Diagonal Modulation Research Articles

Incommensurate nematic fluctuations in the two-dimensional Hubbard model

We analyze effective d-wave interactions in the two-dimensional extended Hubbard model at weak coupling and small to moderate doping. The interactions are computed from a renormalization group flow. Attractive d-wave interactions are generated via antiferromagnetic spin fluctuations in the pairing and charge channels. Above Van Hove filling, the d-wave charge interaction is maximal at incommensurate diagonal wave vectors, corresponding to nematic fluctuations with a diagonal modulation. Below Van Hove filling a modulation along the crystal axes can be favored. The nematic fluctuations are enhanced by the nearest-neighbor interaction in the extended Hubbard model, but they always remain smaller than the dominant antiferromagnetic, pairing, or charge density wave fluctuations.

Scaling Properties of Energy Gap in Dimerized Spinless FermionChains with Quasiperiodic Modulation

By employing the exact diagonalization method, we investigate thelow-energy behaviour of the dimerized spinless fermion chains withquasiperiodic modulation. It is found that for off-diagonal modulation,the energy gap with length N scales as exp(−cNω) withnonuniversal exponent ω if the dimerization is nonzero. However,for diagonal modulation, there may exist a critical dimerizationδc beyond which the system exhibits a metal–insulatortransition.

The use of MR B+1 imaging for validation of FDTD electromagnetic simulations of human anatomies

In this study, MR B+1 imaging is employed to experimentally verify the validity of FDTD simulations of electromagnetic field patterns in human anatomies. Measurements and FDTD simulations of the B+1 field induced by a 3 T MR body coil in a human corpse were performed. It was found that MR B+1 imaging is a sensitive method to measure the radiofrequency (RF) magnetic field inside a human anatomy with a precision of approximately 3.5%. A good correlation was found between the B+1 measurements and FDTD simulations. The measured B+1 pattern for a human pelvis consisted of a global, diagonal modulation pattern plus local B+1 heterogeneties. It is believed that these local B+1 field variations are the result of peaks in the induced electric currents, which could not be resolved by the FDTD simulations on a 5 mm3 simulation grid. The findings from this study demonstrate that B+1 imaging is a valuable experimental technique to gain more knowledge about the dielectric interaction of RF fields with the human anatomy.For more information on this article, see medicalphysicsweb.org

Continuous charge modulated diagonal phase in manganites.

We present a novel ground state that explains the continuous charge modulated diagonal order recently observed in manganese oxides, at hole concentrations x larger than one-half. In this diagonal phase the charge is modulated with a predominant Fourier component inversely proportional to 1-x. Magnetically this state consists of antiferromagnetically coupled zigzag chains. For a wide range of physical parameters such as electron-phonon coupling, antiferromagnetic interaction between Mn ions, and on-site Coulomb repulsion, the diagonal phase is the ground state of the system. Also we find that the diagonal modulation of the electron density is only a small fraction of the average charge, a much smaller modulation than the one obtained by distributing Mn+3 and Mn+4 ions. We discuss also the spin and orbital structure properties of this new diagonal phase.

The Forms and Representations of the Lie algebra sl 2 (ℤ)

We study the structure of integral p-adic forms of the splitting three-dimensional simple Lie algebra over the field of p-adic numbers. We discuss the questions of diagonalizability of such forms and description for maximal diagonal ideals. We consider torsion-free finite-dimensional modules over the splitting three-dimensional simple Lie algebra with integral and p-adic integral coefficients. We describe diagonal modules, demonstrate finiteness of the number of modules in each dimension, and prove a local-global principle for irreducible modules.

Quasiperiodic Hubbard Chains

Low-energy properties of half-filled Fibonacci Hubbard models are studied by weak-coupling renormalization group and density matrix renormalization group methods. In the case of diagonal modulation, weak Coulomb repulsion is irrelevant and the system behaves as a free Fibonacci chain, while for strong Coulomb repulsion the charge sector becomes a Mott insulator and the spin sector behaves as a uniform Heisenberg antiferromagnetic chain. The off-diagonal modulation always drives the charge sector to a Mott insulator and the spin sector to a Fibonacci antiferromagnetic Heisenberg chain.

Origin of the T1.3 power law of pure dephasing for impurity electronic transitions in amorphous solids

The standard tunnel model together with the diagonal modulation theory of dephasing has previously been used to explain the above power law for impurity electronic transitions for T ≲ 10 K. Crude approximations made in averaging over the two-level system (TLS) parameters led to the widely held view that the exponent is 1 + μ where μ is the exponent of the TLS density of states, ρ( E) = ρ 0 E μ. We show that this is incorrect and that the temperature dependence is determined by the average value of the TLS asymmetry parameter 〈Δ〉 E , where E is the tunnel splitting.

Peierls stabilization of magnetic-flux states of two-dimensional lattice electrons.

The problem of two-dimensional lattice electrons in a uniform magnetic field is reduced to a one-dimensional Peierls system which is an off-diagonal tight-binding model. Thus, this off-diagonal equation and the usual Harper equation, which has a diagonal modulation, give the same energy spectrum. The stabilization of the state in which the magnetic field gives one flux unit per electron is formally equivalent to the Peierls instability of lattice deformation.

Optical dephasing of impurity transitions in amorphous solids from diagonal modulation: application of nonphenomenological distribution functions

The diagonal modulation mechanism for optical dephasing of impurity transitions in amorphous solids is reexamined by utilizing nonphenomenological distribution functions for the two-level systems (TLS). With reasonable variations in the distribution function parameters, it is shown that the power law ..delta..omega proportional to T/sup n/ with 1.2 approximately less than or equal to n approximately less than or equal to 2 holds for a dipole-dipole interaction. However, off-diagonal modulation provides the same range in n. The problem of distinguishing between dephasing mechanisms is discussed.

Analysis of observables in diagonally polarized light scattering experiments

We analyze the polarization of light scattered from an ensemble of stationary, randomly oriented molecules of arbitrary size, for ‘‘diagonally’’ polarized incident light (linearly polarized at ±45° to the scattering plane). The results are expressed in terms of the ‘‘diagonal modulation,’’ the change in scattered intensity when the incident polarization switches from +45° to −45°. We use the exact nonrelativistic operators for the interaction of light and matter, and we use no wave function approximations. The diagonal modulation is expressed as a superposition of four independent observables, which may be separated by polarization filters in the observation channel. Three of these are found to posses interesting characteristics after the classical orientation average is taken: (a) they vanish when the damping constants of the molecule are set to zero; and (b) they vanish in the limit of a dipolar scattering tensor. This analysis therefore suggests experiments in which combinations of damping constants and higher transition multipoles might be directly observed.

Anomalous Optical Homogeneous Linewidths in Glasses

The homogeneous linewidth is calculated for resonant optical transitions in glasses. The width is due to modulation of the optical levels caused by coupling to the diagonal as well as off-diagonal elements of the two-level modes. The main contribution arises from diagonal modulation of abundant but weakly coupled two-level modes. For a flat density of states of two-level modes and a dipole-quadrupole coupling a large ${T}^{1.75}$ behavior is found in good agreement with recent data.