings, t'/t = - 0.25, and band fillings where the Fermi surface is inside the Umklapp surface, only the d-pairing susceptibility diverges at low temperatures. When the Fermi surface intersects the Umklapp surface close to the saddle points, d-wave pairing, d-density wave, antiferromagnetic and, to a weaker extent, d-wave Fermi surface deformation susceptibilities grow together when the interactions flow to strong coupling. We interpret these findings as indications for a non-trivial strongly coupled phase with short-ranged superconducting and antiferromagnetic correlations, in close analogy with the spin liquid ground state in the well-understood two-leg Hubbard ladder. Received 23 January 2002     

Rotation reversal bifurcation and energy confinement saturation in tokamak Ohmic L-mode plasmas

Direction reversals of intrinsic toroidal rotation have been observed in diverted Alcator C-Mod Ohmic L-mode plasmas following electron density ramps. For low density discharges, the core rotation is directed cocurrent, and reverses to countercurrent following an increase in the density above a certain threshold. Such reversals occur together with a decrease in density fluctuations with 2 cm(-1)≤k(θ)≤11 cm(-1) and frequencies above 70 kHz. There is a strong correlation between the reversal density and the density at which the Ohmic L-mode energy confinement changes from the linear to the saturated regime.     

Self-consistent Monte Carlo simulations of the electron and ion distributions of inhomogeneous liquid alkali metals. II. Longitudinal and transverse density distributions in the liquid-vapor interface of binary metallic alloys

We present the results of Monte Carlo simulations of the liquid-vapor interface of sodium-cesium alloys. The longitudinal density profile of each alloy shows that the liquid-vapor interface consists of a well-defined monolayer of cesium sitting on top of a slab of the bulk alloy. Underneath the monolayer there is a slight excess of sodium. A comparison with a van der Waals analog of one of the alloys shows that the presence of the well-defined monolayer of cesium on the outside of the liquid-vapor interface is a feature peculiar to metallic mixtures. The transverse pair correlation functions of the cesium monolayer are insensitive to the composition of the bulk of the slab.     

Absorption of shocked benzene

Abstract

We used absorption spectroscopy to observe decomposition of benzene (C₆H₆) subjected to the passage of strong shock waves generated by projectile impact. These measurements were made using a recently developed double-beam, double-pass, fiber-optic-coupled apparatus. Near 13 GPa, we observe absorption throughout most of the visible region, with strong absorption occurring for wavelengths below 400 nm. The absorption is most likely due to a combination of molecular absorption and Mie scattering from carbon particles formed as a result of shock decomposition.     

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.     

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.