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Articles 31 - 60 of 78
Full-Text Articles in Physics
Magnetic Memory And Current Amplification Devices Using Moving Domain Walls, Stewart E. Barnes, J. Ieda, S. Maekawa
Magnetic Memory And Current Amplification Devices Using Moving Domain Walls, Stewart E. Barnes, J. Ieda, S. Maekawa
Physics Articles and Papers
A moving magnetic domain wall produces an electromotive force (emf). It is therefore possible to read the state of a magnetic memory device via the emf it produces when subjected to an interrogation pulse. It is also possible to amplify currents in pulse circuits, opening up the possibility of all magnetic logic circuits.
Comment On “Theory Of Current-Driven Domain Wall Motion: Spin Transfer Versus Momentum Transfer”, S. E. Barnes
Comment On “Theory Of Current-Driven Domain Wall Motion: Spin Transfer Versus Momentum Transfer”, S. E. Barnes
Physics Articles and Papers
A Comment on the Letter by Gen Tatara and Hiroshi Kohno, Phys. Rev. Lett. 92 086601 (2004). The authors of the Letter offer a Reply.
Coherence And Polarization Properties Of Far Fields Generated By Quasi-Homogeneous Planar Electromagnetic Sources, Olga Korotkova, Brian G. Hoover, Victor L. Gamiz, Emil Wolf
Coherence And Polarization Properties Of Far Fields Generated By Quasi-Homogeneous Planar Electromagnetic Sources, Olga Korotkova, Brian G. Hoover, Victor L. Gamiz, Emil Wolf
Physics Articles and Papers
In studies of radiation from partially coherent sources the so-called quasi-homogeneous (QH) model sources have been very useful, for instance in elucidating the behavior of fields produced by thermal sources. The analysis of the fields generated by such sources has, however, been largely carried out in the framework of scalar wave theory. In this paper we generalize the concept of the QH source to the domain of the electromagnetic theory, and we derive expressions for the elements of the cross-spectral density matrix, for the spectral density, the spectral degree of coherence, the degree of polarization, and the Stokes parameters of …
Generalized Stokes Parameters Of Random Electromagnetic Beams, Olga Korotkova, Emil Wolf
Generalized Stokes Parameters Of Random Electromagnetic Beams, Olga Korotkova, Emil Wolf
Physics Articles and Papers
A generalization of the Stokes parameters of a random electromagnetic beam is introduced. Unlike the usual Stokes parameters, which depend on one spatial variable, the generalized Stokes parameters, depend on two spatial variables. They obey precise laws of propagation, both in free space and in any linear medium, whether deterministic or random. With the help of the generalized Stokes parameters, the changes in the ordinary Stokes parameters upon propagation can be determined. Numerical examples of such changes are presented. The generalized Stokes parameters contain information not only about the polarization properties of the beam but also about its coherence properties. …
Current-Spin Coupling For Ferromagnetic Domain Walls In Fine Wires, S. E. Barnes, S. Maekawa
Current-Spin Coupling For Ferromagnetic Domain Walls In Fine Wires, S. E. Barnes, S. Maekawa
Physics Articles and Papers
The coupling between a current and a domain wall is examined. In the presence of a finite current and in the absence of a potential which breaks the translational symmetry, there is a perfect transfer of angular momentum from the conduction electrons to the wall. As a result, the ground state is in uniform motion and this remains the case even when relaxation is included. This is described by, appropriately modified, Landau-Lifshitz-Gilbert equations. The results for a simple pinning model are compared with experiment.
Spectral Degree Of Coherence Of A Random Three-Dimensional Electromagnetic Field, Olga Korotkova, Emil Wolf
Spectral Degree Of Coherence Of A Random Three-Dimensional Electromagnetic Field, Olga Korotkova, Emil Wolf
Physics Articles and Papers
The complex spectral degree of coherence of a general random, statistically stationary electromagnetic field is introduced in a manner similar to the way it is defined for a beamlike field, namely, by means of Young’s interference experiment. Both its modulus and its phase are measurable. We illustrate the definition by applying it to blackbody radiation emerging from a cavity. The results are of particular interest for near-field optics.
Beam Conditions For Radiation Generated By An Electromagnetic Gaussian Schell-Model Source, Olga Korotkova, Mohamed Salem, Emil Wolf
Beam Conditions For Radiation Generated By An Electromagnetic Gaussian Schell-Model Source, Olga Korotkova, Mohamed Salem, Emil Wolf
Physics Articles and Papers
It was shown recently that the basic properties of a fluctuating electromagnetic beam can be derived from knowledge of a 2×2 cross-spectral density matrix of the electric field in the source plane. However, not every such matrix represents a source that will generate a beamlike field. We derive conditions that the matrix must satisfy for the source to generate an electromagnetic Gaussian Schell-model beam.
Model For A Partially Coherent Gaussian Beam In Atmospheric Turbulence With Application In Lasercom, Olga Korotkova, Larry C. Andrews, Ronald L. Phillips
Model For A Partially Coherent Gaussian Beam In Atmospheric Turbulence With Application In Lasercom, Olga Korotkova, Larry C. Andrews, Ronald L. Phillips
Physics Articles and Papers
Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen with Gaussian spectrum (Rytov theory and ABCD ray matrices are applied). The relation between the second- and fourth-order statistics for a beam with any degree of coherence in the atmosphere is introduced with the help of “effective” beam parameters, deduced from the free-space MCF. In particular, the …
Intermediate Spin, Schrödinger Cat States, And Nanomagnets, S. E. Barnes
Intermediate Spin, Schrödinger Cat States, And Nanomagnets, S. E. Barnes
Physics Articles and Papers
Quantum tunneling in nanomagnets finds a natural description in terms of intermediate spin. Periodic magnetic effects correspond to a change of flux by the flux quantum Φ0. Schrödinger cat states with different superpositions of the applied magnetic field occur. The molecular magnet Fe8 is discussed and new effects are predicted for Mn12.
Quantum Coherence In Small Antiferromagnets, S. E. Barnes, R. Ballou, B. Barbara, J. Strelen
Quantum Coherence In Small Antiferromagnets, S. E. Barnes, R. Ballou, B. Barbara, J. Strelen
Physics Articles and Papers
An antiferromagnetic particle with an excess spin j⃗ has 2 degrees of freedom. The one associated with j⃗ is ferromagnetic, while the antiferromagnetic behavior involves Néel vector n⃗. It is shown that the spin-parity effect described by Loss et al. has to do with the ferromagnetic degree of freedom and does not imply the localization of n⃗ for small enough particles for half-integer spin. It is also shown that the tunnel splitting associated with n⃗ is insensitive to applied fields.
The Possibility Of Flux Flow Spectroscopy, S. E. Barnes, J. L. Cohn, F. Zuo
The Possibility Of Flux Flow Spectroscopy, S. E. Barnes, J. L. Cohn, F. Zuo
Physics Articles and Papers
A novel spectroscopic technique applicable to the study of spin excitations in magnetic materials is described. The probe frequency and wave vector are determined by the moving Abrikosov flux lattice of a type II superconductor in the mixed state. The feasibility of employing oxide superconductors and relevant material parameters are discussed.
Implications Of Fumi’S Theorem For Auxiliary Particle Methods, S. E. Barnes
Implications Of Fumi’S Theorem For Auxiliary Particle Methods, S. E. Barnes
Physics Articles and Papers
Fumi’s theorem in conjunction with some general theorems associated with auxiliary particle methods dictates the functional form of the on-shell auxiliary particle self-energy Σ for a certain class of impurity scattering problems. An explicit calculation which confirms the result is described and the implications for various approaches to the Kondo effect discussed.
Thermodynamics Of Integrable Chains With Alternating Spins, H. J. De Vega, Luca Mezincescu, Rafael I. Nepomechie
Thermodynamics Of Integrable Chains With Alternating Spins, H. J. De Vega, Luca Mezincescu, Rafael I. Nepomechie
Physics Articles and Papers
We consider a two-parameter (c¯,c̃) family of quantum integrable isotropic Hamiltonians for a chain of alternating spins of spin s=1/2 and s=1. We determine the thermodynamics for low-temperature T and small external magnetic field H, with T≪H. In the antiferromagnetic (c¯>0,c̃>0) case, the model has two gapless excitations. In particular, for c¯=c̃, the model is conformally invariant and has central charge cvir=2. When one of these parameters is zero, the Bethe ansatz equations admit an infinite number of solutions with lowest energy.
Auger Spectra And Band Structure Of La1.85Sr0.15Cuo4 And La1.85Ba0.15Cuo4, R. Bar-Deroma, J. Felsteiner, R. Brener, J. Ashkenazi, D. Van Der Marel
Auger Spectra And Band Structure Of La1.85Sr0.15Cuo4 And La1.85Ba0.15Cuo4, R. Bar-Deroma, J. Felsteiner, R. Brener, J. Ashkenazi, D. Van Der Marel
Physics Articles and Papers
High-resolution Auger electron spectra of La1.85Sr0.15CuO4 and La1.85Ba0.15CuO4 have been measured. The experimental Auger line shapes of the oxygen KLL and copper L2,3VV transitions are compared with those generated from local-density-approximation linear-muffin-tin-orbital band-structure calculations. An analysis based on this comparison is used to determine the Coulomb parameters Up and Ud of the oxygen and copper atoms, respectively. For this purpose, an extended version of Cini and Sawatzky’s expression for the theoretical Auger line shape is used, taking into account the multiplet structure of the Auger final …
Magnetism With Generalized-Gradient-Approximation Density Functionals, D.J. Singh, J. Ashkenazi
Magnetism With Generalized-Gradient-Approximation Density Functionals, D.J. Singh, J. Ashkenazi
Physics Articles and Papers
We report generalized-gradient-approximation (GGA) density-functional studies of Cr, V, and Pd, using an all-electron, general-potential linearized-augmented-plane-wave technique. These calculations were undertaken in order to assess the utility of the GGA as an improvement over the local-density approximation in describing transition-metal magnetism. It is found that although the tendency towards magnetism is enhanced, both V and Pd are correctly predicted to be nonmagnetic with the recent GGA functional of Perdew (PW GGA-II) as with the Langreth-Mehl-Hu (LM) functional. Calculations for commensurate antiferromagnetic Cr show that the PW GGA-II predicts an overly large moment of 1.4μB while the LM functional yields …
Nonuniversality In The Kondo Effect, M. Naghashpour, S. E. Barnes
Nonuniversality In The Kondo Effect, M. Naghashpour, S. E. Barnes
Physics Articles and Papers
We have analytically evaluated, up to fourth order, all logarithmic terms in the perturbation expansion of the magnetization scrM for the Kondo model. All nonlogarithmic terms have been evaluated up to third order. When these results are compared with the claimed universal function scrM(H/TH) calculated with the Bethe ansatz, we find that the two cannot be made to agree whatever the form of TH. We conclude that such universal functions do not exist.
Resistivity And Thermoelectric-Power Measurements Of PrXY1-XBa2Cu3O7-Δ Up To 1200 K And An Electronic-Structure Analysis, B. Fisher, J. Genossar, L. Patlagan, J. Ashkenazi
Resistivity And Thermoelectric-Power Measurements Of PrXY1-XBa2Cu3O7-Δ Up To 1200 K And An Electronic-Structure Analysis, B. Fisher, J. Genossar, L. Patlagan, J. Ashkenazi
Physics Articles and Papers
We report measurements of absolute thermoelectric power and resistivity in PrxY1-xBa2Cu3O7-δ, as function of x and δ, over a wide temperature range. At high temperature, the substitution of Pr for Y does not affect the thermopower and resistivity; Pr behaves then as a trivalent ion: Pr3+. For small x and δ the effects of substitution of Y by Pr and of oxygen deficiency on the low-temperature resistivity are additive. A simple electronic-structure model is proposed to account for thermopower as function of temperature, x, and δ.
Flux States For The Infinite-U Limit Of The Hubbard Model, S. E. Barnes
Flux States For The Infinite-U Limit Of The Hubbard Model, S. E. Barnes
Physics Articles and Papers
For finite doping, δ, relative to half filling, it is shown, within the confines of a mean-field auxiliary-operator formulation, for the strictly infinite-U, i.e., J=0, limit of the two-dimensional Hubbard model, that the spinons generate a fictitious magnetic flux to which holons are subject and vice versa. For small doping, a flux state is the absolute ground state and has 1-δ=2p/q, where p/q is a rational fraction and the flux per plaquette is Φ=(p/q)Φ0, where Φ0 is the flux quantum. This phenomenon can be viewed upon as an all-electron Peierls instability or as broken statistical symmetry.
All-Electronic Peierls Instability In The Infinite-U Mean-Field Theory For The Hubbard Model, S. E. Barnes
All-Electronic Peierls Instability In The Infinite-U Mean-Field Theory For The Hubbard Model, S. E. Barnes
Physics Articles and Papers
It is shown, for a small hole doping relative to half filling, that the large-U limit of the Hubbard model (or J=0 limit of the t-J model) exhibits an all-electron Peierls instability towards a state with a charge-density wave. The resulting gap in the spinon spectrum renders the model an insulator.
Spin-Disordered Ground State For A Nearly Half-Filled Hubbard Model, S. E. Barnes
Spin-Disordered Ground State For A Nearly Half-Filled Hubbard Model, S. E. Barnes
Physics Articles and Papers
Following Nagaoka, the ground state of the large-U nearly half-filled Hubbard model is ‘‘ferromagnetic.’’ However, because of the anisotropic exchange energy JA, the physical Nagaoka ground state is nondegenerate and comprises a totally disordered spin state, i.e., a linear combination of all possible spin arrangements. For this ground state the susceptibility is Pauli-like but of a magnitude associated with a narrow band of width ∼JA while the holons, i.e., the charge excitations, correspond to spinless fermions with a band of width 2zt. Associated with the spinon sector are very poorly defined spin waves, i.e., bosons, with a …
Application Of The Linear Muffin-Tin-Orbital Band-Structure Method To Calculate Optical Properties Of Solids, R. Zemach, J. Ashkenazi, E. Ehrenfreund
Application Of The Linear Muffin-Tin-Orbital Band-Structure Method To Calculate Optical Properties Of Solids, R. Zemach, J. Ashkenazi, E. Ehrenfreund
Physics Articles and Papers
Optical matrix elements have been derived for crystals by applying the theory of optical transitions within the formalism of the linear muffin-tin-orbital band-structure method and using Racah algebra. A Brillouin-zone integration by the tetrahedron method allows straightforward numerical computation of the optical function ε2(ω) [or σ(ω)]. Other optical functions may be derived by using a Kramers-Kronig analysis.
Calculation Of Optical Properties Of Trans-Polyacetylene By The Linear Muffin-Tin-Orbital Method, R. Zemach, J. Ashkenazi, E. Ehrenfreund
Calculation Of Optical Properties Of Trans-Polyacetylene By The Linear Muffin-Tin-Orbital Method, R. Zemach, J. Ashkenazi, E. Ehrenfreund
Physics Articles and Papers
The basic optical properties of trans-(CH)x in the P21/n and the P21/a space groups are derived from first principles. The relevant optical matrix elements are calculated within the self-consistent linear muffin-tin-orbital band-structure method. The derived optical response functions are compared with the results from a variety of optical as well as electron-energy-loss experimental studies. It is shown that the measured optical constants are consistent with the P21/n structure.
Ground State Of Trans-Polyacetylene And The Peierls Mechanism - Ashkenazi Et Al. Reply, J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, S. R. Chubb
Ground State Of Trans-Polyacetylene And The Peierls Mechanism - Ashkenazi Et Al. Reply, J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, S. R. Chubb
Physics Articles and Papers
Ashkenazi et al. reply to Original Article: J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, and S. R. Chubb, Ground state of trans-polyacetylene and the Peierls mechanism, Physical Review Letters 62, 2016 (1989).
Ground State Of Trans-Polyacetylene And The Peierls Mechanism - Errata, J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, S. R. Chubb
Ground State Of Trans-Polyacetylene And The Peierls Mechanism - Errata, J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, S. R. Chubb
Physics Articles and Papers
Errata to Original Article: J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, and S. R. Chubb, Ground state of trans-polyacetylene and the Peierls mechanism, Physical Review Letters, 62, 2016 (1989).
Ground State Of Trans-Polyacetylene And The Peierls Mechanism, J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, S. R. Chubb
Ground State Of Trans-Polyacetylene And The Peierls Mechanism, J. Ashkenazi, W. E. Pickett, H. Krakauer, C. S. Wang, B. M. Klein, S. R. Chubb
Physics Articles and Papers
It is shown formally that the Peierls expression is inadequate for studying the instabilities of a physically realizable one-dimensional chain of atoms, due to the long-range nature of the Coulomb interaction. Detailed total-energy studies of trans-polyacetylene, for both suggested crystal structures (P21/n,P21/a) and for the one-dimensional chain, establish that no appreciable total-energy minimum occurs for nonzero dimerization amplitude within the local-density approximation. The measured dimerization is discussed within the context of recent models of electron correlation.
Excitations In A Nearly Half-Filled Hubbard Model With U=∞, S. E. Barnes
Excitations In A Nearly Half-Filled Hubbard Model With U=∞, S. E. Barnes
Physics Articles and Papers
It is shown that, even near half filling, the elementary excitations of the large-U Hubbard model form wide bands of width ∼t, the intersite hopping integral, and that the slave-boson-based mean-field approximation is qualitatively wrong in the relevant limit. This large-U limit has no Kondo-type divergences.
Critical Dimensions For Chiral Bosons, Luca Mezincescu, Rafael I. Nepomechie
Critical Dimensions For Chiral Bosons, Luca Mezincescu, Rafael I. Nepomechie
Physics Articles and Papers
We give the Lagrangian formulation of a Bose model in 1+1 dimensions which describes a free chiral Lie-algebra-valued current. This model is a non-Abelian generalization of the chiral scalar model of Siegel. Both the Abelian and non-Abelian actions have a gauge invariance, which becomes anomalous when the models are quantized. The condition that this anomaly be canceled coincides with the string no-ghost condition.
Alternative Feynman-Diagram Method For The General-Spin Kondo Problem, S. E. Barnes
Alternative Feynman-Diagram Method For The General-Spin Kondo Problem, S. E. Barnes
Physics Articles and Papers
We introduce a new method of replacing spin operators with pseudofermions. For a spin value S there are 2S pseudofermions each of which can have spin ‘‘up’’ or ‘‘down’’. In contrast to the standard Abrikosov method which has 2S+1 inequivalent fermions, i.e., one for each Sz value, the present pairs are all equivalent. It follows that the general spin expression is obtained by multiplying the S=(1/2 result by a ceratin combinational factor. This aids in the construction of rotationally invariant solutions for the general-spin Kondo problem for the case when n≤2S, where n is the number of conduction-electron scattering …
Non-Abelian Bosonization: Current Correlation Functions, Lowell S. Brown, Rafael I. Nepomechie
Non-Abelian Bosonization: Current Correlation Functions, Lowell S. Brown, Rafael I. Nepomechie
Physics Articles and Papers
We demonstrate that, in 1+1 dimensions, the effective action produced by massless fermions moving in external gauge potentials is identical with that produced by a suitably defined σ model which includes a Wess-Zumino term. Hence the current correlation functions of the fermion and boson theories are identical.
Non-Abelian Bosonization: Stress-Energy Correlation Functions, Lowell S. Brown, Gary J. Goldberg, Chaiho P. Rim, Rafael I. Nepomechie
Non-Abelian Bosonization: Stress-Energy Correlation Functions, Lowell S. Brown, Gary J. Goldberg, Chaiho P. Rim, Rafael I. Nepomechie
Physics Articles and Papers
We demonstrate that, in 1+1 dimensions, the effective action produced by massless fermions moving in external gravitational and gauge fields is identical with that produced by a suitably defined σ model which includes a Wess-Zumino term. This equivalence holds provided that the two models have the same conformal anomaly, a condition that we investigate in detail.