Magneto-Roton Modes of the Ultra Quantum Crystal: Numerical Study

The Field Induced Spin Density Wave phases observed in quasi-one-dimensional conductors of the Bechgaard salts family under magnetic field exhibit both Spin Density Wave order and a Quantized Hall Effect, which may exhibit sign reversals. The original nature of the condensed phases is evidenced by the collective mode spectrum. Besides the Goldstone modes, a quasi periodic structure of Magneto-Roton modes, predicted to exist for a monotonic sequence of Hall Quantum numbers, is confirmed, and a second mode is shown to exist within the single particle gap. We present numerical estimates of the Magneto-Roton mode energies in a generic case of the monotonic sequence. The mass anisotropy of the collective mode is calculated. We show how differently the MR spectrum evolves with magnetic field at low and high fields. The collective mode spectrum should have specific features, in the sign reversed "Ribault Phase", as compared to modes of the majority sign phases. We investigate numerically the collective mode in the Ribault Phase.Comment: this paper incorporates material contained in a previous cond-mat preprint cond-mat/9709210, but cannot be described as a replaced version, because it contains a significant amount of new material dealing with the instability line and with the topic of Ribault Phases. It contains 13 figures (.ps files

Refined a posteriori error estimation for classical and pressure-robust Stokes finite element methods

Recent works showed that pressure-robust modifications of mixed finite element methods for the Stokes equations outperform their standard versions in many cases. This is achieved by divergence-free reconstruction operators and results in pressure independent velocity error estimates which are robust with respect to small viscosities. In this paper we develop a posteriori error control which reflects this robustness. The main difficulty lies in the volume contribution of the standard residual-based approach that includes the $L^2$-norm of the right-hand side. However, the velocity is only steered by the divergence-free part of this source term. An efficient error estimator must approximate this divergence-free part in a proper manner, otherwise it can be dominated by the pressure error. To overcome this difficulty a novel approach is suggested that uses arguments from the stream function and vorticity formulation of the Navier--Stokes equations. The novel error estimators only take the $\mathrm{curl}$ of the right-hand side into account and so lead to provably reliable, efficient and pressure-independent upper bounds in case of a pressure-robust method in particular in pressure-dominant situations. This is also confirmed by some numerical examples with the novel pressure-robust modifications of the Taylor--Hood and mini finite element methods

Quantum Hall effect anomaly and collective modes in the magnetic-field-induced spin-density-wave phases of quasi-one-dimensional conductors

We study the collective modes in the magnetic-field-induced spin-density-wave (FISDW) phases experimentally observed in organic conductors of the Bechgaard salts family. In phases that exhibit a sign reversal of the quantum Hall effect (Ribault anomaly), the coexistence of two spin-density waves gives rise to additional collective modes besides the Goldstone modes due to spontaneous translation and rotation symmetry breaking. These modes strongly affect the charge and spin response functions. We discuss some experimental consequences for the Bechgaard salts.Comment: Final version (LaTex, 8 pages, no figure), to be published in Europhys. Let

TAM and the World Wide Web

The purpose of this research-in-progress is to test the Technology Acceptance Model (TAM) with the World Wide Web as the users\u27 application. The investigation will validate, extend, or refute TAM. It will thus help identify guidelines for developing and using Web site

Aquilegia, Vol. 24 No. 1, January-February 2000: Newsletter of the Colorado Native Plant Society

https://epublications.regis.edu/aquilegia/1178/thumbnail.jp

Spin-excitations of the quantum Hall ferromagnet of composite fermions

The spin-excitations of a fractional quantum Hall system are evaluated within a bosonization approach. In a first step, we generalize Murthy and Shankar's Hamiltonian theory of the fractional quantum Hall effect to the case of composite fermions with an extra discrete degree of freedom. Here, we mainly investigate the spin degrees of freedom, but the proposed formalism may be useful also in the study of bilayer quantum-Hall systems, where the layer index may formally be treated as an isospin. In a second step, we apply a bosonization scheme, recently developed for the study of the two-dimensional electron gas, to the interacting composite-fermion Hamiltonian. The dispersion of the bosons, which represent quasiparticle-quasihole excitations, is analytically evaluated for fractional quantum Hall systems at \nu = 1/3 and \nu = 1/5. The finite width of the two-dimensional electron gas is also taken into account explicitly. In addition, we consider the interacting bosonic model and calculate the lowest-energy state for two bosons. Besides a continuum describing scattering states, we find a bound-state of two bosons. This state is interpreted as a pair excitation, which consists of a skyrmion of composite fermions and an antiskyrmion of composite fermions. The dispersion relation of the two-boson state is evaluated for \nu = 1/3 and \nu = 1/5. Finally, we show that our theory provides the microscopic basis for a phenomenological non-linear sigma-model for studying the skyrmion of composite fermions.Comment: Revised version, 14 pages, 4 figures, accepted to Phys. Rev.

Sign reversals of the quantum Hall effect and helicoidal magnetic-field-induced spin-density waves in quasi-one-dimensional organic conductors

We study the effect of umklapp scattering on the magnetic-field-induced spin-density-wave phases, which are experimentally observed in the quasi-one-dimensional organic conductors of the Bechgaard salts family. Within the framework of the quantized nesting model, we show that umklapp processes may naturally explain sign reversals of the quantum Hall effect (QHE) observed in these conductors. Moreover, umklapp scattering can change the polarization of the spin-density wave (SDW) from linear (sinusoidal SDW) to circular (helicoidal SDW). The QHE vanishes in the helicoidal phases, but a magnetoelectric effect appears. These two characteristic properties may be utilized to detect the magnetic-field-induced helicoidal SDW phases experimentally.Comment: 4 pages, latex, 3 figure

Surfactant protein A and D polymorphisms and methylprednisolone pharmacogenetics in donor lungs

Objective: Surfactant proteins A and D are important molecules involved in lung allograft innate immunity. Genetic polymorphisms of surfactant proteins A and D are associated with various lung diseases. In this study, surfactant protein A and D expression responses were investigated during pharmacogenetics upon methylprednisolone treatment as observed during lung transplantation. Methods: A human cell line (NCI-H441) and precision-cut lung slices from 16 human donors were incubated with methylprednisolone, and surfactant protein A1, surfactant protein A2, and surfactant protein D messenger RNA and surfactant protein A protein expression were assayed. Surfactant protein A1, A2, and D polymorphisms and surfactant protein A gene and protein expressions were determined. Results: In NCI-H441 cells, methylprednisolone treatment at 10−5 M and 10−6 M reduced surfactant protein A1 and surfactant protein A2 messenger RNA and surfactant protein A protein expression (P <.05). A pharmacogenetic relationship was observed in human donor precision-cut lung slices between the surfactant protein A2 (1Ax) variants: Surfactant protein A1, A2, and D messenger RNA expression were greater for 1A0 versus 1A1 (P <.05); surfactant protein A1/surfactant protein A2 genotype 6A26A2/1A01A0 (n = 5) showed greater surfactant protein A1, A2, and D messenger RNA expression and surfactant protein A protein expression compared with the other surfactant protein A1/surfactant protein A2 genotypes (n = 11) (P <.05). Conclusions: The surfactant protein A genotype and methylprednisolone stimuli influence donor lung surfactant protein A and D expression. Lungs carrying the surfactant protein A2 variant 1A0 have a greater expression of surfactant protein A when treated with methylprednisolone. Surfactant protein A polymorphisms could be used to personalize immunosuppressive regimens

Quantum interference from sums over closed paths for electrons on a three-dimensional lattice in a magnetic field: total energy, magnetic moment, and orbital susceptibility

We study quantum interference effects due to electron motion on a three-dimensional cubic lattice in a continuously-tunable magnetic field of arbitrary orientation and magnitude. These effects arise from the interference between magnetic phase factors associated with different electron closed paths. The sums of these phase factors, called lattice path-integrals, are ``many-loop" generalizations of the standard ``one-loop" Aharonov-Bohm-type argument. Our lattice path integral calculation enables us to obtain various important physical quantities through several different methods. The spirit of our approach follows Feynman's programme: to derive physical quantities in terms of ``sums over paths". From these lattice path-integrals we compute analytically, for several lengths of the electron path, the half-filled Fermi-sea ground-state energy of noninteracting spinless electrons in a cubic lattice. Our results are valid for any strength of the applied magnetic field in any direction. We also study in detail two experimentally important quantities: the magnetic moment and orbital susceptibility at half-filling, as well as the zero-field susceptibility as a function of the Fermi energy.Comment: 14 pages, RevTe

Effect of umklapp scattering on the magnetic-field-induced spin-density waves in quasi-one-dimensional organic conductors

We study the effect of umklapp scattering on the magnetic-field-induced spin-density-wave (FISDW) phases which are experimentally observed in the quasi-one-dimensional organic conductors of the Bechgaard salts family. Within the framework of the quantized nesting model, we show that the transition temperature is determined by a modified Stoner criterion which includes the effect of umklapp scattering. We determine the SDW polarization (linear or circular) by analyzing the Ginzburg-Landau expansion of the free energy. We also study how umklapp processes modify the quantum Hall effect (QHE) and the spectrum of the FISDW phases. We find that umklapp scattering stabilizes phases which exhibit a sign reversal of the QHE, as experimentally observed in the Bechgaard salts. These ``negative'' phases are characterized by the simultaneous existence of two SDWs with comparable amplitudes. As the umklapp scattering strength increases, they may become helicoidal (circularly polarized SDWs). The QHE vanishes in the helicoidal phases, but a magnetoelectric effect appears. These two characteristic properties may be utilized to detect the magnetic-field-induced helicoidal SDW phases experimentally.Comment: Revtex, 27 pages, 9 figure