Realistic Couplings Research Articles

Orbital order, anisotropic spin couplings, and the spin-wave spectrum of the ferromagnetic Mott insulator YTiO3

After discovering a subtle sign error in our previous work we have redone our calculations of the superexchange couplings in the ferromagnetic Mott insulator YTiO3. In contrast to the earlier result, the corrected calculations give realistic ferromagnetic Heisenberg couplings for Ti–Ti bonds in the crystallographic ab planes as well as along the c direction. The calculated antisymmetric (Dzyaloshinskii-Moriya) and symmetric anisotropy terms of the superexchange Hamiltonian are also modified. Based on the calculated superexchange couplings we determine the magnetic order of the Ti ions and the spin-wave spectrum, and both are found in acceptable agreement with experiment. Our model, in which the orbital degrees of freedom are frozen by the crystal field, also reproduces the experimentally observed orbital ordering pattern of the Ti ions. Therefore, this model provides a consistent picture of the orbital and of the magnetic ordering mechanisms in YTiO3. These mechanisms are coupled together, and have been a subject of considerable debate.

Alfven Waves and Turbulence in the Solar Atmosphere and Solar Wind

We solve the problem of propagation and dissipation of Alfvenic turbulence in a model solar atmosphere consisting of a static photosphere and chromosphere, transition region, and open corona and solar wind, using a phenomenological model for the turbulent dissipation based on wave reflection. We show that most of the dissipation for a given wave-frequency spectrum occurs in the lower corona, and the overall rms amplitude of the fluctuations evolves in a way consistent with observations. The frequency spectrum, for a Kolmogorov-like slope, is not found to change dramatically from the photosphere to the solar wind, however it does preserve signatures of transmission throughout the lower atmospheric layers, namely oscillations in the spectrum at high frequencies reminiscent of the resonances found in the linear case. These may disappear once more realistic couplings for the non-linear terms are introduced, or if time-dependent variability of the lower atmospheric layer is introduced.

Orbital order, anisotropic spin couplings, and the spin‐wave spectrum of the ferromagnetic Mott insulator YTiO3*

AbstractAfter discovering a subtle sign error in our previous work we have redone our calculations of the superexchange couplings in the ferromagnetic Mott insulator YTiO3. In contrast to the earlier result, the corrected calculations give realistic ferromagnetic Heisenberg couplings for Ti–Ti bonds in the crystallographic ab planes as well as along the c direction. The calculated antisymmetric (Dzyaloshinskii‐Moriya) and symmetric anisotropy terms of the superexchange Hamiltonian are also modified. Based on the calculated superexchange couplings we determine the magnetic order of the Ti ions and the spin‐wave spectrum, and both are found in acceptable agreement with experiment. Our model, in which the orbital degrees of freedom are frozen by the crystal field, also reproduces the experimentally observed orbital ordering pattern of the Ti ions. Therefore, this model provides a consistent picture of the orbital and of the magnetic ordering mechanisms in YTiO3. These mechanisms are coupled together, and have been a subject of considerable debate.

THE FRIEDRICHS-MODEL WITH FERMION-BOSON COUPLINGS

In this work we present an extended version of the Friedrichs Model (FM), which includes fermions. The standard FM describes the coupling between discrete and continuous components of a boson field. It is an exactly solvable model which allows for a clear identification of Gamow Resonances. In this extended version a set of fermion bound states is coupled to the boson field. As a result of the coupling some of the fermion states become resonant states. The structure of the solutions of the model is discussed analytically. The contact with nuclear structure calculations, where the model can be used to grasp basic features of the solutions of purely numerical treatments, is established by introducing realistic couplings constants.

Radiation-induced magnetotransport in high-mobility two-dimensional systems: Role of electron heating

Effects of microwave radiation on magnetoresistance are analyzed in a balance-equation scheme that covers regimes of inter- and intra-Landau level processes and takes account of photon-asissted electron transitions as well as radiation-induced change of the electron distribution for high mobility two-dimensional systems. Short-range scatterings due to background impurities and defects are shown to be the dominant direct contributors to the photoresistance oscillations. The electron temperature characterizing the system heating due to irradiation, is derived by balancing the energy absorption from the radiation field and the energy dissipation to the lattice through realistic electron-phonon couplings, exhibiting resonant oscillation. Microwave modulations of Shubnikov de Haas oscillation amplitude are produced together with microwave-induced resistance oscillations, in agreement with experimental findings. In addition, the suppression of the magnetoresistance caused by low-frequency radiation in the higher magnetic field side is also demonstrated.

Microwave modulation of electron heating and Shubnikov–de Haas oscillation in two-dimensional electron systems

Recently discovered modulations of Shubnikov–de Haas oscillations in microwave-irradiated two-dimensional electron systems are shown to arise from electron heating induced by the radiation. The electron temperature, obtained by balancing the energy absorption from the microwave field and the energy dissipation to the lattice through realistic electron-phonon couplings, exhibits resonance. The modulation of the Shubnikov–de Haas oscillation and the suppression of magnetoresistance are demonstrated together with microwave-induced resistance oscillation, in agreement with experimental findings.

Theoretical study of half-doped models for manganites: Fragility of CE phase with disorder, two types of colossal magnetoresistance, and charge-ordered states for electron-doped materials

A comprehensive analysis of half-doped manganites is presented using Monte Carlo simulations applied to the double-exchange model with cooperative Jahn-Teller lattice distortions in two dimensions. A variety of novel results are reported: (i) The phase diagram is established in the lambda-J_ AF plane, with lambda the electron-phonon coupling and J_AF the antiferromagnetic exchange between classical t_2g spins. The results include standard phases, such as the CE-insulating and FM-metallic regimes, but they also unveil novel states, such as a ferromagnetic charge-ordered (CO) orbital-ordered phase compatible with recent experimental results by Loudon et al. (ii) For realistic couplings, it was observed that the charge disproportionation delta of the CO phase is far from the widely accepted extreme limit delta=0.5 of a 3+/4+ charge separation. A far smaller delta appears more realistic, in agreement with recent experiments by Garcia et al. and Daoud-Aladine it et al. (iii) Colossal magnetoresistance (CMR) effects are found in calculations of cluster resistances using the Landauer formalism. This occurs near the ubiquitous first-order phase transitions between the insulating and metallic states. (iv) The CE-state is found to be (very sensitive to disorder) since its long-range order rapidly disappears when quenched-disorder is introduced, contrary to the FM state which is more robust. This is also in qualitative agreement with recent experiments by Akahoshi et al. and Nakajima et al. (v) The phase diagram in the half-doped (electron doping) regime is briefly discussed as well. A charge-ordered state is found which is the analog of the x=0.5 CE phase. It contains a 3+/2+ charge arrangement at large lambda.

Microwave cavity searches for dark-matter axions

Recent determinations of cosmological parameters point to a flat Universe, whose total energy density is composed of about two-thirds vacuum energy and one-third matter. Ordinary baryonic matter is relegated to a small fraction of the latter, within which the luminous part is an order of magnitude smaller yet. Particle dark matter, i.e., one or more relic particle species from the big bang, is thus strongly suggested as the dominant component of matter in the Universe. The axion, a hypothetical elementary pseudoscalar arising from the Peccei-Quinn solution to the strong-$\mathrm{CP}$ problem, is a well-motivated candidate. If the axion exists, it must be extremely light, in the mass range of ${10}^{\ensuremath{-}6}--{10}^{\ensuremath{-}3}\mathrm{eV},$ and possess extraordinarily feeble couplings to matter and radiation. Nevertheless, as proposed by Sikivie in 1983, the axion's two-photon coupling lends itself to a feasible search strategy with currently available technology. In this scheme, axions resonantly convert to single microwave photons by a Primakoff interaction, in a tunable microwave cavity permeated by a strong magnetic field. Present experiments utilizing heterostructure transistor microwave amplifiers have achieved total system noise temperatures of $\ensuremath{\sim}3\mathrm{K}$ and represent the world's quietest spectral radio receivers. Exclusion regions have already been published well into the band of realistic axion model couplings, within the lowest decade of mass range. Recent breakthroughs in the development of near-quantum-limited superconducting quantum interference device amplifiers should reduce the system noise temperature to $\ensuremath{\sim}100\mathrm{mK}$ or less. Ongoing research into using Rydberg-atom single-quantum detectors as the detector in a microwave cavity experiment could further reduce the effective noise temperature. In parallel with improvements in amplifier technology, promising concepts for higher-frequency cavity resonators are being explored to open up the higher decades in mass range. Definitive experiments to find or exclude the axion may therefore be at hand in the next few years. As the microwave cavity technique measures the total energy of the axion, a positive discovery could well reveal fine structure of the signal due to flows of nonthermalized axions. Manifesting diurnal and sidereal modulation, such detailed features would contain a wealth of information about the history, structure, and dynamics of our Milky Way galaxy.

The inflationary brane-antibrane universe

We show how the motion through the extra dimensions of a gas of branes and antibranes can, under certain circumstances, produce an era of inflation as seen by observers trapped on a 3-brane, with the inflaton being the inter-brane separation. Although most of our discussion refers to arbitrary p-branes, when we need to be specific we assume that they are D-branes of Type II or Type I string theory. For realistic brane couplings, such as those arising in string theory, the inter-brane potentials are too steep to inflate the universe for acceptably long times. However, for special regions of the parameter space of brane-antibrane positions the brane motion is slow enough for there to be sufficient inflation. Inflation would be more generic in models where the inter-brane interactions are much weaker. The spectrum of primordial density fluctuations predicted has index n slightly less than 1, and an acceptable amplitude, provided that the extra dimensions have linear size 1/r ~ 10^{12} GeV. Reheating occurs as in hybrid inflation, with the tachyonic instability of the brane-antibrane system taking over for small separations. The tachyon field can induce a cascade mechanism within which higher-dimension branes annihilate into lower-dimension ones. We argue that such a cascade naturally stops with the production of 3-branes in 10-dimensional string theory.

Indications of spin-charge separation at short distance and stripe formation in the extendedt-Jmodel on ladders and planes

The recently discussed tendency of holes to generate nontrivial spin environments in the extended two-dimensional $t\ensuremath{-}J$ model [G. Martins, R. Eder, and E. Dagotto, Phys. Rev. B 60, R3716 (1999)] is here investigated using computational techniques applied to ladders with several number of legs. This tendency is studied also with the help of analytic spin-polaron approaches directly in two dimensions. Our main result is that the presence of robust antiferromagnetic correlations between spins located at both sides of a hole either along the x or y axis, observed before numerically on square clusters, is also found using ladders, as well as applying techniques based on a string-basis expansion. This so-called ``across-the-hole'' nontrivial structure exists even in the two-leg spin-gapped ladder system, and leads to an effective reduction in dimensionality and spin-charge separation at short-distances, with a concomitant drastic reduction in the quasiparticle weight Z. In general, it appears that holes tend to induce one-dimensional-like spin arrangements to improve their mobility. Using ladders it is also shown that the very small $J/t\ensuremath{\sim}0.1$ regime of the standard $t\ensuremath{-}J$ model may be more realistic than anticipated in previous investigations, since such regime shares several properties with those found in the extended model at realistic couplings. Another goal of the present paper is to provide additional information on the recently discussed tendencies to stripe formation and spin incommensurability reported for the extended $t\ensuremath{-}J$ model. These tendencies are illustrated with several examples.

Looking for effects of topology in the Dirac spectrum of staggered fermions

We classify SU(3) gauge field configurations in different topological sectors by the smearing technique. In each sector we compute the distribution of low lying eigenvalues of the staggered Dirac operator. In all sectors we find perfect agreement with the predictions for the sector of topological charge zero. The smallest Dirac operator eigenvalues of staggered fermions at presently realistic lattice couplings are thus insensitive to gauge field topology. On the smeared configurations, 4ν eigenvalues go to zero in agreement with the index theorem.

Staggered fermions and gauge field topology

Based on a large number of smearing steps, we classify SU(3) gauge field configurations in different topological sectors. For each sector we compare the exact analytical predictions for the microscopic Dirac operator spectrum of quenched staggered fermions. In all sectors we find perfect agreement with the predictions for the sector of topological charge zero, showing explicitly that the smallest Dirac operator eigenvalues of staggered fermions at presently realistic lattice couplings are insensitive to gauge field topology. On the smeared configurations, 4\ensuremath{\nu} eigenvalues clearly separate out from the rest on configurations of topological charge \ensuremath{\nu}, and move towards zero in agreement with the index theorem.

Nonequilibrium Kondo impurity: Perturbation about an exactly solvable point

We perturb about an exactly solvable point for the nonequilibrium Kondo problem. In each of the three independent directions in parameter space, the differential conductance evolves smoothly as one goes away from the solvable point, and the lowest-order correction contains the logarithm of the band width, or cutoff. Perturbing towards physically realistic exchange couplings yields differential-conductance curves which more closely resemble experimental data than at the solvable point. The leading coefficient which describes the low-temperature and low-voltage scaling changes as one perturbs away from the solvable point, indicating nonuniversal behavior; however, it is restored to the solvable-point value in the limit of an infinite band width.

Superconducting critical temperature for realistic couplings.

We have calculated exactly the critical temperature, within the conventional Eliashberg framework, for realistic couplings. Our more precise analytic calculation leads naturally to a ${\mathit{T}}_{\mathit{c}}$ higher than that given by McMillan's formula, which is generally used. We give an accurate formula for ${\mathit{T}}_{\mathit{c}}$ in the coupling region \ensuremath{\lambda}\ensuremath{\le}1.

Vertex correction to pairing at a Van Hove singularity.

We calculate in a k-averaging approximation, the vertex corrections for pairing at a Van Hove singularity mediated by an Einstein phonon. The vertex correction leads to a modest reduction in ${\mathit{T}}_{\mathit{c}}$, which can still reach the experimental values for realistic phonon couplings, and to a significant deepening of the dip in the isotope shift at the ${\mathit{T}}_{\mathit{c}}$ maximum. Interference between first- and second-order vertex graphs diminishes the effect of the Coulomb interaction on ${\mathit{T}}_{\mathit{c}}$. The isotope-shift dip is shallower for repulsive Coulomb interaction, and deeper in the case of an attractive interaction introduced to model an excitonic contribution to pairing.

States of the phase-damped oscillator.

In the modeling of damped quantum systems by coupling to a heat bath, it has been usual to assume the initial factorization of the density matrix into system and bath components. For realistic couplings, this does not represent a physically reasonable state. One of the simplest such models is that of phase damping, and an alternative characterization of its initial states can be obtained exactly and without ambiguity. The consequences of these new states are particularly interesting when the system is considered as a primitive model of number measurement: The local system cannot, with a finite excitation energy, be put into a superposition of number states. This is a stronger result than from previous treatment of this system, which only showed that such superpositions would decay to mixtures. Similar conclusions can be drawn for other quantum nondemolition measurements.

Mass scales of the string unification.

Although mass scales associated with string tension, compactification of extra dimension, and Newtonian gravity are a priori independent, these scales must be approximately equal to each other in any unified string theory with realistic couplings. Consequently, the compactification cannot be adequately described in terms of ten-dimensional field theory.

Charge-Symmetry Breaking in the Nucleon-Nucleon Interaction

New calculations of charge-symmetry breaking in the NN interaction lead to radically altered predictions for the difference between nn and pp scattering lengths. Inclusion of the eta' meson and use of realistic magnetic couplings for the vector meson changes the sign as well as the magnitude of earlier results. This sensitivity arises in part from cancellations within the pseudoscalar and vector contributions separately, making precise calculations difficult. Meson mixing is determined from quark mass differences, and the finite width of the rho meson is included.