Reply to 'Comment on 'Primordial magnetic seed field amplification by gravitational waves''
Betschart, Gerold; Zunckel, Caroline; Dunsby, Peter K S; Marklund, Mattias
2007-04-15
Here we respond to the comment by Tsagas on our earlier paper. We show that the results in that comment are flawed and cannot be used for drawing conclusions about the nature of magnetic field amplification by gravitational waves and give further support that the results of our earlier paper are correct.
Magnetic braking: Improved theory
NASA Astrophysics Data System (ADS)
Heald, Mark A.
1988-06-01
An alternative analysis is presented for the magnetic braking experiment of Wiederick et al., taking into account the fringing streamlines of eddy currents for a rectangular ``footprint'' of the magnetic field.
Theory of nuclear magnetic relaxation
NASA Technical Reports Server (NTRS)
Mcconnell, J.
1983-01-01
A theory of nuclear magnetic interaction is based on the study of the stochastic rotation operator. The theory is applied explicitly to relaxation by anisotropic chemical shift and to spin-rotational interactions. It is applicable also to dipole-dipole and quadrupole interactions.
Planetary magnetism. [emphasizing dynamo theories
NASA Technical Reports Server (NTRS)
Stevenson, D.
1974-01-01
The origin and maintenance of planetary magnetic fields are discussed. The discussion is not limited to dynamo theories, although these are almost universally favored. Thermoelectric currents are found to be a possible alternative for Jupiter. Two energy sources for dynamos are considered: convection and precessionally induced fluid flow. The earth is the most favorable planet for precessionally driven dynamo, although Neptune is a possibility. Jupiter is likely to have a convectionally driven dynamo, as may Saturn, but the relevant properties of Saturn are not yet well known. Conclusions for each planet are given.
Planetary magnetism. [emphasizing dynamo theories
NASA Technical Reports Server (NTRS)
Stevenson, D.
1974-01-01
The origin and maintenance of planetary magnetic fields are discussed. The discussion is not limited to dynamo theories, although these are almost universally favored. Thermoelectric currents are found to be a possible alternative for Jupiter. Two energy sources for dynamos are considered: convection and precessionally induced fluid flow. The earth is the most favorable planet for precessionally driven dynamo, although Neptune is a possibility. Jupiter is likely to have a convectionally driven dynamo, as may Saturn, but the relevant properties of Saturn are not yet well known. Conclusions for each planet are given.
Theories for the origin of lunar magnetism
NASA Technical Reports Server (NTRS)
Daily, W. D.; Dyal, P.
1979-01-01
This paper reviews the major theories which have been proposed to explain the remanent magnetism found in the lunar crust. A total of nine different mechanisms for lunar magnetism are discussed and evaluated in light of the theoretical and experimental constraints pertinent to lunar magnetism. It is concluded that none of these theories in their present state of development satisfy all the known constraints. However, the theories which agree best with the present understanding of the moon are meteorite impact magnetization, thermoelectric dynamo field generation, and an early solar wind field.
Theories for the origin of lunar magnetism
NASA Technical Reports Server (NTRS)
Daily, W. D.; Dyal, P.
1979-01-01
This paper reviews the major theories which have been proposed to explain the remanent magnetism found in the lunar crust. A total of nine different mechanisms for lunar magnetism are discussed and evaluated in light of the theoretical and experimental constraints pertinent to lunar magnetism. It is concluded that none of these theories in their present state of development satisfy all the known constraints. However, the theories which agree best with the present understanding of the moon are meteorite impact magnetization, thermoelectric dynamo field generation, and an early solar wind field.
Magnetic monopoles in field theory and cosmology.
Rajantie, Arttu
2012-12-28
The existence of magnetic monopoles is predicted by many theories of particle physics beyond the standard model. However, in spite of extensive searches, there is no experimental or observational sign of them. I review the role of magnetic monopoles in quantum field theory and discuss their implications for particle physics and cosmology. I also highlight their differences and similarities with monopoles found in frustrated magnetic systems.
Theory of antiskyrmions in magnets
Koshibae, Wataru; Nagaosa, Naoto
2016-01-01
Skyrmions and antiskyrmions are swirling topological magnetic textures realized as emergent particles in magnets. A skyrmion is stabilized by the Dzyaloshinskii–Moriya interaction in chiral magnets and/or a dipolar interaction in thin film magnets, which prefer the twist of the magnetic moments. Here we show by a numerical simulation of the Landau–Lifshitz–Gilbert equation that pairs of skyrmions and antiskyrmions are created from the helix state as the magnetic field is increased. Antiskyrmions are unstable and disappear immediately in chiral magnets, whereas they are metastable and survive in dipolar magnets. The collision between a skyrmion and an antiskyrmion in a dipolar magnet is also studied. It is found that the collision depends on their relative direction, and the pair annihilation occurs in some cases and only the antiskyrmion is destroyed in the other cases. These results indicate that the antiskyrmion offers a unique opportunity to study particles and antiparticles in condensed-matter systems. PMID:26821932
Generation of sunspot and polar faculae butterflies using bipolar and quadripolar seed fields
NASA Astrophysics Data System (ADS)
Callebaut, D. K.; Khater, A. H.
The exact solution of the evolution equation for the magnetic field in ideal MHD, Callebaut (2006), with an azimuthal velocity which is function of r and θ only (spherical coordinates) is applied to a bipolar magnetic seed field and to a quadripolar field. Resistivity and α-effect are not yet taken into account, but the extensions are possible. From the surface observations we had derived an approximate analytic expression for the differential rotation in order to work fully analytically in the application. Qualitatively the results for a quadripolar field are as for a bipolar seed field. The main features are the same: for some latitudes the field may increase by two orders of magnitude, the separation between sunspots and polar faculae is clearcut, there is, relatively speaking, a too strong amplification in the polar regions (the latter occurs in other models too). The hypothesis that the seed fields are situated at the tachocline is not required: the amplification is active throughout the whole convective zone, albeit with different strengths, and thus during the transit of the flux tubes from tachocline to the solar surface too.
Magnetically insulated ion flow theory
Ottinger, P.F.; Schumer, J.W.
2006-06-15
Initiation of an anode plasma and ion emission into a magnetically insulated transmission line can cause serious current losses unless the ions are magnetically insulated as well as the electrons. A model for magnetically insulated ion flow in a vacuum transmission line is developed. Particle-in-cell simulations are presented that show that this model accurately predicts properties of this flow. The model is applied to determine the current required to magnetically insulate ion flow for a given voltage and vacuum line impedance. Relevance of this work to system designs for Z-pinch-driven inertial confinement fusion is discussed.
Magnetic Catalysis in Graphene Effective Field Theory
NASA Astrophysics Data System (ADS)
DeTar, Carleton; Winterowd, Christopher; Zafeiropoulos, Savvas
2016-12-01
We report on the first calculation of magnetic catalysis at zero temperature in a fully nonperturbative simulation of the graphene effective field theory. Using lattice gauge theory, a nonperturbative analysis of the theory of strongly interacting, massless, (2 +1 )-dimensional Dirac fermions in the presence of an external magnetic field is performed. We show that in the zero-temperature limit, a nonzero value for the chiral condensate is obtained which signals the spontaneous breaking of chiral symmetry. This result implies a nonzero value for the dynamical mass of the Dirac quasiparticle.
A new theory of lunar magnetism.
Runcorn, S K; Urey, H C
1973-05-11
In the hypothesis advanced here it is supposed that the field, in which rocks at the lunar surface acquired the remanent magnetization found through the Apollo project, arose from permanent magnetization of the deep interior of the moon. This theory involves the assumption that the moon, apart from a surface shell, accreted cold and remained below the Curie point of iron until sometime later than 3 x 10(9) years ago. The magnetization was acquired as the moon formed in a gas sphere in the strong magnetic field of the early sun.
Theory of orbital magnetization in disordered systems
NASA Astrophysics Data System (ADS)
Zhu, Guobao; Yang, Shengyuan A.; Fang, Cheng; Liu, W. M.; Yao, Yugui
2012-12-01
We present a general formula of the orbital magnetization of disordered systems based on the Keldysh Green's function theory in the gauge-covariant Wigner space. In our approach, the gauge invariance of physical quantities is ensured from the very beginning, and the vertex corrections are easily included. Our formula applies not only for insulators but also for metallic systems where the quasiparticle behavior is usually strongly modified by the disorder scattering. In the absence of disorders, our formula recovers the previous results obtained from the semiclassical theory and the perturbation theory. As an application, we calculate the orbital magnetization of a weakly disordered two-dimensional electron gas with Rashba spin-orbit coupling. We find that for the short-range disorder scattering, its major effect is to the shifting of the distribution of orbital magnetization corresponding to the quasiparticle energy renormalization.
Stellar magnetic structure and activity /theory/
NASA Astrophysics Data System (ADS)
Weiss, N. O.
Both the overall behavior of the solar cycle and the underlying fine structure of magnetic fields in the sun have been studied mathematically in some detail. These theories are summarized and different phenomenological models of the solar cycle are reviewed. In order to provide a description of the magnetic fields in late-type stars it is necessary to extrapolate boldly from what is known about the sun. In this way field strengths and configurations can be estimated.
Magnetic Soliton, Homotopy and Higgs Theory,
1986-04-24
OD-AL67 366 NAGETIC SOLITON ONOTOPY ND HIGGS THEORY(U) FOREIGNI n1/ 1TECHNOLOGY D V NRIGHT-PATTERSON AFD ON Y LI ET AL. UNCLSSIIED24 APR 86 FTD-ID...MAGNETIC SOLITON, HOMOTOPY AND HIGGS THEORY by Li Yuanjie and Lei Shizu *. . * . .%..**% . . .-..C./ ~~~Approved for public release; -," Distribution...HOMOTOPY AND HIGGS THEORY By: Li Yuanjie and Lei Shizu English pages: 9 Source: Huazhong Gongxueyuan Xuebao, Vol. 11, Nr. 6, 1983, pp. 65-70 Country of
Magnetic fields and density functional theory
Salsbury Jr., Freddie
1999-02-01
A major focus of this dissertation is the development of functionals for the magnetic susceptibility and the chemical shielding within the context of magnetic field density functional theory (BDFT). These functionals depend on the electron density in the absence of the field, which is unlike any other treatment of these responses. There have been several advances made within this theory. The first of which is the development of local density functionals for chemical shieldings and magnetic susceptibilities. There are the first such functionals ever proposed. These parameters have been studied by constructing functionals for the current density and then using the Biot-Savart equations to obtain the responses. In order to examine the advantages and disadvantages of the local functionals, they were tested numerically on some small molecules.
Effective field theory for magnetic compactifications
NASA Astrophysics Data System (ADS)
Buchmuller, Wilfried; Dierigl, Markus; Dudas, Emilian; Schweizer, Julian
2017-04-01
Magnetic flux plays an important role in compactifications of field and string theories in two ways, it generates a multiplicity of chiral fermion zero modes and it can break supersymmetry. We derive the complete four-dimensional effective action for N = 1 supersymmetric Abelian and non-Abelian gauge theories in six dimensions compactified on a torus with flux. The effective action contains the tower of charged states and it accounts for the mass spectrum of bosonic and fermionic fields as well as their level-dependent interactions. This allows us to compute quantum corrections to the mass and couplings of Wilson lines. We find that the one-loop corrections vanish, contrary to the case without flux. This can be traced back to the spontaneous breaking of symmetries of the six-dimensional theory by the background gauge field, with the Wilson lines as Goldstone bosons.
Effects of non-linearities on magnetic field generation
Nalson, Ellie; Malik, Karim A.; Christopherson, Adam J. E-mail: achristopherson@gmail.com
2014-09-01
Magnetic fields are present on all scales in the Universe. While we understand the processes which amplify the fields fairly well, we do not have a ''natural'' mechanism to generate the small initial seed fields. By using fully relativistic cosmological perturbation theory and going beyond the usual confines of linear theory we show analytically how magnetic fields are generated. This is the first analytical calculation of the magnetic field at second order, using gauge-invariant cosmological perturbation theory, and including all the source terms. To this end, we have rederived the full set of governing equations independently. Our results suggest that magnetic fields of the order of 10{sup -30}- 10{sup -27} G can be generated (although this depends on the small scale cut-off of the integral), which is largely in agreement with previous results that relied upon numerical calculations. These fields are likely too small to act as the primordial seed fields for dynamo mechanisms.
THEORY OF MAGNETIC AND ELECTRONIC PROPERTIES OF SOLIDS.
The research dealt with Field theory in one spatial dimension; Magnetic semiconductors; and Anomalous (Kondo-type) effects of magnetic impurities in non magnetic metals. A summary of our findings is given as well as a bibliography of 14 scientific publications which have resulted from this work. (Author)
Magnetic Multipoles in Theory and Practice.
ERIC Educational Resources Information Center
Smith, D. G.
1980-01-01
A magnetic multipole apparatus suitable for the physics teaching laboratory is described. The apparatus enables the student to measure the magnetic field configuration of a single large coil, and of systems of one or more small coils. (Author/DS)
Magnetism of metals in the dynamic spin-fluctuation theory
NASA Astrophysics Data System (ADS)
Melnikov, N. B.; Reser, B. I.
2016-12-01
We overview new developments in spin-fluctuation theory, which describes magnetic properties of ferromagnetic metals at finite temperatures. We present a detailed analysis of the underlying techniques and compare numerical results with experiment.
NASA Astrophysics Data System (ADS)
Hanke, J.-P.; Freimuth, F.; Nandy, A. K.; Zhang, H.; Blügel, S.; Mokrousov, Y.
2016-09-01
We address the importance of the modern theory of orbital magnetization for spintronics. Based on an all-electron first-principles approach, we demonstrate that the predictive power of the routinely employed "atom-centered" approximation is limited to materials like elemental bulk ferromagnets, while the application of the modern theory of orbital magnetization is crucial in chemically or structurally inhomogeneous systems such as magnetic thin films, and materials exhibiting nontrivial topology in reciprocal and real space, e.g., Chern insulators or noncollinear systems. We find that the modern theory is particularly crucial for describing magnetism in a class of materials that we suggest here—topological orbital ferromagnets.
The theory of an active magnetic regenerative refrigerator
NASA Technical Reports Server (NTRS)
Barclay, J. A.
1983-01-01
The adiabatic temperature change with field which is limited to about 2 K/Tesla for ferromagnets near their Curie temperatures by the change of magnetization with temperature and the lattice heat capacity is discussed. Practical magnetic refrigerators operate on a regenerative cycle such as the Brayton cycle. This cycle can be executed through the use of an active magnetic regenerator, i.e., a regenerator composed of magnetic material that is cycled in an out of a magnetic field with appropriate fluid flows. The theory of these devices is predicted by solving the partial differential equations that describe fluid and the magnetic solid. The active magnetic regenerator is described along with the method of calculation. Temperature profiles for a normal regenerator and a magnetic regenerative refrigerator are shown.
TOPICS IN THE THEORY OF MAGNETIC METALS,
transition metals is reviewed. (3) Antiferromagnetic in simple metals is discussed. (4) The distribution of magnetism and superconductivity in the periodic ... table is treated qualitatively. (5) The electronic configuration of the alkali metals is discussed in relation to their optical properties, phase diagrams and the possibility of magnetism. (Author)
Theory of magnetic oscillations in Weyl semimetals
NASA Astrophysics Data System (ADS)
Ashby, Phillip E. C.; Carbotte, Jules P.
2014-04-01
Weyl semimetals are a new class of Dirac material that possesses bulk energy nodes in three dimensions, in contrast to two dimensional graphene. In this paper, we study a Weyl semimetal subject to an applied magnetic field. We find distinct behavior that can be used to identify materials containing three dimensional Dirac fermions. We derive expressions for the density of states, electronic specific heat, and the magnetization. We focus our attention on the quantum oscillations in the magnetization. We find phase shifts in the quantum oscillations that distinguish the Weyl semimetal from conventional three dimensional Schrödinger fermions, as well as from two dimensional Dirac fermions. The density of states as a function of energy displays a sawtooth pattern which has its origin in the dispersion of the three dimensional Landau levels. At the same time, the spacing in energy of the sawtooth spike goes like the square root of the applied magnetic field which reflects the Dirac nature of the fermions. These features are reflected in the specific heat and magnetization. Finally, we apply a simple model for disorder and show that this tends to damp out the magnetic oscillations in the magnetization at small fields.
Theory of Magnetization in Bloch Electron Systems
NASA Astrophysics Data System (ADS)
Ogata, Masao
2017-04-01
The exact formulas for magnetization and magnetic susceptibility are derived for Bloch electrons in terms of Bloch wave functions. They are extensions of the previous work to general cases where the spin-orbit interaction as well as the Zeeman term is included, the potential is noncentrosymmetric, and the time-reversal symmetry is broken. The obtained magnetization for Bloch electrons is a natural generalization of the free-electron magnetic moment including the effect of the spin-orbit interaction. The obtained susceptibility has six contributions and the physical meaning of each term is clarified. The new formula contains the Landau-Peierls, Pauli, and Van Vleck susceptibilities, the atomic diamagnetism, and contributions from the "Berry curvature". In the atomic limit, the obtained formula reduces to two contributions: the atomic diamagnetism and a generalized form of the Van Vleck susceptibility modified by the spin-orbit interaction. It is also found that, in general cases, the Pauli, Van Vleck (interband), and Berry curvature susceptibilities are closely related to common magnetization matrix elements, which is in sharp contrast to previous studies. A general form of the off-diagonal magnetic susceptibility is also derived.
Ab-initio theory of spin fluctuations in magnets
NASA Astrophysics Data System (ADS)
Antropov, Vladimir; Ke, Liqin; van Schilfgaarde, Mark; Katsnelson, Mikhael
2011-03-01
We propose a framework for a true ab initio theory of magnetism, based on many-body perturbation theory (MPBT). It fits in naturally with methods based MPBT such as the GW approximation; but the approach can be implemented as an extension to any existing static method for electronic structure such as the local spin density approximation to density functional theory, to include spin fluctuations. Initially we calculated the spin fluctuation contributions using random phase approximation. The self consistency procedure similar to the one used in Moryia-Kawabata theory can be naturally implemented. The fluctuation dissipation theorem is used to calculate the reduction of the mean field magnetic moment in itinerant magnets. The applications of the technique includes traditional 3d ferromagnetic metals, their alloys and compounds and 5f systems.
Nuclear Magnetic Resonance and the BCS Theory
NASA Astrophysics Data System (ADS)
Slichter, Charles P.
The author describes the inspiration for the experiment by Hebel and Slichter to measure the nuclear spin-lattice relaxation time in super-conductors, the design considerations for the experiment, the surprising experimental results, their theoretical treatment using the Bardeen-Cooper-Schrieffer theory, and how comparing the nuclear relaxation results with those for ultrasound absorption confirmed the central idea of the BCS theory, the BCS pair wave function.
Asymptotic theory of relativistic, magnetized jets
Lyubarsky, Yuri
2011-01-15
The structure of a relativistically hot, strongly magnetized jet is investigated at large distances from the source. Asymptotic equations are derived describing collimation and acceleration of the externally confined jet. Conditions are found for the transformation of the thermal energy into the fluid kinetic energy or into the Poynting flux. Simple scalings are presented for the jet collimation angle and Lorentz factors.
Asymptotic theory of relativistic, magnetized jets.
Lyubarsky, Yuri
2011-01-01
The structure of a relativistically hot, strongly magnetized jet is investigated at large distances from the source. Asymptotic equations are derived describing collimation and acceleration of the externally confined jet. Conditions are found for the transformation of the thermal energy into the fluid kinetic energy or into the Poynting flux. Simple scalings are presented for the jet collimation angle and Lorentz factors.
Magnetic properties of a nanoribbon: An effective-field theory
NASA Astrophysics Data System (ADS)
Wang, Jiu-Ming; Jiang, Wei; Zhou, Chen-Long; Shi, Zuo; Wu, Chuang
2017-02-01
An effective-field theory is proposed to study magnetic properties of a nanoribbon. The model consists of a core spin-3/2 and shell spin-2 with a ferrimagnetic exchange coupling, which is described by transverse Ising model with the anisotropy. Based on the differential operator technique, the magnetization and the susceptibility formulas of the nanoribbon are given. Numerical results of the magnetization, the susceptibility, the hysteresis loop of the system are discussed for specific values of the parameters. Magnetization plateaus exhibits on the magnetization curves at low temperature. The exchange coupling, the anisotropy and the transverse field have important roles in the magnetic properties for the nanoribbon. Results may provide some guidance to design in the nanoribbons.
Neutral Vlasov kinetic theory of magnetized plasmas
Tronci, Cesare; Camporeale, Enrico
2015-02-15
The low-frequency limit of Maxwell equations is considered in the Maxwell-Vlasov system. This limit produces a neutral Vlasov system that captures essential features of plasma dynamics, while neglecting radiation effects. Euler-Poincaré reduction theory is used to show that the neutral Vlasov kinetic theory possesses a variational formulation in both Lagrangian and Eulerian coordinates. By construction, the new model recovers all collisionless neutral models employed in plasma simulations. Then, comparisons between the neutral Vlasov system and hybrid kinetic-fluid models are presented in the linear regime.
Dudley's dilemma: Magnetic moments in relativistic theories
NASA Astrophysics Data System (ADS)
McNeil, J. A.
1986-10-01
In 1975 L. Dudley Miller showed how the basic phenomenology of the major shell and spin-orbit splittings constrained the relativistic scalar/vector structure model to values of the potentials incompatible with the observed magnetic moments of nuclei one nucleon away from closed shell [1]. In this talk the resolution of this problem is presented from three different perspectives. First a self-consistent Landau-Migdal approach is used to define the single particle isoscalar current in infinite nuclear matter. The constraint of self-consistency provides a vector suppression factor to the single particle current which returns the current to its nonrelativistic form and resolves the problem. The same suppression factor is shown to follow as well from either a consideration of gauge invariance or (equivalently) the relativistic random phase approximation. Local density approximation calculations of isoscalar magnetic moments of nuclei one nucleon away from closed shell recover the Schmidt values, thus resolving this longstanding problem.
Lattice Study of Magnetic Catalysis in Graphene Effective Field Theory
NASA Astrophysics Data System (ADS)
Winterowd, Christopher; Detar, Carleton; Zafeiropoulos, Savvas
2016-03-01
The discovery of graphene ranks as one of the most important developments in condensed matter physics in recent years. As a strongly interacting system whose low-energy excitations are described by the Dirac equation, graphene has many similarities with other strongly interacting field theories, particularly quantum chromodynamics (QCD). Graphene, along with other relativistic field theories, have been predicted to exhibit spontaneous symmetry breaking (SSB) when an external magnetic field is present. Using nonperturbative methods developed to study QCD, we study the low-energy effective field theory (EFT) of graphene subject to an external magnetic field. We find strong evidence supporting the existence of SSB at zero-temperature and characterize the dependence of the chiral condensate on the external magnetic field. We also present results for the mass of the Nambu-Goldstone boson and the dynamically generated quasiparticle mass that result from the SSB.
Two-Fluid Theory for Spin Superfluidity in Magnetic Insulators
NASA Astrophysics Data System (ADS)
Flebus, Benedetta; Bender, Scott; Tserkovnyak, Yaroslav; Duine, Rembert; UU Team; UCLA Team
We investigate coupled spin and heat transport in easy-plane magnetic insulators. These materials display a continuous phase transition between normal and condensate states that is controlled by an external magnetic field. Using hydrodynamic equations supplemented by Gross-Pitaevski phenomenology and magnetoelectric circuit theory, we derive a two-fluid model to describe the dynamics of thermal and condensed magnons, and the appropriate boundary conditions in a hybrid normal-metal|magnetic-insulator|normal-metal heterostructure. We discuss how the emergent spin superfluidity can be experimentally probed via a spin Seebeck effect measurement.
Two-Fluid Theory for Spin Superfluidity in Magnetic Insulators
NASA Astrophysics Data System (ADS)
Flebus, B.; Bender, S. A.; Tserkovnyak, Y.; Duine, R. A.
2016-03-01
We investigate coupled spin and heat transport in easy-plane magnetic insulators. These materials display a continuous phase transition between normal and condensate states that is controlled by an external magnetic field. Using hydrodynamic equations supplemented by Gross-Pitaevski phenomenology and magnetoelectric circuit theory, we derive a two-fluid model to describe the dynamics of thermal and condensed magnons, and the appropriate boundary conditions in a hybrid normal-metal-magnetic-insulator-normal-metal heterostructure. We discuss how the emergent spin superfluidity can be experimentally probed via a spin Seebeck effect measurement.
Magnetic Alignment Theory and the Interpretation of Polarization
NASA Technical Reports Server (NTRS)
Cugnon, P.
1984-01-01
The theory of magnetic spinning alignment is outlined. It is shown that a classical enhanced thermal Davis and Greenstein mechanism may be considered as a limiting case of Purcell's suprathermal spinning alignment theory when the number of active sites on a given grain becomes very important and/or for very short lived suprathermal sites. The results are applied to the interstellar medium and some problems in which it may be important are discussed.
Effective-medium theory for anisotropic magnetic metamaterials
NASA Astrophysics Data System (ADS)
Jin, Junfeng; Liu, Shiyang; Lin, Zhifang; Chui, S. T.
2009-09-01
We have developed an effective-medium theory within the coherent-potential approximation, which is especially suitable to retrieve the effective constitutive parameters (permittivity and permeability) of the anisotropic magnetic metamaterials consisting of the ferrite rods. The anisotropy originates from the gyromagnetic property of the ferrite material whose permeability is a tensor with nonzero off-diagonal components. To confirm the validity of our method the photonic band structures of the two-dimensional periodic magnetic metamaterials are calculated, which are in agreement with the effective-medium theory in the long wavelength limit, in addition, even when a/λ0˜0.4 the effective-medium theory can still be applied, where a and λ0 are the lattice constant and the vacuum wavelength, respectively. The simulations on the electric field patterns for a plane wave illuminated on the magnetic metamaterials and the equal-size effective scattering objects are performed, the results corroborate the effectiveness of the effective-medium theory once again. We also perform the simulation for the metamaterial composed of disordered ferrite rods, which is still in agreement with the effective-medium theory, suggesting the powerfulness of the effective-medium theory. Moreover, our results suggest that the anisotropy must be considered exactly in order to retrieve the effective constitutive parameters accurately.
Magnetic expansion of Nekrasov theory: The SU(2) pure gauge theory
He Wei; Miao Yangang
2010-07-15
It is recently claimed by Nekrasov and Shatashvili that the N=2 gauge theories in the {Omega} background with {epsilon}{sub 1}=({h_bar}/2{pi}), {epsilon}{sub 2}=0 are related to the quantization of certain algebraic integrable systems. We study the special case of SU(2) pure gauge theory; the corresponding integrable model is the A{sub 1} Toda model, which reduces to the sine-Gordon quantum mechanics problem. The quantum effects can be expressed as the WKB series written analytically in terms of hypergeometric functions. We obtain the magnetic and dyonic expansions of the Nekrasov theory by studying the property of hypergeometric functions in the magnetic and dyonic regions on the moduli space. We also discuss the relation between the electric-magnetic duality of gauge theory and the action-action duality of the integrable system.
Theory of ferromagnetism in diluted magnetic semiconductor heterostructures
NASA Astrophysics Data System (ADS)
Lee, Byounghak
In this thesis, we explore the magnetic and transport properties of ferromagnetic semiconductors and their heterostructures. First, we introduce diluted magnetic semiconductors and their properties. The unique features of diluted magnetic semiconductors are originated from their position in the parameter space. We compare diluted magnetic semiconductors with similar physical systems and discuss the difference between them. Next, we briefly review the background theories, such as k · p methods, density functional methods, and envelope function methods. These methods serve as the background on which our theory is based. Third, we develop a mean-field theory in the framework of the self-consistent field method. Our approach improves the RKKY model, allowing spatial inhomogeneity of the system, free-carrier spin polarization, finite temperatures, and free-carrier exchange and correlation to be accounted for self-consistently. As an example, we calculate the electronic structure of a MnxGa 1-xAs/GaAs superlattice with alternating ferromagnetic and paramagnetic layers and demonstrate the possibility of semiconductor magnetoresistance systems with designed properties. Fourth, we present a mean-field theory of ferromagnetism in diluted magnetic semiconductor quantum wells. Ignoring subband mixing due to exchange interactions between quantum well free carriers and magnetic impurities, we obtain an analytic result for the dependence of the critical temperature and the spontaneous magnetization on the distribution of magnetic impurities and on the quantum well width. Fifth, we predict that a novel bias-voltage assisted magnetization reversal process will occur in Mn doped II-VI semiconductor quantum wells or heterojunctions with carrier induced ferromagnetism. We analyze the interplay between kinetic exchange coupling and quantum confinement effects, and show that magnetization reversal is due to strong exchange-coupling induced subband mixing. Finally, we apply the mean
The Theory of Magnetic Reconnection: Past, Present, and Future
NASA Astrophysics Data System (ADS)
Cassak, P. A.
2008-05-01
Magnetic reconnection underlies the energy release observed in eruptive events in the solar corona (such as solar flares and coronal mass ejections) and in the Earth's magnetosphere. The theory of magnetic reconnection was originally developed to understand observations by Ron Giovanelli, who discovered that solar flares occur where the coronal magnetic field changes directions. Pioneers in space plasma theory such as James Dungey, Peter Sweet, Eugene Parker, and Harry Petschek first elucidated the underlying physical effects that lead to this massive energy release. Since then, much effort has been made to understand what process or processes cause magnetic reconnection to be fast enough to be consistent with observations, such as anomalous resistivity, secondary instabilities, and the Hall effect. However, a thorough understanding of this important process remains a topic of intense study. In celebration of the 50th anniversary of Parker's paper predicting the high-speed solar wind, this talk will review the history of the theory of magnetic reconnection. The present status of the field will be discussed, and remaining unanswered questions will be summarized.
Theory of neutron scattering by electrons in magnetic materials
NASA Astrophysics Data System (ADS)
Lovesey, S. W.
2015-10-01
A theory of neutron scattering by magnetic materials is reviewed with emphasis on the use of electronic multipoles that have universal appeal, because they are amenable to calculation and appear in theories of many other experimental techniques. The conventional theory of magnetic neutron scattering, which dates back to Schwinger (1937 Phys. Rev. 51 544) and Trammell (1953 Phys. Rev. 92 1387), yields an approximation for the scattering amplitude in terms of magnetic dipoles formed with the spin (S) and orbital angular momentum (L) of valence electrons. The so-called dipole-approximation has been widely adopted by researchers during the past few decades that has seen neutron scattering develop to its present status as the method of choice for investigations of magnetic structure and excitations. Looking beyond the dipole-approximation, however, reveals a wealth of additional information about electronic degrees of freedom conveniently encapsulated in magnetic multipoles. In this language, the dipole-approximation retains electronic axial dipoles, S and L. At the same level of approximation are polar dipoles—called anapoles or toroidal dipoles—allowed in the absence of a centre of inversion symmetry. Anapoles are examples of magneto-electric multipoles, time-odd and parity-odd irreducible tensors, that have come to the fore as signatures of electronic complexity in materials.
Magnetic and antimagnetic rotation in covariant density functional theory
Zhao, P. W.; Liang, H. Z.; Peng, J.; Ring, P.; Zhang, S. Q.; Meng, J.
2012-10-20
Progress on microscopic and self-consistent description of the magnetic rotation and antimagnetic rotation phenomena in tilted axis cranking relativistic mean-field theory based on a point-coupling interaction are briefly reviewed. In particular, the microscopic pictures of the shears mechanism in {sup 60}Ni and the two shears-like mechanism in {sup 105}Cd are discussed.
Magnetic Elements at Finite Temperature and Large Deviation Theory
NASA Astrophysics Data System (ADS)
Kohn, R. V.; Reznikoff, M. G.; vanden-Eijnden, E.
2005-08-01
We investigate thermally activated phenomena in micromagnetics using large deviation theory and concepts from stochastic resonance. We give a natural mathematical definition of finite-temperature astroids, finite-temperature hysteresis loops, etc. Generically, these objects emerge when the (generalized) Arrhenius timescale governing the thermally activated barrier crossing event of magnetic switching matches the timescale at which the magnetic element is pulsed or ramped by an external field; in the special and physically relevant case of multiple-pulse experiments, on the other hand, short-time switching can lead to non-Arrhenius behavior. We show how large deviation theory can be used to explain some properties of the astroids, like their shrinking and sharpening as the number of applied pulses is increased. We also investigate the influence of the dynamics, in particular the relative importance of the gyromagnetic and the damping terms. Finally, we discuss some issues and open questions regarding spatially nonuniform magnetization.
Rock magnetism: Studies in theory, data manipulation, and application
NASA Astrophysics Data System (ADS)
Carter-Stiglitz, Brian Scott
This collection of studies in rock magnetism encompasses theoretical modeling, data analysis, and an environmental magnetism application. Rock magnetism is essentially the study of the magnetic properties of naturally occurring material for application in the geosciences or for its own sake as a basic science. Over the last two decades low-temperature magnetic measurements have become common in rock magnetism studies. Some applications include identification and quantification of specific target minerals and nanophase material (ferrimagnetic or antiferromagnetic); low-temperature demagnetization (in, e.g., paleointensity studies); applications in biomagnetism (e.g., the magnetic detection of magnetotactic bacteria). Despite the wide range of applications a strong theoretical understanding of the low-temperature magnetism of even the most common terrestrial magnetic materials has been slow in coming. Theoretical modeling of the low-temperature magnetic behavior of single-domain magnetite (chapters 1 and 2) yield results that are in general agreement with empirical data. In particular magnetosome chains (as produced by magnetotactic bacteria) are studied. The observed magnetic behavior can be explained, at least in part, by maghemitization. A new study of the low-temperature magnetic properties of multidomain magnetite (chapter 4) shows behavior that seems to be a unique to "true" multidomain magnetite; this is explained with a combination of "twinning pinning" and easy axis bias effects. A new technique to "unmix" mixtures of magnetic materials (chapter 5) is based on fitting data with empirical basis functions, assuming linearity of magnetic moments as a function of mixing ratio. This assumption and the technique are successfully tested on a set of artificial mixtures. The technique is then applied to a small set of natural samples. Finally an environmental magnetism study of Argentine loess (chapter 6) combines rock magnetic measurements, geochemistry, and
Holographic equilibration in confining gauge theories under external magnetic fields
NASA Astrophysics Data System (ADS)
Demircik, T.; Gürsoy, U.
2017-06-01
We investigate the effect of external magnetic fields on equilibration in the improved holographic QCD theory in the deconfined phase using the AdS/CFT correspondence. In particular we calculate the quasinormal mode spectra in the corresponding black brane solutions and study their dependence on temperature, momentum and magnetic field, both in the scalar and the shear channels. We find complex patterns in the motion of quasinormal modes on the complex plane, including certain cross overs between the lowest lying modes under varying magnetic field, momentum and temperature. We also discover a critical value of the magnetic field Bc above which the hydrodynamic approximation breaks down, as the imaginary part of the first excited quasi-normal mode in the shear channel becomes smaller than that of the hydro mode.
On the theory of phase transitions in magnetic fluids
Zubarev, A. Yu. Iskakova, L. Yu.
2007-11-15
Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.
Lattice field theory studies of magnetic catalysis in graphene
NASA Astrophysics Data System (ADS)
Winterowd, Christopher R.
Consisting of a single two-dimensional layer of Carbon atoms arranged in a hexagonal lattice, graphene represents one of the most exciting recent developments in condensed matter physics. With novel electronic and mechancial properties, graphene not only has great potential with respect to technological applications, but also displays phenomena that typically appear in relativistic quantum field theory. The low-energy electronic excitations of graphene consist of two identical species of massless Dirac particles. Due to the small Fermi velocity, these particles are strongly coupled through the Coulomb interaction. Although various perturbative approaches have succeeded in elucidating many of the electronic properties of graphene, one would still like a nonperturbative study to address various questions. In particular, the spontaneous breaking of chiral symmetry in the presence of an external magnetic field, commonly known as magnetic catalysis, is one of these questions. Early studies of this phenomenon in model relativistic field theories have posited the mechanism to be universal. More recently, this mechanism of spontaneous symmetry breaking has been studied in low-dimensional condensed matter systems. Due to the strongly-coupled nature of the low-energy effective field theory of graphene, nonperturbative methods of lattice gauge theory can be used which are well suited to studying chiral symmetry breaking. Most notably used to study the theory of the strong interactions, quantum chromodynamics, these methods have proven successful in elucidating nonperturbative phenomena in cases where perturbative methods fail. In this thesis, using these methods, evidence in favor of magnetic catalysis in the graphene effective field theory will be presented.
Planetary radio emissions from low magnetic latitudes - Observations and theories
NASA Astrophysics Data System (ADS)
Jones, Dyfrig
Recent observations of planetary radiations from low magnetic latitudes are reviewed. At Earth a major source of nonthermal continuum is Terrestrial Myriametric Radiation (TMR) from the equatorial plasmapause and from the magnetopause. The theories proposed for the production of TMR are listed and their predictions are compared with satellite observations. The application of the theories to Jovian Kilometric Radiation (KOM), the radio emission at Jupiter which has been suggested to be the analogue of TMR, is reviewed. The implications of the TMR and KOM results for radiations observed at Saturn and Uranus are briefly considered.
Random matrix theory in biological nuclear magnetic resonance spectroscopy.
Lacelle, S
1984-01-01
The statistical theory of energy levels or random matrix theory is presented in the context of the analysis of chemical shifts of nuclear magnetic resonance (NMR) spectra of large biological systems. Distribution functions for the spacing between nearest-neighbor energy levels are discussed for uncorrelated, correlated, and random superposition of correlated energy levels. Application of this approach to the NMR spectra of a vitamin, an antibiotic, and a protein demonstrates the state of correlation of an ensemble of energy levels that characterizes each system. The detection of coherent and dissipative structures in proteins becomes feasible with this statistical spectroscopic technique. PMID:6478032
Magnetic MAX phases from theory and experiments; a review.
Ingason, A S; Dahlqvist, M; Rosen, J
2016-11-02
This review presents MAX phases (M is a transition metal, A an A-group element, X is C or N), known for their unique combination of ceramic/metallic properties, as a recently uncovered family of novel magnetic nanolaminates. The first created magnetic MAX phases were predicted through evaluation of phase stability using density functional theory, and subsequently synthesized as heteroepitaxial thin films. All magnetic MAX phases reported to date, in bulk or thin film form, are based on Cr and/or Mn, and they include (Cr,Mn)2AlC, (Cr,Mn)2GeC, (Cr,Mn)2GaC, (Mo,Mn)2GaC, (V,Mn)3GaC2, Cr2AlC, Cr2GeC and Mn2GaC. A variety of magnetic properties have been found, such as ferromagnetic response well above room temperature and structural changes linked to magnetic anisotropy. In this paper, theoretical as well as experimental work performed on these materials to date is critically reviewed, in terms of methods used, results acquired, and conclusions drawn. Open questions concerning magnetic characteristics are discussed, and an outlook focused on new materials, superstructures, property tailoring and further synthesis and characterization is presented.
Electrical polarization and orbital magnetization: the modern theories.
Resta, Raffaele
2010-03-31
Macroscopic polarization P and magnetization M are the most fundamental concepts in any phenomenological description of condensed media. They are intensive vector quantities that intuitively carry the meaning of dipole per unit volume. But for many years both P and the orbital term in M evaded even a precise microscopic definition, and severely challenged quantum-mechanical calculations. If one reasons in terms of a finite sample, the electric (magnetic) dipole is affected in an extensive way by charges (currents) at the sample boundary, due to the presence of the unbounded position operator in the dipole definitions. Therefore P and the orbital term in M--phenomenologically known as bulk properties--apparently behave as surface properties; only spin magnetization is problemless. The field has undergone a genuine revolution since the early 1990s. Contrary to a widespread incorrect belief, P has nothing to do with the periodic charge distribution of the polarized crystal: the former is essentially a property of the phase of the electronic wavefunction, while the latter is a property of its modulus. Analogously, the orbital term in M has nothing to do with the periodic current distribution in the magnetized crystal. The modern theory of polarization, based on a Berry phase, started in the early 1990s and is now implemented in most first-principle electronic structure codes. The analogous theory for orbital magnetization started in 2005 and is partly work in progress. In the electrical case, calculations have concerned various phenomena (ferroelectricity, piezoelectricity, and lattice dynamics) in several materials, and are in spectacular agreement with experiments; they have provided thorough understanding of the behaviour of ferroelectric and piezoelectric materials. In the magnetic case the very first calculations are appearing at the time of writing (2010). Here I review both theories on a uniform ground in a density functional theory (DFT) framework, pointing out
Cheng, Lan; Xiao, Yunlong; Liu, Wenjian
2009-12-28
It is recognized only recently that the incorporation of the magnetic balance condition is absolutely essential for four-component relativistic theories of magnetic properties. Another important issue to be handled is the so-called gauge problem in calculations of, e.g., molecular magnetic shielding tensors with finite bases. It is shown here that the magnetic balance can be adapted to distributed gauge origins, leading to, e.g., magnetically balanced gauge-including atomic orbitals (MB-GIAOs) in which each magnetically balanced atomic orbital has its own local gauge origin placed on its center. Such a MB-GIAO scheme can be combined with any level of theory for electron correlation. The first implementation is done here at the coupled-perturbed Dirac-Kohn-Sham level. The calculated molecular magnetic shielding tensors are not only independent of the choice of gauge origin but also converge rapidly to the basis set limit. Close inspections reveal that (zeroth order) negative energy states are only important for the expansion of first order electronic core orbitals. Their contributions to the paramagnetism are therefore transferable from atoms to molecule and are essentially canceled out for chemical shifts. This allows for simplifications of the coupled-perturbed equations.
Relativistic transport theory for a two-temperature magnetized plasma
Metens, T.; Balescu, R. )
1990-09-01
The relativistic kinetic theory of linear transport is worked out within the framework of a new moment method. A complete analytical study of the transport in a two-temperature inhomogeneous magnetized fusion plasma is given. The transport relations and coefficients are derived from the kinetic equation with the full relativistic Beliaev--Budker collision operator and the impact of relativistic effects on the confinement are investigated.
Disorder-induced magnetic memory: Experiments and theories
NASA Astrophysics Data System (ADS)
Pierce, M. S.; Buechler, C. R.; Sorensen, L. B.; Kevan, S. D.; Jagla, E. A.; Deutsch, J. M.; Mai, T.; Narayan, O.; Davies, J. E.; Liu, Kai; Zimanyi, G. T.; Katzgraber, H. G.; Hellwig, O.; Fullerton, E. E.; Fischer, P.; Kortright, J. B.
2007-04-01
Beautiful theories of magnetic hysteresis based on random microscopic disorder have been developed over the past ten years. Our goal was to directly compare these theories with precise experiments. To do so, we first developed and then applied coherent x-ray speckle metrology to a series of thin multilayer perpendicular magnetic materials. To directly observe the effects of disorder, we deliberately introduced increasing degrees of disorder into our films. We used coherent x rays, produced at the Advanced Light Source at Lawrence Berkeley National Laboratory, to generate highly speckled magnetic scattering patterns. The apparently “random” arrangement of the speckles is due to the exact configuration of the magnetic domains in the sample. In effect, each speckle pattern acts as a unique fingerprint for the magnetic domain configuration. Small changes in the domain structure change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed. Our experiments quickly answered one long-standing question: How is the magnetic domain configuration at one point on the major hysteresis loop related to the configurations at the same point on the loop during subsequent cycles? This is called microscopic return-point memory (RPM). We found that the RPM is partial and imperfect in the disordered samples, and completely absent when the disorder is below a threshold level. We also introduced and answered a second important question: How are the magnetic domains at one point on the major loop related to the domains at the complementary point, the inversion symmetric point on the loop, during the same and during subsequent cycles? This is called microscopic complementary-point memory (CPM). We found that the CPM is also partial and imperfect in the disordered samples and completely absent when the disorder is not present. In addition, we found that the RPM is always a little larger than the
Micromagnetic theory of antiferromagnetically coupled magnetic recording media
NASA Astrophysics Data System (ADS)
Schabes, Manfred
2001-03-01
The micromagnetic theory of antiferromagnetically coupled magnetic recording media (AFC media) is discussed for the case of AFC media consisting of two ferromagnetic layers and a polarization layer.[1-2] Attention is focused on AFC media where the top layer governs the remanent magnetization while the bottom layer acts as a slave layer. Micromagnetic simulations of recorded bit transitions demonstrate a good anticorrelation of the layer magnetizations at bit transitions for this case. Calculation of MR readback pulses shows that these AFC media have pulse widths of a thin single layer magnetic recording medium with an effective MrT that is approximately given by the difference in MrT's of the top and bottom layers. Magnetic stability is set to first order by the thicker top layer and the interactions between the top and bottom layers. [1] E.E. Fullerton, D.T. Margulies, M.E. Schabes, M. Carey, B. Gurney, A. Moser, M. Best, G. Zeltzer, K. Rubin, H. Rosen, Appl. Phys. Lett. 77 (2000), 3806. [2] E.N. Abarra, A. Inomata, H. Sato, I. Okamoto, Y. Mizoshita, Appl. Phys. Lett. 77 (2000), 2581.
Magnetic Separations with Magnetite: Theory, Operation, and Limitations
G. B. Cotten
2000-08-01
This dissertation documents the theory development and experimental plan followed to describe how a magnetite-based column under the influence of an external magnetic field functions as a magnetic separator. Theoretical simulations predict that weekly paramagnetic particles in the sub-micron range can be magnetically separated while diamagnetic particles as large as 2 microns in diameter may pass. Magnetite-based columns were evaluated as magnetically-controllable enhanced filtration devices. There was no evidence of enhanced filtration for diamagnetic particles by the magnetite-based bed. Magnetite-based magnetic separators have proven to be effective in specific laboratory experiments, indicating a potential feasibility for scale-up operations. Column media-filter type filtration effects indicate a magnetite-based column would not be suitable for treatment of a waste stream with a high diamagnetic solids content or high volume throughput requirements. Specific applications requiring removal of sub-micron para- or ferromagnetic particles under batch or Stokes flow conditions would be most applicable.
Toward the Theory of Turbulence in Magnetized Plasmas
Boldyrev, Stanislav
2013-07-26
The goal of the project was to develop a theory of turbulence in magnetized plasmas at large scales, that is, scales larger than the characteristic plasma microscales (ion gyroscale, ion inertial scale, etc.). Collisions of counter-propagating Alfven packets govern the turbulent cascade of energy toward small scales. It has been established that such an energy cascade is intrinsically anisotropic, in that it predominantly supplies energy to the modes with mostly field-perpendicular wave numbers. The resulting energy spectrum of MHD turbulence, and the structure of the fluctuations were studied both analytically and numerically. A new parallel numerical code was developed for simulating reduced MHD equations driven by an external force. The numerical setting was proposed, where the spectral properties of the force could be varied in order to simulate either strong or weak turbulent regimes. It has been found both analytically and numerically that weak MHD turbulence spontaneously generates a “condensate”, that is, concentration of magnetic and kinetic energy at small k{sub {parallel}}. A related topic that was addressed in the project is turbulent dynamo action, that is, generation of magnetic field in a turbulent flow. We were specifically concentrated on the generation of large-scale magnetic field compared to the scales of the turbulent velocity field. We investigate magnetic field amplification in a turbulent velocity field with nonzero helicity, in the framework of the kinematic Kazantsev-Kraichnan model.
Primordial magnetic field and kinetic theory with Berry curvature
NASA Astrophysics Data System (ADS)
Bhatt, Jitesh R.; Pandey, Arun Kumar
2016-08-01
We study the generation of a magnetic field in primordial plasma of standard model particles at a temperature T >80 TeV —much higher than the electroweak scale. It is assumed that there is an excess number of right-handed electrons compared to left-handed positrons in the plasma. Using the Berry-curvature modified kinetic theory to incorporate the effect of the Abelian anomaly, we show that this chiral imbalance leads to the generation of a hypermagnetic field in the plasma in both the collision dominated and collisionless regimes. It is shown that, in the collision dominated regime, the chiral-vorticity effect can generate finite vorticity in the plasma together with the magnetic field. Typical strength of the generated magnetic field is 1 027 G at T ˜80 TeV with the length scale 1 05/T , whereas the Hubble length scale is 1 013/T . Furthermore, the instability can also generate a magnetic field of the order 1 031 G at a typical length scale 10 /T . But there may not be any vorticity generation in this regime. We show that the estimated values of the magnetic field are consistent with the bounds obtained from current observations.
Ambient-temperature passive magnetic bearings: Theory and design equations
Post, R.F.; Ryutov, D.D.
1997-12-30
Research has been underway at the Lawrence Livermore National Laboratory to build a theoretical and experimental base for the design of ambient-temperature passive magnetic bearings for a variety of possible applications. in the approach taken the limitations imposed by Earnshaw`s theorem with respect to the stability of passive magnetic bearing systems employing axially symmetric permanent-magnet elements are overcome by employing special combinations of elements, as follows: Levitating and restoring forces are provided by combinations of permanent-magnet-excited elements chosen to provide positive stiffnesses (negative force derivatives) for selected displacements (i.e., those involving translations or angular displacement of the axis of rotation). As dictated by Eamshaw`s theorem, any bearing system thus constructed will be statically unstable for at least one of the remaining possible displacements. Stabilization against this displacement is accomplished by using periodic arrays (`Halbach arrays`) of permanent magnets to induce currents in close-packed inductively loaded circuits, thereby producing negative force derivatives stabilizing the system while in rotation. Disengaging mechanical elements stabilize the system when at rest and when below a low critical speed. The paper discusses theory and equations needed for the design of such systems.
Consistent Chiral Kinetic Theory in Weyl Materials: Chiral Magnetic Plasmons.
Gorbar, E V; Miransky, V A; Shovkovy, I A; Sukhachov, P O
2017-03-24
We argue that the correct definition of the electric current in the chiral kinetic theory for Weyl materials should include the Chern-Simons contribution that makes the theory consistent with the local conservation of the electric charge in electromagnetic and strain-induced pseudoelectromagnetic fields. By making use of such a kinetic theory, we study the plasma frequencies of collective modes in Weyl materials in constant magnetic and pseudomagnetic fields, taking into account the effects of dynamical electromagnetism. We show that the collective modes are chiral plasmons. While the plasma frequency of the longitudinal collective mode coincides with the Langmuir one, this mode is unusual because it is characterized not only by oscillations of the electric current density, but also by oscillations of the chiral current density. The latter are triggered by a dynamical version of the chiral electric separation effect. We also find that the plasma frequencies of the transverse modes split up in a magnetic field. This finding suggests an efficient means of extracting the chiral shift parameter from the measurement of the plasma frequencies in Weyl materials.
Consistent Chiral Kinetic Theory in Weyl Materials: Chiral Magnetic Plasmons
NASA Astrophysics Data System (ADS)
Gorbar, E. V.; Miransky, V. A.; Shovkovy, I. A.; Sukhachov, P. O.
2017-03-01
We argue that the correct definition of the electric current in the chiral kinetic theory for Weyl materials should include the Chern-Simons contribution that makes the theory consistent with the local conservation of the electric charge in electromagnetic and strain-induced pseudoelectromagnetic fields. By making use of such a kinetic theory, we study the plasma frequencies of collective modes in Weyl materials in constant magnetic and pseudomagnetic fields, taking into account the effects of dynamical electromagnetism. We show that the collective modes are chiral plasmons. While the plasma frequency of the longitudinal collective mode coincides with the Langmuir one, this mode is unusual because it is characterized not only by oscillations of the electric current density, but also by oscillations of the chiral current density. The latter are triggered by a dynamical version of the chiral electric separation effect. We also find that the plasma frequencies of the transverse modes split up in a magnetic field. This finding suggests an efficient means of extracting the chiral shift parameter from the measurement of the plasma frequencies in Weyl materials.
Advances in Theory of Solid-State Nuclear Magnetic Resonance
Mananga, Eugene S.; Moghaddasi, Jalil; Sana, Ajaz; Akinmoladun, Andrew; Sadoqi, Mostafa
2015-01-01
Recent advances in theory of solid state nuclear magnetic resonance (NMR) such as Floquet-Magnus expansion and Fer expansion, address alternative methods for solving a time-dependent linear differential equation which is a central problem in quantum physics in general and solid-state NMR in particular. The power and the salient features of these theoretical approaches that are helpful to describe the time evolution of the spin system at all times are presented. This review article presents a broad view of manipulations of spin systems in solid-state NMR, based on milestones theories including the average Hamiltonian theory and the Floquet theory, and the approaches currently developing such as the Floquet-Magnus expansion and the Fer expansion. All these approaches provide procedures to control and describe the spin dynamics in solid-state NMR. Applications of these theoretical methods to stroboscopic and synchronized manipulations, non-synchronized experiments, multiple incommensurated frequencies, magic-angle spinning samples, are illustrated. We also reviewed the propagators of these theories and discussed their convergences. Note that the FME is an extension of the popular Magnus Expansion and Average Hamiltonian Theory. It aims is to bridge the AHT to the Floquet Theorem but in a more concise and efficient formalism. Calculations can then be performed in a finite-dimensional Hilbert space instead of an infinite dimensional space within the so-called Floquet theory. We expected that the FME will provide means for more accurate and efficient spin dynamics simulation and for devising new RF pulse sequence. PMID:26878063
Advances in Theory of Solid-State Nuclear Magnetic Resonance.
Mananga, Eugene S; Moghaddasi, Jalil; Sana, Ajaz; Akinmoladun, Andrew; Sadoqi, Mostafa
Recent advances in theory of solid state nuclear magnetic resonance (NMR) such as Floquet-Magnus expansion and Fer expansion, address alternative methods for solving a time-dependent linear differential equation which is a central problem in quantum physics in general and solid-state NMR in particular. The power and the salient features of these theoretical approaches that are helpful to describe the time evolution of the spin system at all times are presented. This review article presents a broad view of manipulations of spin systems in solid-state NMR, based on milestones theories including the average Hamiltonian theory and the Floquet theory, and the approaches currently developing such as the Floquet-Magnus expansion and the Fer expansion. All these approaches provide procedures to control and describe the spin dynamics in solid-state NMR. Applications of these theoretical methods to stroboscopic and synchronized manipulations, non-synchronized experiments, multiple incommensurated frequencies, magic-angle spinning samples, are illustrated. We also reviewed the propagators of these theories and discussed their convergences. Note that the FME is an extension of the popular Magnus Expansion and Average Hamiltonian Theory. It aims is to bridge the AHT to the Floquet Theorem but in a more concise and efficient formalism. Calculations can then be performed in a finite-dimensional Hilbert space instead of an infinite dimensional space within the so-called Floquet theory. We expected that the FME will provide means for more accurate and efficient spin dynamics simulation and for devising new RF pulse sequence.
Nuclear chiral and magnetic rotation in covariant density functional theory
NASA Astrophysics Data System (ADS)
Meng, Jie; Zhao, Pengwei
2016-05-01
Excitations of chiral rotation observed in triaxial nuclei and magnetic and/or antimagnetic rotations (AMR) seen in near-spherical nuclei have attracted a lot of attention. Unlike conventional rotation in well-deformed or superdeformed nuclei, here the rotational axis is not necessary coinciding with any principal axis of the nuclear density distribution. Thus, tilted axis cranking (TAC) is mandatory to describe these excitations self-consistently in the framework of covariant density functional theory (CDFT). We will briefly introduce the formalism of TAC-CDFT and its application for magnetic and AMR phenomena. Configuration-fixed CDFT and its predictions for nuclear chiral configurations and for favorable triaxial deformation parameters are also presented, and the discoveries of the multiple chiral doublets in 133Ce and 103Rh are discussed.
The role of magnetic helicity in cosmic ray transport theory
NASA Technical Reports Server (NTRS)
Goldstein, M. L.; Matthaeus, W. H.
1981-01-01
It has been found that the computed values of the mean free path for pitch angle scattering of low rigidity cosmic rays computed from weak turbulence theory are systematically larger than the values inferred from analyses of solar particle events. Reasons for this discrepancy are considered, taking into account components of the spectral tensor of the magnetic field fluctuations which can contribute to pitch angle scattering. The contributions of the additional terms can be estimated from the interplanetary magnetic field. It is shown that these additional contributions can be significant. A description is given of a formalism which includes the possibility that the field magnitude is constant. The formalism provides a formal basis for the heuristic arguments made by Goldstein (1980).
Kinetic theory of weak turbulence in magnetized plasmas: Perpendicular propagation
Yoon, Peter H.
2015-08-15
The present paper formulates a weak turbulence theory in which electromagnetic perturbations are assumed to propagate in directions perpendicular to the ambient magnetic field. By assuming that all wave vectors lie in one direction transverse to the ambient magnetic field, the linear solution and second-order nonlinear solutions to the equation for the perturbed distribution function are obtained. Nonlinear perturbed current from the second-order nonlinearity is derived in general form, but the limiting situation of cold plasma temperature is taken in order to derive an explicit nonlinear wave kinetic equation that describes three-wave decay/coalescence interactions among X and Z modes. A potential application of the present formalism is also discussed.
Testing chameleon theories with light propagating through a magnetic field
Brax, Philippe; Davis, Anne-Christine; Mota, David F.
2007-10-15
It was recently argued that the observed PVLAS anomaly can be explained by chameleon field theories in which large deviations from Newton's law can be avoided. Here we present the predictions for the dichroism and the birefringence induced in the vacuum by a magnetic field in these models. We show that chameleon particles behave very differently from standard axionlike particles (ALPs). We find that, unlike ALPs, the chameleon particles are confined within the experimental setup. As a consequence, the birefringence is always bigger than the dichroism in PVLAS-type experiments.
Diamagnetic boundary layers - A kinetic theory. [for collisionless magnetized plasmas
NASA Technical Reports Server (NTRS)
Lemaire, J.; Burlaga, L. F.
1976-01-01
A kinetic theory is presented for boundary layers associated with MHD tangential 'discontinuities' in a collisionless magnetized plasma, such as those observed in the solar wind. The theory consists of finding self-consistent solutions of Vlasov's equation and Maxwell's equation for stationary one-dimensional boundary layers separating two Maxwellian plasma states. Layers in which the current is carried by electrons are found to have a thickness of the order of a few electron gyroradii, but the drift speed of the current-carrying electrons is found to exceed the Alfven speed, and accordingly such layers are not stable. Several types of layers in which the current is carried by protons are discussed; in particular, cases are considered in which the magnetic-field intensity, direction, or both, changed across the layer. In every case, the thickness was of the order of a few proton gyroradii, and the field changed smoothly, although the characteristics depended somewhat on the boundary conditions. The drift speed was always less than the Alfven speed, consistent with stability of such structures. These results are consistent with observations of boundary layers in the solar wind near 1 AU.
An Introduction to the Theory of the Chiral Magnetic Effect
NASA Astrophysics Data System (ADS)
Liao, Jinfeng
2016-09-01
Anomalous chiral transport processes, with the notable example of the Chiral Magnetic Effect (CME), are remarkable phenomena that manifest microscopic quantum anomaly of chiral fermions in a macroscopic many-body setting. Significant progress has been achieved both in their theoretical understanding and in their experimental search. In this talk, an elementary introduction will be given for the CME as well as other anomalous chiral effects such as the Chiral Magnetic Wave (CMW) and the vorticity-driven effects. The theoretical foundation for describing them is rapidly emerging, including the anomalous hydrodynamics framework for (nearly) equilibrated systems while the chiral kinetic theory for out-of-equilibrium systems. The most recent results along these lines will be highlighted, with an emphasis on their applications to heavy ion collision experiments. Finally the currently pressing issues and anticipated future developments, in the context of recently formed Beam Energy Scan Theory (BEST) Collaboration, will be envisioned. The speaker acknowledges partial support from NSF, DOE, and RIKEN BNL Research Center.
Disorder-induced magnetic memory: experiments and theories
Pierce, M.S.; Buechler, C.R.; Sorensen, L.B.; Kevan, S.D.; Jagla,E.A.; Deutsch, J.M.; Mai, T.; Narayan, O.; Davies, J.E.; Liu, K.; Zimanyi, G.T.; Katzgraber, H.G.; Hellwig, O.; Fullerton, E.E.; Fischer,P.; Kortright, J.B.
2007-01-04
Beautiful theories of magnetic hysteresis based on randommicroscopic disorder have been developed over the past ten years. Ourgoal was to directly compare these theories with precise experiments. Todo so, we first developed and then applied coherent x-ray specklemetrology to a series of thin multilayer perpendicular magneticmaterials. To directly observe the effects of disorder, we deliberatelyintroduced increasing degrees of disorder into our films. We usedcoherent x rays, produced at the Advanced Light Source at LawrenceBerkeley National Laboratory, to generate highly speckled magneticscattering patterns. The apparently "random" arrangement of the specklesis due to the exact configuration of the magnetic domains in the sample.In effect, each speckle pattern acts as a unique fingerprint for themagnetic domain configuration. Small changes in the domain structurechange the speckles, and comparison of the different speckle patternsprovides a quantitative determination of how much the domain structurehas changed. Our experiments quickly answered one longstanding question:How is the magnetic domain configuration at one point on the majorhysteresis loop related to the configurations at the same point on theloop during subsequent cycles? This is called microscopic return-pointmemory "RPM". We found that the RPM is partial and imperfect in thedisordered samples, and completely absent when the disorder is below athreshold level. We also introduced and answered a second importantquestion: How are the magnetic domains at one point on the major looprelated to the domains at the complementary point, the inversionsymmetric point on the loop, during the same and during subsequentcycles? This is called microscopic complementary-point memory "CPM". Wefound that the CPM is also partial and imperfect in the disorderedsamples and completely absent when the disorder is not present. Inaddition, we found that the RPM is always a little larger than the CPM.We also studied the
The impact of renormalization group theory on magnetism
NASA Astrophysics Data System (ADS)
Köbler, U.; Hoser, A.
2007-11-01
The basic issues of renormalization group (RG) theory, i.e. universality, crossover phenomena, relevant interactions etc. are verified experimentally on magnetic materials. Universality is demonstrated on account of the saturation of the magnetic order parameter for T ↦ 0. Universal means that the deviations with respect to saturation at T = 0 can perfectly be described by a power function of absolute temperature with an exponent ɛ that is independent of spin structure and lattice symmetry. Normally the Tɛ function holds up to ~0.85Tc where crossover to the critical power function occurs. Universality for T ↦ 0 cannot be explained on the basis of the material specific magnon dispersions that are due to atomistic symmetry. Instead, continuous dynamic symmetry has to be assumed. The quasi particles of the continuous symmetry can be described by plane waves and have linear dispersion in all solids. This then explains universality. However, those quasi particles cannot be observed using inelastic neutron scattering. The principle of relevance is demonstrated using the competition between crystal field interaction and exchange interaction as an example. If the ratio of crystal field interaction to exchange interaction is below some threshold value the local crystal field is not relevant under the continuous symmetry of the ordered state and the saturation moment of the free ion is observed for T ↦ 0. Crossover phenomena either between different exponents or between discrete changes of the pre-factor of the Tɛ function are demonstrated for the spontaneous magnetization and for the heat capacity.
Floquet theory of radical pairs in radiofrequency magnetic fields
NASA Astrophysics Data System (ADS)
Hiscock, Hamish G.; Kattnig, Daniel R.; Manolopoulos, David E.; Hore, P. J.
2016-09-01
We present a new method for calculating the product yield of a radical pair recombination reaction in the presence of a weak time-dependent magnetic field. This method successfully circumvents the computational difficulties presented by a direct solution of the Liouville-von Neumann equation for a long-lived radical pair containing many hyperfine-coupled nuclear spins. Using a modified formulation of Floquet theory, treating the time-dependent magnetic field as a perturbation, and exploiting the slow radical pair recombination, we show that one can obtain a good approximation to the product yield by considering only nearly degenerate sub-spaces of the Floquet space. Within a significant parameter range, the resulting method is found to give product yields in good agreement with exact quantum mechanical results for a variety of simple model radical pairs. Moreover it is considerably more efficient than the exact calculation, and it can be applied to radical pairs containing significantly more nuclear spins. This promises to open the door to realistic theoretical investigations of the effect of radiofrequency electromagnetic radiation on the photochemically induced radical pair recombination reactions in the avian retina which are believed to be responsible for the magnetic compass sense of migratory birds.
Mean-field theory for Bose-Hubbard model under a magnetic field
Oktel, M. Oe.; Tanatar, B.; Nita, M.
2007-01-15
We consider the superfluid-insulator transition for cold bosons under an effective magnetic field. We investigate how the applied magnetic field affects the Mott transition within mean-field theory and find that the critical hopping strength (t/U){sub c} increases with the applied field. The increase in the critical hopping follows the bandwidth of the Hofstadter butterfly at the given value of the magnetic field. We also calculate the magnetization and superfluid density within mean-field theory.
Stability Theory for Interfacial Patterns in Magnetic Pulse Welding
NASA Astrophysics Data System (ADS)
Nassiri, Ali; Chini, Gregory; Kinsey, Brad; UNH Team
2013-11-01
Magnetic Pulse Welding (MPW) is a solid state, high strain-rate joining process in which a weld of dissimilar or similar materials can be created via high-speed oblique impact of two workpieces. Experiments routinely show the emergence of a distinctive wavy pattern, with a well defined amplitude and wavelength of approximately 20 and 70 micrometers, respectively, at the interface between the two welded materials. Although the origin of the wavy pattern has been the subject of much investigation, a unique fundamental physical theory for this phenomenon is as yet not widely accepted. Some researchers have proposed that the interfacial waves are formed in a process akin to Kelvin-Helmholtz instability, with relative shear movement of the flyer and base plates providing the energy source. Here, we employ a linear stability analysis to investigate whether the wavy pattern could be the signature of a shear-driven high strain-rate instability of an elastic-plastic solid material. Preliminary results confirm that an instability giving rise to a wavy interfacial pattern is possible.
On the unsteady-motion theory of magnetic forces for maglev
Chen, S.S.; Zhu, S.; Cai, Y.
1993-11-01
Motion-dependent magnetic forces are the key elements in the study of magnetically levitated vehicle (maglev) system dynamics. In the past, most maglev-system designs were based on a quasisteady-motion theory of magnetic forces. This report presents an experimental and analytical study that will enhance our understanding of the role of unsteady-motion-dependent magnetic forces and demonstrate an experimental technique that can be used to measure those unsteady magnetic forces directly. The experimental technique provides a useful tool to measure motion-dependent magnetic forces for the prediction and control of maglev systems.
Theory and simulation of anisotropic pair correlations in ferrofluids in magnetic fields.
Elfimova, Ekaterina A; Ivanov, Alexey O; Camp, Philip J
2012-05-21
Anisotropic pair correlations in ferrofluids exposed to magnetic fields are studied using a combination of statistical-mechanical theory and computer simulations. A simple dipolar hard-sphere model of the magnetic colloidal particles is studied in detail. A virial-expansion theory is constructed for the pair distribution function (PDF) which depends not only on the length of the pair separation vector, but also on its orientation with respect to the field. A detailed comparison is made between the theoretical predictions and accurate simulation data, and it is found that the theory works well for realistic values of the dipolar coupling constant (λ = 1), volume fraction (φ ≤ 0.1), and magnetic field strength. The structure factor is computed for wavevectors either parallel or perpendicular to the field. The comparison between theory and simulation is generally very good with realistic ferrofluid parameters. For both the PDF and the structure factor, there are some deviations between theory and simulation at uncommonly high dipolar coupling constants, and with very strong magnetic fields. In particular, the theory is less successful at predicting the behavior of the structure factors at very low wavevectors, and perpendicular Gaussian density fluctuations arising from strongly correlated pairs of magnetic particles. Overall, though, the theory provides reliable predictions for the nature and degree of pair correlations in ferrofluids in magnetic fields, and hence should be of use in the design of functional magnetic materials.
ON THE THEORY OF POLARIZATION TRANSFER IN INHOMOGENEOUS MAGNETIZED PLASMAS,
PLASMA MEDIUM, ELECTROMAGNETIC RADIATION ), (* ELECTROMAGNETIC RADIATION , POLARIZATION), TRANSFER FUNCTIONS, ASTROPHYSICS, WAVE FUNCTIONS, MAGNETIC FIELDS, MAGNETOOPTICS, PHASE SHIFT CIRCUITS, DIFFERENTIAL EQUATIONS
Mathematical developments regarding the general theory of the Earth magnetism
NASA Technical Reports Server (NTRS)
Schmidt, A.
1983-01-01
A literature survey on the Earth's magnetic field, citing the works of Gauss, Erman-Petersen, Quintus Icilius and Neumayer is presented. The general formulas for the representation of the potential and components of the Earth's magnetic force are presented. An analytical representation of magnetic condition of the Earth based on observations is also made.
Quantum theory of the dielectric constant of a magnetized plasma and astrophysical applications. I.
NASA Technical Reports Server (NTRS)
Canuto, V.; Ventura, J.
1972-01-01
A quantum mechanical treatment of an electron plasma in a constant and homogeneous magnetic field is considered, with the aim of (1) defining the range of validity of the magnetoionic theory (2) studying the deviations from this theory, in applications involving high densities, and intense magnetic field. While treating the magnetic field exactly, a perturbation approach in the photon field is used to derive general expressions for the dielectric tensor. Numerical estimates on the range of applicability of the magnetoionic theory are given for the case of the 'one-dimensional' electron gas, where only the lowest Landau level is occupied.
Quantum theory of the dielectric constant of a magnetized plasma and astrophysical applications. I.
NASA Technical Reports Server (NTRS)
Canuto, V.; Ventura, J.
1972-01-01
A quantum mechanical treatment of an electron plasma in a constant and homogeneous magnetic field is considered, with the aim of (1) defining the range of validity of the magnetoionic theory (2) studying the deviations from this theory, in applications involving high densities, and intense magnetic field. While treating the magnetic field exactly, a perturbation approach in the photon field is used to derive general expressions for the dielectric tensor. Numerical estimates on the range of applicability of the magnetoionic theory are given for the case of the 'one-dimensional' electron gas, where only the lowest Landau level is occupied.
Magnetization of RFe3 intermetallic compounds: Molecular field theory analysis
NASA Astrophysics Data System (ADS)
Herbst, J. F.; Croat, J. J.
1982-06-01
We report magnetization measurements of all RFe3 intermetallic compounds known to form (R = Y, Sm, Gd, Tb, Dy, Ho, Er, Tm). A two-sublattice molecular field model is employed to analyze the data except in the case of YFe3, for which one magnetic sublattice is assumed. In general, the model adequately describes the temperature dependence of the magnetization. For SmFe3 our results suggest that the samarium and iron moments are ferromagnetically coupled.
Theory and Application of Magnetic Flux Leakage Pipeline Detection.
Shi, Yan; Zhang, Chao; Li, Rui; Cai, Maolin; Jia, Guanwei
2015-12-10
Magnetic flux leakage (MFL) detection is one of the most popular methods of pipeline inspection. It is a nondestructive testing technique which uses magnetic sensitive sensors to detect the magnetic leakage field of defects on both the internal and external surfaces of pipelines. This paper introduces the main principles, measurement and processing of MFL data. As the key point of a quantitative analysis of MFL detection, the identification of the leakage magnetic signal is also discussed. In addition, the advantages and disadvantages of different identification methods are analyzed. Then the paper briefly introduces the expert systems used. At the end of this paper, future developments in pipeline MFL detection are predicted.
Theory and Application of Magnetic Flux Leakage Pipeline Detection
Shi, Yan; Zhang, Chao; Li, Rui; Cai, Maolin; Jia, Guanwei
2015-01-01
Magnetic flux leakage (MFL) detection is one of the most popular methods of pipeline inspection. It is a nondestructive testing technique which uses magnetic sensitive sensors to detect the magnetic leakage field of defects on both the internal and external surfaces of pipelines. This paper introduces the main principles, measurement and processing of MFL data. As the key point of a quantitative analysis of MFL detection, the identification of the leakage magnetic signal is also discussed. In addition, the advantages and disadvantages of different identification methods are analyzed. Then the paper briefly introduces the expert systems used. At the end of this paper, future developments in pipeline MFL detection are predicted. PMID:26690435
Theory of zwitterionic molecular-based organic magnets
NASA Astrophysics Data System (ADS)
Shelton, William A.; Aprà, Edoardo; Sumpter, Bobby G.; Saraiva-Souza, Aldilene; Souza Filho, Antonio G.; Nero, Jordan Del; Meunier, Vincent
2011-08-01
We describe a class of organic molecular magnets based on zwitterionic molecules (betaine derivatives) possessing donor, π bridge, and acceptor groups. Using extensive electronic structure calculations we show the electronic ground-state in these systems is magnetic. In addition, we show that the large energy differences computed for the various magnetic states indicate a high Neel temperature. The quantum mechanical nature of the magnetic properties originates from the conjugated π bridge (only p electrons) in cooperation with the molecular donor-acceptor character. The exchange interactions between electron spin are strong, local, and independent on the length of the π bridge.
Theory of using magnetic deflections to combine charged particle beams
Steckbeck, Mackenzie K.; Doyle, Barney Lee
2014-09-01
Several radiation effects projects in the Ion Beam Lab (IBL) have recently required two disparate charged particle beams to simultaneously strike a single sample through a single port of the target chamber. Because these beams have vastly different mass–energy products (MEP), the low-MEP beam requires a large angle of deflection toward the sample by a bending electromagnet. A second electromagnet located further upstream provides a means to compensate for the small angle deflection experienced by the high-MEP beam during its path through the bending magnet. This paper derives the equations used to select the magnetic fields required by these two magnets to achieve uniting both beams at the target sample. A simple result was obtained when the separation of the two magnets was equivalent to the distance from the bending magnet to the sample, and the equation is given by: B_{s}= 1/2(r_{c}/r_{s}) B_{c}, where B_{s} and B_{c} are the magnetic fields in the steering and bending magnet and r_{c}/r_{s} is the ratio of the radii of the bending magnet to that of the steering magnet. This result is not dependent upon the parameters of the high MEP beam, i.e. energy, mass, charge state. Therefore, once the field of the bending magnet is set for the low-MEP beam, and the field in the steering magnet is set as indicted in the equation, the trajectory path of any high-MEP beam will be directed into the sample.
Ab Initio Theory of Coherent Laser-Induced Magnetization in Metals
NASA Astrophysics Data System (ADS)
Berritta, Marco; Mondal, Ritwik; Carva, Karel; Oppeneer, Peter M.
2016-09-01
We present the first materials specific ab initio theory of the magnetization induced by circularly polarized laser light in metals. Our calculations are based on nonlinear density matrix theory and include the effect of absorption. We show that the induced magnetization, commonly referred to as inverse Faraday effect, is strongly materials and frequency dependent, and demonstrate the existence of both spin and orbital induced magnetizations which exhibit a surprisingly different behavior. We show that for nonmagnetic metals (such as Cu, Au, Pd, Pt) and antiferromagnetic metals the induced magnetization is antisymmetric in the light's helicity, whereas for ferromagnetic metals (Fe, Co, Ni, FePt) the imparted magnetization is only asymmetric in the helicity. We compute effective optomagnetic fields that correspond to the induced magnetizations and provide guidelines for achieving all-optical helicity-dependent switching.
Magnetic particle motions within living cells. Physical theory and techniques.
Valberg, P A; Butler, J P
1987-01-01
Body tissues are not ferromagnetic, but ferromagnetic particles can be present as contaminants or as probes in the lungs and in other organs. The magnetic domains of these particles can be aligned by momentary application of an external magnetic field; the magnitude and time course of the resultant remanent field depend on the quantity of magnetic material and the degree of particle motion. The interpretation of magnetometric data requires an understanding of particle magnetization, agglomeration, random motion, and both rotation and translation in response to magnetic fields. We present physical principles relevant to magnetometry and suggest models for intracellular particle motion driven by thermal, elastic, or cellular forces. The design principles of instrumentation for magnetizing intracellular particles and for detecting weak remanent magnetic fields are described. Such magnetic measurements can be used for noninvasive studies of particle clearance from the body or of particle motion within body tissues and cells. Assumptions inherent to this experimental approach and possible sources of artifact are considered and evaluated. PMID:3676435
Theory of lattice effects on magnetic interactions in solids
NASA Astrophysics Data System (ADS)
Meskine, Hakim
This dissertation focuses on studying the effect of lattice distortions on the magnetic properties of nickelates and manganites. These two families of materials have great potential in industrial applications in the fields of magnetic (superdense hard-drives, fast memory) and charge storage (batteries). The introduction and methods sections present the main ideas of the dissertation and discuss the various techniques used. Electron-lattice coupling is first examined in chapter three for a two-site model where we estimate the magnitude of the isotope effect on the critical temperature and show that it decreases magnetic exchange. In the next part we study electronic structure and magnetism of NaNiO2 and show that inter-planar exchange is reduced by lattice coupling. In the fifth chapter we examine the magnetic polaron and discuss the effect of static lattice coupling on its binding energy, and find it to further stabilize the polaron.
Magnetized domain wall in f(R, T) theory of gravity
NASA Astrophysics Data System (ADS)
Agrawal, P. K.; Pawar, D. D.
2017-07-01
We studied Bianchi type-V space-time using magnetic domain wall in f(R, T) theory of gravity and deciphered the exact solutions of the corresponding field equations. In this study, we discussed the physical behavior of the resultant cosmological model in the presence and absence of magnetic field with the help of few physical parameters.
Rayleigh-taylor instability with magnetic fluids: experiment and theory
Pacitto; Flament; Bacri; Widom
2000-12-01
We present experiments showing the Rayleigh-Taylor instability at the interface between a dense magnetic liquid and an immiscible less dense liquid. The liquids are confined in a Hele-Shaw cell and a magnetic field is applied perpendicular to the cell. We measure the wavelength and the growth rate at the onset of the instability as a function of the external magnetic field. The wavelength decreases as the field increases. The amplitude of the interface deformation grows exponentially with time in the early stage, and the growth rate is an increasing function of the field. These results are compared to theoretical predictions given in the framework of linear stability analysis.
GravitoMagnetic Field in Tensor-Vector-Scalar Theory
Exirifard, Qasem
2013-04-01
We study the gravitomagnetism in the TeVeS theory. We compute the gravitomagnetic field that a slow moving mass distribution produces in its Newtonian regime. We report that the consistency between the TeVeS gravitomagnetic field and that predicted by the Einstein-Hilbert theory leads to a relation between the vector and scalar coupling constants of the theory. We translate the Lunar Laser Ranging measurement's data into a constraint on the deviation from this relation.
Astrophysical dynamos and the growth of magnetic fields in high-redshift galaxies
NASA Astrophysics Data System (ADS)
Rieder, Michael; Teyssier, Romain
2015-08-01
The origin and evolution of magnetic fields in the Universe is still an open question. Observations of galaxies at high-redshift give evidence for strong galactic magnetic fields even in the early Universe which are consistently measured at later times up to the present age. However, primordial magnetic fields and seed field generation by battery processes cannot explain such high field strengths, suggesting the presence of a rapid growth mechanism in those high-redshift galaxies and subsequent maintenance against decay. Astrophysical dynamo theory provides efficient means of field amplification where even weak initial fields can grow exponentially on sufficiently fast timescales, driving the conversion of kinetic energy into magnetic energy. We investigate the role which feedback mechanisms play in the creation of the turbulence necessary for dynamos to operate. Performing magnetohydrodynamic simulations of cooling halos of dwarf and Milky Way-like high-redshift progenitors, we compare the magnetic field evolution of weak seed fields with various topologies and stellar feedback mechanisms. We find that strong feedback can drive galactic gas turbulence which gives rise to velocity fields with fast exponential magnetic field growth. The simulations display a high gas fraction and a clumpy morphology with kinematics resembling Kolmogorov turbulence and magnetic energy spectra as predicted by Kazantsev dynamo theory. Magnetic fields reach equipartition with $\\mu$G field strength. In a final quiescent phase where feedback is turned off, gas turbulence is reduced and a quadrupole symmetry is observed in the magnetic field. These findings support the theory of rapid magnetic field amplification inside high-redshift galaxies, when the Universe was still young.
Verification of a magnetic island in gyro-kinetics by comparison with analytic theory
Zarzoso, D. Casson, F. J.; Poli, E.; Hornsby, W. A.; Peeters, A. G.
2015-02-15
A rotating magnetic island is imposed in the gyrokinetic code GKW, when finite differences are used for the radial direction, in order to develop the predictions of analytic tearing mode theory and understand its limitations. The implementation is verified against analytics in sheared slab geometry with three numerical tests that are suggested as benchmark cases for every code that imposes a magnetic island. The convergence requirements to properly resolve physics around the island separatrix are investigated. In the slab geometry, at low magnetic shear, binormal flows inside the island can drive Kelvin-Helmholtz instabilities which prevent the formation of the steady state for which the analytic theory is formulated.
Relativistic theory of magnetic inertia in ultrafast spin dynamics
NASA Astrophysics Data System (ADS)
Mondal, Ritwik; Berritta, Marco; Nandy, Ashis K.; Oppeneer, Peter M.
2017-07-01
The influence of possible magnetic inertia effects has recently drawn attention in ultrafast magnetization dynamics and switching. Here we derive rigorously a description of inertia in the Landau-Lifshitz-Gilbert equation on the basis of the Dirac-Kohn-Sham framework. Using the Foldy-Wouthuysen transformation up to the order of 1 /c4 gives the intrinsic inertia of a pure system through the second order time derivative of magnetization in the dynamical equation of motion. Thus, the inertial damping I is a higher order spin-orbit coupling effect, ˜1 /c4 , as compared to the Gilbert damping Γ that is of order 1 /c2 . Inertia is therefore expected to play a role only on ultrashort timescales (subpicoseconds). We also show that the Gilbert damping and inertial damping are related to one another through the imaginary and real parts of the magnetic susceptibility tensor, respectively.
Electric-magnetic dualities in non-abelian and non-commutative gauge theories
NASA Astrophysics Data System (ADS)
Ho, Jun-Kai; Ma, Chen-Te
2016-08-01
Electric-magnetic dualities are equivalence between strong and weak coupling constants. A standard example is the exchange of electric and magnetic fields in an abelian gauge theory. We show three methods to perform electric-magnetic dualities in the case of the non-commutative U (1) gauge theory. The first method is to use covariant field strengths to be the electric and magnetic fields. We find an invariant form of an equation of motion after performing the electric-magnetic duality. The second method is to use the Seiberg-Witten map to rewrite the non-commutative U (1) gauge theory in terms of abelian field strength. The third method is to use the large Neveu Schwarz-Neveu Schwarz (NS-NS) background limit (non-commutativity parameter only has one degree of freedom) to consider the non-commutative U (1) gauge theory or D3-brane. In this limit, we introduce or dualize a new one-form gauge potential to get a D3-brane in a large Ramond-Ramond (R-R) background via field redefinition. We also use perturbation to study the equivalence between two D3-brane theories. Comparison of these methods in the non-commutative U (1) gauge theory gives different physical implications. The comparison reflects the differences between the non-abelian and non-commutative gauge theories in the electric-magnetic dualities. For a complete study, we also extend our studies to the simplest abelian and non-abelian p-form gauge theories, and a non-commutative theory with the non-abelian structure.
QED theory of the nuclear magnetic shielding in hydrogenlike ions.
Yerokhin, V A; Pachucki, K; Harman, Z; Keitel, C H
2011-07-22
The shielding of the nuclear magnetic moment by the bound electron in hydrogenlike ions is calculated ab initio with inclusion of relativistic, nuclear, and quantum electrodynamics (QED) effects. The QED correction is evaluated to all orders in the nuclear binding strength parameter and, independently, to the first order in the expansion in this parameter. The results obtained lay the basis for the high-precision determination of nuclear magnetic dipole moments from measurements of the g factor of hydrogenlike ions.
Bamba, Kazuharu; Ohta, Nobuyoshi; Tsujikawa, Shinji
2008-08-15
We estimate the strength of large-scale magnetic fields produced during inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis is sufficiently general in the sense that it covers most of conformal symmetry breaking theories in which the electromagnetic field is coupled to a scalar field. In DBI theories there is an additional factor associated with the speed of sound, which allows a possibility to lead to an extra amplification of the magnetic field in a ultrarelativistic region. We clarify the conditions under which seed magnetic fields to feed the galactic dynamo mechanism at a decoupling epoch as well as present magnetic fields on galactic scales are sufficiently generated to satisfy observational bounds.
Phonon and magnetic structure in δ-plutonium from density-functional theory.
Söderlind, Per; Zhou, F; Landa, A; Klepeis, J E
2015-10-30
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.
Phonon and magnetic structure in δ-plutonium from density-functional theory
Söderlind, Per; Zhou, F.; Landa, A.; Klepeis, J. E.
2015-10-30
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment (DLM) method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, However, the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.
Magnetic-Field-Induced Insulator-Conductor Transition in SU(2) Quenched Lattice Gauge Theory
Buividovich, P.V.; Kharzeev, D.; Chernodub, M.N., Kalaydzhyan, T., Luschevskaya, E.V., and M.I. Polikarpov
2010-09-24
We study the correlator of two vector currents in quenched SU(2) lattice gauge theory with a chirally invariant lattice Dirac operator with a constant external magnetic field. It is found that in the confinement phase the correlator of the components of the current parallel to the magnetic field decays much slower than in the absence of a magnetic field, while for other components the correlation length slightly decreases. We apply the maximal entropy method to extract the corresponding spectral function. In the limit of zero frequency this spectral function yields the electric conductivity of quenched theory. We find that in the confinement phase the external magnetic field induces nonzero electric conductivity along the direction of the field, transforming the system from an insulator into an anisotropic conductor. In the deconfinement phase the conductivity does not exhibit any sizable dependence on the magnetic field.
Phonon and magnetic structure in δ-plutonium from density-functional theory
Söderlind, Per; Zhou, F.; Landa, A.; Klepeis, J. E.
2015-01-01
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure and (ii) the disordered-local-moment method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, but the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments. PMID:26514238
Magnetic Fields Around the Heliosphere: Theory vs Observations
NASA Astrophysics Data System (ADS)
Pogorelov, Nikolai
2016-07-01
Voyager in situ measurements of the magnetic field around the heliosphere are the source of invaluable information about the interface between the solar wind (SW) and local interstellar medium (LISM). On the other hand, they are quite challenging for theoretical analysis unless accompanied by remote observations of neutral atoms the Interstellar Boundary Explorer (IBEX) and Ulysses missions. Of particular interest is the fine structure of the heliopause due to its instability and possible magnetic reconnection. Both phenomena may have contributed to the remarkable changes in the galactic and anomalous cosmic ray fluxes observed by Voyager 1 within a one-month period of 2012 after which the spacecraft penetrated into the LISM. Draping of the heliopause by the interstellar magnetic field affects the position of the bright ribbon of enhanced ENA flux observed by IBEX on the celestial sphere and 2-3 kHz radio emission caused by shock propagation through the outer heliosheath observed by Voyager 1. Interstellar magnetic field determines the structure of the bow wave in front of the heliopause. Moreover, magnetic fields define the orientation and shape of the heliotail, the features of which have been observed by IBEX. Recent numerical simulations show that the details of the large-scale interstellar magnetic field modification caused by the presence of the heliotail may be the source of the observed 1-10 TeV cosmic ray anisotropy studied in detail in numerous air shower measurements around the world. In this paper, an overview will be given of the recent theoretical and simulations results describing the magnetic field distribution around the heliosphere. The objective of the talk is to connect observational and theoretical results, and outline challenges that are going to inspire the heliospheric community in the coming years.
Szalai, I; Nagy, S; Dietrich, S
2013-11-20
The influence of polydispersity on the magnetization of ferrofluids is studied based on a previously published magnetization equation of state (Szalai and Dietrich, 2011 J. Phys.: Condens. Matter 23 326004) and computer simulations. The polydispersity of the particle diameter is described by the gamma distribution function. Canonical ensemble Monte Carlo simulations have been performed in order to test these theoretical results for the initial susceptibility and the magnetization. The results for the magnetic properties of the polydisperse systems turn out to be in quantitative agreement with our present simulation data. In addition, we find good agreement between our theory and experimental data for magnetite-based ferrofluids.
Theory of Magnetization Plateaux in the Shastry-Sutherland Model
Dorier, J.; Mila, F.; Schmidt, K. P.
2008-12-19
Motivated by the remarkable properties of SrCu{sub 2}(BO{sub 3}){sub 2} in a magnetic field, we use perturbative continuous unitary transformations to determine the magnetization plateaux of the Shastry-Sutherland model, unveiling an unexpected sequence of plateaux progressively appearing at 2/9, 1/6, 1/9, and 2/15 upon increasing the interdimer coupling. We predict that a 1/6 plateau should be present in SrCu{sub 2}(BO{sub 3}){sub 2}, even if residual interactions beyond the Shastry-Sutherland are strong enough to modify the other plateaux below 1/3. The method is extended to calculate the magnetization profile within the plateaux, leading to a local structure around triplons that agrees with NMR results on SrCu{sub 2}(BO{sub 3}){sub 2}.
Theory of quantum oscillations of magnetization in Kondo insulators
NASA Astrophysics Data System (ADS)
Ram, Panch; Kumar, Brijesh
2017-08-01
The Kondo lattice model of spin-1/2 local moments coupled to the conduction electrons at half filling is studied for its orbital response to magnetic field on bipartite lattices. Through an effective charge dynamics, in a canonical representation of electrons that appropriately describes the Kondo insulating ground state, the magnetization is found to show de Haas-van Alphen oscillations from intermediate to weak Kondo coupling. These oscillations are ascribed to the inversion of a dispersion of the gapped charge quasiparticles, whose chemical potential surface is measured by the oscillation frequency. Such oscillations are also predicted to occur in spin-density wave insulators.
Theory of electromagnetic fluctuations for magnetized multi-species plasmas
Navarro, Roberto E. Muñoz, Víctor; Araneda, Jaime; Moya, Pablo S.; Viñas, Adolfo F.; Valdivia, Juan A.
2014-09-15
Analysis of electromagnetic fluctuations in plasma provides relevant information about the plasma state and its macroscopic properties. In particular, the solar wind persistently sustains a small but detectable level of magnetic fluctuation power even near thermal equilibrium. These fluctuations may be related to spontaneous electromagnetic fluctuations arising from the discreteness of charged particles. Here, we derive general expressions for the plasma fluctuations in a multi-species plasma following arbitrary distribution functions. This formalism, which generalizes and includes previous works on the subject, is then applied to the generation of electromagnetic fluctuations propagating along a background magnetic field in a plasma of two proton populations described by drifting bi-Maxwellians.
Theory of Magnetic Edge States in Chiral Graphene Nanoribbons
NASA Astrophysics Data System (ADS)
Capaz, Rodrigo; Yazyev, Oleg; Louie, Steven
2011-03-01
Using a model Hamiltonian approach including electron Coulomb interactions, we systematically investigate the electronic structure and magnetic properties of chiral graphene nanoribbons. We show that the presence of magnetic edge states is an intrinsic feature of any smooth graphene nanoribbons with chiral edges, and discover a number of structure-property relations. Specifically, we describe how the edge-state energy gap, zone-boundary edge-state energy splitting, and magnetic moment per edge length depend on the nanoribbon width and chiral angle. The role of environmental screening effects is also studied. Our results address a recent experimental observation of signatures of magnetic ordering at smooth edges of chiral graphene nanoribbons and provide an avenue towards tuning their properties via the structural and environmental degrees of freedom. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and the ONR MURI program. RBC acknowledges financial support from Brazilian agencies CNPq, FAPERJ and INCT-Nanomateriais de Carbono.
Nuclear conversion theory: molecular hydrogen in non-magnetic insulators
NASA Astrophysics Data System (ADS)
Ilisca, Ernest; Ghiglieno, Filippo
2016-09-01
The hydrogen conversion patterns on non-magnetic solids sensitively depend upon the degree of singlet/triplet mixing in the intermediates of the catalytic reaction. Three main `symmetry-breaking' interactions are brought together. In a typical channel, the electron spin-orbit (SO) couplings introduce some magnetic excitations in the non-magnetic solid ground state. The electron spin is exchanged with a molecular one by the electric molecule-solid electron repulsion, mixing the bonding and antibonding states and affecting the molecule rotation. Finally, the magnetic hyperfine contact transfers the electron spin angular momentum to the nuclei. Two families of channels are considered and a simple criterion based on the SO coupling strength is proposed to select the most efficient one. The denoted `electronic' conversion path involves an emission of excitons that propagate and disintegrate in the bulk. In the other denoted `nuclear', the excited electron states are transients of a loop, and the electron system returns to its fundamental ground state. The described model enlarges previous studies by extending the electron basis to charge-transfer states and `continui' of band states, and focuses on the broadening of the antibonding molecular excited state by the solid conduction band that provides efficient tunnelling paths for the hydrogen conversion. After working out the general conversion algebra, the conversion rates of hydrogen on insulating and semiconductor solids are related to a few molecule-solid parameters (gap width, ionization and affinity potentials) and compared with experimental measures.
Nuclear conversion theory: molecular hydrogen in non-magnetic insulators.
Ilisca, Ernest; Ghiglieno, Filippo
2016-09-01
The hydrogen conversion patterns on non-magnetic solids sensitively depend upon the degree of singlet/triplet mixing in the intermediates of the catalytic reaction. Three main 'symmetry-breaking' interactions are brought together. In a typical channel, the electron spin-orbit (SO) couplings introduce some magnetic excitations in the non-magnetic solid ground state. The electron spin is exchanged with a molecular one by the electric molecule-solid electron repulsion, mixing the bonding and antibonding states and affecting the molecule rotation. Finally, the magnetic hyperfine contact transfers the electron spin angular momentum to the nuclei. Two families of channels are considered and a simple criterion based on the SO coupling strength is proposed to select the most efficient one. The denoted 'electronic' conversion path involves an emission of excitons that propagate and disintegrate in the bulk. In the other denoted 'nuclear', the excited electron states are transients of a loop, and the electron system returns to its fundamental ground state. The described model enlarges previous studies by extending the electron basis to charge-transfer states and 'continui' of band states, and focuses on the broadening of the antibonding molecular excited state by the solid conduction band that provides efficient tunnelling paths for the hydrogen conversion. After working out the general conversion algebra, the conversion rates of hydrogen on insulating and semiconductor solids are related to a few molecule-solid parameters (gap width, ionization and affinity potentials) and compared with experimental measures.
Nuclear conversion theory: molecular hydrogen in non-magnetic insulators
NASA Astrophysics Data System (ADS)
Ilisca, Ernest; Ghiglieno, Filippo
2016-09-01
The hydrogen conversion patterns on non-magnetic solids sensitively depend upon the degree of singlet/triplet mixing in the intermediates of the catalytic reaction. Three main `symmetry-breaking' interactions are brought together. In a typical channel, the electron spin-orbit (SO) couplings introduce some magnetic excitations in the non-magnetic solid ground state. The electron spin is exchanged with a molecular one by the electric molecule-solid electron repulsion, mixing the bonding and antibonding states and affecting the molecule rotation. Finally, the magnetic hyperfine contact transfers the electron spin angular momentum to the nuclei. Two families of channels are considered and a simple criterion based on the SO coupling strength is proposed to select the most efficient one. The denoted `electronic' conversion path involves an emission of excitons that propagate and disintegrate in the bulk. In the other denoted `nuclear', the excited electron states are transients of a loop, and the electron system returns to its fundamental ground state. The described model enlarges previous studies by extending the electron basis to charge-transfer states and `continui' of band states, and focuses on the broadening of the antibonding molecular excited state by the solid conduction band that provides efficient tunnelling paths for the hydrogen conversion. After working out the general conversion algebra, the conversion rates of hydrogen on insulating and semiconductor solids are related to a few molecule-solid parameters (gap width, ionization and affinity potentials) and compared with experimental measures.
Transport Theory for Plasmas that are Strongly Magnetized and Strongly Coupled
NASA Astrophysics Data System (ADS)
Baalrud, Scott; Daligault, Jerome
2016-10-01
Plasmas with components that are magnetized, strongly coupled, or both arise in a variety of frontier plasma physics experiments including magnetized dusty plasmas, nonneutral plasmas, magnetized ICF concepts, as well as from self-generated fields in ICF. Here, a species is considered strongly magnetized if the gyroradius is smaller than the spatial scale over which Coulomb interactions occur. A theory for transport properties is described that treats a wide range of both coupling and magnetization strengths. The approach is based on an extension of the recent effective potential transport theory to include a strong magnetic field. The underlying kinetic theory is based on an extension of the Boltzmann equation to include a strong magnetic field in the dynamics of binary scattering events. Corresponding magnetohydrodynamic equations are derived by solving the kinetic equation using a Chapman-Enskog like spectral method. Results are compared with classical molecular dynamics simulations of self-diffusion of the one component plasmas, and with simulations of parallel to perpendicular temperature equilibration of an initially anisotropic distribution. This material is based upon work supported by AFOSR Award FA9550-16-1-0221 and DOE OFES Award DE-SC0016159.
Airborne Bacteria in the Atmospheric Surface Layer: Temporal Distribution above a Grass Seed Field.
Lighthart, B; Shaffer, B T
1995-04-01
Temporal airborne bacterial concentrations and meteorological conditions were measured above a grass seed field in the Willamette River Valley, near Corvallis, Oreg., in the summer of 1993. The concentration of airborne bacteria had a maximum of 1,368.5 CFU/m(sup3), with a coefficient of variation of 90.5% and a mean of 121.3 CFU/m(sup3). The lowest concentration of bacteria occurred during the predawn hours, with an average of 32.2 CFU/m(sup3), while sunrise and early evening hours had the highest averages (164.7 and 158.1 CFU/m(sup3), respectively). The concentrations of bacteria in the atmosphere varied greatly, with a maximum difference between two 2-min samples of 1,995 CFU/m(sup3). The concentrations of bacteria in the atmosphere could be divided into five time periods during the day that were thought to be related to the local diurnal sea breeze and Pacific Coast monsoon weather conditions as follows: (i) the nighttime minimum concentration, i.e., 2300 to 0600 h; (ii) the sunrise peak concentration, i.e., 0600 to 0800 h; (iii) the midday accumulating concentration, i.e., 0800 to 1515 h; (iv) the late-afternoon sea breeze trough concentration, i.e., 1515 to 1700 h; and (v) the evening decrease to the nighttime minimum concentration, i.e., 1700 to 2300 h. The sunrise peak concentration (period ii) is thought to be a relatively general phenomenon dependent on ground heating by the sun, while the afternoon trough concentration is thought to be a relatively local phenomenon dependent on the afternoon sea breeze. Meteorological conditions are thought to be an important regulating influence on airborne bacterial concentrations in the outdoor atmosphere in the Willamette River Valley.
Airborne Bacteria in the Atmospheric Surface Layer: Temporal Distribution above a Grass Seed Field
Lighthart, B.; Shaffer, B. T.
1995-01-01
Temporal airborne bacterial concentrations and meteorological conditions were measured above a grass seed field in the Willamette River Valley, near Corvallis, Oreg., in the summer of 1993. The concentration of airborne bacteria had a maximum of 1,368.5 CFU/m(sup3), with a coefficient of variation of 90.5% and a mean of 121.3 CFU/m(sup3). The lowest concentration of bacteria occurred during the predawn hours, with an average of 32.2 CFU/m(sup3), while sunrise and early evening hours had the highest averages (164.7 and 158.1 CFU/m(sup3), respectively). The concentrations of bacteria in the atmosphere varied greatly, with a maximum difference between two 2-min samples of 1,995 CFU/m(sup3). The concentrations of bacteria in the atmosphere could be divided into five time periods during the day that were thought to be related to the local diurnal sea breeze and Pacific Coast monsoon weather conditions as follows: (i) the nighttime minimum concentration, i.e., 2300 to 0600 h; (ii) the sunrise peak concentration, i.e., 0600 to 0800 h; (iii) the midday accumulating concentration, i.e., 0800 to 1515 h; (iv) the late-afternoon sea breeze trough concentration, i.e., 1515 to 1700 h; and (v) the evening decrease to the nighttime minimum concentration, i.e., 1700 to 2300 h. The sunrise peak concentration (period ii) is thought to be a relatively general phenomenon dependent on ground heating by the sun, while the afternoon trough concentration is thought to be a relatively local phenomenon dependent on the afternoon sea breeze. Meteorological conditions are thought to be an important regulating influence on airborne bacterial concentrations in the outdoor atmosphere in the Willamette River Valley. PMID:16534998
Quasi-neutral Vlasov theory of magnetized plasmas
NASA Astrophysics Data System (ADS)
Tronci, Cesare; Camporeale, Enrico
2015-11-01
The low-frequency limit of Maxwell equations is considered in the Maxwell-Vlasov system. This limit produces a quasi-neutral Vlasov system that captures essential features of plasma dynamics, while neglecting radiation effects. Euler-Poincaré reduction theory is used to show that the quasi-neutral Vlasov theory possesses a variational formulation in both Lagrangian and Eulerian coordinates. By construction, the new model recovers all collisionless neutral models employed in plasma simulations. Then, comparisons between the quasi-neutral Vlasov system and hybrid kinetic-fluid models are presented in the linear regime. Financial support by the Leverhulme Trust Research Project Grant 2014-112 is greatly acknowledged.
Scaling of asymmetric magnetic reconnection: General theory and collisional simulations
Cassak, P. A.; Shay, M. A.
2007-10-15
A Sweet-Parker-type scaling analysis for asymmetric antiparallel reconnection (in which the reconnecting magnetic field strengths and plasma densities are different on opposite sides of the dissipation region) is performed. Scaling laws for the reconnection rate, outflow speed, the density of the outflow, and the structure of the dissipation region are derived from first principles. These results are independent of the dissipation mechanism. It is shown that a generic feature of asymmetric reconnection is that the X-line and stagnation point are not colocated, leading to a bulk flow of plasma across the X-line. The scaling laws are verified using two-dimensional resistive magnetohydrodynamics numerical simulations for the special case of asymmetric magnetic fields with symmetric density. Observational signatures and applications to reconnection in the magnetosphere are discussed.
Theory of fluorescence polarization in magnetically oriented photosynthetic systems.
Knox, R S; Davidovich, M A
1978-01-01
Many cells and cell fragments are known to assume specific alignments with respect to an applied magnetic field. One indicator of this alignment is a difference between the intensities of fluorescence observed in polarizations parallel and perpendicular to the magnetic filed. We calculate these two intensities using a model that assumes axially symmetric membranes and that covers a wide variety of shapes from flat disk to right cylinder. The fluorescence is assumed to originate at chromophores randomly exicted but nonrandomly oriented in the membranes. The membrane alignment is assumed to be due to the net torque on a nonrandom distribution of diamagnetically anisotropic molecules. The predicted results are consistent with most magnetoorientation data from green cells, but we are able to show that Chlorella data are not consistent with the hypothesis that the membranes have, and maintain, a cuplike configuration. Images FIGURE 4 FIGURE 5 PMID:737283
Contribution to the Theory of Magnetoelectric Effect in Magnetic Semiconductors
NASA Astrophysics Data System (ADS)
Kuivalainen, P.; Sinkkonen, J.; Stubb, T.
1980-01-01
A generation of a d.c. electric field at ferromagnetic resonance or the magnetoelectric effect in magnetic semiconductors is studied theoretically in the whole magnon wave vector space. The magnetoelectric effect in the classical regime kl < 1 and contributions of different electron-magnon interactions to the magnon drag effect in the quantum regime kl > 1 are calculated. All the calculated static electric fields are compared with experimental results. At low-microwave power levels the non-linear electrodynamic interaction due to a mixing of the oscillating magnetization (k = 0) and the induced electric field seems to dominate the magnetoelectric effect. In some cases the classical drag effect may also be significant. At sufficiently high power levels the quantum mechanical magnon drag can make the largest contribution. It is shown that in the magnon drag effect the induced static electric field is proportional to the damping coefficient of magnons.
2011-01-01
Background Magnetic Particle Imaging is a novel method for medical imaging. It can be used to measure the local concentration of a tracer material based on iron oxide nanoparticles. While the resulting images show the distribution of the tracer material in phantoms or anatomic structures of subjects under examination, no information about the tissue is being acquired. To expand Magnetic Particle Imaging into the detection of soft tissue properties, a new method is proposed, which detects acoustic emissions caused by magnetization changes in superparamagnetic iron oxide. Methods Starting from an introduction to the theory of acoustically detected Magnetic Particle Imaging, a comparison to magnetically detected Magnetic Particle Imaging is presented. Furthermore, an experimental setup for the detection of acoustic emissions is described, which consists of the necessary field generating components, i.e. coils and permanent magnets, as well as a calibrated microphone to perform the detection. Results The estimated detection limit of acoustic Magnetic Particle Imaging is comparable to the detection limit of magnetic resonance imaging for iron oxide nanoparticles, whereas both are inferior to the theoretical detection limit for magnetically detected Magnetic Particle Imaging. Sufficient data was acquired to perform a comparison to the simulated data. The experimental results are in agreement with the simulations. The remaining differences can be well explained. Conclusions It was possible to demonstrate the detection of acoustic emissions of magnetic tracer materials in Magnetic Particle Imaging. The processing of acoustic emission in addition to the tracer distribution acquired by magnetic detection might allow for the extraction of mechanical tissue parameters. Such parameters, like for example the velocity of sound and the attenuation caused by the tissue, might also be used to support and improve ultrasound imaging. However, the method can also be used to perform
Gleich, Bernhard; Weizenecker, Jürgen; Borgert, Jörn
2011-06-29
Magnetic Particle Imaging is a novel method for medical imaging. It can be used to measure the local concentration of a tracer material based on iron oxide nanoparticles. While the resulting images show the distribution of the tracer material in phantoms or anatomic structures of subjects under examination, no information about the tissue is being acquired. To expand Magnetic Particle Imaging into the detection of soft tissue properties, a new method is proposed, which detects acoustic emissions caused by magnetization changes in superparamagnetic iron oxide. Starting from an introduction to the theory of acoustically detected Magnetic Particle Imaging, a comparison to magnetically detected Magnetic Particle Imaging is presented. Furthermore, an experimental setup for the detection of acoustic emissions is described, which consists of the necessary field generating components, i.e. coils and permanent magnets, as well as a calibrated microphone to perform the detection. The estimated detection limit of acoustic Magnetic Particle Imaging is comparable to the detection limit of magnetic resonance imaging for iron oxide nanoparticles, whereas both are inferior to the theoretical detection limit for magnetically detected Magnetic Particle Imaging. Sufficient data was acquired to perform a comparison to the simulated data. The experimental results are in agreement with the simulations. The remaining differences can be well explained. It was possible to demonstrate the detection of acoustic emissions of magnetic tracer materials in Magnetic Particle Imaging. The processing of acoustic emission in addition to the tracer distribution acquired by magnetic detection might allow for the extraction of mechanical tissue parameters. Such parameters, like for example the velocity of sound and the attenuation caused by the tissue, might also be used to support and improve ultrasound imaging. However, the method can also be used to perform imaging on its own.
Nuclear conversion theory: molecular hydrogen in non-magnetic insulators
Ghiglieno, Filippo
2016-01-01
The hydrogen conversion patterns on non-magnetic solids sensitively depend upon the degree of singlet/triplet mixing in the intermediates of the catalytic reaction. Three main ‘symmetry-breaking’ interactions are brought together. In a typical channel, the electron spin–orbit (SO) couplings introduce some magnetic excitations in the non-magnetic solid ground state. The electron spin is exchanged with a molecular one by the electric molecule–solid electron repulsion, mixing the bonding and antibonding states and affecting the molecule rotation. Finally, the magnetic hyperfine contact transfers the electron spin angular momentum to the nuclei. Two families of channels are considered and a simple criterion based on the SO coupling strength is proposed to select the most efficient one. The denoted ‘electronic’ conversion path involves an emission of excitons that propagate and disintegrate in the bulk. In the other denoted ‘nuclear’, the excited electron states are transients of a loop, and the electron system returns to its fundamental ground state. The described model enlarges previous studies by extending the electron basis to charge-transfer states and ‘continui’ of band states, and focuses on the broadening of the antibonding molecular excited state by the solid conduction band that provides efficient tunnelling paths for the hydrogen conversion. After working out the general conversion algebra, the conversion rates of hydrogen on insulating and semiconductor solids are related to a few molecule–solid parameters (gap width, ionization and affinity potentials) and compared with experimental measures. PMID:27703681
Robiche, J.; Rax, J.-M.; Bonnaud, G.; Gremillet, L.
2010-03-15
The collisional dynamics of a relativistic electron jet in a magnetized plasma are investigated within the framework of kinetic theory. The relativistic Fokker-Planck equation describing slowing down, pitch angle scattering, and cyclotron rotation is derived and solved. Based on the solution of this Fokker-Planck equation, an analytical formula for the root mean square spot size transverse to the magnetic field is derived and this result predicts a reduction in radial transport. Some comparisons with particle-in-cell simulation are made and confirm striking agreement between the theory and the simulation. For fast electron with 1 MeV typical kinetic energy interacting with a solid density hydrogen plasma, the energy deposition density in the transverse direction increases by a factor 2 for magnetic field of the order of 1 T. Along the magnetic field, the energy deposition profile is unaltered compared with the field-free case.
Electronic structure theory of wide gap dilute magnetic semiconductors
NASA Astrophysics Data System (ADS)
Ye, Linhui; Freeman, A. J.
2007-03-01
The recent exciting reports that wide gap semiconductors, most notably ZnO, TiO2 and GaN, when doped with transition metal elements, may have Tc's that are higher than room temperature have attracted great interest. When interpreted with care, highly precise first principles FLAPW calculations such as used here, are now providing insights into the nature of their strong ferromagnetism (FM). Here, we present an analysis to the electronic structures of several typical wide gap DMS's and illustrate how first principles calculations can lead to correct predictions of their magnetic properties for both Cr:TiO2 and Mn:GaN. The results demonstrate the importance of defect compensation in the determination of the magnetism. A comparison between Mn:ZnO and Co:ZnO highlights the fundamental difference in their electronic structures which explains why their FM is dependent on carriers of different polarity. Correct predictions of their magnetism are found to be due to the correct treatment of the LDA band gap problem. Finally, we provide semi-quantitative discussions of Co doped TiO2, and illustrate why it is highly non- trivial to fully explain its FM based on first principles calculations. E.Wimmer,H.Krakauer,M.Weinert,A.J.Freeman, PRB 24, 864(1981)
Prasanna, A.R.
1982-05-15
In this brief paper we present a complete exact solution for the external magnetic field (dipolar at infinity) of a static magnetic star on the spherically symmetric background metric solution of Rosen's bimetric theory of gravity. Unlike in general relativity the field is well behaved throughout the manifold except at r = 0, and thus allows one to consider the field for stars collapsed beyond 2m.
Non-Abelian Dual Superconductivity in SU(3) Yang-Mills Theory due to Non-Abelian Magnetic Monopoles
NASA Astrophysics Data System (ADS)
Kondo, Kei-Ichi; Shibata, Akihiro; Kato, Seikou; Shinohara, Toru
We give numerical evidences for the non-Abelian dual superconductivity due to non-Abelian magnetic monopoles in SU(3) Yang-Mills theory as a mechanism for quark confinement, based on our new formulation of lattice gauge theory.
Non-Abelian discrete flavor symmetries of 10D SYM theory with magnetized extra dimensions
NASA Astrophysics Data System (ADS)
Abe, Hiroyuki; Kobayashi, Tatsuo; Ohki, Hiroshi; Sumita, Keigo; Tatsuta, Yoshiyuki
2014-06-01
We study discrete flavor symmetries of the models based on a ten-dimensional supersymmetric Yang-Mills (10D SYM) theory compactified on magnetized tori. We assume non-vanishing non-factorizable fluxes as well as the orbifold projections. These setups allow model-building with more various flavor structures. Indeed, we show that there exist various classes of non-Abelian discrete flavor symmetries. In particular, we find that S 3 flavor symmetries can be realized in the framework of the magnetized 10D SYM theory for the first time.
Theory and application of maximum magnetic energy in toroidal plasmas
Chu, T.K.
1992-02-01
The magnetic energy in an inductively driven steady-state toroidal plasma is a maximum for a given rate of dissipation of energy (Poynting flux). A purely resistive steady state of the piecewise force-free configuration, however, cannot exist, as the periodic removal of the excess poloidal flux and pressure, due to heating, ruptures the static equilibrium of the partitioning rational surfaces intermittently. The rupture necessitates a plasma with a negative q{prime}/q (as in reverse field pinches and spheromaks) to have the same {alpha} in all its force-free regions and with a positive q{prime}/q (as in tokamaks) to have centrally peaked {alpha}'s.
Theory and application of maximum magnetic energy in toroidal plasmas
Chu, T.K.
1992-02-01
The magnetic energy in an inductively driven steady-state toroidal plasma is a maximum for a given rate of dissipation of energy (Poynting flux). A purely resistive steady state of the piecewise force-free configuration, however, cannot exist, as the periodic removal of the excess poloidal flux and pressure, due to heating, ruptures the static equilibrium of the partitioning rational surfaces intermittently. The rupture necessitates a plasma with a negative q{prime}/q (as in reverse field pinches and spheromaks) to have the same {alpha} in all its force-free regions and with a positive q{prime}/q (as in tokamaks) to have centrally peaked {alpha}`s.
Magnetically insulated theory with both electron and ion flows
NASA Astrophysics Data System (ADS)
Wang, Huihui; Meng, Lin; Liu, Dagang; Liu, Laqun; Yang, Chao
2012-10-01
Both the ion emission from anode surface and the electron emission from cathode surface may occur in the magnetically insulated transmission line (MITL) with a very high pulsed power and a very large current density. A model for the MITL with both electron and ion flow is developed. In this model, physical quantities (such as space-charge sheath thicknesses and flow currents) in the MITL are theoretically analyzed, and the specific expression for the voltage on the line by the terms of currents is derived. Furthermore, particle-in-cell simulations are carried out to verify the theoretical results.
Magnetically insulated theory with both electron and ion flows
Wang Huihui; Meng Lin; Liu Dagang; Liu Laqun; Yang Chao
2012-10-15
Both the ion emission from anode surface and the electron emission from cathode surface may occur in the magnetically insulated transmission line (MITL) with a very high pulsed power and a very large current density. A model for the MITL with both electron and ion flow is developed. In this model, physical quantities (such as space-charge sheath thicknesses and flow currents) in the MITL are theoretically analyzed, and the specific expression for the voltage on the line by the terms of currents is derived. Furthermore, particle-in-cell simulations are carried out to verify the theoretical results.
Mean-field theory for confinement transitions and magnetization plateaux in spin ice
NASA Astrophysics Data System (ADS)
Powell, Stephen
2017-03-01
We study phase transitions in classical spin ice at nonzero magnetization, by introducing a mean-field theory designed to capture the interplay between confinement and topological constraints. The method is applied to a model of spin ice in an applied magnetic field along the ≤ft[1 0 0\\right] crystallographic direction and yields a phase diagram containing the Coulomb phase as well as a set of magnetization plateaux. We argue that the lobe structure of the phase diagram, strongly reminiscent of the Bose–Hubbard model, is generic to Coulomb spin liquids.
NASA Astrophysics Data System (ADS)
Santos, Jander P.; Sá Barreto, F. C.
2017-10-01
Thermodynamic and magnetic properties of a trilayer nanostructure of hexagonal lattices described by the spin-1/2 Ising model are investigated by the use of the effective-field theory (EFT) with correlations. The results for the magnetization, the free energy, the internal energy, the entropy, the specific heat and the critical frontiers were obtained. The critical temperature and the compensation temperature are investigated with a negative interlayer coupling, in order to clarify the distinction between the ferromagnetic and ferrimagnetic behaviors. From the thermal variations of the total magnetization, the six compensation types can be found, i.e., L-, Q-, R-, S-, P-, and N-types.
Modelling and analysis of magnetic memory testing method based on the density functional theory
NASA Astrophysics Data System (ADS)
Liu, Bin; Fu, Ying; Jian, Ren
2015-01-01
Metal magnetic memory (MMM) method is a novel, passive magnetic method for inspecting mechanical degradation of ferromagnetic components. To promote a further understanding of the MMM testing mechanism, the relationship between stress concentration and the self-magnetic leakage field measured by MMM effect was quantitatively interpreted using a density functional theory based on the generalised gradient approximation. Meanwhile, the influence of doping effect on MMM signal was calculated. Interestingly, the theoretical approach is in very good agreement with the experimental observations. A new research programme for quantitative interpretation of the MMM effect was initiated.
Exact two-component relativistic theory for nuclear magnetic resonance parameters.
Sun, Qiming; Liu, Wenjian; Xiao, Yunlong; Cheng, Lan
2009-08-28
An exact two-component (X2C) relativistic theory for nuclear magnetic resonance parameters is obtained by first a single block-diagonalization of the matrix representation of the Dirac operator in a magnetic-field-dependent basis and then a magnetic perturbation expansion of the resultant two-component Hamiltonian and transformation matrices. Such a matrix formulation is not only simple but also general in the sense that the various ways of incorporating the field dependence can be treated in a unified manner. The X2C dia- and paramagnetic terms agree individually with the corresponding four-component ones up to machine accuracy for any basis.
Quark confinement due to non-Abelian magnetic monopoles in SU(3) Yang-Mills theory
Kondo, Kei-Ichi; Shibata, Akihiro; Shinohara, Toru; Kato, Seikou
2012-10-23
We present recent results on quark confinement: in SU(3) Yang-Mills theory, confinement of fundamental quarks is obtained due to the dual Meissner effect originated from non-Abelian magnetic monopoles defined in a gauge-invariant way, which is distinct from the well-known Abelian projection scenario. This is achieved by using a non-Abelian Stokes theorem for the Wilson loop operator and a new reformulation of the Yang-Mills theory.
Electromagnetic fluctuations in magnetized plasmas. I. The rigorous relativistic kinetic theory
Schlickeiser, R. E-mail: yoonp@umd.edu; Yoon, P. H. E-mail: yoonp@umd.edu
2015-07-15
Using the system of the Klimontovich and Maxwell equations, the general linear fluctuation theory for magnetized plasmas is developed. General expressions for the electromagnetic fluctuation spectra (electric and magnetic fields) from uncorrelated plasma particles in plasmas with a uniform magnetic field are derived, which are covariantly correct within the theory of special relativity. The general fluctuation spectra hold for plasmas of arbitrary composition, arbitrary momentum dependences of the plasma particle distribution functions, and arbitrary orientations of the wave vector with respect to the uniform magnetic field. Moreover, no restrictions on the values of the real and the imaginary parts of the frequency are made. The derived fluctuation spectra apply to both non-collective fluctuations and collective plasma eigenmodes in magnetized plasmas. In the latter case, kinetic equations for the components of fluctuating electric and magnetic fields in magnetized plasmas are derived that include the effect of spontaneous emission and absorption. In the limiting case of an unmagnetized plasmas, the general fluctuation spectra correctly reduce to the unmagnetized fluctuation spectra derived before.
Non-linear magnetization effects within the Kosterlitz-Thouless theory
NASA Astrophysics Data System (ADS)
Benfatto, Lara; Castellani, Claudio; Giamarchi, Thierry
2008-03-01
Recent experiments in cuprate superconductors have attracted the attention on the role of vortex fluctuations. Measurements of the field-induced magnetization showed that the correlation length diverge exponentially, as predicted within the Kosterlitz-Thouless (KT) theory. However, it is somehow puzzling thepersistence of strong non-linear magnetization effects at low field. Here we address this issue by means of a new theoretical approach to the KT transition at finite magnetic field, based on the sine-Gordon model. This approach is particularly useful in two respects. First, it leads to a straightforward definition of the field-induced magnetization as a function of the external magnetic field H instead of the magnetic induction B, which is crucial to get a consistent description of the Meissner phase. Second, it allows us to identify the cross-over field Hcr from linear to non-linear magnetization both below and above the transition. Above TKT Hcr turns out to scale as the inverse correlation length, so that it decreases as the transition is approached. As a consequence, the fact that only the non-linear regime is accessible experimentally should be interpreted as a typical signature of the fast divergence of the correlation length within the KT theory. L.Benfatto, C.Castellani and T.Giamarchi, Phys. Rev. Lett. 99, 207002 (2007)
Electromagnetic fluctuations in magnetized plasmas. I. The rigorous relativistic kinetic theory
NASA Astrophysics Data System (ADS)
Schlickeiser, R.; Yoon, P. H.
2015-07-01
Using the system of the Klimontovich and Maxwell equations, the general linear fluctuation theory for magnetized plasmas is developed. General expressions for the electromagnetic fluctuation spectra (electric and magnetic fields) from uncorrelated plasma particles in plasmas with a uniform magnetic field are derived, which are covariantly correct within the theory of special relativity. The general fluctuation spectra hold for plasmas of arbitrary composition, arbitrary momentum dependences of the plasma particle distribution functions, and arbitrary orientations of the wave vector with respect to the uniform magnetic field. Moreover, no restrictions on the values of the real and the imaginary parts of the frequency are made. The derived fluctuation spectra apply to both non-collective fluctuations and collective plasma eigenmodes in magnetized plasmas. In the latter case, kinetic equations for the components of fluctuating electric and magnetic fields in magnetized plasmas are derived that include the effect of spontaneous emission and absorption. In the limiting case of an unmagnetized plasmas, the general fluctuation spectra correctly reduce to the unmagnetized fluctuation spectra derived before.
Theory of the negative magnetoresistance in magnetic metallic multilayers
Hood, R.Q.; Falicov, L.M. |
1993-04-01
The Boltzman equation is solved for a system consisting of alternating ferromagnetic normal metallic layers. The in-plane conductance of the film is calculated for two configurations: successive ferromagnetic layers aligned parallel and antiparallel to each other. Results explain the giant negative magnetoresistance encountered in these systems when an initial antiparallel arrangement is changed into a parallel configuration by application of an extemal magnetic field. The calculation depends on geometric parameters (the thicknesses of the layers); intrinsic metal parameters (number of conduction electrons, magnetization and effective masses in the layers); bulk sample properties (conductivity relaxation times); and interface scattering properties (diffuse scattering versus potential scattering at the interfaces). It is found that a large negative magnetoresistance requires, in general, considerable asymmetry in the interface scattering for the two spin orienmtions. All qualitative features of the experiments are reproduced. Quantitative agreement can be achieved with sensible values of the parameters. The effect can be conceptually explained based on considerations of phase-space availability for an electron of a given spin orientation as it travels through the multilayer sample in the various configurations and traverses the interfaces.
Separation of magnetization precession in 3He-B into two magnetic domains. Theory
NASA Astrophysics Data System (ADS)
Fomin, I. A.
It is shown that even small deviations of the magnetic field from uniformity can substantially modify the magnetization precession in 3He-B. Specifically, a two-domain structure forms if the magnetic-field non-uniformity is linear. The magnetization makes an angle ˜ 104° with the field in one of the domains and is parallel to it in the other. These domains can explain the anomalously long persistence of the induction signal in 3He-B; moreover, the change in the induction-signal frequency with time discovered and investigated by Borovik-Romanov et al. [JETP Lett. 40, 1033 (1984)] is a consequence of the relaxation of the domain structure.
Effective spin theories for edge magnetism in graphene zigzag ribbons
NASA Astrophysics Data System (ADS)
Koop, Cornelie; Schmidt, Manuel J.
2015-09-01
We report a thorough study of the reducibility of edge correlation effects in graphene to much simplified effective models for the edge states. The latter have been used before in specially tailored geometries. By a systematic investigation of corrections due to the bulk states in second-order perturbation theory, we show that the reduction to pure edge state models is well justified in general geometries. The framework of reduced models discussed here enables the study of non-mean-field correlation physics for system sizes far beyond the reach of conventional methods, such as, e.g., quantum Monte Carlo.
Theory of the jitter radiation in a magnetized plasma accompanying a temperature gradient
NASA Astrophysics Data System (ADS)
Hattori, Makoto; Fujiki, Kazushiro
2016-04-01
The linear stability of a magnetized plasma accompanying a temperature gradient is reexamined by using plasma kinetic theory. We propose that the anisotropic velocity distribution function should be decomposed into two components. One is proportional to the temperature gradient parallel to the background magnetic field. The other is proportional to the temperature gradient perpendicular to the background magnetic field. Since the amplitude of the anisotropic velocity distribution function is proportional to the heat conductivity, and the heat conductivity perpendicular to the magnetic field is strongly reduced, the first component of the anisotropic velocity distribution function is predominant. The anisotropic velocity distribution function induced by the temperature gradient along the background magnetic field drives plasma kinetic instability and circular polarized magnetic plasma waves are excited. We show that the instability is almost identical to the Weibel instability in the weakly magnetized plasma. However, in the case of the instability caused by the temperature gradient, whether wave vectors of modes are parallel to or antiparallel to the background magnetic field, the growth rate of one mode is suppressed and the growth rate of the other mode is enhanced due to the background magnetic field. In the strongly magnetized plasma, one mode is stabilized and only one of the modes remains unstable. The formulae for the jitter radiation spectrum emitted by relativistic electrons when they travel through the magnetized plasma with the plasma waves driven by the instability are deduced at the first time. We show that the synchrotron emission and the jitter radiation are simultaneously emitted from the same relativistic electron. The jitter radiation is expected to be circularly polarized but with a very small polarization degree since almost the same amounts of left-handed and right-handed circular polarized magnetic waves are excited by the instability.
Kinetic theory of a two-dimensional magnetized plasma.
NASA Technical Reports Server (NTRS)
Vahala, G.; Montgomery, D.
1971-01-01
Several features of the equilibrium and nonequilibrium statistical mechanics of a two-dimensional plasma in a uniform dc magnetic field are investigated. The charges are assumed to interact only through electrostatic potentials. The problem is considered both with and without the guiding-center approximation. With the guiding-center approximation, an appropriate Liouville equation and BBGKY hierarchy predict no approach to thermal equilibrium for the spatially uniform case. For the spatially nonuniform situation, a guiding-center Vlasov equation is discussed and solved in special cases. For the nonequilibrium, nonguiding-center case, a Boltzmann equation, and a Fokker-Planck equation are derived in the appropriate limits. The latter is more tractable than the former, and can be shown to obey conservation laws and an H-theorem, but contains a divergent integral which must be cut off on physical grounds. Several unsolved problems are posed.
Kinetic theory of a two-dimensional magnetized plasma.
NASA Technical Reports Server (NTRS)
Vahala, G.; Montgomery, D.
1971-01-01
Several features of the equilibrium and nonequilibrium statistical mechanics of a two-dimensional plasma in a uniform dc magnetic field are investigated. The charges are assumed to interact only through electrostatic potentials. The problem is considered both with and without the guiding-center approximation. With the guiding-center approximation, an appropriate Liouville equation and BBGKY hierarchy predict no approach to thermal equilibrium for the spatially uniform case. For the spatially nonuniform situation, a guiding-center Vlasov equation is discussed and solved in special cases. For the nonequilibrium, nonguiding-center case, a Boltzmann equation, and a Fokker-Planck equation are derived in the appropriate limits. The latter is more tractable than the former, and can be shown to obey conservation laws and an H-theorem, but contains a divergent integral which must be cut off on physical grounds. Several unsolved problems are posed.
Lemons, Don S.
2012-01-15
We develop a Markov process theory of charged particle scattering from stationary, transverse, magnetic waves. We examine approximations that lead to quasilinear theory, in particular the resonant diffusion approximation. We find that, when appropriate, the resonant diffusion approximation simplifies the result of the weak turbulence approximation without significant further restricting the regime of applicability. We also explore a theory generated by expanding drift and diffusion rates in terms of a presumed small correlation time. This small correlation time expansion leads to results valid for relatively small pitch angle and large wave energy density - a regime that may govern pitch angle scattering of high-energy electrons into the geomagnetic loss cone.
Xiao, Yunlong; Liu, Wenjian; Cheng, Lan; Peng, Daoling
2007-06-07
Both formal and numerical analyses have been carried out on various exact and approximate variants of the four-component relativistic theory for nuclear magnetic shielding constants. These include the standard linear response theory (LRT), the full or external field-dependent unitary transformations of the Dirac operator, as well as the orbital decomposition approach. In contrast with LRT, the latter schemes take explicitly into account both the kinetic and magnetic balances between the large and small components of the Dirac spinors, and are therefore much less demanding on the basis sets. In addition, the diamagnetic contributions, which are otherwise "missing" in LRT, appear naturally in the latter schemes. Nevertheless, the definitions of paramagnetic and diamagnetic terms are not the same in the different schemes, but the difference is only of O(c(-2)) and thus vanishes in the nonrelativistic limit. It is shown that, as an operator theory, the full field-dependent unitary transformation approach cannot be applied to singular magnetic fields such as that due to the magnetic point dipole moment of a nucleus. However, the inherent singularities can be avoided by the corresponding matrix formulation (with a partial closed summation). All the schemes are combined with the Dirac-Kohn-Sham ansatz for ground state calculations, and by using virtually complete basis sets a new and more accurate set of absolute nuclear magnetic resonance shielding scales for the rare gases He-Rn have been established.
Ginzburg-Landau theory for skyrmions in inversion-symmetric magnets with competing interactions
Lin, Shi-Zeng; Hayami, Satoru
2016-02-01
Magnetic skyrmions have attracted considerable attention recently for their huge potential in spintronic applications. Generally skyrmions are big compared to the atomic lattice constant, which allows for the Ginzburg-Landau type description in the continuum limit. This description successfully captures the main experimental observations on skyrmions in B20 compound without inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets with competing interactions. Here, we derive a general Ginzburg-Landau theory for skyrmions in these magnets valid in the long-wavelength limit. We study the unusual static and dynamical properties of skyrmions based on the derived Ginzburg-Landau theory. We show that an easy axismore » spin anisotropy is sufficient to stabilize a skyrmion lattice. Interestingly, the skyrmion in inversion-symmetric magnets has a new internal degree of freedom associated with the rotation of helicity, i.e., the “spin” of the skyrmion as a particle, in addition to the usual translational motion of skyrmions (orbital motion). The orbital and spin degree of freedoms of an individual skyrmion can couple to each other, and give rise to unusual behavior that is absent for the skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. Finally, the derived Ginzburg-Landau theory provides a convenient and general framework to discuss skyrmion physics and will facilitate the search for skyrmions in inversion-symmetric magnets.« less
Ginzburg-Landau theory for skyrmions in inversion-symmetric magnets with competing interactions
Lin, Shi-Zeng; Hayami, Satoru
2016-02-01
Magnetic skyrmions have attracted considerable attention recently for their huge potential in spintronic applications. Generally skyrmions are big compared to the atomic lattice constant, which allows for the Ginzburg-Landau type description in the continuum limit. This description successfully captures the main experimental observations on skyrmions in B20 compound without inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets with competing interactions. Here, we derive a general Ginzburg-Landau theory for skyrmions in these magnets valid in the long-wavelength limit. We study the unusual static and dynamical properties of skyrmions based on the derived Ginzburg-Landau theory. We show that an easy axis spin anisotropy is sufficient to stabilize a skyrmion lattice. Interestingly, the skyrmion in inversion-symmetric magnets has a new internal degree of freedom associated with the rotation of helicity, i.e., the “spin” of the skyrmion as a particle, in addition to the usual translational motion of skyrmions (orbital motion). The orbital and spin degree of freedoms of an individual skyrmion can couple to each other, and give rise to unusual behavior that is absent for the skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. Finally, the derived Ginzburg-Landau theory provides a convenient and general framework to discuss skyrmion physics and will facilitate the search for skyrmions in inversion-symmetric magnets.
A Compact, TIM-Based, Pulsed-Power System for Magnetized Target Experiments on OMEGA
NASA Astrophysics Data System (ADS)
Gotchev, O. V.; Barbero, M. D.; Jang, N. W.; Knauer, J. P.; Betti, R.
2006-10-01
By magnetizing the target and then compressing the magnetic flux to levels sufficient to inhibit thermal transport in the hot spot, one can trigger ignition in massive cryogenic shells imploded with low velocity. The reduction in thermal-conduction losses leads to increased hot-spot temperatures at lower implosion velocities, thus relaxing the energy requirements for ignition. This work describes a compact, pulsed-power system for the generation of a macroscopic seed magnetic field and its integration into such flux-compression experiments on OMEGA. Magnetohydrodynamic simulations predict compression of a 10-T seed field to multimegagauss values. A fast (100-ns) current pulse (up to 60 kA), driven by a TIM-based energy-delivery system, is discharged into a low-mass, double coil that surrounds the laser target. A working prototype has generated a >11-T seed field utilizing a <100-J capacitor bank, laser-triggered spark gap, and a low-impedance (<1-φ) stripline. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. N. W. Jang et al., ``Theory and Simulation of Laser-Driven Magnetic Field Compression,'' this conference.
Bianchi type-I magnetized radiating cosmological model in self creation theory of gravitation
NASA Astrophysics Data System (ADS)
Jain, Vimal Chand; Jain, Nikhil
2015-06-01
We have investigated Bianchi type-I cosmological model in the presence of magnetized field with disordered radiation in Barber's second self-creation theory of gravitation. To obtain exact solution we assume that the component of shear tensor is proportional to expansion ( θ). Some geometrical and physical properties of the model have also been discussed.
NASA Technical Reports Server (NTRS)
Sung, C. C.
1973-01-01
The longitudinal and transverse spin-correlation functions of local paramagnetic impurities are solved in the long-time limit on the basis of the Blume-Hubbard theory. The magnetic field dependence of the nuclear spin-lattice relaxation via paramagnetic centers is in good agreement with the experimental data by McHenry et al.
Charge, magnetic dipole, and lense-thirring effect in the generalized theory of gravitation
Arutyunyan, G.G.; Papoyan, V.V.
1985-05-01
Three physical problems are solved in the framework of the generalized theory of gravitation. The gravitational field of a point charged mass and an expression for the vector potential for a magnetic field of dipole nature are found, and the angular velocity of frame dragging by the rotation of a central body is calculated.
Some properties of magnetized Bonnor-dihole solution in Brans-Dicke theory
NASA Astrophysics Data System (ADS)
Kirezli, Pinar; Delice, Ozgur
2017-02-01
Some properties of magnetized Bonnor-dihole solution in Brans-Dicke (BD) theory is discussed. Conical deficit angle and equatiorial geodesics of timelike and null particles are investigated in detail for different values of the parameters of the BD solution.
Topics in the Theory of Josephson Arrays and Disordered Magnetic Systems
NASA Astrophysics Data System (ADS)
Porter, Christopher D.
This thesis consists of two parts. In the first part, we discuss several topics in the theory of Josephson junction arrays. The second part is concerned with two problems in the theory of magnetic systems: charge transfer ferromagnetism, and the clustering of Fe adatoms on graphene. In the field of Josephson arrays we consider three topics. First, the effects of a current bias on arrays of underdamped junctions are considered, for several junction geometries including both 2D and 3D systems. Approximate phase diagrams are constructed for various values of the ratio of charging energy to Josephson coupling. The effects of finite temperature are also discussed. Next, we examine the rich response of Josephson arrays to magnetic fields in the case of Josephson ladders with nonuniform spacing, known as superconducting quantum interference filters (SQIFs). Such ladders are already used for the detection of DC magnetic fields, but here their applicability to detecting AC fields is also discussed. It is shown that, for sufficiently low frequencies, the voltage produced by an AC field is equivalent to a convolution of the DC voltage response with the sinusoidal field oscillation. These SQIFs are studied in an effort to greatly increase the period of their response to external magnetic fields. Finally, we investigate underdamped 2D and 3D arrays with a special inhomogeneity. Specifically, insulating regions of varying width are sandwiched between superconducting regions of the arrays. The phase ordering in the superconducting regions is shown to penetrate into the insulating regions, leading to an unusual type of proximity effect. Our calculations for these arrays are done using mean field and perturbation theory, mean field theory and numerical methods, and our results are quantitatively confirmed by quantum Monte Carlo calculations. The superconducting correlation length is calculated inside the insulating region and it is found that the structure yields multiple layers of
Nonlinear theory of a "shear-current" effect and mean-field magnetic dynamos.
Rogachevskii, Igor; Kleeorin, Nathan
2004-10-01
The nonlinear theory of a "shear-current" effect in a nonrotating and nonhelical homogeneous turbulence with an imposed mean velocity shear is developed. The shear-current effect is associated with the W x J term in the mean electromotive force and causes the generation of the mean magnetic field even in a nonrotating and nonhelical homogeneous turbulence (where W is the mean vorticity and J is the mean electric current). It is found that there is no quenching of the nonlinear shear-current effect contrary to the quenching of the nonlinear alpha effect, the nonlinear turbulent magnetic diffusion, etc. During the nonlinear growth of the mean magnetic field, the shear-current effect only changes its sign at some value B (*) of the mean magnetic field. The magnitude B (*) determines the level of the saturated mean magnetic field which is less than the equipartition field. It is shown that the background magnetic fluctuations due to the small-scale dynamo enhance the shear-current effect and reduce the magnitude B (*) . When the level of the background magnetic fluctuations is larger than 1/3 of the kinetic energy of the turbulence, the mean magnetic field can be generated due to the shear-current effect for an arbitrary exponent of the energy spectrum of the velocity fluctuations.
Microscopic theory for the Markovian decay of magnetization fluctuations in nanomagnets
NASA Astrophysics Data System (ADS)
Rousochatzakis, Ioannis
2007-12-01
We present a microscopic theory for the phonon-driven decay of the magnetization fluctuations in a wide class of nanomagnets where the dominant energy is set by isotropic exchange and/or uniaxial anisotropy. Based on the Zwanzig-Mori projection formalism, the theory reveals that the magnetization fluctuations are governed by a single decay rate ωc , which we further identify with the zero-frequency portion of the associated self-energy. This dynamical decoupling from the remaining slow degrees of freedom is attributed to a conservation law and the discreteness of the energy spectrum and explains the omnipresent monoexponential decay of the magnetization over several decades in time, as observed experimentally. A physically transparent analytical expression for ωc is derived which highlights the three specific mechanisms of the slowing-down effect which are known so far in nanomagnets.
Stochastic field-line wandering in magnetic turbulence with shear. I. Quasi-linear theory
Shalchi, A.; Negrea, M.; Petrisor, I.
2016-07-15
We investigate the random walk of magnetic field lines in magnetic turbulence with shear. In the first part of the series, we develop a quasi-linear theory in order to compute the diffusion coefficient of magnetic field lines. We derive general formulas for the diffusion coefficients in the different directions of space. We like to emphasize that we expect that quasi-linear theory is only valid if the so-called Kubo number is small. We consider two turbulence models as examples, namely, a noisy slab model as well as a Gaussian decorrelation model. For both models we compute the field line diffusion coefficients and we show how they depend on the aforementioned Kubo number as well as a shear parameter. It is demonstrated that the shear effect reduces all field line diffusion coefficients.
Magnetic exchange couplings from noncollinear perturbation theory: dinuclear CuII complexes.
Phillips, Jordan J; Peralta, Juan E
2014-08-07
To benchmark the performance of a new method based on noncollinear coupled-perturbed density functional theory [J. Chem. Phys. 138, 174115 (2013)], we calculate the magnetic exchange couplings in a series of triply bridged ferromagnetic dinuclear Cu(II) complexes that have been recently synthesized [Phys. Chem. Chem. Phys. 15, 1966 (2013)]. We find that for any basis-set the couplings from our noncollinear coupled-perturbed methodology are practically identical to those of spin-projected energy-differences when a hybrid density functional approximation is employed. This demonstrates that our methodology properly recovers a Heisenberg description for these systems, and is robust in its predictive power of magnetic couplings. Furthermore, this indicates that the failure of density functional theory to capture the subtle variation of the exchange couplings in these complexes is not simply an artifact of broken-symmetry methods, but rather a fundamental weakness of current approximate density functionals for the description of magnetic couplings.
Tellgren, E I; Teale, A M; Furness, J W; Lange, K K; Ekström, U; Helgaker, T
2014-01-21
We present a novel implementation of Kohn-Sham density-functional theory utilizing London atomic orbitals as basis functions. External magnetic fields are treated non-perturbatively, which enable the study of both magnetic response properties and the effects of strong fields, using either standard density functionals or current-density functionals-the implementation is the first fully self-consistent implementation of the latter for molecules. Pilot applications are presented for the finite-field calculation of molecular magnetizabilities, hypermagnetizabilities, and nuclear magnetic resonance shielding constants, focusing on the impact of current-density functionals on the accuracy of the results. Existing current-density functionals based on the gauge-invariant vorticity are tested and found to be sensitive to numerical details of their implementation. Furthermore, when appropriately regularized, the resulting magnetic properties show no improvement over standard density-functional results. An advantage of the present implementation is the ability to apply density-functional theory to molecules in very strong magnetic fields, where the perturbative approach breaks down. Comparison with high accuracy full-configuration-interaction results show that the inadequacies of current-density approximations are exacerbated with increasing magnetic field strength. Standard density-functionals remain well behaved but fail to deliver high accuracy. The need for improved current-dependent density-functionals, and how they may be tested using the presented implementation, is discussed in light of our findings.
Stoner-Type Theory of Magnetism in Silicon MOSFETs
NASA Astrophysics Data System (ADS)
Golosov, D. I.
2017-08-01
We consider a quasi-two-dimensional gas of electrons in a typical Si-MOSFET, assuming repulsive contact interaction between electrons. Magnetisation and susceptibility are evaluated within the mean-field approach. Finite thickness of the inversion layer results in an interaction-induced electron wave function change, not found in both purely two-dimensional and three-dimensional (bulk) cases. Taking this self-consistent change into account leads to an increased susceptibility and ultimately to a ferromagnetic transition deep in the high-density metallic regime. We further find that in the paramagnetic state, magnetisation increases sublinearly with increasing in-plane magnetic field. In the opposite limit of low carrier densities, the effects of long-range interaction become important and can be included phenomenologically via bandwidth renormalisation. Our treatment then suggests that with decreasing density, the metal-insulator transition is preceded by a ferromagnetic instability. Results are discussed in the context of the available experimental data, and arguments for the validity of our mean-field scheme are presented.
Theory and simulations of electron vortices generated by magnetic pushing
Richardson, A. S.; Angus, J. R.; Swanekamp, S. B.; Schumer, J. W.; Ottinger, P. F.
2013-08-15
Vortex formation and propagation are observed in kinetic particle-in-cell (PIC) simulations of magnetic pushing in the plasma opening switch. These vortices are studied here within the electron-magnetohydrodynamic (EMHD) approximation using detailed analytical modeling. PIC simulations of these vortices have also been performed. Strong v×B forces in the vortices give rise to significant charge separation, which necessitates the use of the EMHD approximation in which ions are fixed and the electrons are treated as a fluid. A semi-analytic model of the vortex structure is derived, and then used as an initial condition for PIC simulations. Density-gradient-dependent vortex propagation is then examined using a series of PIC simulations. It is found that the vortex propagation speed is proportional to the Hall speed v{sub Hall}≡cB{sub 0}/4πn{sub e}eL{sub n}. When ions are allowed to move, PIC simulations show that the electric field in the vortex can accelerate plasma ions, which leads to dissipation of the vortex. This electric field contributes to the separation of ion species that has been observed to occur in pulsed-power experiments with a plasma-opening switch.
A compact theory of magnetic nerve stimulation: predicting how to aim
2014-01-01
Background A compact theory that predicts quantitatively when and where magnetic neurostimulation will occur is needed as a guide to therapy, ideally providing a single equation that defines the target volume of tissue excited by single or dual coils. Methods A first-principles analysis of magnetic stimulation incorporating a simplified description of electromagnetic fields and a simplified cable theory of the axon yields a mathematical synthesis predicting how to aim. Results Nerve stimulation produced by a single circular coil having one or more closely packed turns occurs in donut shaped volume of tissue beneath the coil. Axons spanning several millimeters are the sites of magnetic stimulation. The sites of maximal transmembrane depolarization in nerve fibers correspond to points where the axons enter or exit this volume of magnetically induced voltage and current. The axonal membrane at one end is depolarized locally during the rising phase of current in the coil. The axonal membrane at the opposite end is depolarized locally during the falling phase of current in the coil. Penetration depths of several centimeters from the skin surface or approximately one to two coil radii are practical. With two coils placed in a figure-of-eight configuration the separate clockwise and counterclockwise currents generate magnetic fields that add, producing maximal stimulation of a spindle shaped volume, centered at a depth of one-third to one-half coil radius from the body surface. Conclusions This condensed synthesis of electromagnetic theory and cable theories of axon physiology provides a partial solution to the targeting problem in peripheral and in transcranial magnetic stimulation. PMID:24885299
Basic theory for polarized, astrophysical maser radiation in a magnetic field
NASA Technical Reports Server (NTRS)
Watson, William D.
1994-01-01
Fundamental alterations in the theory and resulting behavior of polarized, astrophysical maser radiation in the presence of a magnetic field have been asserted based on a calculation of instabilities in the radiative transfer. I reconsider the radiative transfer and find that the relevant instabilities do not occur. Calculational errors in the previous investigation are identified. In addition, such instabilities would have appeared -- but did not -- in the numerous numerical solutions to the same radiative transfer equations that have been presented in the literature. As a result, all modifications that have been presented in a recent series of papers (Elitzur 1991, 1993) to the theory for polarized maser radiation in the presence of a magnetic field are invalid. The basic theory is thus clarified.
Giant-spin nonlinear response theory of magnetic nanoparticle hyperthermia: A field dependence study
NASA Astrophysics Data System (ADS)
Carrião, M. S.; Aquino, V. R. R.; Landi, G. T.; Verde, E. L.; Sousa, M. H.; Bakuzis, A. F.
2017-05-01
Understanding high-field amplitude electromagnetic heat loss phenomena is of great importance, in particular, in the biomedical field, because the heat-delivery treatment plans might rely on analytical models that are only valid at low field amplitudes. Here, we develop a nonlinear response model valid for single-domain nanoparticles of larger particle sizes and higher field amplitudes in comparison to the linear response theory. A nonlinear magnetization expression and a generalized heat loss power equation are obtained and compared with the exact solution of the stochastic Landau-Lifshitz-Gilbert equation assuming the giant-spin hypothesis. The model is valid within the hyperthermia therapeutic window and predicts a shift of optimum particle size and distinct heat loss field amplitude exponents, which is often obtained experimentally using a phenomenological allometric function. Experimental hyperthermia data with distinct ferrite-based nanoparticles and third harmonic magnetization data support the nonlinear model, which also has implications for magnetic particle imaging and magnetic thermometry.
Casanova, S.; Schlickeiser, R.
2012-02-01
Recently, a new transport theory of cosmic rays in magnetized space plasmas extending the quasilinear approximation to the particle orbit has been developed for the case of an axisymmetric incompressible magnetic turbulence. Here, we generalize the approach to the important physical case of a compressible plasma. As previously obtained in the case of an incompressible plasma, we allow arbitrary gyrophase deviations from the unperturbed spiral orbits in the uniform magnetic field. For the case of quasi-stationary and spatially homogeneous magnetic turbulence we derive, in the small Larmor radius approximation, gyrophase-averaged cosmic-ray Fokker-Planck coefficients. Upper limits for the perpendicular and pitch-angle Fokker-Planck coefficients and for the perpendicular and parallel spatial diffusion coefficients are presented.
Phonon and magnetic structure in δ-plutonium from density-functional theory
Söderlind, Per; Zhou, F.; Landa, A.; ...
2015-10-30
We present phonon properties of plutonium metal obtained from a combination of density-functional-theory (DFT) electronic structure and the recently developed compressive sensing lattice dynamics (CSLD). The CSLD model is here trained on DFT total energies of several hundreds of quasi-random atomic configurations for best possible accuracy of the phonon properties. The calculated phonon dispersions compare better with experiment than earlier results obtained from dynamical mean-field theory. The density-functional model of the electronic structure consists of disordered magnetic moments with all relativistic effects and explicit orbital-orbital correlations. The magnetic disorder is approximated in two ways: (i) a special quasi-random structure andmore » (ii) the disordered-local-moment (DLM) method within the coherent potential approximation. Magnetism in plutonium has been debated intensely, However, the present magnetic approach for plutonium is validated by the close agreement between the predicted magnetic form factor and that of recent neutron-scattering experiments.« less
Magnetic Anisotropies in Rhombic Lanthanide(III) Complexes Do Not Conform to Bleaney's Theory.
Castro, Goretti; Regueiro-Figueroa, Martín; Esteban-Gómez, David; Pérez-Lourido, Paulo; Platas-Iglesias, Carlos; Valencia, Laura
2016-04-04
We report a complete set of magnetic susceptibilities of lanthanide complexes with a macrocyclic ligand based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing four hydroxyethyl pendant arms (L(1)). The [LnL(1)](3+) complexes are isostructural along the lanthanide series from Ce(3+) to Yb(3+), with the only structural change observed along the series being the monotonous shortening of the Ln-donor distances due to lanthanide contraction. The (1)H NMR spectra point to a D2 symmetry of the [LnL(1)](3+) complexes in aqueous solution, which provides a unique opportunity for analysis of the rhombic magnetic anisotropies with an unequivocal location of the magnetic axes. The contact contributions for the observed paramagnetic shifts have been estimated with density functional theory calculations on the [GdL(1)](3+) complex. Subsequently, the pseudocontact shifts could be factored out, thereby giving access to the axial and rhombic contributions of the magnetic susceptibility tensor. Our results show that the calculated magnetic anisotropies do not follow the trends predicted by Bleaney's theory, particularly in the case of Ho(3+) and Er(3+) complexes.
Sketch of a unifying auroral theory. [based on magnetic mirroring of magnetic disturbances
NASA Technical Reports Server (NTRS)
Lennartsson, W.
1975-01-01
On the basis of field and particle observations, it is suggested that a bright auroral display is a part of a magnetosphere-ionosphere current system which is fed by a charge-separation process in the outer magnetosphere (or the solar wind). The upward magnetic-field-aligned current is flowing out of the display, carried mainly by down-flowing electrons from the hot-particle populations in the outer magnetosphere (the ambient cold electrons being depleted at high altitudes). As a result of the magnetic mirroring of these downflowing current carriers, a large potential drop is set up along the magnetic field, increasing both the number flux and the kinetic energy of the precipitating electrons. It is found that this simple basic model, when combined with wave-particle interactions, may be able to explain a highly diversified selection of auroral particle observations. It may thus be possible to explain both inverted-V events and auroral rays in terms of a static parallel electric field, and the electric field may be compatible with a strongly variable pitch-angle distribution of the precipitating electrons, including distributions peaked at 90 deg as well as 0 deg. This model may also provide a simple explanation of the simultaneous precipitation of electrons and collimated positive ions.
NASA Technical Reports Server (NTRS)
Mazuruk, Konstantin; Grugel, Richard N.
2003-01-01
A magnetohydrodynamic model that examines the effect of rotating an electrically conducting cylinder with a uniform external magnetic field applied orthogonal to its axis is presented. Noting a simple geometry, it can be classified as a fundamental dynamo problem. For the case of an infinitely long cylinder, an analytical solution is obtained and analyzed in detail. A semi-analytical model was developed that considers a finite cylinder. Experimental data from a spinning brass wheel in the presence of Earth's magnetic field were compared to the proposed theory and found to fit well.
From liquid crystal models to the guiding-center theory of magnetized plasmas
Tronci, Cesare
2016-08-15
Upon combining Northrop’s picture of charged particle motion with modern liquid crystal theories, this paper provides a new description of guiding center dynamics (to lowest order). This new perspective is based on a rotation gauge field (gyrogauge) that encodes rotations around the magnetic field. In liquid crystal theory, an analogue rotation field is used to encode the rotational state of rod-like molecules. Instead of resorting to sophisticated tools (e.g. Hamiltonian perturbation theory and Lie series expansions) that still remain essential in higher-order gyrokinetics, the present approach combines the WKB method with a simple kinematical ansatz, which is then replaced into the charged particle Lagrangian. The latter is eventually averaged over the gyrophase to produce the guiding-center equations. A crucial role is played by the vector potential for the gyrogauge field. A similar vector potential is related to liquid crystal defects and is known as wryness tensor in Eringen’s micropolar theory.
Magnetic field generation by rotating black holes
NASA Technical Reports Server (NTRS)
Leahy, D. A.; Vilenkin, A.
1981-01-01
A new mechanism of cosmic magnetic field generation is discussed. Neutrinos asymmetrically emitted by rotating black holes scatter on protons and produce a proton current which generates the magnetic field. It is shown that this mechanism can in principle produce a seed field sufficiently strong to account for present galactic fields.
Hartree-Fock Theory with a Self-Generated Magnetic Field
NASA Astrophysics Data System (ADS)
Comelli, Silvia
In the first part of this thesis, we give a brief introduction of the most important theories developed to describe systems of N electrons and K nuclei interacting through the Coulomb potential in the Born-Oppenheimer approximation. After that, we describe how these theories can be adapted to include the interaction of our system with magnetic fields, whether these fields are external or self-generated by the orbital motion of the electrons. Finally, we define the Hartree-Fock ground state problem for Coulomb systems in the presence of self-generated magnetic fields and when spin-field interactions are taken into account. We prove existence of minimizers for neutral or positively charged molecules. Our proof is based on variational techniques applied to the Hartree-Fock energy functional.
A combined theory for magnetohydrodynamic equilibria with anisotropic pressure and magnetic shear
NASA Astrophysics Data System (ADS)
Hodgson, J. D. B.; Neukirch, T.
2017-03-01
We present a new approach to the theory of magnetohydrodynamic equilibria with anisotropic pressure, magnetic shear and translational/rotational invariance. This approach involves combining two existing formalisms in order to eliminate their individual weaknesses. The theoretical aspects of the method are explored in detail along with numerical solutions which make use of the method. Eventually, this method could be applied to model various plasma systems, such as planetary magnetospheres.
Spin Chain in Magnetic Field: Limitations of the Large-N Mean-Field Theory
Wohlfeld, K.; Chen, Cheng-Chien; van Veenendaal, M.; ...
2015-02-01
Motivated by the recent success in describing the spin and orbital spectrum of a spin-orbital chain using a large-N mean-field approximation [Phys. Rev. B 91, 165102 (2015)], we apply the same formalism to the case of a spin chain in the external magnetic field. It occurs that in this case, which corresponds to N=2 in the approximation, the large-N mean-field theory cannot qualitatively reproduce the spin excitation spectra at high magnetic fields, which polarize more than 50% of the spins in the magnetic ground state. This, rather counterintuitively, shows that the physics of a spin chain can under some circumstancesmore » be regarded as more complex than the physics of a spin-orbital chain.« less
Spin Chain in Magnetic Field: Limitations of the Large-N Mean-Field Theory
Wohlfeld, K.; Chen, Cheng-Chien; van Veenendaal, M. ; Devereaux, T. P.
2015-02-01
Motivated by the recent success in describing the spin and orbital spectrum of a spin-orbital chain using a large-N mean-field approximation [Phys. Rev. B 91, 165102 (2015)], we apply the same formalism to the case of a spin chain in the external magnetic field. It occurs that in this case, which corresponds to N=2 in the approximation, the large-N mean-field theory cannot qualitatively reproduce the spin excitation spectra at high magnetic fields, which polarize more than 50% of the spins in the magnetic ground state. This, rather counterintuitively, shows that the physics of a spin chain can under some circumstances be regarded as more complex than the physics of a spin-orbital chain.
Anisotropic heavy quark potential in strongly-coupled N =4 SYM theory in a magnetic field
NASA Astrophysics Data System (ADS)
Rougemont, R.; Critelli, R.; Noronha, J.
2015-03-01
In this work we use the gauge/gravity duality to study the anisotropy in the heavy quark potential in strongly coupled N =4 super-Yang Mills (SYM) theory (both at zero and nonzero temperature) induced by a constant and uniform magnetic field B . At zero temperature, the inclusion of the magnetic field decreases the attractive force between heavy quarks with respect to its B =0 value and the force associated with the parallel potential is the least attractive force. We find that the same occurs at nonzero temperature and, thus, at least in the case of strongly coupled N =4 SYM, the presence of a magnetic field generally weakens the interaction between heavy quarks in the plasma.
Coupled-cluster theory for atoms and molecules in strong magnetic fields
Stopkowicz, Stella Lange, Kai K.; Tellgren, Erik I.; Helgaker, Trygve; Gauss, Jürgen
2015-08-21
An implementation of coupled-cluster (CC) theory to treat atoms and molecules in finite magnetic fields is presented. The main challenges for the implementation stem from the magnetic-field dependence in the Hamiltonian, or, more precisely, the appearance of the angular momentum operator, due to which the wave function becomes complex and which introduces a gauge-origin dependence. For this reason, an implementation of a complex CC code is required together with the use of gauge-including atomic orbitals to ensure gauge-origin independence. Results of coupled-cluster singles–doubles–perturbative-triples (CCSD(T)) calculations are presented for atoms and molecules with a focus on the dependence of correlation and binding energies on the magnetic field.
Negative Magnetic Eddy Diffusivities from the Test-field Method and Multiscale Stability Theory
NASA Astrophysics Data System (ADS)
Andrievsky, Alexander; Brandenburg, Axel; Noullez, Alain; Zheligovsky, Vladislav
2015-10-01
The generation of a large-scale magnetic field in the kinematic regime in the absence of an α-effect is investigated by following two different approaches: the test-field method and the multiscale stability theory relying on the homogenization technique. Our computations of the magnetic eddy diffusivity tensor of the parity-invariant flow IV of G. O. Roberts and the modified Taylor-Green flow confirm the findings of previous studies and also explain some of their apparent contradictions. The two flows have large symmetry groups; this is used to considerably simplify the eddy diffusivity tensor. Finally, a new analytic result is presented: upon expressing the eddy diffusivity tensor in terms of solutions to auxiliary problems for the adjoint operator, we derive relations between the magnetic eddy diffusivity tensors that arise for mutually reverse small-scale flows {\\boldsymbol{v}}(x) and -{\\boldsymbol{v}}(x).
Field theory on R× S 3 topology. IV: Electrodynamics of magnetic moments
NASA Astrophysics Data System (ADS)
Carmeli, M.; Malin, S.
1986-08-01
The equations of electrodynamics for the interactions between magnetic moments are written on R×S3 topology rather than on Minkowskian space-time manifold of ordinary Maxwell's equations. The new field equations are an extension of the previously obtained Klein-Gordon-type, Schrödinger-type, Weyl-type, and Dirac-type equations. The concept of the magnetic moment in our case takes over that of the charge in ordinary electrodynamics as the fundamental entity. The new equations have R×S3 invariance as compared to the Lorentz invariance of Maxwell's equations. The solutions of the new field equations are given. In this theory the divergence of the electric field vanishes whereas that of the magnetic field does not.
Theory of exciton-polaron complexes in pulsed electrically detected magnetic resonance
NASA Astrophysics Data System (ADS)
Keevers, T. L.; Baker, W. J.; McCamey, D. R.
2015-05-01
Several microscopic pathways have been proposed to explain the large magnetic effects observed in organic semiconductors; however, it is difficult to identify and characterize the microscopic process which actually influences the overall magnetic field response in a particular instance. Pulsed electrically detected magnetic resonance provides an ideal platform for this task as it intrinsically monitors the charge carriers of interest and provides dynamical information which is inaccessible through conventional magnetoconductance measurements. Here we develop a general time-domain theory to describe the spin-dependent recombination of exciton-polaron complexes following the coherent manipulation of paramagnetic centers through electron paramagnetic resonance. A general Hamiltonian is treated, and it is shown that the transition frequencies and resonance positions of the exciton-polaron complex can be used to estimate interspecies coupling. This work also provides a general formalism for analyzing multipulse experiments which can be used to extract relaxation and transport rates.
Electron theory of perpendicular magnetic anisotropy of Co-ferrite thin films
Inoue, Jun-ichiro; Yanagihara, Hideto; Kita, Eiji; Niizeki, Tomohiko; Itoh, Hiroyoshi
2014-02-15
We develop an electron theory for the t{sub 2g} electrons of Co{sup 2+} ions to clarify the perpendicular magnetic anisotropy (PMA) mechanism of Co-ferrite thin films by considering the spin-orbit interaction (SOI) and crystal-field (CF) potentials induced by the local symmetry around the Co ions and the global tetragonal symmetry of the film. Uniaxial and in-plane MA constants K{sub u} and K{sub 1} at 0 K, respectively, are calculated for various values of SOI and CF. We show that reasonable parameter values explain the observed PMA and that the orbital moment for the in-plane magnetization reduces to nearly half of that of the out-of-plane magnetization.
Zevenhoven, Koos C. J.; Busch, Sarah; Hatridge, Michael; Öisjöen, Fredrik; Ilmoniemi, Risto J.; Clarke, John
2014-01-01
Eddy currents induced by applied magnetic-field pulses have been a common issue in ultra-low-field magnetic resonance imaging. In particular, a relatively large prepolarizing field—applied before each signal acquisition sequence to increase the signal—induces currents in the walls of the surrounding conductive shielded room. The magnetic-field transient generated by the eddy currents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddy currents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy-current patterns and their time evolution in a previous-generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6-mm-thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy-current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures. PMID:24753629
Theory and simulation of the magnetic island-induced TAE mode
NASA Astrophysics Data System (ADS)
Cook, Carson; Hegna, Chris; Spong, Don; Hirshman, Steve
2013-10-01
In this work, we develop the theory of the shear Alfven continuum in the presence of a magnetic island in a toroidal equilibrium. The shear Alfven spectrum of a magnetically confined plasma influences the stability properties of the system. Discrete Alfven eigenmodes are of particular interest to fusion plasmas. The frequencies of these modes lie in the gaps of the Alfven continuum, and thus the modes do not experience continuum damping and can be driven unstable by energetic particles. The effects of magnetic islands on the ellipticity-induced Alfven eigenmode and the beta-induced Alfven eigenmode have been studied in some detail (see e.g. Biancalani et al., Plasma Phys. Control. Fusion53, 025009 (2011)). However, the effects of islands on the toroidicity-induced Alfven eigenmode (TAE) has not been investigated. The magnetic island-induced TAE (MiTAE) gap will be discussed along with the discrete MiTAE mode. Numerical simulation results using the SIESTA equilibrium code will be presented and compared to theory. Using the Hessian matrix from a SIESTA equilibrium, the Alfven eigenmodes and frequencies can be computed. A simple toroidal equilibrium with an island will be studied and the computed MiTAE structure and frequency will be compared to the analytical prediction. Research supported by the U. S. DOE under grants DE-FG02-99ER54546 and DE-SC0006103.
Liu, Xia; Tan, Yingzi; Li, Xiuling; Wu, Xiaojun; Pei, Yong
2015-08-28
The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to the well-studied gas phase TM-benzene molecular wires. Si-[Mn(styrene)]∞ and Si-[Cr(styrene)]∞ single molecular wires (SMWs) are a ferromagnetic semiconductor and half metal, respectively. Creation of H-atom defects on the silicon surface can introduce an impurity metallic band, which leads to novel half-metallic magnetism of a Si-[Mn(styrene)]∞ system. Moreover, double molecular wires (DMWs) containing two identical or hetero SMWs are theoretically designed. The [Mn(styrene)]∞-[Cr(styrene)]∞ DMW exhibits half-metallic magnetism where the spin-up and spin-down channels are contributed by two single molecular wires. Finally, we demonstrate that introducing a TM-defect may significantly affect the electronic structure and magnetic properties of molecular wires. These studies provide new insights into the structure and properties of surface supported 1-D sandwiched molecular wires and may inspire the future experimental synthesis of substrate confined organometallic sandwiched molecular wires.
The origin of magnetic fields in hot stars
NASA Astrophysics Data System (ADS)
Neiner, Coralie; Mathis, Stéphane; Alecian, Evelyne; Emeriau, Constance; Grunhut, Jason; BinaMIcS; MiMeS Collaborations
2015-10-01
Observations of stable mainly dipolar magnetic fields at the surface of ~7% of single hot stars indicate that these fields are of fossil origin, i.e. they descend from the seed field in the molecular clouds from which the stars were formed. The recent results confirm this theory. First, theoretical work and numerical simulations confirm that the properties of the observed fields correspond to those expected from fossil fields. They also showed that rapid rotation does not modify the surface dipolar magnetic configurations, but hinders the stability of fossil fields. This explains the lack of correlation between the magnetic field properties and stellar properties in massive stars. It may also explain the lack of detections of magnetic fields in Be stars, which rotate close to their break-up velocity. In addition, observations by the BinaMIcS collaboration of hot stars in binary systems show that the fraction of those hosting detectable magnetic fields is much smaller than for single hot stars. This could be related to results obtained in simulations of massive star formation, which show that the stronger the magnetic field in the original molecular cloud, the more difficult it is to fragment massive cores to form several stars. Therefore, more and more arguments support the fossil field theory.
Bogoliubov theory of interacting bosons on a lattice in a synthetic magnetic field
Powell, Stephen; Barnett, Ryan; Sensarma, Rajdeep; Das Sarma, Sankar
2011-01-15
We consider theoretically the problem of an artificial gauge potential applied to a cold atomic system of interacting neutral bosons in a tight-binding optical lattice. Using the Bose-Hubbard model, we show that an effective magnetic field leads to superfluid phases with simultaneous spatial order, which we analyze using Bogliubov theory. This gives a consistent expansion in terms of quantum and thermal fluctuations, in which the lowest order gives a Gross-Pitaevskii equation determining the condensate configuration. We apply an analysis based on the magnetic symmetry group to show how the spatial structure of this configuration depends on commensuration between the magnetic field and the lattice. Higher orders describe the quasiparticle excitations, whose spectrum combines the intricacy of the Hofstadter butterfly with the characteristic features of the superfluid phase. We use the depletion of the condensate to determine the range of validity of our approximations and also to find an estimate for the onset of the Mott insulator phase. Our theory provides concrete experimental predictions for both time-of-flight imagery and Bragg spectroscopy.
Frustrated magnetism and caloric effects in Mn-based antiperovskite nitrides: Ab initio theory
NASA Astrophysics Data System (ADS)
Zemen, J.; Mendive-Tapia, E.; Gercsi, Z.; Banerjee, R.; Staunton, J. B.; Sandeman, K. G.
2017-05-01
We model changes of magnetic ordering in Mn-based antiperovskite nitrides driven by biaxial lattice strain at zero and at finite temperature. We employ a noncollinear spin-polarized density functional theory to compare the response of the geometrically frustrated exchange interactions to a tetragonal symmetry breaking (the so called piezomagnetic effect) across a range of Mn3AN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) at zero temperature. Building on the robustness of the effect we focus on Mn3GaN and extend our study to finite temperature using the disordered local moment (DLM) first-principles electronic structure theory to model the interplay between the ordering of Mn magnetic moments and itinerant electron states. We discover a rich temperature-strain magnetic phase diagram with two previously unreported phases stabilized by strains larger than 0.75% and with transition temperatures strongly dependent on strain. We propose an elastocaloric cooling cycle crossing two of the available phase transitions to achieve simultaneously a large isothermal entropy change (due to the first-order transition) and a large adiabatic temperature change (due to the second-order transition).
NASA Technical Reports Server (NTRS)
Kulsrud, Russell M.; Anderson, Stephen W.
1992-01-01
The fluctuation spectrum that must arise in a mean field dynamo generation of galactic fields if the initial field is weak is considered. A kinetic equation for its evolution is derived and solved. The spectrum evolves by transfer of energy from one magnetic mode to another by interaction with turbulent velocity modes. This kinetic equation is valid in the limit that the rate of evolution of the magnetic modes is slower than the reciprocal decorrelation time of the turbulent modes. This turns out to be the case by a factor greater than 3. Most of the fluctuation energy concentrates on small scales, shorter than the hydrodynamic turbulent scales. The fluctuation energy builds up to equipartition with the turbulent energy in times that are short compared to the e-folding time of the mean field. The turbulence becomes strongly modified before the dynamo amplification starts. Thus, the kinematic assumption of the mean dynamo theory is invalid. Thus, the galactic field must have a primordial origin, although it may subsequently be modified by dynamo action.
Magnetism in undoped ZnS studied from density functional theory
Xiao, Wen-Zhi E-mail: llwang@hun.edu.cn; Rong, Qing-Yan; Xiao, Gang; Wang, Ling-ling E-mail: llwang@hun.edu.cn; Meng, Bo
2014-06-07
The magnetic property induced by the native defects in ZnS bulk, thin film, and quantum dots are investigated comprehensively based on density functional theory within the generalized gradient approximation + Hubbard U (GGA + U) approach. We find the origin of magnetism is closely related to the introduction of hole into ZnS systems. The relative localization of S-3p orbitals is another key to resulting in unpaired p-electron, due to Hund's rule. For almost all the ZnS systems under study, the magnetic moment arises from the S-dangling bonds generated by Zn vacancies. The charge-neutral Zn vacancy, Zn vacancy in 1− charge sate, and S vacancy in the 1+ charge sate produce a local magnetic moment of 2.0, 1.0, and 1.0 μ{sub B}, respectively. The Zn vacancy in the neutral and 1− charge sates are the important cause for the ferromagnetism in ZnS bulk, with a Curie temperature (T{sub C}) above room temperature. For ZnS thin film with clean (111) surfaces, the spins on each surface are ferromagnetically coupled but antiferromagnetically coupled between two surfaces, which is attributable to the internal electric field between the two polar (111) surfaces of the thin film. Only surface Zn vacancies can yield local magnetic moment for ZnS thin film and quantum dot, which is ascribed to the surface effect. Interactions between magnetic moments on S-3p states induced by hole-doping are responsible for the ferromagnetism observed experimentally in various ZnS samples.
NASA Astrophysics Data System (ADS)
Erkan, K.; Jekeli, C.
2009-12-01
Today gravity and magnetic field measurements are acquired in grids with high resolution and accuracy. Magnetic field measurements have already been proven for superior accuracy and practicality. Modern gravity gradiometry instruments have boosted the practicality of gravity field measurements for many subsurface problems. As a result of this, advanced algorithms are needed for quantitative integration of the two fields for a specific subsurface problem. These fields are correlated by Poisson relation as a first order approximation. However, subsurface sources generally show large deviations from the ideal conditions; in this case a generalized Poisson relation may be proposed as a perturbation of the ideal conditions. In this study, we take advantage of the abstraction of the deformation theory between two metric fields, and implement it between the two geophysical fields. In this generalized approach, the different geophysical fields are loosely correlated by Poisson relation; so the calculated deformation reflects the deviations from ideal density/susceptibility relationships for the subsurface structure. The resulting deformation field can then be used for detection of a known target with an expected deformation field. The present method introduces a novel algorithm for integration of the gravity gradiometry and magnetic field data. In this method, the results can be directly interpreted without making individual density and magnetic susceptibility assumptions. The method also intrinsically overcomes the scale problem between the two potential fields.
NASA Astrophysics Data System (ADS)
Peng, J.; Zhao, P. W.
2015-04-01
The self-consistent tilted axis cranking relativistic mean-field (TAC-RMF) theory based on a point-coupling interaction is applied to investigate the observed magnetic and antimagnetic rotations in the nucleus 110Cd . The energy spectra, the relation between the spin and the rotational frequency, the deformation parameters, and the reduced M 1 and E 2 transition probabilities are studied with the various configurations. It is found that the configuration has to be changed to reproduce the energy spectra and the relations between the spin and the rotational frequency for both the magnetic and antimagnetic rotational bands. The shears mechanism for the magnetic rotation and the two-shears-like mechanism for the antimagnetic rotation are examined by investigating the orientation of the neutron and proton angular momenta. The calculated electromagnetic transitions B (M 1 ) and B (E 2 ) are in reasonable agreement with the data, and their tendencies are coincident with the typical characteristics of the magnetic and antimagnetic rotations.
2D joint inversion of CSAMT and magnetic data based on cross-gradient theory
NASA Astrophysics Data System (ADS)
Wang, Kun-Peng; Tan, Han-Dong; Wang, Tao
2017-06-01
A two-dimensional forward and backward algorithm for the controlled-source audio-frequency magnetotelluric (CSAMT) method is developed to invert data in the entire region (near, transition, and far) and deal with the effects of artificial sources. First, a regularization factor is introduced in the 2D magnetic inversion, and the magnetic susceptibility is updated in logarithmic form so that the inversion magnetic susceptibility is always positive. Second, the joint inversion of the CSAMT and magnetic methods is completed with the introduction of the cross gradient. By searching for the weight of the cross-gradient term in the objective function, the mutual influence between two different physical properties at different locations are avoided. Model tests show that the joint inversion based on cross-gradient theory offers better results than the single-method inversion. The 2D forward and inverse algorithm for CSAMT with source can effectively deal with artificial sources and ensures the reliability of the final joint inversion algorithm.
Pérez-Jiménez, Angel J; Pérez-Jordá, José M; Illas, Francesc
2004-01-01
A new method to improve the excess spin density obtained from unrestricted Hartree-Fock wave functions in terms of natural orbitals is proposed. Using this modified excess spin density to evaluate the correlation energy by means of density functionals leads to large improvements in the computed magnetic coupling constants of several materials without need to modify the exchange contribution. This is important because it reconciles the density functional theory description with the one provided by multi-determinant wave functions. Using the present approach, the leading contribution to the magnetic coupling constant arises from electron correlation effects. The performance of the new method is illustrated on various materials including high-critical-temperature superconductors parent compounds.
Quantal density functional theory (QDFT) in the presence of a magnetic field
NASA Astrophysics Data System (ADS)
Pan, Xiaoyin; Yang, Tao; Sahni, Viraht
2011-03-01
We present the QDFT of electrons in an external electrostatic E (r) = - ∇ v (r) and magnetostatic B (r) = ∇ × A (r) field. This is the mapping from the interacting system of electrons to one of noninteracting fermions with the same density ρ (r) and physical current density j (r) . The mapping, based on the `quantal Newtonian' first law, is in terms of `classical' fields and quantal sources, the fields being separately representative of electron correlations due to the Pauli exclusion principle and Coulomb repulsion, and correlation-kinetic and correlation-magnetic effects. The theory is valid for ground and excited states. It is explicated by application to a ground state of the exactly solvable Hooke's atom in the presence of a magnetic field. Supported by National NSF, China and RF CUNY.
Khaira, Jobanpreet S.; Jain, Richa N.; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-24
The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 Å from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 µ{sub B} due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices.
Theory of triplet-triplet annihilation in optically detected magnetic resonance
NASA Astrophysics Data System (ADS)
Keevers, T. L.; McCamey, D. R.
2016-01-01
Triplet-triplet annihilation allows two low-energy photons to be upconverted into a single high-energy photon. By essentially engineering the solar spectrum, this allows solar cells to be made more efficient and even exceed the Shockley-Quiesser limit. Unfortunately, optimizing the reaction pathway is difficult, especially with limited access to the microscopic time scales and states involved in the process. Optical measurements can provide detailed information: triplet-triplet annihilation is intrinsically spin dependent and exhibits substantial magnetoluminescence in the presence of a static magnetic field. Pulsed optically detected magnetic resonance is especially suitable, since it combines high spin sensitivity with coherent manipulation. In this paper, we develop a time-domain theory of triplet-triplet annihilation for complexes with arbitrary spin-spin coupling. We identify unique "Rabi fingerprints" for each coupling regime and show that this can be used to characterize the microscopic Hamiltonian.
Non-local theory of a transverse magnetic mode pumped free electron laser
NASA Astrophysics Data System (ADS)
Sharma, B. S.; Jaiman, N. K.
2008-10-01
A non-local theory is used to study the effects of the corrugation parameter ε of a plasma-filled slow wave structure, the cyclotron frequency of a pumped magnetic field Ω and the relativistic gamma factor γ0 on the instability growth Γ of a free electron laser in the presence of an external finite axial magnetic field. The dispersion relation is derived and the growth rate is formulated in the Raman regime. The growth rate is approximately proportional to ε. There is a considerable decrease in the instability growth when the cyclotron frequency is close to ω0. The growth rate approximately scales inversely as the 19/2 power of the relativistic gamma factor.
NASA Astrophysics Data System (ADS)
Glassmeier, K.-H.; Tsurutani, B. T.
2014-02-01
This is a translation of the Allgemeine Theorie des Erdmagnetismus published by Carl Friedrich Gauss in 1839 in the Resultate aus den Beobachtungen des Magnetischen Vereins im Jahre 1838. The current translation is based on an earlier translation by Elizabeth Juliana Sabine published in 1841. This earlier translation has been revised, corrected, and extended. Numerous biographical comments on the scientists named in the original text have been added as well as further information on the observational material used by Carl Friedrich Gauss. An attempt is made to provide a readable text to a wider scientific community, a text laying the foundation of today's understanding of planetary magnetic fields.
Exact image theory for the problem of dielectric/magnetic slab
NASA Technical Reports Server (NTRS)
Lindell, I. V.
1987-01-01
Exact image method, recently introduced for the exact solution of electromagnetic field problems involving homogeneous half spaces and microstrip-like geometries, is developed for the problem of homogeneous slab of dielectric and/or magnetic material in free space. Expressions for image sources, creating the exact reflected and transmitted fields, are given and their numerical evaluation is demonstrated. Nonradiating modes, guided by the slab and responsible for the loss of convergence of the image functions, are considered and extracted. The theory allows, for example, an analysis of finite ground planes in microstrip antenna structures.
Temesvári, T; De Dominicis, C
2002-08-26
We use the generic replica symmetric cubic field theory to study the transition of short-range Ising spin glasses in a magnetic field around the upper critical dimension. A novel fixed point is found from the application of the renormalization group. In the spin-glass limit, this fixed point governs the critical behavior of a class of systems characterized by a single cubic parameter. For this universality class, the spin-glass susceptibility diverges at criticality, whereas the longitudinal mode remains massive. The third mode, however, behaves unusually. The physical consequences of this unusual behavior are discussed, and a comparison with the conventional de Almeida-Thouless scenario is presented.
Rezende, Carlos A; San Gil, Rosane A S; Borré, Leandro B; Pires, José Ricardo; Vaiss, Viviane S; Resende, Jackson A L C; Leitão, Alexandre A; De Alencastro, Ricardo B; Leal, Katia Z
2016-09-01
The experiments of carvedilol form II, form III, and hydrate by (13)C and (15)N cross-polarization magic-angle spinning (CP MAS) are reported. The GIPAW (gauge-including projector-augmented wave) method from DFT (density functional theory) calculations was used to simulate (13)C and (15)N chemical shifts. A very good agreement was found for the comparison between the global results of experimental and calculated nuclear magnetic resonance (NMR) chemical shifts for carvedilol polymorphs. This work aims a comprehensive understanding of carvedilol crystalline forms employing solution and solid-state NMR as well as DFT calculations. Copyright © 2016. Published by Elsevier Inc.
Murray, James M; Tesanović, Zlatko
2010-07-16
A Ginzburg-Landau approach to fluctuations of a layered superconductor in a magnetic field is used to show that the interlayer coupling can be incorporated within an interacting self-consistent theory of a single layer, in the limit of a large number of neighboring layers. The theory exhibits two phase transitions-a vortex liquid-to-solid transition is followed by a Bose-Einstein condensation into the Abrikosov lattice-illustrating the essential role of interlayer coupling. By using this theory, explicit expressions for magnetization, specific heat, and fluctuation conductivity are derived. We compare our results with recent experimental data on the iron-pnictide superconductors.
Theory of triplon dynamics in the quantum magnet BiCu2PO6
NASA Astrophysics Data System (ADS)
Hwang, Kyusung; Kim, Yong Baek
2016-06-01
We provide a theory of triplon dynamics in the valence bond solid ground state of the coupled spin ladders modeled for BiCu2PO6 . Utilizing the recent high-quality neutron scattering data [K. W. Plumb et al., Nat. Phys. 12, 224 (2016), 10.1038/nphys3566] as guides and a theory of interacting triplons via the bond operator formulation, we determine a minimal spin Hamiltonian for this system. It is shown that the splitting of the low-energy triplon modes and the peculiar magnetic field dependence of the triplon dispersions can be explained by including substantial Dzyaloshinskii-Moriya and symmetric anisotropic spin interactions. Taking into account the interactions between triplons and the decay of the triplons to the two-triplon continuum via anisotropic spin interactions, we provide a theoretical picture that can be used to understand the main features of the recent neutron scattering experimental data.
A unified theory of electrodynamic coupling in coronal magnetic loops - The coronal heating problem
NASA Technical Reports Server (NTRS)
Ionson, J. A.
1984-01-01
The coronal heating problem is studied, and it is demonstrated that Ionson's (1982) LRC approach results in a unified theory of coronal heating which unveils a variety of new heating mechanisms and which links together previously proposed mechanisms. Ionson's LRC equation is rederived, focusing on various aspects that were not clarified in the original article and incorporating new processes that were neglected. A parameterized heating rate is obtained. It is shown that Alfvenic surface wave heating, stochastic magnetic pumping, resonant electrodynamic heating, and dynamical dissipation emerge as special cases of a much more general formalism. This generalized theory is applied to solar coronal loops and it is found that active region and large scale loops are underdamped systems. Young active region loops and (possibly) bright points are found to be overdamped systems.
Computation of magnetic suspension of maglev systems using dynamic circuit theory
NASA Astrophysics Data System (ADS)
He, J. L.; Rote, D. M.; Coffey, H. T.
1992-05-01
Dynamic circuit theory is applied to several magnetic suspensions associated with maglev systems. These suspension systems are the loop-shaped coil guideway, the figure-eight-shaped null-flux coil guideway, and the continuous sheet guideway. Mathematical models, which can be used for the development of computer codes, are provided for each of these suspension systems. The differences and similarities of the models in using dynamic circuit theory are discussed in the paper. The paper emphasizes the transient and dynamic analysis and computer simulation of maglev systems. In general, the method discussed here can be applied to many electrodynamic suspension system design concepts. It is also suited for the computation of the performance of maglev propulsion systems. Numerical examples are presented in the paper.
Quasilinear theory of collisionless Fermi acceleration in a multicusp magnetic confinement geometry
NASA Astrophysics Data System (ADS)
Dewar, R. L.; Ciubotariu, C. I.
1999-12-01
Particle motion in a cylindrical multiple-cusp magnetic field configuration is shown to be highly (though not completely) chaotic, as expected by analogy with the Sinai billiard. This provides a collisionless, linear mechanism for phase randomization during monochromatic wave heating. A general quasilinear theory of collisionless energy diffusion is developed for particles with a Hamiltonian of the form H0+H1, motion in the unperturbed Hamiltonian H0 being assumed chaotic, while the perturbation H1 can be coherent (i.e., not stochastic). For the multicusp geometry, two heating mechanisms are identified-cyclotron resonance heating of particles temporarily mirrortrapped in the cusps, and nonresonant heating of nonadiabatically reflected particles (the majority). An analytically solvable model leads to an expression for a transit-time correction factor, exponentially decreasing with increasing frequency. The theory is illustrated using the geometry of a typical laboratory experiment.
Quasilinear theory of collisionless Fermi acceleration in a multicusp magnetic confinement geometry.
Dewar, R L; Ciubotariu, C I
1999-12-01
Particle motion in a cylindrical multiple-cusp magnetic field configuration is shown to be highly (though not completely) chaotic, as expected by analogy with the Sinai billiard. This provides a collisionless, linear mechanism for phase randomization during monochromatic wave heating. A general quasilinear theory of collisionless energy diffusion is developed for particles with a Hamiltonian of the form H0+H1, motion in the unperturbed Hamiltonian H0 being assumed chaotic, while the perturbation H1 can be coherent (i.e., not stochastic). For the multicusp geometry, two heating mechanisms are identified-cyclotron resonance heating of particles temporarily mirrortrapped in the cusps, and nonresonant heating of nonadiabatically reflected particles (the majority). An analytically solvable model leads to an expression for a transit-time correction factor, exponentially decreasing with increasing frequency. The theory is illustrated using the geometry of a typical laboratory experiment.
USDA-ARS?s Scientific Manuscript database
Slugs are common pests of grass seed fields in western Oregon and are currently controlled using bait pellets that often fail to give adequate protection. Here we demonstrate the loss of bait pellet products to earthworms and its adverse effects on controlling slugs. Three years of field and greenho...
NASA Astrophysics Data System (ADS)
Liu, Xia; Tan, Yingzi; Li, Xiuling; Wu, Xiaojun; Pei, Yong
2015-08-01
The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to the well-studied gas phase TM-benzene molecular wires. Si-[Mn(styrene)]∞ and Si-[Cr(styrene)]∞ single molecular wires (SMWs) are a ferromagnetic semiconductor and half metal, respectively. Creation of H-atom defects on the silicon surface can introduce an impurity metallic band, which leads to novel half-metallic magnetism of a Si-[Mn(styrene)]∞ system. Moreover, double molecular wires (DMWs) containing two identical or hetero SMWs are theoretically designed. The [Mn(styrene)]∞-[Cr(styrene)]∞ DMW exhibits half-metallic magnetism where the spin-up and spin-down channels are contributed by two single molecular wires. Finally, we demonstrate that introducing a TM-defect may significantly affect the electronic structure and magnetic properties of molecular wires. These studies provide new insights into the structure and properties of surface supported 1-D sandwiched molecular wires and may inspire the future experimental synthesis of substrate confined organometallic sandwiched molecular wires.The electronic and magnetic properties of transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional (1D) styrene molecular wires confined on the hydrogen-terminated Si(100) surface are explored for the first time by means of spin-polarized density functional theory, denoted as Si-[TM(styrene)]. It is unveiled that TM atoms bind asymmetrically to the adjacent phenyl rings, which leads to novel electronic and magnetic properties in stark contrast to
Relativistic theory of nuclear magnetic resonance parameters in a Gaussian basis representation
Kutzelnigg, Werner; Liu Wenjian
2009-07-28
The calculation of NMR parameters from relativistic quantum theory in a Gaussian basis expansion requires some care. While in the absence of a magnetic field the expansion in a kinetically balanced basis converges for the wave function in the mean and for the energy with any desired accuracy, this is not necessarily the case for magnetic properties. The results for the magnetizability or the nuclear magnetic shielding are not even correct in the nonrelativistic limit (nrl) if one expands the original Dirac equation in a kinetically balanced Gaussian basis. This defect disappears if one starts from the unitary transformed Dirac equation as suggested by Kutzelnigg [Phys. Rev. A 67, 032109 (2003)]. However, a new difficulty can arise instead if one applies the transformation in the presence of the magnetic field of a point nucleus. If one decomposes certain contributions, the individual terms may diverge, although their sum is regular. A controlled cancellation may become difficult and numerical instabilities can arise. Various ways exist to avoid these singularities and at the same time get the correct nrl. There are essentially three approaches intermediate between the transformed and the untransformed formulation, namely, the bispinor decomposition, the decomposition of the lower component, and the hybrid unitary transformation partially at operator and partially at matrix level. All three possibilities were first considered by Xiao et al. [J. Chem. Phys. 126, 214101 (2007)] in a different context and in a different nomenclature. Their analysis and classification in a more general context are given here for the first time. Use of an extended balanced basis has no advantages and has other drawbacks and is not competitive, while the use of a restricted magnetic balance basis can be justified.
Modeling the Southwood Theory of Rotation-Period Perturbations of a Magnetized Plasma
NASA Astrophysics Data System (ADS)
Kivelson, M.; Jia, X.; Southwood, D. J.
2016-12-01
Many of Saturn's plasma and field properties vary at approximately Saturn's rotation period. The periodic behavior is imposed by a system of rotating currents whose origin remains uncertain. Southwood has proposed an analytical mathematical model that shows that a uniformly magnetized plasma bounded by a rotating conducting plate at its base naturally develops MHD disturbances that produce the rotating currents and vary at the rotation period of the plate. Such rotationally driven MHD perturbations achieve a steady state and remain azimuthally symmetric, a conclusion consistent with the sinusoidal dependence on Saturn's rotation phase found in the data from Cassini spacecraft measurements. The model is designed to represent flux tubes from either polar cap of Saturn (N or S) that link magnetically to the solar wind. The transverse magnetic field component transmits angular momentum but a compressional component may also develop (as observed). To test this model, we have carried out magnetohydrodynamic simulations of a cylinder filled with a uniform plasma and a constant magnetic field bounded at the base by a conducting plate that is set into rotational motion. Magnetic perturbations develop and propagate through the system. However, results of the simulation are highly sensitive to boundary conditions and, with time, our models depart from the quasi-steady conditions that we desire to represent. We describe aspects of the theory that are reproduced by runs using different boundary conditions and where and why they differ. For all initial conditions and boundary conditions used in the simulation, we find that both transverse and compressional perturbations develop before the simulation becomes unstable or in other ways unrealistic. However, in order to set up a relatively stable oscillating system, we continue to test new boundary conditions that come increasingly close to representing the portion of a magnetosphere linked to the polar cap of a rotating planet.
Quantal density-functional theory in the presence of a magnetic field
NASA Astrophysics Data System (ADS)
Yang, Tao; Pan, Xiao-Yin; Sahni, Viraht
2011-04-01
We generalize the quantal density-functional theory (QDFT) of electrons in the presence of an external electrostatic field E(r)=-∇v(r) to include an external magnetostatic field B(r)=∇×A(r), where {v(r),A(r)} are the respective scalar and vector potentials. The generalized QDFT, valid for nondegenerate ground and excited states, is the mapping from the interacting system of electrons to a model of noninteracting fermions with the same density ρ(r) and physical current density j(r), and from which the total energy can be obtained. The properties {ρ(r),j(r)} constitute the basic quantum-mechanical variables because, as proved previously, for a nondegenerate ground state they uniquely determine the potentials {v(r),A(r)}. The mapping to the noninteracting system is arbitrary in that the model fermions may be either in their ground or excited state. The theory is explicated by application to a ground state of the exactly solvable (two-dimensional) Hooke’s atom in a magnetic field, with the mapping being to a model system also in its ground state. The majority of properties of the model are obtained in closed analytical or semianalytical form. A comparison with the corresponding mapping from a ground state of the (three-dimensional) Hooke’s atom in the absence of a magnetic field is also made.
NASA Astrophysics Data System (ADS)
Shore, Robert; Freeman, Mervyn; Gjerloev, Jesper
2017-04-01
We apply the meteorological analysis method of Empirical Orthogonal Functions (EOF) to ground magnetometer measurements, and subsequently use graph theory to classify the results. The EOF method is used to characterise and separate contributions to the variability of the Earth's external magnetic field (EMF) in the northern polar region. EOFs decompose the noisy EMF data into a small number of independent spatio-temporal basis functions, which collectively describe the majority of the magnetic field variance. We use these basis functions (computed monthly) to infill where data are missing, providing a self-consistent description of the EMF at 5-minute resolution spanning 1997—2009 (solar cycle 23). Each of the EOF basis functions can typically be associated with one of the Disturbance Polar (DP)-type current systems (e.g. DP2, DP1, DPY, NBZ), or with the motion of these systems. This association allows us to describe the varying behaviour of the current systems over the 144 months (i.e. 1997—2009) of our reanalysis. However, the EOF basis functions are (by definition) ranked by their contribution to the total variance, and thus a given current system may be described by a different rank of basis vector from month to month. We use graph theory to find clusters of quantifiably-similar spatial basis functions, and thereby track a given pattern throughout the span of 144 months. Via this method, we present the seasonal and solar cycle variations in the polar current systems.
Asmat-Uceda, Martin; Buchanan, Kristen S.; Cheng, Xuemei; Wang, Xiao; Clarke, David J.; Tchernyshyov, Oleg
2015-03-28
Magnetostatic interactions between vortices in closely spaced planar structures are important for applications including vortex-based magnonic crystals and spin torque oscillator networks. Analytical theories that include magnetostatic interaction effects have been proposed but have not yet been rigorously tested. Here, we compare micromagnetic simulations of the dynamics of magnetic vortices confined in three disks in an equilateral triangle configuration to analytical theories that include coupling. Micromagnetic simulations show that the magnetostatic coupling between the disks leads to splitting of the gyrotropic resonance into three modes and that the frequency splitting increases with decreasing separation. The temporal profiles of the magnetization depend on the vortex polarities and chiralities; however, the frequencies depend only on the polarity combinations and will fall into one of two categories: all polarities equal or one polarity opposite to the others, where the latter leads to a larger frequency splitting. Although the magnitude of the splitting observed in the simulations is larger than what is expected based on purely dipolar interactions, a simple analytical model that assumes dipole-dipole coupling captures the functional form of the frequency splitting and the motion patterns just as well as more complex models.
Scherr, Thomas F; Markwalter, Christine F; Bauer, Westley S; Gasperino, David; Wright, David W; Haselton, Frederick R
2017-08-01
In many diagnostic assays, specific biomarker extraction and purification from a patient sample is performed in microcentrifuge tubes using surface-functionalized magnetic beads. Although assay binding times are known to be highly dependent on sample viscosity, sample volume, capture reagent, and fluid mixing, the theoretical mass transport framework that has been developed and validated in engineering has yet to be applied in this context. In this work, we adapt this existing framework for simultaneous mass transfer and surface reaction and apply it to the binding of biomarkers in clinical samples to surface-functionalized magnetic beads. We discuss the fundamental fluid dynamics of vortex mixing within microcentrifuge tubes as well as describe how particles and biomolecules interact with the fluid. The model is solved over a wide range of parameters, and we present scenarios when a simplified analytical expression would be most accurate. Next, we review of some relevant techniques for model parameter estimation. Finally, we apply the mass transfer theory to practical use-case scenarios of immediate use to clinicians and assay developers. Throughout, we highlight where further characterization is necessary to bridge the gap between theory and practical application. Copyright © 2017. Published by Elsevier B.V.
Quantal density-functional theory in the presence of a magnetic field
Yang Tao; Pan Xiaoyin; Sahni, Viraht
2011-04-15
We generalize the quantal density-functional theory (QDFT) of electrons in the presence of an external electrostatic field E(r)=-{nabla}v(r) to include an external magnetostatic field B(r)={nabla}xA(r), where (v(r),A(r)) are the respective scalar and vector potentials. The generalized QDFT, valid for nondegenerate ground and excited states, is the mapping from the interacting system of electrons to a model of noninteracting fermions with the same density {rho}(r) and physical current density j(r), and from which the total energy can be obtained. The properties ({rho}(r),j(r)) constitute the basic quantum-mechanical variables because, as proved previously, for a nondegenerate ground state they uniquely determine the potentials (v(r),A(r)). The mapping to the noninteracting system is arbitrary in that the model fermions may be either in their ground or excited state. The theory is explicated by application to a ground state of the exactly solvable (two-dimensional) Hooke's atom in a magnetic field, with the mapping being to a model system also in its ground state. The majority of properties of the model are obtained in closed analytical or semianalytical form. A comparison with the corresponding mapping from a ground state of the (three-dimensional) Hooke's atom in the absence of a magnetic field is also made.
Three-dimensional linear peeling-ballooning theory in magnetic fusion devices
Weyens, T. Sánchez, R.; García, L.; Loarte, A.; Huijsmans, G.
2014-04-15
Ideal magnetohydrodynamics theory is extended to fully 3D magnetic configurations to investigate the linear stability of intermediate to high n peeling-ballooning modes, with n the toroidal mode number. These are thought to be important for the behavior of edge localized modes and for the limit of the size of the pedestal that governs the high confinement H-mode. The end point of the derivation is a set of coupled second order ordinary differential equations with appropriate boundary conditions that minimize the perturbed energy and that can be solved to find the growth rate of the perturbations. This theory allows of the evaluation of 3D effects on edge plasma stability in tokamaks such as those associated with the toroidal ripple due to the finite number of toroidal field coils, the application of external 3D fields for elm control, local modification of the magnetic field in the vicinity of ferromagnetic components such as the test blanket modules in ITER, etc.
Federrath, Christoph; Klessen, Ralf S.
2012-12-20
The role of turbulence and magnetic fields is studied for star formation in molecular clouds. We derive and compare six theoretical models for the star formation rate (SFR)-the Krumholz and McKee (KM), Padoan and Nordlund (PN), and Hennebelle and Chabrier (HC) models, and three multi-freefall versions of these, suggested by HC-all based on integrals over the log-normal distribution of turbulent gas. We extend all theories to include magnetic fields and show that the SFR depends on four basic parameters: (1) virial parameter {alpha}{sub vir}; (2) sonic Mach number M; (3) turbulent forcing parameter b, which is a measure for the fraction of energy driven in compressive modes; and (4) plasma {beta}=2M{sub A}{sup 2}/M{sup 2} with the Alfven Mach number M{sub A}. We compare all six theories with MHD simulations, covering cloud masses of 300 to 4 Multiplication-Sign 10{sup 6} M{sub Sun} and Mach numbers M=3-50 and M{sub A}=1-{infinity}, with solenoidal (b = 1/3), mixed (b = 0.4), and compressive turbulent (b = 1) forcings. We find that the SFR increases by a factor of four between M=5 and 50 for compressive turbulent forcing and {alpha}{sub vir} {approx} 1. Comparing forcing parameters, we see that the SFR is more than 10 times higher with compressive than solenoidal forcing for M=10 simulations. The SFR and fragmentation are both reduced by a factor of two in strongly magnetized, trans-Alfvenic turbulence compared to hydrodynamic turbulence. All simulations are fit simultaneously by the multi-freefall KM and multi-freefall PN theories within a factor of two over two orders of magnitude in SFR. The simulated SFRs cover the range and correlation of SFR column density with gas column density observed in Galactic clouds, and agree well for star formation efficiencies SFE = 1%-10% and local efficiencies {epsilon} = 0.3-0.7 due to feedback. We conclude that the SFR is primarily controlled by interstellar turbulence, with a secondary effect coming from magnetic fields.
NASA Astrophysics Data System (ADS)
Federrath, Christoph; Klessen, Ralf S.
2012-12-01
The role of turbulence and magnetic fields is studied for star formation in molecular clouds. We derive and compare six theoretical models for the star formation rate (SFR)—the Krumholz & McKee (KM), Padoan & Nordlund (PN), and Hennebelle & Chabrier (HC) models, and three multi-freefall versions of these, suggested by HC—all based on integrals over the log-normal distribution of turbulent gas. We extend all theories to include magnetic fields and show that the SFR depends on four basic parameters: (1) virial parameter αvir (2) sonic Mach number {M}; (3) turbulent forcing parameter b, which is a measure for the fraction of energy driven in compressive modes; and (4) plasma \\beta =2 {M}_A^2/ {M}^2 with the Alfvén Mach number {M}_A. We compare all six theories with MHD simulations, covering cloud masses of 300 to 4 × 106 M ⊙ and Mach numbers {M}=3-50 and {M}_A=1-∞, with solenoidal (b = 1/3), mixed (b = 0.4), and compressive turbulent (b = 1) forcings. We find that the SFR increases by a factor of four between {M}=5 and 50 for compressive turbulent forcing and αvir ~ 1. Comparing forcing parameters, we see that the SFR is more than 10 times higher with compressive than solenoidal forcing for {M}=10 simulations. The SFR and fragmentation are both reduced by a factor of two in strongly magnetized, trans-Alfvénic turbulence compared to hydrodynamic turbulence. All simulations are fit simultaneously by the multi-freefall KM and multi-freefall PN theories within a factor of two over two orders of magnitude in SFR. The simulated SFRs cover the range and correlation of SFR column density with gas column density observed in Galactic clouds, and agree well for star formation efficiencies SFE = 1%-10% and local efficiencies epsilon = 0.3-0.7 due to feedback. We conclude that the SFR is primarily controlled by interstellar turbulence, with a secondary effect coming from magnetic fields.
Nonuniversal gaugino masses in a magnetized toroidal compactification of SYM theories
NASA Astrophysics Data System (ADS)
Sumita, Keigo
2015-10-01
This paper proposes a concrete model of nonuniversal gaugino masses on the basis of higher-dimensional supersymmetric Yang-Mills theories compactified on a magnetized factorizable torus, and we estimate the gauge coupling constants and gaugino masses in the model. In the magnetized toroidal compactifications, the four-dimensional effective action can be obtained analytically identifying its dependence on moduli fields, where the magnetic fluxes are able to yield the flavor structure of the minimal supersymmetric standard model (MSSM). The obtained gauge kinetic functions contains multi moduli fields and their dependence is nonuniversal for the three gauge fields. The nonuniversal gauge kinetic functions can lead to nonuniversal gaugino masses at a certain high energy scale (e.g. compactification scale). Our numerical analysis of them shows that, particular ratios of gaugino masses, which were found to enhance the Higgs boson mass and lead to "natural supersymmetry" in the MSSM, can be realized in our model, while the gauge couplings are unified as is achieved in the MSSM.
NASA Astrophysics Data System (ADS)
Deák, A.; Simon, E.; Balogh, L.; Szunyogh, L.; dos Santos Dias, M.; Staunton, J. B.
2014-06-01
We develop a self-consistent relativistic disordered local moment (RDLM) scheme aimed at describing finite-temperature magnetism of itinerant metals from first principles. Our implementation in terms of the Korringa-Kohn-Rostoker multiple-scattering theory and the coherent potential approximation allows us to relate the orientational distribution of the spins to the electronic structure, thus a self-consistent treatment of the distribution is possible. We present applications for bulk bcc Fe, L10-FePt, and FeRh ordered in the CsCl structure. The calculations for Fe show significant variation of the local moments with temperature, whereas according to the mean-field treatment of the spin fluctuations the Curie temperature is overestimated. The magnetic anisotropy of FePt alloys is found to depend strongly on intermixing between nominally Fe and Pt layers, and it shows a power-law behavior as a function of magnetization for a broad range of chemical disorder. In the case of FeRh we construct a lattice constant vs temperature phase diagram and determine the phase line of metamagnetic transitions based on self-consistent RDLM free-energy curves.
Theory of NMR signal behavior in magnetically inhomogeneous tissues: the static dephasing regime.
Yablonskiy, D A; Haacke, E M
1994-12-01
This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo time TE is less than a known characteristic time tc for a given system, then the signal decays exponentially with an argument which depends quadratically on TE. This is equivalent to an R2* relaxation rate which is a linear function of TE. In the opposite case, when TE is greater than tc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction sigma occupied by the field-creating objects, b) magnetic field Bo or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference delta chi between the objects and the medium.
A Hot Big Bang Theory: Magnetic Fields and the Early Evolution of the Protolunar Disk
NASA Astrophysics Data System (ADS)
Gammie, C. F.; Liao, Wei-Ting; Ricker, P. M.
2016-09-01
The leading theory for the formation of Earth’s Moon invokes a collision between a Mars-sized body and the proto-Earth to produce a disk of orbiting material that later condenses to form the Moon. We show that the disk opacity is large, and cooling is therefore inefficient ({t}{cool}{{Ω }}\\gg 1). In this regime, angular momentum transport in the disk leads to steady heating unless α \\lt {({t}{cool}{{Ω }})}-1\\ll 1. Following earlier work by Charnoz and Michaut, and Carballido et al., we show that once the disk is completely vaporized it is well coupled to the magnetic field. We consider a scenario in which turbulence driven by magnetic fields leads to a brief, hot phase where the disk is geometrically thick, with strong turbulent mixing. The disk cools by spreading until it decouples from the field. We point out that approximately half the accretion energy is dissipated in the boundary layer where the disk meets the Earth’s surface. This creates high entropy material close to the Earth, driving convection and mixing. Finally, a hot magnetized disk could drive bipolar outflows that remove mass and angular momentum from the Earth-Moon system.
Wang, Huai-Qian; Li, Hui-Fang; Wang, Jia-Xian; Kuang, Xiao-Yu
2012-07-01
The application of the ab initio stochastic search procedure with Saunders "kick" method has been carried out for the elucidation of global minimum structures of a series of Al-doped clusters, Nb(n)Al (1 ≤ n ≤ 10). We have studied the structural characters, growth behaviors, electronic and magnetic properties of Nb(n)Al by the density functional theory calculations. Unlike the previous literature reported on Al-doped systems where ground state structures undergo a structural transition from the Al-capped frame to Al-encapsulated structure, we found that Al atom always occupies the surface of Nb(n)Al clusters and structural transition does not take place until n = 10. Note that the fragmentation proceeds preferably by the ejection of an aluminum atom other than niobium atom. According to the natural population analysis, charges always transfer from aluminum to niobium atoms. Furthermore, the magnetic moments of the Nb(n)Al clusters are mainly located on the 4d orbital of niobium atoms, and aluminum atom possesses very small magnetic moments.
Linear theory for fast collisionless magnetic reconnection in the lower-hybrid frequency range
Jovanovic, D.; Shukla, P.K.
2005-05-15
A linear theory is presented for the interplay between the fast collisionless magnetic reconnection and the lower-hybrid waves that has been observed in recent computer simulations [J. F. Drake, M. Swisdak, C. Cattell et al., Science 299, 873 (2003)]. In plasma configurations with a strong guide field and anisotropic electron temperature, the electron dynamics is described within the framework of standard electron magnetohydrodynamic equations, accounting also for the effects of the electron polarization and ion motions in the presence of perpendicular electric fields. In the linear phase, we find two types of instabilities of a thin current sheet with steep edges, corresponding to its filamentation (or tearing) and bending. Using a surface-wave formalism for the perturbations whose wavelength is larger than the thickness of the current sheet, the corresponding growth rates are calculated as the contributions of singularities in the plasma dispersion function. These are governed by the electron inertia and the linear coupling of the reconnecting magnetic field with local plasma modes propagating in the perpendicular direction that are subject to the Buneman instability. The linear surface wave instability may be particularly important as a secondary instability, dissipating the thin current sheets that develop in the course of the fast reconnection in the shear-Alfven and kinetic-Alfven regimes, and providing the anomalous resistivity for the growth of magnetic islands beyond the shear-Alfven and kinetic-Alfven scales.
Theory of magnetic enhancement in strontium hexaferrite through Zn-Sn pair substitution
Liyanage, LSI; Kim, S; Hong, YK; Park, JH; Erwin, SC; Kim, SG
2013-12-01
We study the site occupancy and magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite SrFe12-x(Zn0.5Sn0.5)(x)O-19 with x=1 using first-principles total-energy calculations. We find that in the lowest-energy configuration Zn2+ and Sn4+ ions preferentially occupy the 4f(1) and 4f(2) sites, respectively, in contrast to the model previously suggested by Ghasemi et al. [J. Appl. Phys, 107, 09A734 (2010)], where Zn2+ and Sn4+ ions occupy the 2b and 4f(2) sites. Density-functional theory calculations show that our model has a lower total energy by more than 0.2 eV per unit cell compared to Ghasemi's model. More importantly, the latter does not show an increase in saturation magnetization (M-s) compared to the pure M-type Sr-hexaferrite, in disagreement with the experiment. On the other hand, our model correctly predicts a rapid increase in M-s as well as a decrease in magnetic anisotropy compared to the pure M-type Sr-hexaferrite, consistent with experimental measurements. (c) 2013 Elsevier B.V. All rights reserved.
Novikov, Alexander
2011-02-01
A complete time-dependent physics theory of symmetric unperturbed driven hybrid birdcage resonator was developed for general application. In particular, the theory can be applied for radiofrequency (RF) coil engineering, computer simulations of coil-sample interaction, etc. Explicit time dependence is evaluated for different forms of driving voltage. The major steps of the solution development are shown and appropriate explanations are given. Green's functions and spectral density formula were developed for any form of periodic driving voltage. The concept of distributed power losses based on transmission line theory is developed for evaluation of local losses of a coil. Three major types of power losses are estimated as equivalent series resistances in the circuit of the birdcage resonator. Values of generated resistances in legs and end-rings are estimated. An application of the theory is shown for many practical cases. Experimental curve of B(1) field polarization dependence is measured for eight-sections birdcage coil. It was shown that the steady-state driven resonance frequencies do not depend on damping factor unlike the free oscillation (transient) frequencies. An equivalent active resistance is generated due to interaction of RF electromagnetic field with a sample. Resistance of the conductor (enhanced by skin effect), Eddy currents and dielectric losses are the major types of losses which contribute to the values of generated resistances. A biomedical sample for magnetic resonance imaging and spectroscopy is the source of the both Eddy current and dielectric losses of a coil. As demonstrated by the theory, Eddy current loss is the major effect of coil shielding.
Novikov, Alexander
2010-01-01
A complete time-dependent physics theory of symmetric unperturbed driven Hybrid Birdcage resonator was developed for general application. In particular, the theory can be applied for RF coil engineering, computer simulations of coil-sample interaction, etc. Explicit time dependence is evaluated for different forms of driving voltage. The major steps of the solution development are shown and appropriate explanations are given. Green’s functions and spectral density formula were developed for any form of periodic driving voltage. The concept of distributed power losses based on transmission line theory is developed for evaluation of local losses of a coil. Three major types of power losses are estimated as equivalent series resistances in the circuit of the Birdcage resonator. Values of generated resistances in Legs and End-Rings are estimated. An application of the theory is shown for many practical cases. Experimental curve of B1 field polarization dependence is measured for eight-sections Birdcage coil. It was shown, that the steady-state driven resonance frequencies do not depend on damping factor unlike the free oscillation (transient) frequencies. An equivalent active resistance is generated due to interaction of RF electromagnetic field with a sample. Resistance of the conductor (enhanced by skin effect), Eddy currents and dielectric losses are the major types of losses which contribute to the values of generated resistances. A biomedical sample for magnetic resonance imaging and spectroscopy is the source of the both Eddy current and dielectric losses of a coil. As demonstrated by the theory, Eddy currents losses is the major effect of coil shielding. PMID:20869184
Magnetic shielding of a laboratory Hall thruster. I. Theory and validation
Mikellides, Ioannis G. Katz, Ira; Hofer, Richard R.; Goebel, Dan M.
2014-01-28
We demonstrate a technique by which erosion of the acceleration channel in Hall thrusters can be reduced by at least a few orders of magnitude. The first principles of the technique, now known as “magnetic shielding,” have been derived based on the findings of 2-D numerical simulations. The simulations, in turn, guided the modification of an existing 6-kW laboratory Hall thruster to test the theory and are the main subject of this Part I article. Part II expands on the results of the experiments. Near the walls of the magnetically shielded (MS) thruster theory and experiment agree that (1) the plasma potential has been sustained at values near the discharge voltage, and (2) the electron temperature has been lowered compared to the unshielded thruster. Erosion rates deduced directly from the wall probes show reductions of at least ∼3 orders of magnitude at the MS inner wall when an ion energy threshold of 30.5 V is used in the sputtering yield model of the channel material. At the outer wall the probes reveal that the ion energy was below the assumed threshold. Using a threshold of 25 V, the simulations predict a minimum reduction of ∼600 at the MS inner wall. At the MS outer wall ion energies are found to be below 25 V. When a 50-V threshold is used the computed ion energies are below the threshold at both sides of the channel. Uncertainties, sensitivities, and differences between theory and experiment are also discussed. The elimination of wall erosion in Hall thrusters solves a problem that has remained unsettled for more than five decades.
Prediction of d^0 magnetism in self-interaction corrected density functional theory
NASA Astrophysics Data System (ADS)
Das Pemmaraju, Chaitanya
2010-03-01
Over the past couple of years, the phenomenon of ``d^0 magnetism'' has greatly intrigued the magnetism community [1]. Unlike conventional magnetic materials, ``d^0 magnets'' lack any magnetic ions with open d or f shells but surprisingly, exhibit signatures of ferromagnetism often with a Curie temperature exceeding 300 K. Current research in the field is geared towards trying to understand the mechanism underlying this observed ferromagnetism which is difficult to explain within the conventional m-J paradigm [1]. The most widely studied class of d^0 materials are un-doped and light element doped wide gap Oxides such as HfO2, MgO, ZnO, TiO2 all of which have been put forward as possible d0 ferromagnets. General experimental trends suggest that the magnetism is a feature of highly defective samples leading to the expectation that the phenomenon must be defect related. In particular, based on density functional theory (DFT) calculations acceptor defects formed from the O-2p states in these Oxides have been proposed as being responsible for the ferromagnetism [2,3]. However. predicting magnetism originating from 2p orbitals is a delicate problem, which depends on the subtle interplay between covalency and Hund's coupling. DFT calculations based on semi-local functionals such as the local spin-density approximation (LSDA) can lead to qualitative failures on several fronts. On one hand the excessive delocalization of spin-polarized holes leads to half-metallic ground states and the expectation of room-temperature ferromagnetism. On the other hand, in some cases a magnetic ground state may not be predicted at all as the Hund's coupling might be under estimated. Furthermore, polaronic distortions which are often a feature of acceptor defects in Oxides are not predicted [4,5]. In this presentation, we argue that the self interaction error (SIE) inherent to semi-local functionals is responsible for the failures of LSDA and demonstrate through various examples that beyond
Magnetic flux creep in HTSC and Anderson-Kim theory (Review Article)
NASA Astrophysics Data System (ADS)
Lykov, A. N.
2014-09-01
Theoretical results and experimental data on flux creep in high-temperature superconductors (HTSC) are analyzed in this review paper. When reviewing experimental work, the main attention is paid to the most striking experimental results which have had a major impact on the study of flux creep in HTSC. On the other hand, the analysis of theoretical results is focused on the studies which explain the features of flux creep by introducing modifications to the Anderson-Kim (AK) theory, i.e., on the studies that have not received sufficient attention earlier. However, it turned out that the modified AK theory could explain a number of features of flux creep in HTSC: the scaling behavior of current-voltage curves in HTSC, the finite rate of flux creep at ultralow temperatures, the logarithmic dependence of the effective pinning potential on the transport current and its decrease with temperature. The harmonic potential field which is used in this approach makes it possible to solve accurately both the problem of viscous vortex motion and the problem of thermally activated flux creep in this magnetic field. Moreover, the energy distribution of pinning potential and the interaction of vortices with each other are also taken into account in the approach. Thus, the modification of the AK theory consists, essentially, in its refinement and achieving a more realistic approximation.
W.M. Tang
2005-01-03
The present lecture provides an introduction to the subject of gyrokinetic theory with applications in the area of magnetic confinement research in plasma physics--the research arena from which this formalism was originally developed. It was presented as a component of the ''Short Course in Kinetic Theory within the Thematic Program in Partial Differential Equations'' held at the Fields Institute for Research in Mathematical Science (24 March 2004). This lecture also discusses the connection between the gyrokinetic formalism and powerful modern numerical simulations. Indeed, simulation, which provides a natural bridge between theory and experiment, is an essential modern tool for understanding complex plasma behavior. Progress has been stimulated in particular by the exponential growth of computer speed along with significant improvements in computer technology. The advances in both particle and fluid simulations of fine-scale turbulence and large-scale dynamics have produced increasingly good agreement between experimental observations and computational modeling. This was enabled by two key factors: (i) innovative advances in analytic and computational methods for developing reduced descriptions of physics phenomena spanning widely disparate temporal and spatial scales and (ii) access to powerful new computational resources.
NASA Astrophysics Data System (ADS)
Meinert, Markus; Schmalhorst, Jan-Michael; Klewe, Christoph; Reiss, Günter; Arenholz, Elke; Böhnert, Tim; Nielsch, Kornelius
2011-10-01
Epitaxial thin films of the half-metallic Xa compound Mn2CoGa (Hg2CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of 1.95(5)μB per unit cell were obtained. The L3,2 x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.
NASA Astrophysics Data System (ADS)
D'Elia, Massimo; Mariti, Marco
2017-04-01
We discuss the properties of non-Abelian gauge theories formulated on manifolds with compactified dimensions and in the presence of fermionic fields coupled to magnetic backgrounds. We show that different phases may emerge, corresponding to different realizations of center symmetry and translational invariance, depending on the compactification radius and on the magnitude of the magnetic field. Our discussion then focuses on the case of an S U (3 ) gauge theory in four dimensions with fermions fields in the fundamental representation, for which we provide some exploratory numerical lattice results.
NASA Astrophysics Data System (ADS)
Mendive-Tapia, Eduardo; Staunton, Julie B.
2017-05-01
We describe a disordered local moment theory for long-period magnetic phases and investigate the temperature and magnetic field dependence of the magnetic states in the heavy rare earth elements (HREs), namely, paramagnetic, conical and helical antiferromagnetic (HAFM), fan, and ferromagnetic (FM) states. We obtain a generic HRE magnetic phase diagram which is consequent on the response of the common HRE valence electronic structure to f -electron magnetic moment ordering. The theory directly links the first-order HAFM-FM transition to the loss of Fermi surface nesting, induced by this magnetic ordering, as well as provides a template for analyzing the other phases and exposing where f -electron correlation effects are particularly intricate. Gadolinium, for a range of hexagonal, close-packed lattice constants c and a , is the prototype, described ab initio, and applications to other HREs are made straightforwardly by scaling the effective pair and quartic local moment interactions that emerge naturally from the theory with de Gennes factors and choosing appropriate lanthanide-contracted c and a values.
Kinetic theory of a two-dimensional magnetized plasma. II - Balescu-Lenard limit.
NASA Technical Reports Server (NTRS)
Vahala, G.
1972-01-01
The kinetic theory of a two-dimensional one-species plasma in a uniform dc magnetic field is investigated in the small plasma parameter limit. The plasma consists of charged rods interacting through the logarithmic Coulomb potential. Vahala and Montgomery earlier (1971) derived a Fokker-Planck equation for this system, but it contained a divergent integral, which had to be cut off on physical grounds. This cutoff is compared to the standard cutoff introduced in the two-dimensional unmagnetized Fokker-Planck equation. In the small plasma parameter limit, it is shown that the Balescu-Lenard collision term is zero in the long time average limit if only two-body interactions are considered. The energy transfer from a test particle to an equilibrium plasma is discussed and is also shown to be zero in the long time average limit. This supports the unexpected result of zero Balescu-Lenard collision term.
Bifurcation theory for the L-H transition in magnetically confined fusion plasmas
Weymiens, W.; Blank, H. J. de; Hogeweij, G. M. D.; Valenca, J. C. de
2012-07-15
The mathematical field of bifurcation theory is extended to be applicable to 1-dimensionally resolved systems of nonlinear partial differential equations, aimed at the determination of a certain specific bifurcation. This extension is needed to be able to properly analyze the bifurcations of the radial transport in magnetically confined fusion plasmas. This is of special interest when describing the transition from the low-energy-confinement state to the high-energy-confinement state of the radial transport in fusion plasmas (i.e., the L-H transition), because the nonlinear dynamical behavior during the transition corresponds to the dynamical behavior of a system containing such a specific bifurcation. This bifurcation determines how the three types (sharp, smooth, and oscillating) of observed L-H transitions are organized as function of all the parameters contained in the model.
Density matrix theory of transport and gain in quantum cascade lasers in a magnetic field
NASA Astrophysics Data System (ADS)
Savić, Ivana; Vukmirović, Nenad; Ikonić, Zoran; Indjin, Dragan; Kelsall, Robert W.; Harrison, Paul; Milanović, Vitomir
2007-10-01
A density matrix theory of electron transport and optical gain in quantum cascade lasers in an external magnetic field is formulated. Starting from a general quantum kinetic treatment, we describe the intraperiod and interperiod electron dynamics at the non-Markovian, Markovian, and Boltzmann approximation levels. Interactions of electrons with longitudinal optical phonons and classical light fields are included in the present description. The non-Markovian calculation for a prototype structure reveals a significantly different gain spectra in terms of linewidth and additional polaronic features in comparison to the Markovian and Boltzmann ones. Despite strongly controversial interpretations of the origin of the transport processes in the non-Markovian or Markovian and the Boltzmann approaches, they yield comparable values of the current densities.
The expansion of polarization charge layers into magnetized vacuum - Theory and computer simulations
NASA Technical Reports Server (NTRS)
Galvez, Miguel; Borovsky, Joseph E.
1991-01-01
The formation and evolution of polarization charge layers on cylindrical plasma streams moving in vacuum are investigated using analytic theory and 2D electrostatic particle-in-cell computer simulations. It is shown that the behavior of the electron charge layer goes through three stages. An early time expansion is driven by electrostatic repulsion of electrons in the charge layer. At the intermediate stage, the simulations show that the electron-charge-layer expansion is halted by the positively charged plasma stream. Electrons close to the stream are pulled back to the stream and a second electron expansion follows in time. At the late stage, the expansion of the ion charge layer along the magnetic field lines accompanies the electron expansion to form an ambipolar expansion. It is found that the velocities of these electron-ion expansions greatly exceed the velocities of ambipolar expansions which are driven by plasma temperatures.
NASA Astrophysics Data System (ADS)
Simons, F. J.; Beggan, C.; Saarimaki, J.; Whaler, K. A.; Lewis, K. W.; Plattner, A.
2012-12-01
We elucidate "myths and truths concerning the estimation of power spectra", to paraphrase a paper written by G. Efstathiou. A complete theory and software are available to construct and interpret localized power spectral densities from noisy and incomplete data observed on a spherical surface, but such knowledge has not made it into mainstream geophysical practice. Key are the doubly-orthogonal spherical Slepian functions, and their Cartesian and vectorial relatives. There is confusion in the community on whether to use the Slepian functions as a basis for data approximation, as windows for regularization of power spectral estimates, or hybridly. Each of these choices is legitimate but carries consequences to be characterized statistically. We review the options and present them tutorially, to motivate the scientific analysis of the terrestrial magnetic power spectrum, and that of Mars, which follows. The power spectral density of the lithospheric magnetic field on Earth is broken down in to portions that are geographically limited to the oceans, the continents, and various portions thereof. The different overall power levels and varying character of the power spectra contain much diagnostic information about the generation and preservation of magnetic signatures in the Earth's crust. On Mars, we conducted a parameterized inversion for magnetization strength and decorrelation depth, and compared this to independent, gravity-based estimates for crustal thickness. Both the terrestrial and martian examples are different manifestations of how Slepian functions can be used for power-spectral localization, and in both cases, the uncertainty of the spectral estimates, both on the horizontal axis (degree resolution) and the vertical axis (estimation variance), is easily computed via exact and also approximate relationships. The analytical expressions that are involved may look off-puttingly complex, but the result of the theoretical analysis is a set of simple routines
Electronic and magnetic properties of C-doped Mg 3N 2: A density functional theory study
NASA Astrophysics Data System (ADS)
Niu, C. W.; Yang, Kesong; Lv, Yingbo; Wei, Wei; Dai, Ying; Huang, Baibiao
2010-12-01
Based on density functional theory, we investigate the electronic and spin-polarized properties of C-doped Mg 3N 2 with C at two nonequivalent N sites. Results of our calculations reveal that the electronic properties are sensitive to the doping sites while the magnetic moment is not. The substitution of C by N favors a spin-polarized state with a total magnetic moment of 1.0μ per C, which is equal to the number of holes in the system. Our magnetic coupling calculations also indicate that substantial ferromagnetism is possible in the C-doped Mg 3N 2.
Ising spin-glass transition in a magnetic field outside the limit of validity of mean-field theory.
Leuzzi, L; Parisi, G; Ricci-Tersenghi, F; Ruiz-Lorenzo, J J
2009-12-31
The spin-glass transition in a magnetic field is studied both in and out of the limit of validity of mean-field theory on a diluted one dimensional chain of Ising spins where exchange bonds occur with a probability decaying as the inverse power of the distance. Varying the power in this long-range model corresponds, in a one-to-one relationship, to changing the dimension in spin-glass short-range models. Evidence for a spin-glass transition in a magnetic field is found also for systems whose equivalent dimension is below the upper critical dimension in a zero magnetic field.
Strong-coupling perturbation theory for the two-dimensional Bose-Hubbard model in a magnetic field
NASA Astrophysics Data System (ADS)
Niemeyer, M.; Freericks, J. K.; Monien, H.
1999-07-01
The Bose-Hubbard model in an external magnetic field is investigated with strong-coupling perturbation theory. The lowest-order secular equation leads to the problem of a charged particle moving on a lattice in the presence of a magnetic field, which was first treated by Hofstadter. We present phase diagrams for the two-dimensional square and triangular lattices, showing a change in shape of the phase lobes away from the well-known power-law behavior in zero magnetic field. Some qualitative agreement with experimental work on Josephson-junction arrays is found for the insulating phase behavior at small fields.
Vacancy-induced magnetism in BaTiO3(001) thin films based on density functional theory.
Cao, Dan; Cai, Meng-Qiu; Hu, Wang-Yu; Yu, Ping; Huang, Hai-Tao
2011-03-14
The origin of magnetism induced by vacancies on BaTiO(3)(001) surfaces is investigated systematically by first-principles calculations within density-functional theory. The calculated results show that O vacancy is responsible for the magnetism of the BaO-terminated surface and the magnetism of the TiO(2)-terminated surface is induced by Ti vacancy. For the BaO-terminated surface, the magnetism mainly arises from the unpaired electrons that are localized in the O vacancy basin. In contrast, for the TiO(2)-terminated surface, the magnetism mainly originates from the partially occupied O-2p states of the first nearest neighbor O atoms surrounding the Ti vacancy. These results suggest the possibility of implementing magneto-electric coupling in conventional ferroelectric materials.
A Combined Density Functional Theory and Monte Carlo Study of Manganites for Magnetic Refrigeration
NASA Astrophysics Data System (ADS)
Korotana, Romi; Mallia, Giuseppe; Gercsi, Zsolt; Harrison, Nicholas
2015-03-01
Perovskite oxides are considered to be strong candidates for applications in magnetic refrigeration technology, due to their remarkable properties, in addition to low processing costs. Manganites with the general formula R1-xAxMnO3, particularly for A=Ca and 0 . 2 < x < 0 . 5 , undergo a field driven transition from a paramagnetic to ferromagnetic state, which is accompanied by changes in the lattice and electronic structure. Therefore, one may anticipate a large entropy change across the phase transition due to the first order nature. The present work aims to achieve an understanding of the relevant structural, magnetic, and electronic entropy contributions in the doped compound La0.75Ca0.25MnO3. A combination of thermodynamics and first principles theory is applied to determine individual contributions to the total entropy change of the system. Hybrid-exchange density functional (B3LYP) calculations for La0.75Ca0.25MnO3 predict an anti-Jahn-Teller polaron in the localised hole state, which is influenced by long-range cooperative Jahn-Teller distortions. Through the analysis of individual entropy contributions, it is identified that the electronic and vibrational terms have a deleterious effect on the total entropy change.
Theory of Stochastic Dipolar Recoupling in Solid State Nuclear Magnetic Resonance
Tycko, Robert
2008-01-01
Dipolar recoupling techniques in solid state nuclear magnetic resonance (NMR) consist of radio-frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create non-zero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter fmax) increases; (2) In a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large fmax, with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) Quantum mechanical interferences among non-commuting pairwise dipole-dipole couplings, which are a complicating factor in solid state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large fmax, provided that coupled nuclei have distinct NMR chemical shifts. PMID:18085769
NASA Astrophysics Data System (ADS)
Solovyova, Anna Y.; Goldina, Olga A.; Ivanov, Alexey O.; Lebedev, Aleksandr V.; Elfimova, Ekaterina A.
2016-08-01
Temperature dependencies of the static initial magnetic susceptibility for ferrofluids at various concentrations are studied using experiment and statistical-mechanical theories. Magnetic susceptibility measurements are carried out for twelve samples of magnetite-based fluids stabilized with oleic acid over a wide range of temperatures (210 K ≲T ≲ 390 K); all samples have the same granulometric composition but different volume ferroparticle concentrations (0.2 ≲ φ ≲ 0.5). Experimental results are analyzed using three theories: the second-order modified mean-field theory (MMF2) [A. O. Ivanov and O. B. Kuznetsova, Phys. Rev. E 64, 41405 (2001)]; its correction for polydisperse ferrofluids arising from Mayer-type cluster expansion and taking into account the first terms of the polydisperse second virial coefficient [A. O. Ivanov and E. A. Elfimova, J. Magn. Magn. Mater 374, 327 (2015)]; and a new theory based on MMF2 combined with the first terms of the polydisperse second and third virial contributions to susceptibility. It turns out that the applicability of each theory depends on the experimental sample density. If twelve ferrofluid samples are split into three groups of strong, moderate, and low concentrated fluids, the temperature dependences of the initial magnetic susceptibility in each group are very precisely described by one of the three theories mentioned above. The determination of a universal formula predicting a ferrofluid susceptibility over a broad range of concentrations and temperatures remains as a challenge.
Solovyova, Anna Y; Goldina, Olga A; Ivanov, Alexey O; Lebedev, Aleksandr V; Elfimova, Ekaterina A
2016-08-28
Temperature dependencies of the static initial magnetic susceptibility for ferrofluids at various concentrations are studied using experiment and statistical-mechanical theories. Magnetic susceptibility measurements are carried out for twelve samples of magnetite-based fluids stabilized with oleic acid over a wide range of temperatures (210 K ≲T ≲ 390 K); all samples have the same granulometric composition but different volume ferroparticle concentrations (0.2 ≲ φ ≲ 0.5). Experimental results are analyzed using three theories: the second-order modified mean-field theory (MMF2) [A. O. Ivanov and O. B. Kuznetsova, Phys. Rev. E 64, 41405 (2001)]; its correction for polydisperse ferrofluids arising from Mayer-type cluster expansion and taking into account the first terms of the polydisperse second virial coefficient [A. O. Ivanov and E. A. Elfimova, J. Magn. Magn. Mater 374, 327 (2015)]; and a new theory based on MMF2 combined with the first terms of the polydisperse second and third virial contributions to susceptibility. It turns out that the applicability of each theory depends on the experimental sample density. If twelve ferrofluid samples are split into three groups of strong, moderate, and low concentrated fluids, the temperature dependences of the initial magnetic susceptibility in each group are very precisely described by one of the three theories mentioned above. The determination of a universal formula predicting a ferrofluid susceptibility over a broad range of concentrations and temperatures remains as a challenge.
Electrostatic drift waves in a 2D magnetic current sheet - a new kinetic theory
NASA Astrophysics Data System (ADS)
Fruit, G.; Louarn, P.; Tur, A.
2015-12-01
In the general context of understanding the possible destabilization of the magnetotail before a substorm, a kinetic model for electromagnetic instabilities in resonant interaction with trapped bouncing electrons has been proposed for several years. Fruit et al. 2013 already used it to investigate the possibilities for electrostatic instabilities. Tur et al. 2014 generalizes the model for full electromagnetic perturbations.It turns out that some corrections should be added to the electrostatic version of Fruit et al. 2013. We propose to revist the theory in this present paper.Starting with a modified 2D Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electrostatic fluctuations with period of the order of the electron bounce period (a few seconds). The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electrostatic modes is finally obtained through the quasineutrality condition.The new feature of the present model is the inclusion of diamagnetic drift effects due to the density gradient in the tail. It is well known in MHD theory that drift waves are driven unstable through collisions or other dissipative effects. Here electrostatic drift waves are revisited in this more complete kinetic model including bouncing electrons and finite Larmor radius effects. A new mode has been found with original propagation proprieties. It is moreover mildly unstable due to electron or ion damping (dissipative instability).
SL(2,R) duality-symmetric action for electromagnetic theory with electric and magnetic sources
Lee, Choonkyu; Min, Hyunsoo
2013-12-15
For the SL(2,R) duality-invariant generalization of Maxwell electrodynamics in the presence of both electric and magnetic sources, we formulate a local, manifestly duality-symmetric, Zwanziger-type action by introducing a pair of four-potentials A{sup μ} and B{sup μ} in a judicious way. On the two potentials A{sup μ} and B{sup μ} the SL(2,R) duality transformation acts in a simple linear manner. In quantum theory including charged source fields, this action can be recast as a SL(2,Z)-invariant action. Also given is a Zwanziger-type action for SL(2,R) duality-invariant Born–Infeld electrodynamics which can be important for D-brane dynamics in string theory. -- Highlights: •We formulate a local, manifestly duality-symmetric, Zwanziger-type action. •Maxwell electrodynamics is generalized to include dilaton and axion fields. •SL(2,R) symmetry is manifest. •We formulate a local, manifestly duality-symmetric, nonlinear Born–Infeld action with SL(2,R) symmetry.
Progress in theory and simulation of ion cyclotron emission from magnetic confinement fusion plasmas
NASA Astrophysics Data System (ADS)
Dendy, Richard; Chapman, Ben; Chapman, Sandra; Cook, James; Reman, Bernard; McClements, Ken; Carbajal, Leopoldo
2016-10-01
Suprathermal ion cyclotron emission (ICE) is detected from all large tokamak and stellarator plasmas. Its frequency spectrum has narrow peaks at sequential cyclotron harmonics of the energetic ion population (fusion-born or neutral beam-injected) at the outer edge of the plasma. ICE was the first collective radiative instability driven by confined fusion-born ions observed in deuterium-tritium plasmas in JET and TFTR, and the magnetoacoustic cyclotron instability is the most likely emission mechanism. Contemporary ICE measurements are taken at very high sampling rates from the LHD stellarator and from the conventional aspect ratio KSTAR tokamak. A correspondingly advanced modelling capability for the ICE emission mechanism has been developed using 1D3V PIC and hybrid-PIC codes, supplemented by analytical theory. These kinetic codes simulate the self-consistent full orbit dynamics of energetic and thermal ions, together with the electric and magnetic fields and the electrons. We report recent progress in theory and simulation that addresses: the scaling of ICE intensity with energetic particle density; the transition between super-Alfvénic and sub-Alfvénic regimes for the collectively radiating particles; and the rapid time evolution that is seen for some ICE measurements. This work was supported in part by the RCUK Energy Programme [Grant Number EP/I501045] and by Euratom.
Ottinger, P.F.; Schumer, J.W.
2006-06-15
By relaxing an assumption on the electron density in the flow layer used in magnetically insulated transmission line (MITL) theory, the theory is rescaled to match particle-in-cell (PIC) simulation results, providing a more accurate determination of the line voltage from the measurement of anode and cathode currents over a broad range of parameters. Results from the PIC simulations also show that self-limited flow is not determined by either a minimum-current or a minimum-energy condition, but rather is closer to saturated flow. In addition, analytic expressions are obtained for the first time for the self-limited flow impedance Z{sub f}{sup SL}(V)/Z{sub 0} and the self-limited anode and cathode currents Z{sub 0}I{sub a}{sup SL}(V) and Z{sub 0}I{sub c}{sup SL}(V), where Z{sub 0} is the vacuum impedance of the line and V is the voltage. Similar expressions for both minimum-current flow and minimum-energy flow are also obtained. Results are compared with other models for MITL flow and show that this rescaled MITL flow model is most consistent with the PIC simulation results. Finally, it is shown that a matched load condition can never be satisfied for self-limited (or line-limited) flow.
One-loop gap equations for the magnetic mass in d=3 gauge theory
NASA Astrophysics Data System (ADS)
Cornwall, John M.
1998-03-01
Recently several workers have attempted determinations of the so-called magnetic mass of d=3 non-Abelian gauge theories through a one-loop gap equation, using a free massive propagator as input. Self-consistency is attained only on-shell, because the usual Feynman-graph construction is gauge-dependent off-shell. We examine two previous studies of the pinch technique proper self-energy, which is gauge-invariant at all momenta, using a free propagator as input, and show that it leads to inconsistent and unphysical results. In one case the residue of the pole has the wrong sign (necessarily implying the presence of a tachyonic pole); in the second case the residue is positive, but two orders of magnitude larger than the input residue, which shows that the residue is on the verge of becoming ghost-like. This happens because of the infrared instability of d=3 gauge theory. A possible alternative one-loop determination via the effective action also fails. The lesson is that gap equations must be considered at least at the two-loop level.
Solitons in quantum field theory: Magnetic monopoles, black holes, and supersymmetry
NASA Astrophysics Data System (ADS)
Miller, Christopher M.
We show that spherically-symmetric non-extremal black holes can be obtained as solutions to a set of BPS-like first-order equations. Because the Killing spinor equations for a supergravity theory are also first-order, this suggests that supersymmetry may play a hidden role in black hole solutions. We show that in the effective 1 + 1-dimensional action for the spherically-symmetric ansatz, the black hole is a supersymmetric solution. However, we also show that a non-extremal black hole metric cannot admit a Killing spinor, and hence it cannot be a supersymmetric solution of any supergravity theory. Following this result, we present an analysis of massless monopole dynamics. Massless monopoles are manifested as clouds of non-Abelian magnetic charge that surround massive monopoles. Previous analyses have investigated how these clouds interact with massive monopoles, but this calculation is the first to show how massless monopoles interact with one another. Because of the conjectured duality between magnetic monopole moduli spaces and the corresponding moduli spaces of Nahm data, the moduli space approximation tells us that the geodesics on the space of Nahm data will provide a description of the low-energy dynamics of a multi-monopole configuration. We find an implicit expression for the metric on the forty-dimensional space of Nahm data for a configuration of four massive and six massless monopoles in an SU(6) gauge theory---this space will be referred to as a (2, [2], [2], [2], 2) multi-monopole. We then provide the explicit expression for the metric of a four-dimensional totally geodesic subspace, which corresponds to the subspace of axially symmetric configurations contained within the (2, [2], [2], [2], 2) multi-monopole. Once this metric is calculated, it is possible to obtain the geodesics on the resulting subspace via numerical methods. We find two distinct types of massless monopole clouds, which we refer to as SU(4) and Dancer clouds. In a system containing
Nonuniqueness of magnetic fields and energy derivatives in spin-polarized density functional theory
NASA Astrophysics Data System (ADS)
Gál, T.; Ayers, P. W.; De Proft, F.; Geerlings, P.
2009-10-01
The effect of the recently uncovered nonuniqueness of the external magnetic field B(r⃑) corresponding to a given pair of density n(r⃑) and spin density ns(r⃑) on the derivative of the energy functional of spin-polarized density functional theory, and its implications for the definition of chemical reactivity descriptors, is examined. For ground states, the nonuniqueness of B(r⃑) implies the nondifferentiability of the energy functional Ev,B[n,ns] with respect to ns(r⃑). It is shown, on the other hand, that this nonuniqueness allows the existence of the one-sided derivatives of Ev,B[n,ns] with respect to ns(r⃑). Although the N-electron ground state can always be obtained from the minimization of Ev,B[n,ns] without any constraint on the spin number Ns=∫ns(r⃑)dr⃑, the Lagrange multiplier μs associated with the fixation of Ns does not vanish even for ground states. μs is identified as the left- or right-side derivative of the total energy with respect to Ns, which justifies the interpretation of μs as a (spin) chemical potential. This is relevant not only for the spin-polarized generalization of conceptual density functional theory, the spin chemical potential being one of the elementary reactivity descriptors, but also for the extension of the thermodynamical analogy of density functional theory for the spin-polarized case. For higher-order reactivity indices, B(r⃑)'s nonuniqueness has similar implications as for μs, leading to a split of the indices with respect to Ns into one-sided reactivity descriptors.
Nonuniqueness of magnetic fields and energy derivatives in spin-polarized density functional theory.
Gál, T; Ayers, P W; De Proft, F; Geerlings, P
2009-10-21
The effect of the recently uncovered nonuniqueness of the external magnetic field B(r) corresponding to a given pair of density n(r) and spin density n(s)(r) on the derivative of the energy functional of spin-polarized density functional theory, and its implications for the definition of chemical reactivity descriptors, is examined. For ground states, the nonuniqueness of B(r) implies the nondifferentiability of the energy functional E(v,B)[n,n(s)] with respect to n(s)(r). It is shown, on the other hand, that this nonuniqueness allows the existence of the one-sided derivatives of E(v,B)[n,n(s)] with respect to n(s)(r). Although the N-electron ground state can always be obtained from the minimization of E(v,B)[n,n(s)] without any constraint on the spin number N(s)=integraln(s)(r)dr, the Lagrange multiplier mu(s) associated with the fixation of N(s) does not vanish even for ground states. Mu(s) is identified as the left- or right-side derivative of the total energy with respect to N(s), which justifies the interpretation of mu(s) as a (spin) chemical potential. This is relevant not only for the spin-polarized generalization of conceptual density functional theory, the spin chemical potential being one of the elementary reactivity descriptors, but also for the extension of the thermodynamical analogy of density functional theory for the spin-polarized case. For higher-order reactivity indices, B(r)'s nonuniqueness has similar implications as for mu(s), leading to a split of the indices with respect to N(s) into one-sided reactivity descriptors.
NASA Astrophysics Data System (ADS)
Belozerov, A. S.; Katanin, A. A.; Anisimov, V. I.
2017-08-01
We analyze the momentum and temperature dependences of the magnetic susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by a combination of density functional theory and dynamical mean-field theory (DFT+DMFT). By considering a general derivation of the orbital-resolved effective model for spin degrees of freedom for Hund's metals, we relate momentum-dependent susceptibilities in the paramagnetic phase to the magnetic exchange. We then calculate nonuniform orbital-resolved susceptibilities at high-symmetry wave vectors by constructing appropriate supercells in the DMFT approach. Extracting the irreducible parts of susceptibilities with respect to Hund's exchange interaction, we determine the corresponding orbital-resolved exchange interactions, which are then interpolated to the whole Brillouin zone. Using the spherical model we estimate the temperature dependence of the resulting exchange between local moments.
Orlenko, E. V. Ershova, E. V.; Orlenko, F. E.
2013-10-15
The formalism of exchange perturbation theory is presented with regard to the general principles of constructing an antisymmetric vector with the use of the Young diagrams and tableaux in which the coordinate and spin parts are not separated. The form of the energy and wave function corrections coincides with earlier obtained expressions, which are reduced in the present paper to a simpler form of a symmetry-adapted perturbation operator, which preserves all intercenter exchange contributions. The exchange perturbation theory (EPT) formalism itself is presented in the standard form of invariant perturbation theory that takes into account intercenter electron permutations between overlapping nonorthogonal states. As an example of application of the formalism of invariant perturbation theory, we consider the magnetic properties of perovskite manganites La{sub 1/3}Ca{sub 2/3}MnO{sub 3} that are associated with the charge and spin ordering in magnetic chains of manganese. We try to interpret the experimental results obtained from the study of the effect of doping the above alloys by the model of superexchange interaction in manganite chains that is constructed on the basis of the exchange perturbation theory (EPT) formalism. The model proposed makes it possible to carry out a quantitative analysis of the effect of substitution of manganese atoms by doping elements with different electron configurations on the electronic structure and short-range order in a magnetic chain of manganites.
NASA Astrophysics Data System (ADS)
Murray, James; Tesanovic, Zlatko
2011-03-01
A Ginzburg-Landau approach to fluctuations of a layered superconductor in a magnetic field is used to show that the interlayer coupling can be incorporated within an interacting self-consistent theory of a single layer, in the limit of a large number of neighboring layers. The theory exhibits two phase transitions: a vortex liquid-to- solid transition is followed by a Bose-Einstein condensation into the Abrikosov lattice, illustrating the essential role of interlayer coupling. By using this theory, explicit expressions for magnetization, specific heat, and fluctuation conductivity are derived. We compare our results with recent experimental data on the iron-pnictide superconductors. Supported in part by the Gardner Foundation and the Johns Hopkins-Princeton Institute for Quantum Matter, under Grant No. DE-FG02-08ER46544 by the U.S. Department of Energy, OBES, Division of Materials Sciences and Engineering.
NASA Astrophysics Data System (ADS)
Enemark, Søren; Santos, Ilmar F.
2016-02-01
In this work, the nonlinear dynamic behaviour of a vertical rigid rotor interacting with a flexible foundation by means of two passive magnetic bearings is quantified and evaluated. The quantification is based on theoretical and experimental investigation of the non-uniformity (anisotropy) of the magnetic field and the weak nonlinearity of the magnetic forces. Through mathematical modelling the nonlinear equations of motion are established for describing the shaft and bearing housing lateral dynamics coupled via the nonlinear and non-uniform magnetic forces. The equations of motion are solved in the frequency domain by the methods of Finite Difference and pseudo-arclength continuation. The theoretical findings are validated against experiments carried out using a dedicated test-rig and a special device for characterisation of the magnetic anisotropy. The characterisation of the magnetic anisotropy shows that it can be quantified as magnetic eccentricities having an amplitude and a phase, which result in linear and parametric excitation. The magnetic eccentricities are also determined using the steady-state response of the rotor-bearing system due to forcing from the magnetic anisotropies and several levels of mass imbalance. Discrepancies in the results from the two methods in terms of magnetic eccentricity magnitude are due to additional geometric eccentricities in the shaft. The steady-state system response shows clear nonlinear phenomena, e.g. bent resonance peaks, jump phenomena and nonlinear cross-coupling between the two orthogonal directions, especially during counter-phase motion between shaft and bearings. The clear nonlinear behaviour is facilitated by the lack of damping resulting in relatively large vibrations. The overall nonlinear dynamic behaviour is well captured by the theoretical model, thereby validating the modelling approach.
Droubay, T.; Rosso, K. M.; Chambers, S. A.; Heald, S. M.; McCready, D. E.; Wang, C. M.
2007-03-01
We explore the feasibility of growing epitaxial Ti-doped {alpha}-Fe{sub 2}O{sub 3} hematite in which Ti(IV) substitutes for Fe(III) preferentially in one magnetic sublattice but not the other. Such a structure has been predicted by first-principles theory to be energetically favorable, and is expected to yield interesting and useful magnetic and electronic properties. However, we find experimentally that a majority of Ti dopants disperse and occupy random cation sites in both magnetic sublattices. Density functional theory predicts that the magnetically ordered and magnetically random structures are nearly isoenergetic.
Droubay, T.; Rosso, K. M.; Heald, S. M.; McCready, D. E.; Wang, C. M.; Chambers, S. A.; PNNL
2007-03-16
We explore the feasibility of growing epitaxial Ti-doped {alpha}-Fe{sub 2}O{sub 3} hematite in which Ti(IV) substitutes for Fe(III) preferentially in one magnetic sublattice but not the other. Such a structure has been predicted by first-principles theory to be energetically favorable, and is expected to yield interesting and useful magnetic and electronic properties. However, we find experimentally that a majority of Ti dopants disperse and occupy random cation sites in both magnetic sublattices. Density functional theory predicts that the magnetically ordered and magnetically random structures are nearly isoenergetic.
NASA Astrophysics Data System (ADS)
Balakin, Alexander B.; Lemos, José P. S.; Zayats, Alexei E.
2016-04-01
Alternative theories of gravity and their solutions are of considerable importance since, at some fundamental level, the world can reveal new features. Indeed, it is suspected that the gravitational field might be nonminimally coupled to the other fields at scales not yet probed, bringing into the forefront nonminimally coupled theories. In this mode, we consider a nonminimal Einstein-Yang-Mills theory with a cosmological constant. Imposing spherical symmetry and staticity for the spacetime and a magnetic Wu-Yang ansatz for the Yang-Mills field, we find expressions for the solutions of the theory. Further imposing constraints on the nonminimal parameters, we find a family of exact solutions of the theory depending on five parameters—two nonminimal parameters, the cosmological constant, the magnetic charge, and the mass. These solutions represent magnetic monopoles and black holes in magnetic monopoles with de Sitter, Minkowskian, and anti-de Sitter asymptotics, depending on the sign and value of the cosmological constant Λ . We classify completely the family of solutions with respect to the number and the type of horizons and show that the spacetime solutions can have, at most, four horizons. For particular sets of the parameters, these horizons can become double, triple, and quadruple. For instance, for a positive cosmological constant Λ , there is a critical Λc for which the solution admits a quadruple horizon, evocative of the Λc that appears for a given energy density in both the Einstein static and Eddington-Lemaître dynamical universes. As an example of our classification, we analyze solutions in the Drummond-Hathrell nonminimal theory that describe nonminimal black holes. Another application is with a set of regular black holes previously treated.
NASA Astrophysics Data System (ADS)
Pham, Hung Tan; Cuong, Ngo Tuan; Tam, Nguyen Minh; Lam, Vu Dinh; Tung, Nguyen Thanh
2016-01-01
We study CoxCryOm+ (x + y = 2, 3 and 1 ≤ m ≤ 4) clusters by means of density-functional-theory calculations. It is found that the clusters grow preferentially through maximizing the number of metal-oxygen bonds with a favor on Cr sites. The size- and composition-dependent magnetic behavior is discussed in relation with the local atomic magnetic moments. While doped species show an oscillatory magnetic behavior, the total magnetic moment of pure cobalt and chromium oxide clusters tends to enhance or reduce as increasing the oxygen content, respectively. The dissociation energies for different evaporation channels are also calculated to suggest the stable patterns, as fingerprints for future photofragmentation experiments.
The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory
Cheng, Mu-Jeng; Nielsen, Robert J.; Tahir-Kheli, Jamil; Goddard III, William A.
2011-01-01
We have studied the magnetic structure of the high symmetry vanadyl pyrophosphate ((VO)₂P₂O₇, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_{OPO} = ₋156.8 K and J_{O} = ₋68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_{layer} = 19.2 K and J_{chain} = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_{OPO} originates from a super-exchange interaction through the bridging –O–P–O– unit. In contrast, J_{O} results from a direct overlap of 3d_{x²₋y² } orbitals on two vanadium atoms in the same V_{2}O_{8} motif, making it very sensitive to structural fluctuations. Based on the variations in V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)_{2}–V are covalent and rigid, whereas the bonds of V–(O)_{2}–V are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_{O}, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_{O} to be a constant over the range of 2–300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_{2} can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride.
ERIC Educational Resources Information Center
Wida, Sam
1992-01-01
Uses extremely strong neodymium magnets to demonstrate several principles of physics including electromagnetic induction, Lenz's Law, domain theory, demagnetization, the Curie point, and magnetic flux lines. (MDH)
ERIC Educational Resources Information Center
Wida, Sam
1992-01-01
Uses extremely strong neodymium magnets to demonstrate several principles of physics including electromagnetic induction, Lenz's Law, domain theory, demagnetization, the Curie point, and magnetic flux lines. (MDH)
NASA Astrophysics Data System (ADS)
Frandsen, Benjamin A.; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J.; Staunton, Julie B.; Billinge, Simon J. L.
2016-05-01
We present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ˜1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. The Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.
Benjamin A. Frandsen; Brunelli, Michela; Page, Katharine; ...
2016-05-11
Here, we present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ~1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominatedmore » by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. Furthermore, the Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.« less
Verma, Nishant; Muralidhar, Gautam S; Bovik, Alan C; Cowperthwaite, Matthew C; Burnett, Mark G; Markey, Mia K
2014-10-01
Brain tissue segmentation on magnetic resonance (MR) imaging is a difficult task because of significant intensity overlap between the tissue classes. We present a new knowledge-driven decision theory (KDT) approach that incorporates prior information of the relative extents of intensity overlap between tissue class pairs for volumetric MR tissue segmentation. The proposed approach better handles intensity overlap between tissues without explicitly employing methods for removal of MR image corruptions (such as bias field). Adaptive tissue class priors are employed that combine probabilistic atlas maps with spatial contextual information obtained from Markov random fields to guide tissue segmentation. The energy function is minimized using a variational level-set-based framework, which has shown great promise for MR image analysis. We evaluate the proposed method on two well-established real MR datasets with expert ground-truth segmentations and compare our approach against existing segmentation methods. KDT has low-computational complexity and shows better segmentation performance than other segmentation methods evaluated using these MR datasets.
Three-dimensional brain magnetic resonance imaging segmentation via knowledge-driven decision theory
Verma, Nishant; Muralidhar, Gautam S.; Bovik, Alan C.; Cowperthwaite, Matthew C.; Burnett, Mark G.; Markey, Mia K.
2014-01-01
Abstract. Brain tissue segmentation on magnetic resonance (MR) imaging is a difficult task because of significant intensity overlap between the tissue classes. We present a new knowledge-driven decision theory (KDT) approach that incorporates prior information of the relative extents of intensity overlap between tissue class pairs for volumetric MR tissue segmentation. The proposed approach better handles intensity overlap between tissues without explicitly employing methods for removal of MR image corruptions (such as bias field). Adaptive tissue class priors are employed that combine probabilistic atlas maps with spatial contextual information obtained from Markov random fields to guide tissue segmentation. The energy function is minimized using a variational level-set-based framework, which has shown great promise for MR image analysis. We evaluate the proposed method on two well-established real MR datasets with expert ground-truth segmentations and compare our approach against existing segmentation methods. KDT has low-computational complexity and shows better segmentation performance than other segmentation methods evaluated using these MR datasets. PMID:26158060
NASA Astrophysics Data System (ADS)
Zhang, Weiyi; Chui, S. T.
2009-06-01
We generalize Kirchoff's law for multiply connected wire networks to finite frequencies. We focus on the boundary conditions not present in the conventional Kirchoff's law at joints when more than three wires come together, which is absent in our previous "circuit theory" for the finite frequency properties of metallic wire networks for singly connected structures. These boundary conditions at the joints involve introducing localized boundary electric fields, in addition to the electric fields of inductive and capacitive origins. The boundary fields act as natural "Lagrange multipliers" for imposing the boundary conditions on the circuit currents. In this way the number of equations is the same as the number of unknowns. The eigenmodes determine not only the circuit current and charge profiles, but also the boundary electric fields which supplement such profiles. The application to T- and H-shape metallic wire networks suggests that the basic types of resonances are mainly controlled by the symmetry and the wire dimensions of the networks. The low frequency modes form along the longest connected paths of the wire network while the high frequency modes can be generated via succeedingly adding more nodes along these various wire paths. The characteristic behavior of the electric and magnetic responses can be inferred from the circuit current profile of a given mode, which offers a simple physical picture on circuit design with particular electromagnetic parameters.
Electromagnetic currents and magnetic moments in chiral effective field theory ({chi}EFT)
Pastore, S.; Girlanda, L.; Schiavilla, R.; Viviani, M.; Wiringa, R. B.
2009-09-15
A two-nucleon potential and consistent electromagnetic currents are derived in chiral effective field theory ({chi}EFT) at, respectively, Q{sup 2} (or N{sup 2}LO) and eQ (or N{sup 3}LO), where Q generically denotes the low-momentum scale and e is the electric charge. Dimensional regularization is used to renormalize the pion-loop corrections. A simple expression is derived for the magnetic dipole (M1) operator associated with pion loops, consisting of two terms, one of which is determined, uniquely, by the isospin-dependent part of the two-pion-exchange potential. This decomposition is also carried out for the M1 operator arising from contact currents, in which the unique term is determined by the contact potential. Finally, the low-energy constants entering the N{sup 2}LO potential are fixed by fits to the np S- and P-wave phase shifts up to 100 MeV laboratory energies.
Two-dimensional Kagome phosphorus and its edge magnetism: a density functional theory study.
Yu, Guodong; Jiang, Liwei; Zheng, Yisong
2015-07-01
By means of density functional theory calculations, we predict a new two-dimensional phosphorus allotrope with the Kagome-like lattice(Kagome-P). It is an indirect gap semiconductor with a band gap of 1.64 eV. The gap decreases sensitively with the compressive strain. In particular, shrinking the lattice beyond 13% can drive it into metallic state. In addition, both the AA and AB stacked Kagome-P multi-layer structures exhibit a bandgap much smaller than 1.64 eV. Edges in the Kagome-P monolayer probably suffer from the edge reconstruction. An isolated zigzag edge can induce antiferromagnetic (AF) ordering with a magnetic transition temperature of 23 K. More importantly, when applying a stretching strain beyond 4%, such an edge turns to possess a ferromagnetic ground state. A very narrow zigzag-edged Kagome-P ribbon displays the spin moment distribution similar to the zigzag-edged graphene nanoribbon because of the coupling between the opposites edges. But the inter-edge coupling in the Kagome-P ribbon vanishes more rapidly as the ribbon width increases. These properties make it a promising material in spintronics.
On a theory of an FEL amplifier with circular waveguide and guiding magnetic field
Saldin, E.L.; Schneidmiller, E.A.; Yurkov, M.V.
1995-12-31
We consider an FEL amplifier with an axisymmetric electron beam, circular waveguide, helical undulator and guiding magnetic field. The presented nonlinear theory of the FEL amplifier is based on Hamiltonian description of particle motion and radiation field representation with Green function method. The space charge fields, energy spread and diffraction effects are taken into consideration. Such an FEL amplifier configuration possesses some peculiarities when it operates in a regime with the negative longitudinal mass (i.e. when{mu}{sup -1}{proportional_to}dv{sub z}/dE < 0). It is shown that in the presence of strong space charge fields, the so-called {open_quotes}negative mass{close_quotes} instability may influence significantly on the FEL amplifier operation resulting in a significant increase in the FEL amplifier efficiency. It is proposed in the presented paper to use the effect of the {open_quotes}negative mass instability{close_quotes} to achieve an effective bunching of the CERN Linear Collider (LIC) driving beam.
NASA Astrophysics Data System (ADS)
Jha, Pardeep K.; Jha, Priyanka A.; Srivastava, Geetika; Jha, A. K.; Kotnala, R. K.; Dwivedi, R. K.
2014-01-01
In this study, authors have synthesized bismuth ferrite using (i) solid state route and (ii) mechano-chemical activation technique. The structural studies reveal the formation of bismuth ferrite of AB5O12 type by mechanochemical activation technique and of ABO3 type by conventional solid state route. In addition to the detailed studies on magnetic and electrical properties of both the samples, a comparative analysis has been done using Landau and Berry phase theory. Through this, an effort has been made to establish a relationship between magnetism and electrical polarization vis a vis magneto-electric coupling in these samples.
Krykunov, Mykhaylo; Autschbach, Jochen
2007-01-14
We report implementations and results of time-dependent density functional calculations (i) of the frequency-dependent magnetic dipole-magnetic dipole polarizability, (ii) of the (observable) translationally invariant linear magnetic response, and (iii) of a linear intensity differential (LID) which includes the dynamic dipole magnetizability. The density functional calculations utilized density fitting. For achieving gauge-origin independence we have employed time-periodic magnetic-field-dependent basis functions as well as the dipole velocity gauge, and have included explicit density-fit related derivatives of the Coulomb potential. We present the results of calculations of static and dynamic magnetic dipole-magnetic dipole polarizabilities for a set of small molecules, the LID for the SF6 molecule, and dispersion curves for M-hexahelicene of the origin invariant linear magnetic response as well as of three dynamic polarizabilities: magnetic dipole-magnetic dipole, electric dipole-electric dipole, and electric dipole-magnetic dipole. We have also performed comparison of the linear magnetic response and magnetic dipole-magnetic dipole polarizability over a wide range of frequencies for H2O and SF6.
Detection of microgauss coherent magnetic fields in a galaxy five billion years ago
NASA Astrophysics Data System (ADS)
Mao, S. A.; Carilli, C.; Gaensler, B. M.; Wucknitz, O.; Keeton, C.; Basu, A.; Beck, R.; Kronberg, P. P.; Zweibel, E.
2017-09-01
Magnetic fields play a pivotal role in the physics of interstellar medium in galaxies1, but there are few observational constraints on how they evolve across cosmic time2-7. Spatially resolved synchrotron polarization maps at radio wavelengths reveal well-ordered large-scale magnetic fields in nearby galaxies1,8,9 that are believed to grow from a seed field via a dynamo effect10,11. To directly test and characterize this theory requires magnetic field strength and geometry measurements in cosmologically distant galaxies, which are challenging to obtain due to the limited sensitivity and angular resolution of current radio telescopes. Here, we report the cleanest measurements yet of magnetic fields in a galaxy beyond the local volume, free of the systematics traditional techniques would encounter. By exploiting the scenario where the polarized radio emission from a background source is gravitationally lensed by a foreground galaxy at z = 0.439 using broadband radio polarization data, we detected coherent μG magnetic fields in the lensing disk galaxy as seen 4.6 Gyr ago, with similar strength and geometry to local volume galaxies. This is the highest redshift galaxy whose observed coherent magnetic field property is compatible with a mean-field dynamo origin.
Petit, L.; Paudyal, D.; Mudryk, Y.; ...
2015-11-09
We explain a profound complexity of magnetic interactions of some technologically relevant gadolinium intermetallics using an ab initio electronic structure theory which includes disordered local moments and strong f-electron correlations. The theory correctly finds GdZn and GdCd to be simple ferromagnets and predicts a remarkably large increase of Curie temperature with a pressure of +1.5 K kbar–1 for GdCd confirmed by our experimental measurements of +1.6 K kbar–1. Moreover, we find the origin of a ferromagnetic-antiferromagnetic competition in GdMg manifested by noncollinear, canted magnetic order at low temperatures. As a result, replacing 35% of the Mg atoms with Zn removesmore » this transition, in excellent agreement with long-standing experimental data.« less
Non-Abelian Dual Superconductivity in SU(3) Yang-Mills Theory Due to Non-Abelian Magnetic Monopoles
NASA Astrophysics Data System (ADS)
Shibata, Akihiro; Kondo, Kei-Ichi; Kato, Seikou; Shinohara, Toru
The dual superconductivity is the promising mechanism for quark confinement. We have proposed the non-Abelian dual superconductivity picture in the SU(3) Yang-Mills theory, and already presented numerical evidences for the restricted field dominance and the non-Abelian magnetic monopole dominance in the string tension, by applying our new formulation of Yang-Mills theory to a lattice. In this talk, we focus on the non-Abelian dual Meissner effect and the type of dual superconductivity. We find that the measured chromo-electric flux tube between a quark and antiquark pair strongly supports the non-Abelian dual Meissner effect due to non-Abelian magnetic monopoles. Moreover, we give a remarkable result that the type of the resulting dual superconductor is the type I in SU(3) Yang-Mills, rather than the border between the type I and II, in marked contrast to the SU(2) case.
Fukuda, Ryoichi; Nakatsuji, Hiroshi
2005-07-22
The quasirelativistic (QR) generalized unrestricted Hartree-Fock method for the magnetic shielding constant [R. Fukuda, M. Hada, and H. Nakatsuji, J. Chem. Phys. 118, 1015 (2003); R. Fukuda, M. Hada, and H. Nakatsuji, J. Chem. Phys.118, 1027 (2003)] has been extended to include the electron correlation effect in the level of the second-order Møller-Plesset perturbation theory (MP2). We have implemented the energy gradient and finite-perturbation methods to calculate the magnetic shielding constant at the QR MP2 level and applied to the magnetic shielding constants and the NMR chemical shifts of 125Te nucleus in various tellurium compounds. The calculated magnetic shielding constants and NMR chemical shifts well reproduced the experimental values. The relations of the chemical shifts with the natures of ligands, and the tellurium oxidation states were investigated. The chemical shifts in different valence states were explained by the paramagnetic shielding and spin-orbit terms. The tellurium 5p electrons are the dominant origin of the chemical shifts in the Te I and Te II compounds and the chemical shifts were explained by the p-hole mechanism. The tellurium d electrons also play an important role in the chemical shifts of the hypervalent compounds.
Destabilization of 2D magnetic current sheets by resonance with bouncing electron - a new theory
NASA Astrophysics Data System (ADS)
Fruit, Gabriel; Louarn, Philippe; Tur, Anatoly
2016-07-01
In the general context of understanding the possible destabilization of the magnetotail before a substorm, we propose a kinetic model for electromagnetic instabilities in resonant interaction with trapped bouncing electrons. The geometry is clearly 2D and uses Harris sheet profile. Fruit et al. 2013 already used this model to investigate the possibilities of electrostatic instabilities. Tur et al. 2014 generalizes the model for full electromagnetic perturbations. Starting with a modified Harris sheet as equilibrium state, the linearized gyrokinetic Vlasov equation is solved for electromagnetic fluctuations with period of the order of the electron bounce period (a few seconds). The particle motion is restricted to its first Fourier component along the magnetic field and this allows the complete time integration of the non local perturbed distribution functions. The dispersion relation for electromagnetic modes is finally obtained through the quasi neutrality condition and the Ampere's law for the current density. The present talk will focus on the main results of this theory. The electrostatic version of the model may be applied to the near-Earth environment (8-12 R_{E}) where beta is rather low. It is showed that inclusion of bouncing electron motion may enhance strongly the growth rate of the classical drift wave instability. This model could thus explain the generation of strong parallel electric fields in the ionosphere and the formation of aurora beads with wavelength of a few hundreds of km. In the electromagnetic version, it is found that for mildly stretched current sheet (B_{z} > 0.1 B _{lobes}) undamped modes oscillate at typical electron bounce frequency with wavelength of the order of the plasma sheet thickness. As the stretching of the plasma sheet becomes more intense, the frequency of these normal modes decreases and beyond a certain threshold in B_{z}/B _{lobes}, the mode becomes explosive (pure imaginary frequency) with typical growing rate of a few
The GEM Theory of the Unification of Gravitation and Electro-Magnetism
NASA Astrophysics Data System (ADS)
Brandenburg, J. E.
2012-01-01
The GEM (Gravity Electro-Magnetism), theory is presented as an alloy of Sakharov and Kaluza-Klein approaches to field unification. GEM uses the concept of gravity fields as Poynting fields to postulate that the non-metric portion of the EM stress tensor becomes the metric tensor in strong fields leading to "self-censorship". Covariant formulation of the GEM theory is accomplished through definition of the spacetime metric tensor as a portion of the EM stress tensor normalized by its own trace: gab = 4(FcaFcb )/(FabFab), it is found that this results in a massless ground state vacuum and a Newtonian gravitation potential f=1/2 E2/B2 =GM/r , where E, B and F are part of the vacuum Zero Point Fluctuation (ZPF) and M and r are the mass and distance from the center of a gravitating body and G is the Newton gravitation constant. It is found that a Lorentz flat-space metric is recovered in the limit of a vacuum full spectrum ZPF. The vacuum ZPF energy and vacuum quantities G, h, c, gives birth to particles quantities mp, me, e,-e in a process triggered by the appearance of the Kaluza-Klein fifth dimension, where also the EM and gravity forces split from each other in a process correlated to the splitting apart of protons and electrons. The separate appearance of the proton and electron occurs as the splitting of a light-like spacetime interval of zero-length into a finite space-like portion containing three subdimensions identified with the quarks and a time-like portion identified with the electron. The separation of mass with charge for the electron and proton pair comes about from a U(1) symmetry with a rotation in imaginary angle. A logarithmic variation of charge with mass for the proton-electron pair results and leads to the formula ln(ro/rp) = s, where s = (mp/me)1/2 , where mp and me are the electron and proton masses respectively and where ro =e2/moc2 , and where mo = (mpme)1/2 and where rp is the Planck length . This leads to the formula G=e2/mo2aexp(-2s)=6
Parity nonconservation and the origin of cosmic magnetic fields
NASA Technical Reports Server (NTRS)
Vilenkin, A.; Leahy, D. A.
1982-01-01
Three mechanisms of cosmic magnetic field generation are discussed: (1) asymmetric decay of particles emitted by rotating black holes; (2) asymmetric proton emission by black holes due to weak radiative corrections, and (3) equilibrium parity-violating currents. It is shown that all three mechanisms can produce a seed field sufficiently strong to account for the present galactic fields.
Parity nonconservation and the origin of cosmic magnetic fields
NASA Technical Reports Server (NTRS)
Vilenkin, A.; Leahy, D. A.
1982-01-01
Three mechanisms of cosmic magnetic field generation are discussed: (1) asymmetric decay of particles emitted by rotating black holes; (2) asymmetric proton emission by black holes due to weak radiative corrections, and (3) equilibrium parity-violating currents. It is shown that all three mechanisms can produce a seed field sufficiently strong to account for the present galactic fields.
NASA Astrophysics Data System (ADS)
Singh, Gurpreet; Mohanty, B. P.; Saini, G. S. S.
2016-02-01
Structure, vibrational and nuclear magnetic resonance spectra, and antioxidant action of ascorbic acid towards hydroxyl radicals have been studied computationally and in vitro by ultraviolet-visible, nuclear magnetic resonance and vibrational spectroscopic techniques. Time dependant density functional theory calculations have been employed to specify various electronic transitions in ultraviolet-visible spectra. Observed chemical shifts and vibrational bands in nuclear magnetic resonance and vibrational spectra, respectively have been assigned with the help of calculations. Changes in the structure of ascorbic acid in aqueous phase have been examined computationally and experimentally by recording Raman spectra in aqueous medium. Theoretical calculations of the interaction between ascorbic acid molecule and hydroxyl radical predicted the formation of dehydroascorbic acid as first product, which has been confirmed by comparing its simulated spectra with the corresponding spectra of ascorbic acid in presence of hydrogen peroxide.
Singh, Gurpreet; Mohanty, B P; Saini, G S S
2016-02-15
Structure, vibrational and nuclear magnetic resonance spectra, and antioxidant action of ascorbic acid towards hydroxyl radicals have been studied computationally and in vitro by ultraviolet-visible, nuclear magnetic resonance and vibrational spectroscopic techniques. Time dependant density functional theory calculations have been employed to specify various electronic transitions in ultraviolet-visible spectra. Observed chemical shifts and vibrational bands in nuclear magnetic resonance and vibrational spectra, respectively have been assigned with the help of calculations. Changes in the structure of ascorbic acid in aqueous phase have been examined computationally and experimentally by recording Raman spectra in aqueous medium. Theoretical calculations of the interaction between ascorbic acid molecule and hydroxyl radical predicted the formation of dehydroascorbic acid as first product, which has been confirmed by comparing its simulated spectra with the corresponding spectra of ascorbic acid in presence of hydrogen peroxide. Copyright © 2015 Elsevier B.V. All rights reserved.
Green's function theory of ferromagnetic resonance in magnetic superlattices with damping
NASA Astrophysics Data System (ADS)
Qiu, R. K.; Guo, F. F.; Zhang, Z. D.
2016-02-01
We explore a quantum Green's-function method to study the resonance absorption of magnetic materials. The relationship between the resonance magnon (spin wave) density and the resonance frequency of a superlattice consisting of two magnetic layers with damping and antiferromagnetic interlayer exchange coupling is studied. The effects of temperature, interlayer coupling, anisotropy, external magnetic field and damping on the the resonance frequency and resonance magnon density are investigated. The resonance excitation probability for a magnon is proportional to the resonance magnon density. In the classic methods, the imaginary part of magnetic permeability represents the resonance absorption in magnetic materials. In the quantum approach, the resonance magnon density can be used to estimate the strength of the resonance absorption. In the present work, a quantum approach is developed to study resonance absorption of magnetic materials and the results show the method to obtain a magnetic multilayered materials with both high resonance frequency and high resonance absorption.
Clemens, Benjamin; Regenbogen, Christina; Koch, Kathrin; Backes, Volker; Romanczuk-Seiferth, Nina; Pauly, Katharina; Shah, N. Jon; Schneider, Frank; Habel, Ute; Kellermann, Thilo
2015-01-01
In functional magnetic resonance imaging (fMRI) studies that apply a “subsequent memory” approach, successful encoding is indicated by increased fMRI activity during the encoding phase for hits vs. misses, in areas underlying memory encoding such as the hippocampal formation. Signal-detection theory (SDT) can be used to analyze memory-related fMRI activity as a function of the participant’s memory trace strength (d′). The goal of the present study was to use SDT to examine the relationship between fMRI activity during incidental encoding and participants’ recognition performance. To implement a new approach, post-experimental group assignment into High- or Low Performers (HP or LP) was based on 29 healthy participants’ recognition performance, assessed with SDT. The analyses focused on the interaction between the factors group (HP vs. LP) and recognition performance (hits vs. misses). A whole-brain analysis revealed increased activation for HP vs. LP during incidental encoding for remembered vs. forgotten items (hits > misses) in the insula/temporo-parietal junction (TPJ) and the fusiform gyrus (FFG). Parameter estimates in these regions exhibited a significant positive correlation with d′. As these brain regions are highly relevant for salience detection (insula), stimulus-driven attention (TPJ), and content-specific processing of mnemonic stimuli (FFG), we suggest that HPs’ elevated memory performance was associated with enhanced attentional and content-specific sensory processing during the encoding phase. We provide first correlative evidence that encoding-related activity in content-specific sensory areas and content-independent attention and salience detection areas influences memory performance in a task with incidental encoding of facial stimuli. Based on our findings, we discuss whether the aforementioned group differences in brain activity during incidental encoding might constitute the basis of general differences in memory performance
Theory of light-induced effective magnetic field in Rashba ferromagnets
NASA Astrophysics Data System (ADS)
Qaiumzadeh, Alireza; Titov, Mikhail
2016-07-01
Motivated by recent experiments on all-optical magnetization reversal in conductive ferromagnetic thin films we use nonequilibrium formalism to calculate the effective magnetic field induced in a Rashba ferromagnet by a short laser pulse. The main contribution to the effect originates in the direct optical transitions between spin-split subbands. The resulting effective magnetic field is inversely proportional to the impurity scattering rate and can reach the amplitude of a few Tesla in the systems like Co/Pt bilayers. We show that the total light-induced effective magnetic field in ferromagnetic systems is the sum of two contributions: a helicity dependent term, which is an even function of magnetization, and a helicity independent term, which is an odd function of magnetization. The primary role of the spin-orbit interaction is to widen the frequency range for direct optical transitions.
Theory of magnetic-field-induced polarization flop in spin-spiral multiferroics
NASA Astrophysics Data System (ADS)
Mochizuki, Masahito
2015-12-01
The magnetic-field-induced 90∘ flop of ferroelectric polarization P in a spin-spiral multiferroic material TbMnO3 is theoretically studied based on a microscopic spin model. I find that the direction of the P flop or the choice of +Pa or -Pa after the flop is governed by magnetic torques produced by the applied magnetic field H acting on the Mn spins and thus is selected in a deterministic way, in contradistinction to the naively anticipated probabilistic flop. This mechanism resolves a puzzle of the previously reported memory effect in the P direction depending on the history of the magnetic-field sweep, and enables controlled switching of multiferroic domains by externally applied magnetic fields. My Monte-Carlo analysis also uncovers that the magnetic structure in the P ∥a phase under H ∥b is not a previously anticipated simple a b -plane spin cycloid but a conical spin structure.
Deformed neutron stars due to strong magnetic field in terms of relativistic mean field theories
NASA Astrophysics Data System (ADS)
Yanase, Kota; Yoshinaga, Naotaka
2014-09-01
Some observations suggest that magnetic field intensity of neutron stars that have particularly strong magnetic field, magnetars, reaches values up to 1014-15G. It is expected that there exists more strong magnetic field of several orders of magnitude in the interior of such stars. Neutron star matter is so affected by magnetic fields caused by intrinsic magnetic moments and electric charges of baryons that masses of neutron stars calculated by using Tolman-Oppenheimer-Volkoff equation is therefore modified. We calculate equation of state (EOS) in density-dependent magnetic field by using sigma-omega-rho model that can reproduce properties of stable nuclear matter in laboratory Furthermore we calculate modified masses of deformed neutron stars.
NASA Astrophysics Data System (ADS)
Garibay-Alonso, R.; Dorantes-Dávila, J.; Pastor, G. M.
2015-05-01
A local electronic theory of transition-metal magnetism at finite temperatures is presented, which takes into account longitudinal and transverse spin fluctuations on the same footing. The magnetic properties are determined in the framework of a rotational-invariant d -band model Hamiltonian by applying a four-field Hubbard-Stratonovich functional-integral method in the static approximation. The role of transverse spin excitations on the temperature-dependent magnetic properties is investigated by performing alloy averages in the single-site virtual crystal approximation. Bulk Fe is considered as the representative example for the applications. Results are given for the average magnetization M , for the spin-excitation energies, and for the transverse and longitudinal contributions to the local magnetic moments μl at atom l . The importance of noncollinear spin excitations is quantified by comparison with the corresponding collinear calculations. An important reduction of about 33% of the calculated Curie temperature TC is obtained, which now amounts to 1250 K and is thus relatively close to the experimental value. The longitudinal (transverse) components of μl are found to decrease (increase) as a function of temperature until the full rotational symmetry is reached at TC. This reflects the increasing importance of the transverse spin fluctuations. The origin of the temperature dependence of M and μl is analyzed in terms of the local spin-fluctuation energies.
NASA Astrophysics Data System (ADS)
Arani, A. Ghorbanpour; Jalaei, M. H.
2017-02-01
This research aims to investigate the influence of a longitudinal magnetic field on the dynamic response of single-layered graphene sheet (SLGS) resting on viscoelastic foundation based on the nonlocal sinusoidal shear deformation theory. The present model is capable of capturing both small scale effect and transverse shear deformation effects of nanoplate, and does not require shear correction factors. The material properties of graphene sheet are assumed orthotropic viscoelastic using Kelvin-Voigt model. Utilizing Hamilton's principle governing equations of motion are derived and solved analytically. The parametric study is conducted, focusing on the remarkable effects of the magnetic field, structural damping, stiffness and damping coefficient of the foundation, nonlocal parameter, aspect ratio and length to thickness ratio on the dynamic response of the SLGS. Results indicate that the longitudinal magnetic field exerted on the SLGS decreases the amplitude of dynamic response. In addition, it is observed that the magnetic field effect on the dynamic response is more distinguished as the nonlocal parameter increases while by increasing the foundation and structural damping coefficients, this effect diminishes. The results of this study can be used in design and manufacturing of nanomechanical devices in the presence of magnetic field as a parametric controller.
López-Urías, F; Muñoz-Sandoval, E; Reyes-Reyes, M; Romero, A H; Terrones, M; Morán-López, J L
2005-06-03
We report a novel magnetic phenomenon consisting of the formation of helical spin configurations during the magnetization of densely packed ferromagnetic nanowires encapsulated inside carbon nanotubes. We studied the hysteresis loops when the magnetic fields are applied parallel and perpendicular to the nanotubes axes. We also performed theoretical calculations on aligned nanowire arrays that clearly indicate the creation of helical spin vortices in the hysteresis loops. The latter are caused by the presence of strong dipolar interactions among neighboring wires.
Chaotic structures of nonlinear magnetic fields. I - Theory. II - Numerical results
NASA Technical Reports Server (NTRS)
Lee, Nam C.; Parks, George K.
1992-01-01
A study of the evolutionary properties of nonlinear magnetic fields in flowing MHD plasmas is presented to illustrate that nonlinear magnetic fields may involve chaotic dynamics. It is shown how a suitable transformation of the coupled equations leads to Duffing's form, suggesting that the behavior of the general solution can also be chaotic. Numerical solutions of the nonlinear magnetic field equations that have been cast in the form of Duffing's equation are presented.
Chaotic structures of nonlinear magnetic fields. I - Theory. II - Numerical results
NASA Technical Reports Server (NTRS)
Lee, Nam C.; Parks, George K.
1992-01-01
A study of the evolutionary properties of nonlinear magnetic fields in flowing MHD plasmas is presented to illustrate that nonlinear magnetic fields may involve chaotic dynamics. It is shown how a suitable transformation of the coupled equations leads to Duffing's form, suggesting that the behavior of the general solution can also be chaotic. Numerical solutions of the nonlinear magnetic field equations that have been cast in the form of Duffing's equation are presented.
NASA Astrophysics Data System (ADS)
Forest, M. Gregory; Sircar, Sarthok; Wang, Qi; Zhou, Ruhai
2006-10-01
We establish reciprocity relations of the Doi-Hess kinetic theory for rigid rod macromolecular suspensions governed by the strong coupling among an excluded volume potential, linear flow, and a magnetic field. The relation provides a reduction of the flow and field driven Smoluchowski equation: from five parameters for coplanar linear flows and magnetic field, to two field parameters. The reduced model distinguishes flows with a rotational component, which map to simple shear (with rate parameter) subject to a transverse magnetic field (with strength parameter), and irrotational flows, for which the reduced model consists of a triaxial extensional flow (with two extensional rate parameters). We solve the Smoluchowski equation of the reduced model to explore: (i) the effect of introducing a coplanar magnetic field on each sheared monodomain attractor of the Doi-Hess kinetic theory and (ii) the coupling of coplanar extensional flow and magnetic fields. For (i), we show each sheared attractor (steady and unsteady, with peak axis in and out of the shearing plane, periodic and chaotic orbits) undergoes its own transition sequence versus magnetic field strength. Nonetheless, robust predictions emerge: out-of-plane degrees of freedom are arrested with increasing field strength, and a unique flow-aligning or tumbling/wagging limit cycle emerges above a threshold magnetic field strength or modified geometry parameter value. For (ii), irrotational flows coupled with a coplanar magnetic field yield only steady states. We characterize all (generically biaxial) equilibria in terms of an explicit Boltzmann distribution, providing a natural generalization of analytical results on pure nematic equilibria [P. Constantin, I. Kevrekidis, and E. S. Titi, Arch. Rat. Mech. Anal. 174, 365 (2004); P. Constantin, I. Kevrekidis, and E. S. Titi, Discrete and Continuous Dynamical Systems 11, 101 (2004); P. Constantin and J. Vukadinovic, Nonlinearity 18, 441 (2005); H. Liu, H. Zhang, and P
Peng, Yan; Bodenstein, Tilmann; Fink, Karin; Mereacre, Valeriu; Anson, Christopher E; Powell, Annie K
2016-11-02
The single ion magnetic properties of Co(ii) are affected by the details of the coordination geometry of the ion. Here we show that a geometry close to trigonal prismatic which arises when the ligand 6,6'-((1Z)-((piperazine-1,4-diylbis(propane-3,1-diyl))bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol) coordinates to Co(ii) does indeed lead to enhanced single-ion behaviour as has previously been predicted. Synthesis of the compound, structural information, and static as well as dynamic magnetic data are presented along with an analysis using quantum chemical ab initio calculations. Though the complex shows a slight deviation from an ideal trigonal prismatic coordination, the zero-field splitting as well as the g-tensor are strongly axial with D = -41 cm(-1) and E < 0.01 cm(-1). For the lowest Kramers doublet (S = 1/2) g∥ = 7.86 and g⊥ < 0.05 were found. In contrast, the second Kramers doublet possesses a rhombic g-tensor with g∥ = 2.75 and g⊥ = 4.35. Due to large spin-orbit coupling resulting in very different g tensors, it is not possible to simulate the temperature dependence of the magnetic susceptibility with a spin Hamiltonian of the form H = D(Sz(2) - S(S + 1)/3) + E(Sx(2) - Sy(2)) + μBgS·B using an effective spin S = 3/2. Calculations on model complexes show the influence of the coordinating atoms and the deviation from the ideal trigonal prismatic coordination. As the distortion is reduced towards idealised D3h, the zero field splitting increases and the g-tensor of the second Kramers doublet also becomes axial.
Difficulties in Learning the Introductory Magnetic Field Theory in the First Years of University
ERIC Educational Resources Information Center
Guisasola, Jenaro; Almudi, Jose M.; Zubimendi, Jose L.
2004-01-01
This study examined university engineering and physical science students' misconceptions of the nature of magnetic field. It is assumed that a significant knowledge of the sources of magnetic field is a basic prerequisite when students have to think about electromagnetic phenomena. To analyze students' conceptions, we have taken into account the…
Magnetic Control of Solutal Buoyancy-driven Convection. Part 1; Theory and Experiments
NASA Technical Reports Server (NTRS)
Ramachandran, N.; Leslie, F. W.
2003-01-01
Experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented.
Magnetic Control of Solutal Buoyancy-driven Convection. Part 1; Theory and Experiments
NASA Technical Reports Server (NTRS)
Ramachandran, N.; Leslie, F. W.
2003-01-01
Experiments on solutal convection in a paramagnetic fluid were conducted in a strong magnetic field gradient using a dilute solution of Manganese Chloride. The observed flows indicate that the magnetic field can completely counter the settling effects of gravity locally and are consistent with the theoretical predictions presented.
Callen, J.D.
1993-10-01
The research performed under this grant during the current year has concentrated on few tokamak plasma confinement issues: applications of our new Chapman-Enskog-like approach for developing hybrid fluid/kinetic descriptions of tokamak plasmas; multi-faceted studies as part of our development of a new interacting island paradigm for the tokamak equilibrium`` and transport; investigations of the resolution power of BES and ECE diagnostics for measuring core plasma fluctuations; and studies of net transport in the presence of fluctuating surfaces. Recent progress and publications in these areas, and in the management of the NERSC node and the fusion theory workstations are summarized briefly in this report.
NASA Astrophysics Data System (ADS)
Sutter, Kiplangat
This thesis illustrates the utilization of Density functional theory (DFT) in calculations of gas and solution phase Nuclear Magnetic Resonance (NMR) properties of light and heavy nuclei. Computing NMR properties is still a challenge and there are many unknown factors that are still being explored. For instance, influence of hydrogen-bonding; thermal motion; vibration; rotation and solvent effects. In one of the theoretical studies of 195Pt NMR chemical shift in cisplatin and its derivatives illustrated in Chapter 2 and 3 of this thesis. The importance of representing explicit solvent molecules explicitly around the Pt center in cisplatin complexes was outlined. In the same complexes, solvent effect contributed about half of the J(Pt-N) coupling constant. Indicating the significance of considering the surrounding solvent molecules in elucidating the NMR measurements of cisplatin binding to DNA. In chapter 4, we explore the Spin-Orbit (SO) effects on the 29Si and 13C chemical shifts induced by surrounding metal and ligands. The unusual Ni, Pd, Pt trends in SO effects to the 29Si in metallasilatrane complexes X-Si-(mu-mt)4-M-Y was interpreted based on electronic and relativistic effects rather than by structural differences between the complexes. In addition, we develop a non-linear model for predicting NMR SO effects in a series of organics bonded to heavy nuclei halides. In chapter 5, we extend the idea of "Chemist's orbitals" LMO analysis to the quantum chemical proton NMR computation of systems with internal resonance-assisted hydrogen bonds. Consequently, we explicitly link the relationship between the NMR parameters related to H-bonded systems and intuitive picture of a chemical bond from quantum calculations. The analysis shows how NMR signatures characteristic of H-bond can be explained by local bonding and electron delocalization concepts. One shortcoming of some of the anti-cancer agents like cisplatin is that they are toxic and researchers are looking for
Vélez, Ederley; Alberola, Antonio; Polo, Víctor
2009-12-17
The magnetic exchange coupling constants between two Mn(II) centers for a set of five inverse crown structures have been investigated by means of a methodology based on broken-symmetry unrestricted density functional theory. These novel and highly unstable compounds present superexchange interactions between two Mn centers, each one with S = 5/2 through anionic "guests" such as oxygen, benzene, or hydrides or through the cationic ring formed by amide ligands and alkali metals (Na, Li). Magnetic exchange couplings calculated at B3LYP/6-31G(d,p) level yield strong antiferromagnetic couplings for compounds linked via an oxygen atom or hydride and very small antiferromagnetic couplings for those linked via a benzene molecule, deprotonated in either 1,4- or 1,3- positions. Analysis of the magnetic orbitals and spin polarization maps provide an understanding of the exchange mechanism between the Mn centers. The dependence of J with respect to 10 different density functional theory potentials employed and the basis set has been analyzed.
NASA Astrophysics Data System (ADS)
Kenney, John W.; Boatz, Jerry A.; Terrill Vosbein, Heidi A.
The history of magnetic circular dichroism (MCD) spectroscopy in the study of alkali metal/rare gas (M/Rg) cryogenic systems is reviewed in the context of developing a better understanding of alkali metal/hydrogen systems of current interest to the U.S. Air Force as enhanced-performance cryogenic rocket propellants. A new theory for simulating the MCD spectra of M/Rg systems is presented together with a careful discussion of the theory's implicit and explicit approximations and their implications. This theory uses a classical Monte Carlo (MC) simulation scheme to model the perturbing effects of the Rg environment on the 2S → 2P MCD-active transition of the M atom. The theory sets up the MC-MCD simulation as a 6 × 6 matrix eigenvalue/eigenvector problem in the 2P manifold in which are included the effects of M-Rg interactions, metal atom spin-orbit coupling in the 2P manifold, magnetic Zeeman perturbations of the 2S and 2P manifolds, Boltzmann temperature factors, and electric dipole transition moment integrals for left circularly polarized (LCP) and right circularly polarized (RCP) light. The theory may be applied to any type of trapping site of the host M in the guest Rg matrix; a single atom substitutional metal atom trapping site (one host Rg atom is replaced by one guest M atom) is modeled in this study for M = Na and Rg = Ar. Two temperature factors are used in these simulations; a lattice temperature to model the mobility of the Rg lattice and a magnetic temperature to model Boltzmann factors in the 2S ground manifold. The 6 × 6 eigenvalue/eigenvector problem is solved for a number of randomly generated and suitably averaged Rg configurations to yield the simulated MC-MCD spectrum for the single substitutional Na/Ar system. The MC-MCD simulations of Na/Ar give the characteristic triplet MCD spectrum with the correct Boltzmann temperature dependence. The simulated MC-MCD spectrum correctly inverts when the direction of the applied magnetic field is
NASA Astrophysics Data System (ADS)
Golub, R.; Rohm, Ryan M.; Swank, C. M.
2011-02-01
There is an extensive literature on magnetic-gradient-induced spin relaxation. Cates, Schaefer, and Happer, in a seminal publication, have solved the problem in the regime where diffusion theory (the Torrey equation) is applicable using an expansion of the density matrix in diffusion equation eigenfunctions and angular momentum tensors. McGregor has solved the problem in the same regime using a slightly more general formulation using the Redfield theory formulated in terms of the autocorrelation function of the fluctuating field seen by the spins and calculating the correlation functions using the diffusion-theory Green’s function. The results of both calculations were shown to agree for a special case. In the present work, we show that the eigenfunction expansion of the Torrey equation yields the expansion of the Green’s function for the diffusion equation, thus showing the identity of this approach with that of the Redfield theory. The general solution can also be obtained directly from the Torrey equation for the density matrix. Thus, the physical content of the Redfield and Torrey approaches are identical. We then introduce a more general expression for the position autocorrelation function of particles moving in a closed cell, extending the range of applicability of the theory.
NASA Astrophysics Data System (ADS)
Vikas, Hash(0xb7f6e60)
2012-01-01
Hydrogen molecule in a strong ultrashort magnetic field is investigated through a current-density functional theory (CDFT) and quantum fluid dynamics (QFD) based approach employing current-density dependent vector exchange-correlation potential and energy density functional derived with a vorticity variable. The numerical computations through the CDFT based approach are performed for the H2 molecule, starting initially from its field-free ground state, in a parallel internuclear axis and magnetic field-axis configuration with the internuclear separation R ranging from 0.1 a.u. to 14.0 a.u., and the strength of the time-dependent (TD) magnetic field varying between 0-1011 G over a few femtoseconds. The numerical results are compared with that obtained using an approach based on the current-density independent approximation under similar computational constraints but employing only scalar exchange-correlation potential dependent on the electronic charge-density alone. The current-density based approach yields exchange- and correlation energy as well as electronic charge-density of the H2 molecule drastically different from that obtained using current-independent approach, in particular, at TD magnetic field-strengths >109 G during a typical time-period of the field when the magnetic-field had attained maximum applied field-strength and is switched to a decreasing ramp function. This nonadiabatic behavior of the TD electronic charge-density is traced to the TD vorticity-dependent vector exchange-correlation potential of the CDFT based approach. The interesting electron dynamics of the H2 molecule in strong TD magnetic field is further elucidated by treating electronic charge-density as an `electron-fluid'. The present work also reveals interesting real-time dynamics on the attosecond time-scale in the electronic charge-density distribution of the hydrogen molecule.
On the origin of cosmic magnetic fields
NASA Astrophysics Data System (ADS)
Kulsrud, Russell M.; Zweibel, Ellen G.
2008-04-01
and most popular mechanism is the α-Ω mean field dynamo theory developed by a number of people in the late sixties. This theory and its application to galactic magnetic fields is discussed in considerable detail in this review. We point out certain difficulties with this theory that make it seem unlikely that this is the whole story. The main difficulty with this as the only such amplification mechanism is rooted in the fact that, on galactic scales, flux is constant and is frozen in the interstellar medium. This implies that flux must be removed from the galactic discs, as is well recognized by the standard theory. For our Galaxy this turns out to be a major problem, since unless the flux and the interstellar mass are somehow separated, some interstellar mass must also be removed from the deep galactic gravitational well. This is very difficult. It is pointed out that unless the field has a substantial field strength, much larger than that of the seed fields, this separation can hardly happen. And of course, it must if the α-Ω dynamo is to start from the ultra weak seed field. (It is our philosophy, expressed in this review, that if an origin theory is unable to create the magnetic field in our Galaxy it is essentially incomplete.) Thus, it is more reasonable for the first and largest amplification to occur before the Galaxy forms, and the matter embedded in the field is gravitationally trapped. Two such mechanisms are discussed for such a pregalactic origin; (1) they are generated in the turbulence of the protogalaxy and (2) the fields come from giant radio jets. Several arguments against a primordial origin are also discussed, as are ways around them. Our conclusion as to the most likely origin of cosmic magnetic fields is that they are first produced at moderate field strengths by primordial mechanisms and then changed and their strength increased to their present value and structure by a galactic disc dynamo. The primordial mechanisms have not yet been
NASA Astrophysics Data System (ADS)
de Andrade, L. C. G.
2016-01-01
A generalized dynamo equation in the first order torsion Garcia de Andrade L C (2012 Phys. Lett. B 711 143) has previously been derived. From this equation it is shown that for the 10 kpc scale, torsion gravity is not able to help seed galactic dynamos since the dynamo time is not long enough to take into account structure formation. In this paper, the dynamo equation is extended to second-order torsion terms—but unfortunately, the situation is even worse and the torsion does not seem to help dynamo efficiency. Nevertheless, in the intergalactic magnetic field scale of 1 mpc, the efficiency of the self-induction equation with torsion changes, and even in the first-order torsion case, one obtains large-scale magnetic fields with 109 yr dynamo efficiency. Dynamo efficiency in the case of interstellar matter (ISM) reaches a diffusion time of 1013 yr. This seems to be in contrast with a recent investigation by Bamba et al (2012 J. Cosmol. Astropart. Phys. JCAP05(2010)08) where they obtained, from another type of torsion theory called teleparallelism (A Einstein, Math Annalen (1922)), a large scale intergalactic magnetic field of 10-9 G. If this is not a model-dependent result, there is an apparent contradiction that has to be addressed. It is shown that for dynamo efficiency in astrophysical flow without shear, a strong seed field of 10-11 G is obtained, which is suitable for seeding galactic dynamos. As an example of a non-parity-violating dynamo equation, a magnetic field of the order of 10-27G is obtained as a seed field for the galactic dynamo from the theory of Einstein’s unified teleparallelism. This shows that in certain gravity models, torsion is able to enhance cosmological magnetic fields in view of obtaining better dynamo efficiency. To better compare our work with Bamba et al (2012 J. Cosmol. Astropart. Phys. JCAP05(2010)08), we consider the slow decay of magnetic fields in the teleparallel model. This observation is due to an anonymous referee who
Kirschvink, Joseph L; Winklhofer, Michael; Walker, Michael M
2010-04-06
The first demonstrations of magnetic effects on the behaviour of migratory birds and homing pigeons in laboratory and field experiments, respectively, provided evidence for the longstanding hypothesis that animals such as birds that migrate and home over long distances would benefit from possession of a magnetic sense. Subsequent identification of at least two plausible biophysical mechanisms for magnetoreception in animals, one based on biogenic magnetite and another on radical-pair biochemical reactions, led to major efforts over recent decades to test predictions of the two models, as well as efforts to understand the ultrastructure and function of the possible magnetoreceptor cells. Unfortunately, progress in understanding the magnetic sense has been challenged by: (i) the availability of a relatively small number of techniques for analysing behavioural responses to magnetic fields by animals; (ii) difficulty in achieving reproducible results using the techniques; and (iii) difficulty in development and implementation of new techniques that might bring greater experimental power. As a consequence, laboratory and field techniques used to study the magnetic sense today remain substantially unchanged, despite the huge developments in technology and instrumentation since the techniques were developed in the 1950s. New methods developed for behavioural study of the magnetic sense over the last 30 years include the use of laboratory conditioning techniques and tracking devices based on transmission of radio signals to and from satellites. Here we consider methodological developments in the study of the magnetic sense and present suggestions for increasing the reproducibility and ease of interpretation of experimental studies. We recommend that future experiments invest more effort in automating control of experiments and data capture, control of stimulation and full blinding of experiments in the rare cases where automation is impossible. We also propose new
Kirschvink, Joseph L.; Winklhofer, Michael; Walker, Michael M.
2010-01-01
The first demonstrations of magnetic effects on the behaviour of migratory birds and homing pigeons in laboratory and field experiments, respectively, provided evidence for the longstanding hypothesis that animals such as birds that migrate and home over long distances would benefit from possession of a magnetic sense. Subsequent identification of at least two plausible biophysical mechanisms for magnetoreception in animals, one based on biogenic magnetite and another on radical-pair biochemical reactions, led to major efforts over recent decades to test predictions of the two models, as well as efforts to understand the ultrastructure and function of the possible magnetoreceptor cells. Unfortunately, progress in understanding the magnetic sense has been challenged by: (i) the availability of a relatively small number of techniques for analysing behavioural responses to magnetic fields by animals; (ii) difficulty in achieving reproducible results using the techniques; and (iii) difficulty in development and implementation of new techniques that might bring greater experimental power. As a consequence, laboratory and field techniques used to study the magnetic sense today remain substantially unchanged, despite the huge developments in technology and instrumentation since the techniques were developed in the 1950s. New methods developed for behavioural study of the magnetic sense over the last 30 years include the use of laboratory conditioning techniques and tracking devices based on transmission of radio signals to and from satellites. Here we consider methodological developments in the study of the magnetic sense and present suggestions for increasing the reproducibility and ease of interpretation of experimental studies. We recommend that future experiments invest more effort in automating control of experiments and data capture, control of stimulation and full blinding of experiments in the rare cases where automation is impossible. We also propose new
NASA Astrophysics Data System (ADS)
Williams, P. Stephen; Carpino, Francesca; Zborowski, Maciej
2009-05-01
Quadrupole magnetic field-flow fractionation (QMgFFF) is a separation and characterization technique for magnetic nanoparticles such as those used for cell labeling and for targeted drug therapy. A helical separation channel is used to efficiently exploit the quadrupole magnetic field. The fluid and sample components therefore have angular and longitudinal components to their motion in the thin annular space occupied by the helical channel. The retention ratio is defined as the ratio of the times for non-retained and a retained material to pass through the channel. Equations are derived for the respective angular and longitudinal components to retention ratio.
A complete theory for the magnetism of an ideal gas of electrons
Biswas, Shyamal; Jana, Debnarayan; Sen, Swati
2013-05-15
We have explored Pauli paramagnetism, Landau diamagnetism, and de Haas-van Alphen effect in a single framework, and unified these three effects for all temperatures as well as for all strengths of magnetic field. Our result goes beyond Pauli-Landau result on the magnetism of the 3-D ideal gas of electrons, and is able to describe crossover of the de Haas-van Alphen oscillation to the saturation of magnetization. We also have obtained a novel asymptotic series expansion for the low temperature properties of the system.
Microscopic analytical theory of a correlated, two-dimensional N-electron gas in a magnetic field
NASA Astrophysics Data System (ADS)
Johnson, Neil F.; Quiroga, Luis
1997-07-01
We present a microscopic, analytical theory describing a confined N-electron gas in two dimensions subject to an external magnetic field. The number of electrons N and strength of the electron - electron interaction can be arbitrarily large, and all Landau levels are included implicitly. For any value of the magnetic field B, the correlated N-electron states are determined by the solution to a universal effective problem which resembles that of a fictitious particle moving in a multi-dimensional space, without a magnetic field, occupied by potential minima corresponding to the classical N-electron equilibrium configurations. Introducing the requirement of total wavefunction antisymmetry selects out the allowed minimum-energy N-electron states. It is shown that low-energy minima can exist at filling factors 0953-8984/9/27/018/img5 where p and n are any positive integers. These filling factors correspond to the experimentally observed fractional (FQHE) and integer (IQHE) quantum Hall effects. The energy gaps calculated analytically at 0953-8984/9/27/018/img6 are found to be consistent with experimental data as a function of magnetic field, over a range of samples.
NASA Astrophysics Data System (ADS)
Trojnar, Anna H.; Kadantsev, Eugene S.; Korkusiński, Marek; Hawrylak, Pawel
2011-12-01
A theory of the fine structure of correlated exciton states in self-assembled parabolic semiconductor quantum dots in a magnetic field perpendicular to the quantum dot plane is presented. The correlated exciton wave function is expanded in configurations consisting of products of electron and heavy-hole 2D harmonic oscillator states (HO) in a magnetic field and the electron spin Sz=±1/2 and a heavy-hole spin τz=±3/2 states. Analytical expressions for the short- and long-range electron-hole exchange Coulomb interaction matrix elements are derived in the HO and spin basis for arbitrary magnetic field. This allows the incorporation of short- and long-range electron-hole exchange, direct electron-hole interaction, and quantum dot anisotropy in the exact diagonalization of the exciton Hamiltonian. The fine structure of ground and excited correlated exciton states as a function of a number of confined shells, quantum dot anisotropy, and magnetic field is obtained using exact diagonalization of the many-body Hamiltonian. The effects of correlations are shown to significantly affect the energy splitting of the two bright exciton states.
NASA Astrophysics Data System (ADS)
Reid, J.-Ph.; O'Neill, Chris; Walker, Alex; Lithgow, Calum; Abdul-Jabbar, Gino; Yelland, Edward; Sokolov, Dmitry A.; Huxley, Andrew D.
The ferromagnet PrPtAl is unlike any other. At the phase boundary between paramagnetism and ferromagnetism the fluctuations of the order parameter are so strong that energetically favourable phases of novel modulated magnetism emerge. In fact, it's the lack of order (the 'disorder') that is pivotal to promote a new 'order'. This mechanism is referred to as 'order by disorder' and is the centre of numerous theoretical studies. In this seminar, following an introduction on the topic of ferromagnetic materials, I will show how we can use both electrical and thermal conductivities to learn everything about these phases of modulated magnetism and to validate the predictions of the 'order by disorder' theory.
Wieser, R
2017-05-04
A self-consistent mean field theory is introduced and used to investigate the thermodynamics and spin dynamics of an S = 1 quantum spin system with a magnetic Skyrmion. The temperature dependence of the Skyrmion profile as well as the phase diagram are calculated. In addition, the spin dynamics of a magnetic Skyrmion is described by solving the time dependent Schrödinger equation with additional damping term. The Skyrmion annihilation process driven by an electric field is used to compare the trajectories of the quantum mechanical simulation with a semi-classical description for the spin expectation values using a differential equation similar to the classical Landau-Lifshitz-Gilbert equation.
To the theory of elastic properties of isotropic magnetic gels. Effect of interparticle interaction
NASA Astrophysics Data System (ADS)
Lopez-Lopez, M. T.; Iskakova, L. Yu; Zubarev, A. Yu; Borin, D. Yu
2017-09-01
The paper deals with theoretical study of elastic shear properties of a magnetic gel, consisting of spherical magnetizable particles, randomly (gas-like) distributed in an elastic matrix. We suppose that the composite is placed in a magnetic field, perpendicular to the direction of the sample shear. In order to get mathematically rigorous results, we have restricted ourselves by the analysis of the system with a low concentration of the particles. Magnetic and elastic (through the matrix deformation) interactions between them are considered in the framework of the regular pair approximation. Analysis shows that external magnetic field decreases the macroscopic shear modulus of the composite with a low concentration of the particles. The decreasing dependence of the modulus on the macroscopic shear is estimated. We believe that the suggested rigorous approach can be a robust background for the study of the systems with a high concentration of the particles.
Chinks in Solar Dynamo Theory: Turbulent Diffusion, Dynamo Waves and Magnetic Helicity
NASA Technical Reports Server (NTRS)
DeLuca, E. E.; Wagner, William J. (Technical Monitor)
2001-01-01
We have investigated the generation of magnetic fields in the Sun using two-dimensional and three-dimensional numerical simulations. The results of our investigations have been presented at scientific meetings and published.
Rusz, Ján; Idrobo, Juan Carlos
2016-03-24
It was recently proposed that electron magnetic circular dichroism (EMCD) can be measured in scanning transmission electron microscopy (STEM) with atomic resolution by tuning the phase distribution of a electron beam. Here, we describe the theoretical and practical aspects for the detection of out-of-plane and in-plane magnetization utilizing atomic size electron probes. Here we present the calculated optimized astigmatic probes and discuss how to achieve them experimentally.
NASA Astrophysics Data System (ADS)
Engelhardt, Larry
2015-12-01
We discuss how computers can be used to solve the ordinary differential equations that provide a quantum mechanical description of magnetic resonance. By varying the parameters in these equations and visually exploring how these parameters affect the results, students can quickly gain insights into the nature of magnetic resonance that go beyond the standard presentation found in quantum mechanics textbooks. The results were generated using an IPython notebook, which we provide as an online supplement with interactive plots and animations.
Rusz, Ján; Idrobo, Juan Carlos
2016-03-24
It was recently proposed that electron magnetic circular dichroism (EMCD) can be measured in scanning transmission electron microscopy (STEM) with atomic resolution by tuning the phase distribution of a electron beam. Here, we describe the theoretical and practical aspects for the detection of out-of-plane and in-plane magnetization utilizing atomic size electron probes. Here we present the calculated optimized astigmatic probes and discuss how to achieve them experimentally.
NASA Astrophysics Data System (ADS)
Valvidares, S. M.; Álvarez-Prado, L. M.; Martín, J. I.; Alameda, J. M.
2001-10-01
The magnetization reversal processes in magnetic bilayers with individual uniaxial anisotropies have been studied, both theoretically and experimentally, to analyze the possible existence of inverted hysteresis loops, that is, with negative remanent magnetization (Mr). Kerr effect measurements in amorphous YCo2/YCo2 bilayers and alternating gradient magnetometry in polycrystalline FeNi/FeNi samples reveal that Mr<0 can be observed for certain directions of the applied magnetic field in the sample plane. This property has also been found in CoNbZr films annealed under an applied field. Our theoretical approach shows that the behavior of these magnetic heterogeneous systems with two coupled uniaxial anisotropies can be understood in terms of two competing effective anisotropies, one biaxial (with Kbiax) and one uniaxial (with Kuniax). In particular, a phase diagram has been deduced for the conditions on Kbiax and Kuniax that can produce negative remanence. This description indicates that, under those anisotropy conditions, inverted hysteresis loops can be observed for an applied field close to the hard axis of the effective uniaxial anisotropy, when magnetization reversal is driven by rotations and not by domain nucleation and wall movement. To consider the real situation in a YCo2/YCo2 bilayer sample, the predictions of this phenomenological model have been further improved by micromagnetic calculations, which are in very good agreement with the magneto-optical measurements.
Backhopping effect in magnetic tunnel junctions: Comparison between theory and experiment
Skowroński, Witold Wrona, Jerzy; Stobiecki, Tomasz; Ogrodnik, Piotr; Świrkowicz, Renata; Barnaś, Józef; Reiss, Günter; Dijken, Sebastiaan van
2013-12-21
We report on magnetic switching and backhopping effects due to spin-transfer-torque in magnetic tunnel junctions. Experimental data on current-induced switching in junctions with a MgO tunnel barrier reveal random back-and-forth switching between magnetization states, which appears when the current direction favors the parallel magnetic configuration. The effect depends on the barrier thickness t{sub b} and is not observed in tunnel junctions with very thin MgO tunnel barriers, t{sub b} < 0.95 nm. The switching dependence on bias voltage and barrier thickness is explained in terms of the macrospin model, with the magnetization dynamics described by the modified Landau-Lifshitz-Gilbert equation. Numerical simulations indicate that the competition between in-plane and out-of-plane torque components can result in a non-deterministic switching behavior at high bias voltages, in agreement with experimental observations. When the barrier thickness is reduced, the overall coupling between the magnetic layers across the barrier becomes ferromagnetic, which suppresses the backhopping effect.
Microscopic theory of the Coulomb based exchange coupling in magnetic tunnel junctions.
Udalov, O G; Beloborodov, I S
2017-05-04
We study interlayer exchange coupling based on the many-body Coulomb interaction between conduction electrons in magnetic tunnel junction. This mechanism complements the known interaction between magnetic layers based on virtual electron hopping (or spin currents). We find that these two mechanisms have different behavior on system parameters. The Coulomb based coupling may exceed the hopping based exchange. We show that the Coulomb based exchange interaction, in contrast to the hopping based coupling, depends strongly on the dielectric constant of the insulating layer. The dependence of the interlayer exchange interaction on the dielectric properties of the insulating layer in magnetic tunnel junction is similar to magneto-electric effect where electric and magnetic degrees of freedom are coupled. We calculate the interlayer coupling as a function of temperature and electric field for magnetic tunnel junction with ferroelectric layer and show that the exchange interaction between magnetic leads has a sharp decrease in the vicinity of the ferroelectric phase transition and varies strongly with external electric field.
Microscopic theory of magnetism in the magnetocaloric material Fe2P1-xTx (T=B and Si)
NASA Astrophysics Data System (ADS)
Delczeg-Czirjak, E. K.; Bergqvist, L.; Eriksson, O.; Gercsi, Z.; Nordblad, P.; Szunyogh, L.; Johansson, B.; Vitos, L.
2012-07-01
Landau phenomenological theory in combination with first-principles calculations was used to reveal the origin of the metamagnetic nature and the unusually strong dependence of the ordering temperature with doping of the Fe2P compound. We show that the magnetism of the two sublattices occupied by Fe atoms has an entwined codependency, which is strongly influenced by alloying. We furthermore demonstrate that a constrained disordered local moment approach combined with Monte Carlo simulations can only reproduce the experimental ordering temperatures in these technologically important prototype alloys for magnetocaloric refrigeration.
Ertas, Mehmet; Keskin, Mustafa; Deviren, Bayram
2010-12-23
The dynamic phase transitions are studied in the spin-2 Ising model under a time-dependent oscillating magnetic field by using the effective-field theory with correlations. The effective-field dynamic equation is derived by employing the Glauber transition rates and the phases in the system are obtained by solving this dynamic equation. The nature (first- or second-order) of the dynamic phase transition is characterized by investigating the thermal behavior of the dynamic order parameter and the dynamic phase transition temperatures are obtained. The dynamic phase diagrams are presented in (T/zJ, h/zJ) plane.
NASA Technical Reports Server (NTRS)
Mikellides, Ioannis G.; Katz, Ira; Hofer, Richard R.; Goebel, Dan M.
2012-01-01
A proof-of-principle effort to demonstrate a technique by which erosion of the acceleration channel in Hall thrusters of the magnetic-layer type can be eliminated has been completed. The first principles of the technique, now known as "magnetic shielding," were derived based on the findings of numerical simulations in 2-D axisymmetric geometry. The simulations, in turn, guided the modification of an existing 6-kW laboratory Hall thruster. This magnetically shielded (MS) thruster was then built and tested. Because neither theory nor experiment alone can validate fully the first principles of the technique, the objective of the 2-yr effort was twofold: (1) to demonstrate in the laboratory that the erosion rates can be reduced by >order of magnitude, and (2) to demonstrate that the near-wall plasma properties can be altered according to the theoretical predictions. This paper concludes the demonstration of magnetic shielding by reporting on a wide range of comparisons between results from numerical simulations and laboratory diagnostics. Collectively, we find that the comparisons validate the theory. Near the walls of the MS thruster, theory and experiment agree: (1) the plasma potential has been sustained at values near the discharge voltage, and (2) the electron temperature has been lowered by at least 2.5-3 times compared to the unshielded (US) thruster. Also, based on carbon deposition measurements, the erosion rates at the inner and outer walls of the MS thruster are found to be lower by at least 2300 and 1875 times, respectively. Erosion was so low along these walls that the rates were below the resolution of the profilometer. Using a sputtering yield model with an energy threshold of 25 V, the simulations predict a reduction of 600 at the MS inner wall. At the outer wall ion energies are computed to be below 25 V, for which case we set the erosion to zero in the simulations. When a 50-V threshold is used the computed ion energies are below the threshold at both
NASA Technical Reports Server (NTRS)
Mikellides, Ioannis G.; Katz, Ira; Hofer, Richard R.; Goebel, Dan M.
2012-01-01
A proof-of-principle effort to demonstrate a technique by which erosion of the acceleration channel in Hall thrusters of the magnetic-layer type can be eliminated has been completed. The first principles of the technique, now known as "magnetic shielding," were derived based on the findings of numerical simulations in 2-D axisymmetric geometry. The simulations, in turn, guided the modification of an existing 6-kW laboratory Hall thruster. This magnetically shielded (MS) thruster was then built and tested. Because neither theory nor experiment alone can validate fully the first principles of the technique, the objective of the 2-yr effort was twofold: (1) to demonstrate in the laboratory that the erosion rates can be reduced by >order of magnitude, and (2) to demonstrate that the near-wall plasma properties can be altered according to the theoretical predictions. This paper concludes the demonstration of magnetic shielding by reporting on a wide range of comparisons between results from numerical simulations and laboratory diagnostics. Collectively, we find that the comparisons validate the theory. Near the walls of the MS thruster, theory and experiment agree: (1) the plasma potential has been sustained at values near the discharge voltage, and (2) the electron temperature has been lowered by at least 2.5-3 times compared to the unshielded (US) thruster. Also, based on carbon deposition measurements, the erosion rates at the inner and outer walls of the MS thruster are found to be lower by at least 2300 and 1875 times, respectively. Erosion was so low along these walls that the rates were below the resolution of the profilometer. Using a sputtering yield model with an energy threshold of 25 V, the simulations predict a reduction of 600 at the MS inner wall. At the outer wall ion energies are computed to be below 25 V, for which case we set the erosion to zero in the simulations. When a 50-V threshold is used the computed ion energies are below the threshold at both
Bammer, Roland; Stollberger, Rudolf
2012-01-01
Counterexamples are used to motivate the revision of the established theory of tracer transport. Then dynamic contrast enhanced magnetic resonance imaging in particular is conceptualized in terms of a fully distributed convection–diffusion model from which a widely used convolution model is derived using, alternatively, compartmental discretizations or semigroup theory. On this basis, applications and limitations of the convolution model are identified. For instance, it is proved that perfusion and tissue exchange states cannot be identified on the basis of a single convolution equation alone. Yet under certain assumptions, particularly that flux is purely convective at the boundary of a tissue region, physiological parameters such as mean transit time, effective volume fraction, and volumetric flow rate per unit tissue volume can be deduced from the kernel. PMID:17429633
Dark energy, non-minimal couplings and the origin of cosmic magnetic fields
Jiménez, Jose Beltrán; Maroto, Antonio L. E-mail: maroto@fis.ucm.es
2010-12-01
In this work we consider the most general electromagnetic theory in curved space-time leading to linear second order differential equations, including non-minimal couplings to the space-time curvature. We assume the presence of a temporal electromagnetic background whose energy density plays the role of dark energy, as has been recently suggested. Imposing the consistency of the theory in the weak-field limit, we show that it reduces to standard electromagnetism in the presence of an effective electromagnetic current which is generated by the momentum density of the matter/energy distribution, even for neutral sources. This implies that in the presence of dark energy, the motion of large-scale structures generates magnetic fields. Estimates of the present amplitude of the generated seed fields for typical spiral galaxies could reach 10{sup −9} G without any amplification. In the case of compact rotating objects, the theory predicts their magnetic moments to be related to their angular momenta in the way suggested by the so called Schuster-Blackett conjecture.
Epov, Vladimir N
2011-08-07
A novel approach is suggested to investigate the mechanisms of chemical complexation reactions based on the results of Fujii with co-workers; they have experimentally observed that several metals and metalloids demonstrate mass-independent isotope fractionation during the reactions with the DC18C6 crown ether using solvent-solvent extraction. In this manuscript, the isotope fractionation caused by the magnetic isotope effect is used to understand the mechanisms of chemical exchange reactions. Due to the rule that reactions are allowed for certain electron spin states, and forbidden for others, magnetic isotopes show chemical anomalies during these reactions. Mass-independent fractionation is suggested to take place due to the hyperfine interaction of the nuclear spin with the electron spin of the intermediate product. Moreover, the sign of the mass-independent fractionation is found to be dependent on the element and its species, which is also explained by the magnetic isotope effect. For example, highly negative mass-independent isotope fractionation of magnetic isotopes was observed for reactions of DC18C6 with SnCl(2) species and with several Ru(III) chloro-species, and highly positive for reactions of this ether with TeCl(6)(2-), and with several Cd(II) and Pd(II) species. The atomic radius of an element is also a critical parameter for the reaction with crown ether, particularly the element ions with [Kr]4d(n)5s(m) electron shell fits the best with the DC18C6 crown ring. It is demonstrated that the magnetic isotope effect in combination with the theory of orbital hybridization can help to understand the mechanism of complexation reactions. The suggested approach is also applied to explain previously published mass-independent fractionation of Hg isotopes in other types of chemical exchange reactions.
NASA Technical Reports Server (NTRS)
Stepinski, Tomasz F.; Reyes-Ruiz, Mauricio; Vanhala, Harri A. T.
1993-01-01
A hydromagnetic dynamo provides the best mechanism for contemporaneously producing magnetic fields in a turbulent solar nebula. We investigate the solar nebula in the framework of a steady-state accretion disk model and establish the criteria for a viable nebular dynamo. We have found that typically a magnetic gap exists in the nebula, the region where the degree of ionization is too small for the magnetic field to couple to the gas. The location and width of this gap depend on the particular model; the supposition is that gaps cover different parts of the nebula at different evolutionary stages. We have found, from several dynamical constraints, that the generated magnetic field is likely to saturate at a strength equal to equipartition with the kinetic energy of turbulence. Maxwell stress arising from a large-scale magnetic field may significantly influence nebular structure, and Maxwell stress due to small-scale fields can actually dominate other stresses in the inner parts of the nebula. We also argue that the bulk of nebular gas, within the scale height from the midplane, is stable against Balbus-Hawley instability.
NASA Technical Reports Server (NTRS)
Stepinski, Tomasz F.; Reyes-Ruiz, Mauricio; Vanhala, Harri A. T.
1993-01-01
A hydromagnetic dynamo provides the best mechanism for contemporaneously producing magnetic fields in a turbulent solar nebula. We investigate the solar nebula in the framework of a steady-state accretion disk model and establish the criteria for a viable nebular dynamo. We have found that typically a magnetic gap exists in the nebula, the region where the degree of ionization is too small for the magnetic field to couple to the gas. The location and width of this gap depend on the particular model; the supposition is that gaps cover different parts of the nebula at different evolutionary stages. We have found, from several dynamical constraints, that the generated magnetic field is likely to saturate at a strength equal to equipartition with the kinetic energy of turbulence. Maxwell stress arising from a large-scale magnetic field may significantly influence nebular structure, and Maxwell stress due to small-scale fields can actually dominate other stresses in the inner parts of the nebula. We also argue that the bulk of nebular gas, within the scale height from the midplane, is stable against Balbus-Hawley instability.
On the theory of magnetic field generation by relativistically strong laser radiation
Berezhiani, V.I.; Shatashvili, N.L.; Mahajan, S.M. |
1996-07-01
The authors consider the interaction of subpicosecond relativistically strong short laser pulses with an underdense cold unmagnetized electron plasma. It is shown that the strong plasma inhomogeneity caused by laser pulses results in the generation of a low frequency (quasistatic) magnetic field. Since the electron density distribution is determined completely by the pump wave intensity, the generated magnetic field is negligibly small for nonrelativistic laser pulses but increases rapidly in the ultrarelativistic case. Due to the possibility of electron cavitation (complete expulsion of electrons from the central region) for narrow and intense beams, the increase in the generated magnetic field slows down as the beam intensity is increased. The structure of the magnetic field closely resembles that of the field produced by a solenoid; the field is maximum and uniform in the cavitation region, then it falls, changes polarity and vanishes. In extremely dense plasmas, highly intense laser pulses in the self-channeling regime can generate magnetic fields {approximately} 100 Mg and greater.
Nonisothermal theory of the positive column of an electric discharge in the axial magnetic field
Ul'yanov, D. K. Ul'yanov, K. N.
2013-01-15
A nonisothermal model of the positive column allowing for electron energy balance is analyzed. The influence of the axial magnetic field on the characteristics of the cylindrical positive column of a low-pressure discharge is investigated in the hydrodynamic approximation. It is shown that the magnetic field affects the plasma density distribution, plasma velocity, and electron energies. The radial dependences of the plasma density, electron energy, and plasma velocity, as well as the azimuthal velocities of electrons and ions, are calculated for helium at different values of the magnetic field strength. It is established that inertia should be taken into account in the equations for the azimuthal motion of electrons and ions. The results obtained in the hydrodynamic approximation differ significantly from those obtained in the framework of the common diffusion model of the positive column in the axial magnetic field. It is shown that the distributions of the plasma density and radial plasma velocity in the greater part of the positive column tend to those obtained in the diffusion approximation at higher values of the axial magnetic field and gas density, although substantial differences remain in the near-wall region.
NASA Technical Reports Server (NTRS)
Gary, S. P.; Tokar, R. L.
1985-01-01
The present investigation is concerned with the application of a second-order theory for electromagnetic instabilities in a collisionless plasma to two modes which resonate with hot ion beams. The application of the theory is strictly limited to the linear growth phase. However, the application of the theory may be extended to obtain a description of the beam at postsaturation if the wave-beam resonance is sufficiently broad in velocity space. Under the considered limitations, it is shown that, as in the cold beam case, the fluctuating fields do not gain appreciable momentum and that the primary exchange of momentum is between the beam and main component.
Probing Magnetic Fields of Early Galaxies
NASA Astrophysics Data System (ADS)
Kohler, Susanna
2017-06-01
How do magnetic fields form and evolve in early galaxies? A new study has provided some clever observations to help us answer this question.The Puzzle of Growing FieldsDynamo theory is the primary model describing how magnetic fields develop in galaxies. In this picture, magnetic fields start out as weak seed fields that are small and unordered. These fields then become ordered and amplified by large-scale rotation and turbulence in galaxy disks and halos, eventually leading to the magnetic fields we observe in galaxies today.Schematic showinghow to indirectly measure protogalactic magnetic fields. The measured polarization of a background quasar is altered by the fields in a foreground protogalaxy. Click for a closer look! [Farnes et al. 2017/Adolf Schaller/STSCI/NRAO/AUI/NSF]To test this model, we need observations of the magnetic fields in young protogalaxies. Unfortunately, we dont have the sensitivity to be able to measure these fields directly but a team of scientists led by Jamie Farnes (Radboud University in the Netherlands) have come up with a creative alternative.The key is to find early protogalaxies that absorb the light of more distant background objects. If a protogalaxy lies between us and a distant quasar, then magnetic fields of the protogalaxy if present will affect the polarization measurements of the background quasar.Observing Galactic Building BlocksTop: Redshift distribution for the background quasars in the authors sample. Bottom: Redshift distribution for the foreground protogalaxies the authors are exploring. [Farnes et al. 2017]Farnes and collaborators examined two types of foreground protogalaxies: Damped Lyman-Alpha Absorbers (DLAs) and Lyman Limit Systems (LLSs). They obtained polarimetric data for a sample of 114 distant quasars with nothing in the foreground (the control sample), 19 quasars with DLAs in the foreground, and 27 quasars with LLSs in the foreground. They then used statistical analysis techniques to draw conclusions about
Extended gyrokinetic field theory for time-dependent magnetic confinement fields
Sugama, H.; Watanabe, T.-H.; Nunami, M.
2014-01-15
A gyrokinetic system of equations for turbulent toroidal plasmas in time-dependent axisymmetric background magnetic fields is derived from the variational principle. Besides governing equations for gyrocenter distribution functions and turbulent electromagnetic fields, the conditions which self-consistently determine the background magnetic fields varying on a transport time scale are obtained by using the Lagrangian, which includes the constraint on the background fields. Conservation laws for energy and toroidal angular momentum of the whole system in the time-dependent background magnetic fields are naturally derived by applying Noether's theorem. It is shown that the ensemble-averaged transport equations of particles, energy, and toroidal momentum given in the present work agree with the results from the conventional recursive formulation with the WKB representation except that collisional effects are disregarded here.
Theory of spin wave modes in tangentially magnetized thin cylindrical dots: A variational approach
NASA Astrophysics Data System (ADS)
Zivieri, R.; Stamps, R. L.
2006-04-01
We present a theoretical study of the quantized spin wave spectrum in tangentially magnetized cylindrical thin magnetic dots. Low-energy spin waves in magnetic dots may be subdivided into four families: Damon-Eshbach like, backward like, mixed, and end modes. Frequencies and mode profiles are found using a variational approach based on carefully chosen trial functions. The variational method has the advantage that it can be used for large dots that are not practical to treat using numerical finite-element methods. Results for small dots generated using the variational method compare well with micromagnetic results. The variational method is demonstrated with an analysis of data obtained from experimental Brillouin light scattering data from saturated thin cylindrical Permalloy dots. Our approach allows for the definition of parameters describing important contributions to the spin wave energies. As an example, we show that a variational parameter γ provides a measure of spin wave localization near the dot border for one class of modes.
Theory of magnetic field line random walk in noisy reduced magnetohydrodynamic turbulence
Ruffolo, D.; Matthaeus, W. H.
2013-01-15
When a magnetic field consists of a mean part and fluctuations, the stochastic wandering of its field lines is often treated as a diffusive process. Under suitable conditions, a stable value is found for the mean square transverse displacement per unit parallel displacement relative to the mean field. Here, we compute the associated field line diffusion coefficient for a highly anisotropic 'noisy' reduced magnetohydrodynamic model of the magnetic field, which is useful in describing low frequency turbulence in the presence of a strong applied DC mean magnetic field, as may be found, for example, in the solar corona, or in certain laboratory devices. Our approach is nonperturbative, based on Corrsin's independence hypothesis, and makes use of recent advances in understanding factors that control decorrelation over a range of parameters described by the Kubo number. Both Bohm and quasilinear regimes are identified.
Extended gyrokinetic field theory for time-dependent magnetic confinement fields
NASA Astrophysics Data System (ADS)
Sugama, H.; Watanabe, T.-H.; Nunami, M.
2014-01-01
A gyrokinetic system of equations for turbulent toroidal plasmas in time-dependent axisymmetric background magnetic fields is derived from the variational principle. Besides governing equations for gyrocenter distribution functions and turbulent electromagnetic fields, the conditions which self-consistently determine the background magnetic fields varying on a transport time scale are obtained by using the Lagrangian, which includes the constraint on the background fields. Conservation laws for energy and toroidal angular momentum of the whole system in the time-dependent background magnetic fields are naturally derived by applying Noether's theorem. It is shown that the ensemble-averaged transport equations of particles, energy, and toroidal momentum given in the present work agree with the results from the conventional recursive formulation with the WKB representation except that collisional effects are disregarded here.
Structure of small-scale magnetic fields in the kinematic dynamo theory.
Schekochihin, Alexander; Cowley, Steven; Maron, Jason; Malyshkin, Leonid
2002-01-01
A weak fluctuating magnetic field embedded into a a turbulent conducting medium grows exponentially while its characteristic scale decays. In the interstellar medium and protogalactic plasmas, the magnetic Prandtl number is very large, so a broad spectrum of growing magnetic fluctuations is excited at small (subviscous) scales. The condition for the onset of nonlinear back reaction depends on the structure of the field lines. We study the statistical correlations that are set up in the field pattern and show that the magnetic-field lines possess a folding structure, where most of the scale decrease is due to the field variation across itself (rapid transverse direction reversals), while the scale of the field variation along itself stays approximately constant. Specifically, we find that, though both the magnetic energy and the mean-square curvature of the field lines grow exponentially, the field strength and the field-line curvature are anticorrelated, i.e., the curved field is relatively weak, while the growing field is relatively flat. The detailed analysis of the statistics of the curvature shows that it possesses a stationary limiting distribution with the bulk located at the values of curvature comparable to the characteristic wave number of the velocity field and a power tail extending to large values of curvature where it is eventually cut off by the resistive regularization. The regions of large curvature, therefore, occupy only a small fraction of the total volume of the system. Our theoretical results are corroborated by direct numerical simulations. The implication of the folding effect is that the advent of the Lorentz back reaction occurs when the magnetic energy approaches that of the smallest turbulent eddies. Our results also directly apply to the problem of statistical geometry of the material lines in a random flow.
Pfuetzner, H. . Bioelectricity and Magnetism Division)
1994-09-01
Rotational magnetization of grain oriented SiFe sheets involves three mechanisms: anisotropy, hysteresis and eddy currents. Apart from describing the respective physical background, the paper is focused on a separation of mechanisms. It discusses dependencies between field quantities which in the dynamic case are complicated by the fact that a three-dimensional field problem arises here. It is demonstrated that within a plane inner surface of a sheet, the magnetic behavior is independent of frequency in approximation. On the other hand, eddy currents yield phase shifts between individual surfaces. Respective rotational losses and their portions are discussed on the basis of Poynting's theorem.
Magnetic phenomena at and near nu = 1 / 2 and 1 / 4: theory, experiment, and interpretation
Shankar
2000-04-24
I show that the Hamiltonian theory of composite fermions is capable of yielding a unified description in fair agreement with recent experiments on polarization P and relaxation rate 1/T1 in quantum Hall states at filling nu = p/(2ps+1), at and near nu = 1 / 2 and 1 / 4 ( s = 1,2) at zero and nonzero temperatures. I show how rotational invariance and two dimensionality can make the underlying interacting theory behave like a free one in a limited context.
NASA Astrophysics Data System (ADS)
Champel, Thierry; Florens, Serge
2010-07-01
We study theoretically the energy and spatially resolved local density of states (LDoS) in graphene at high perpendicular magnetic field. For this purpose, we extend from the Schrödinger to the Dirac case a semicoherent-state Green’s-function formalism, devised to obtain in a quantitative way the lifting of the Landau-level degeneracy in the presence of smooth confinement and smooth disordered potentials. Our general technique, which rigorously describes quantum-mechanical motion in a magnetic field beyond the semiclassical guiding center picture of vanishing magnetic length (both for the ordinary two-dimensional electron gas and graphene), is connected to the deformation (Weyl) quantization theory in phase space developed in mathematical physics. For generic quadratic potentials of either scalar (i.e., electrostatic) or mass (i.e., associated with coupling to the substrate) types, we exactly solve the regime of large magnetic field (yet at finite magnetic length, formally, this amounts to considering an infinite Fermi velocity) where Landau-level mixing becomes negligible. Hence, we obtain a closed-form expression for the graphene Green’s function in this regime, providing analytically the discrete energy spectra for both cases of scalar and mass parabolic confinement. Furthermore, the coherent-state representation is shown to display a hierarchy of local energy scales ordered by powers of the magnetic length and successive spatial derivatives of the local potential, which allows one to devise controlled approximation schemes at finite temperature for arbitrary and possibly disordered potential landscapes. As an application, we derive general analytical nonperturbative expressions for the LDoS, which may serve as a good starting point for interpreting experimental studies. For instance, we are able to account for many puzzling features of the LDoS recently observed by high magnetic field scanning tunneling spectroscopy experiments on graphene, such as a
Shiau, T.N.; Sheu, G.J.; Yang, C.D.
1997-01-01
The vibration and active control of a flexible rotor system with magnetic bearings are investigated using Hybrid Method (HM) and H{sup {infinity}} control theory with consideration of gyroscopic effect. The hybrid method, which combines the merits of the finite element method (FEM) and generalized polynomial expansion method (GPEM) is employed to model the flexible rotor system with small order of plant. The mixed sensitivity problem of H{sup {infinity}} control theory is applied to design the control of system vibration with spillover phenomena for the reduced order plant. The H{sub 2} control design is also employed for comparison with the H{sup {infinity}} design. The experimental simulation is used to illustrate the effects of control design. It is shown that the H{sup {infinity}} controller design can be very effective to suppress spillover phenomena. In addition, the H{sup {infinity}} control design has robustness to the variation of the model parameters. The application of the hybrid method (HM) together with H{sup {infinity}} control design is highly recommended for vibration control of flexible rotor systems with magnetic bearings.
Maruyama, Tomoyuki; Cheoun, Myung-Ki; Kajino, Toshitaka; ...
2016-03-26
We study pion production by proton synchrotron radiation in the presence of a strong magnetic field when the Landau numbers of the initial and final protons are n(i, f) similar to 10(4)-10(5). We find in our relativistic field theory calculations that the pion decay width depends only on the field strength parameter which previously was only conjectured based upon semi-classical arguments. Moreover, we also find new results that the decay width satisfies a robust scaling relation, and that the polar angular distribution of emitted pion momenta is very narrow and can be easily obtained. This scaling implies that one canmore » infer the decay width in more realistic magnetic fields of 10(15) G, where n(i, f) similar to 10(12)-10(13), from the results for n(i, f) similar to 10(4)-10(5). The resultant pion intensity and angular distributions for realistic magnetic field strengths are presented and their physical implications discussed. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP(3).« less
NASA Astrophysics Data System (ADS)
Kiani, Keivan
2014-03-01
The alteration of the frequencies of nanostructures via an external field is of great importance in the design of nanomechanical devices whose vibrations should be appropriately controlled. Herein, free in-plane and out-of-plane vibration behaviors of conducting rectangular nanoplates subjected to unidirectional in-plane steady magnetic fields are of concern. To this end, the body forces exerted on the nanoplate based on the hypotheses of Kirchhoff, Mindlin, and higher-order plate theories are obtained. Subsequently, the nonlocal versions of the equations of motion of the conducting nanoplate for the suggested plate models are extracted. The presented formulations show that the small-scale parameter is incorporated into the exerted forces on the nanoplate due to the applied magnetic field. For the proposed models, the frequencies pertinent to the in-plane and out-of-plane vibrations of the nanoplates are evaluated. In the continuing, the roles of the length-to-thickness ratio, length-to-width ratio, small-scale parameter, and magnetic field strength on both in-plane and out-of-plane frequencies are addressed. The capabilities of the proposed models in predicting such frequencies are also explained and discussed.
Maruyama, Tomoyuki; Cheoun, Myung-Ki; Kajino, Toshitaka; Mathews, Grant J.
2016-03-26
We study pion production by proton synchrotron radiation in the presence of a strong magnetic field when the Landau numbers of the initial and final protons are n(i, f) similar to 10(4)-10(5). We find in our relativistic field theory calculations that the pion decay width depends only on the field strength parameter which previously was only conjectured based upon semi-classical arguments. Moreover, we also find new results that the decay width satisfies a robust scaling relation, and that the polar angular distribution of emitted pion momenta is very narrow and can be easily obtained. This scaling implies that one can infer the decay width in more realistic magnetic fields of 10(15) G, where n(i, f) similar to 10(12)-10(13), from the results for n(i, f) similar to 10(4)-10(5). The resultant pion intensity and angular distributions for realistic magnetic field strengths are presented and their physical implications discussed. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP(3).
NASA Astrophysics Data System (ADS)
Chang, Zhiwei; Halle, Bertil
2016-02-01
In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. We have embarked on a systematic program to develop, from the stochastic Liouville equation, a general and rigorous theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole coupling strengths, and Larmor frequencies. Here, we present a general theoretical framework applicable to spin systems of arbitrary size with symmetric or asymmetric exchange. So far, the dipolar EMOR theory is only available for a two-spin system with symmetric exchange. Asymmetric exchange, when the spin system is fragmented by the exchange, introduces new and unexpected phenomena. Notably, the anisotropic dipole couplings of non-exchanging spins break the axial symmetry in spin Liouville space, thereby opening up new relaxation channels in the locally anisotropic sites, including longitudinal-transverse cross relaxation. Such cross-mode relaxation operates only at low fields; at higher fields it becomes nonsecular, leading to an unusual inverted relaxation dispersion that splits the extreme-narrowing regime into two sub-regimes. The general dipolar EMOR theory is illustrated here by a detailed analysis of the asymmetric two-spin case, for which we present relaxation dispersion profiles over a wide range of conditions as well as analytical results for integral relaxation rates and time-dependent spin modes in the zero-field and motional-narrowing regimes. The general theoretical framework presented here will enable a quantitative analysis of frequency-dependent water-proton longitudinal relaxation in model systems with immobilized macromolecules and, ultimately, will provide a rigorous link between relaxation-based magnetic resonance image contrast and molecular parameters.
Ott, T; Baiko, D A; Kählert, H; Bonitz, M
2013-04-01
Two different approaches to the calculation of the wave spectra of magnetized strongly coupled liquid one-component plasmas are analzyed: the semianalytical quasilocalized charge approximation (QLCA) and the angle-averaged harmonic lattice (AAHL) theory. Both theories are benchmarked against the numerical evidence obtained from molecular dynamics simulations. It is found that not too far from the melting transition (Γ≳100), the AAHL theory is superior to the QLCA, while further away from the transition, the QLCA performs comparably to or better than the AAHL theory.
Brandenburg, J. E.
2008-01-21
Progress on the GEMS (Gravity Electro-Magnetism-Strong), theory is presented as well as its application to space problems. The GEMS theory is now validated through the Standard Model of physics. Derivation of the value of the Gravitation constant based on the observed variation of {alpha} with energy: results in the formula G congruent with ({Dirac_h}/2{pi})c/M{sub {eta}}{sub c}{sup 2} exp(-1/(1.61{alpha})), where {alpha} is the fine structure constant,({Dirac_h}/2{pi}), is Planck's constant, c, is the speed of light, and M{sub {eta}}{sub c} is the mass of the {eta}{sub cc} Charmonium meson that is shown to be identical to that derived from the GEM postulates. Covariant formulation of the GEM theory is now possible through definition of the spacetime metric tensor as a portion of the EM stress tensor normalized by its own trace: g{sub ab} = 4(F{sup c}{sub a}F{sub cb})/(F{sup ab}F{sub ab}), it is found that this results in a massless ground state vacuum and a Newtonian gravitation potential {phi} = 1/2 E{sup 2}/B{sup 2}. It is also found that a Lorentz or flat-space metric is recovered in the limit of a full spectrum ZPF.
On a theory of surface waves in a smoothly inhomogeneous plasma in an external magnetic field
Kuzelev, M. V. Orlikovskaya, N. G.
2016-12-15
A theory of surface waves in a magnetoactive plasma with smooth boundaries has been developed. A dispersion equation for surface waves has been derived for a linear law of density change at the plasma boundary. The frequencies of surface waves and their collisionless damping rates have been determined. A generalization to an arbitrary density profile at the plasma boundary is given. The collisions have been taken into account, and the application of the Landau rule in the theory of surface wave damping in a spatially inhomogeneous magnetoactive collisional plasma has been clarified.
Meinert, M.; Schmalhorst, J; Klewe, C.; Reiss, G.; Arenholz, E.; Bohnert, T.; Nielsch, K.
2011-08-08
Epitaxial thin films of the half-metallic X{sub a}-compound Mn{sub 2}CoGa (Hg{sub 2}CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality lms with a bulk magnetization of 1.95(5) {mu}{sub }B per unit cell were obtained. The average Mn magnetic moment and the Co moment are parallel, in agreement with theory. The x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism allows to distinguish between itinerant and localized Mn moments. It is shown that one of the two Mn moments has localized character, whereas the other Mn moment and the Co moment are itinerant.
Chinks in Solar Dynamo Theory: Turbulent Diffusion, Dynamo Waves and Magnetic Helicity
NASA Technical Reports Server (NTRS)
DeLuca, E. E.; Hurlburt, N.
1998-01-01
In this first year of our investigation we explored the role of compressibility and stratification in the dissipation of magnetic fields. The predictions of Mean Field Electrodynamics have been questioned because of the strong feedback of small scale magnetic structure on the velocity fields. In 2-D, this nonlinear feedback results in a lengthening of the turbulent decay time. In 3-D alpha-quenching is predicted. Previous studies assumed a homogeneous fluid. This first year we present recent results from 2-D compressible MHD decay simulations in a highly stratified atmosphere that more closely resembles to solar convection zone. We have applied for NCCS T3E time to assist in the performance of our 3-D calculations.
Electronic and magnetic properties of substituted BN sheets: A density functional theory study
Zhou, Yungang; Yang, Ping; Wang, Zhiguo; Zu, Xiaotao T.; Xiao, Hai Yan; Sun, Xin; Khaleel, Mohammad A.; Gao, Fei
2011-04-15
Using density functional calculations, we investigate the geometries, electronic structures and magnetic properties of hexagonal BN sheets with 3d transition metal (TM) and nonmetal atoms embedded in three types of vacancies: VB, VN, and VB+N. We show that some embedded configurations, except TM atoms in VN vacancy, are stable in BN sheet and yield interesting phenomena. For instance, the band gaps and magnetic moments of BN sheet can be tuned depending on the embedded dopant species and vacancy type. In particular, embedment such as Cr in VB+N, Co in VB, and Ni in VB leads to half-metallic BN sheets interesting for spin filter applications. From the investigation of Mn-chain (CMn) embedments, a regular 1D structure can be formed in BN sheet as an electron waveguide, a metal nanometer wire with a single atom thickness.
NASA Astrophysics Data System (ADS)
Sornchamni, T.; Jovanovic, G.; Atwater, J.; Akse, J.; Wheeler, R.
Typically, the operation of a conventional fluidized bed relies on the balance of gravitational, buoyancy, and drag forces. In the absence of normal gravity, or under microgravity and variable gravity conditions, the gravitational force must be replaced with an alternative force to restore fluidization. Our work has shown that, given a suitable variable magnetic field design, the resulting magnetic field gradient can create sufficient magnetic force acting upon the ferromagnetic particles to replace or supplement the gravitational force. Therefore, the ferromagnetic granular media can be fluidized in either microgravity or hypogravity. In this paper, we present our experimental and theoretical work leading to a) development of theoretical model based on fundamental principles for the design of the Gradient Magnetically Assisted Fluidized Bed (G-MAFB), and b) practical implementation of the G-MAFB in the filtration and destruction of solid biowaste particles from liquid streams. The G-MAFB system consists of a fluidization column and series of Helmholtz electromagnetic coils, with DC power supply. Each Helmholtz ring is powered and controlled separately. Experiments are performed in both 0g (on board NASA KC- 135) and 1g (laboratory) environments. The experiments in 0g are conducted in a two-dimensional, square cross-section, tapered fluidization column. The tapered shape is introduced to provide additional stability to the fluidization particles. The experiments in 0g prove that the magnetic force has a significant role in keeping the particles from extruding out of the bed. Without the magnetic force, it is impossible to have fluidization in space. Solid waste destruction technologies are needed to support long duration human habitation in space. The current technologies, including supercritical water oxidation (SCWO), microwave powered combustion and fluidized bed incineration, have been applied to the destruction of solid wastes, but none are compatible with
Comment on "Extended Born-Infeld theory and the bouncing magnetic universe"
NASA Astrophysics Data System (ADS)
García-Salcedo, Ricardo; Gonzalez, Tame; Horta-Rangel, Antonio; Quiros, Israel
2014-12-01
In a recent paper [Phys. Rev. D 85, 023528 (2012), 10.1103/PhysRevD.85.023528] the authors proposed a generalized Born-Infeld electrodynamics coupled to general relativity which produces a nonsingular bouncing universe. For a magnetic universe the resulting cosmic evolution inevitably interpolates between asymptotic de Sitter states. Here we show that (i) the conclusion about the inevitability of the asymptotic vacuum regime in a magnetic universe is wrong, (ii) a sudden curvature singularity—not better than the big bang—arises, but for spaces of positive curvature, (iii) the square sound speed can be a negative quantity, signaling instability against small perturbations of the background energy density, and (iv) the speed of sound can exceed the speed of light, which raises a causality issue.
Effective Hamiltonian theory for nonreciprocal light propagation in magnetic Rashba conductor
NASA Astrophysics Data System (ADS)
Kawaguchi, Hideo; Tatara, Gen
2016-12-01
Rashba spin-orbit interaction leads to a number of electromagnetic cross-correlation effects by inducing a mixing of electric and magnetic degrees of freedom. In this study, we investigate the optical properties of a magnetic Rashba conductor by deriving an effective Hamiltonian based on an imaginary-time path-integral formalism. We show that the effective Hamiltonian can be described in terms of toroidal and quadrupole moments, as has been argued in the case of insulator multiferroics. The toroidal moment turns out to coincide with the spin gauge field induced by the Rashba field. It causes Doppler shift by inducing intrinsic spin current, resulting in anisotropic light propagation (directional dichroism) irrespective of the polarization. The quadrupole moment on the other hand results in a magneto-optical phenomenon such as a Faraday effect for circularly polarized waves.
Magnetic fields with photon beams: dose calculation using electron multiple-scattering theory.
Jette, D
2000-08-01
Strong transverse magnetic fields can produce large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. We have developed a new equation of motion for the transport of charged particles in an arbitrary magnetic field, incorporating both energy loss and multiple scattering. Key to modeling the latter process is a new concept, that of "typical scattered particles." The formulas which we have arrived at are particularly applicable to the transport of, and deposition of energy by, Compton electrons and pair-production electrons and positrons generated within a medium by a photon beam, and we have shown qualitatively how large dose enhancements and reductions can occur. A companion article examines this dose modification effect through systematic Monte Carlo simulations.
3D MHD Coronal Oscillations about a Magnetic Null Point: Application of WKB Theory
NASA Astrophysics Data System (ADS)
McLaughlin, J. A.; Ferguson, J. S. L.; Hood, A. W.
2008-09-01
This paper is a demonstration of how the WKB approximation can be used to help solve the linearised 3D MHD equations. Using Charpit’s method and a Runge Kutta numerical scheme, we have demonstrated this technique for a potential 3D magnetic null point, B=[ x, ɛ y,-( ɛ+1) z]. Under our cold-plasma assumption, we have considered two types of wave propagation: fast magnetoacoustic and Alfvén waves. We find that the fast magnetoacoustic wave experiences refraction towards the magnetic null point and that the effect of this refraction depends upon the Alfvén speed profile. The wave and thus the wave energy accumulate at the null point. We have found that current buildup is exponential and the exponent is dependent upon ɛ. Thus, for the fast wave there is preferential heating at the null point. For the Alfvén wave, we find that the wave propagates along the field lines. For an Alfvén wave generated along the fan plane, the wave accumulates along the spine. For an Alfvén wave generated across the spine, the value of ɛ determines where the wave accumulation will occur: fan plane ( ɛ=1), along the x-axis (0< ɛ<1) or along the y-axis ( ɛ>1). We have shown analytically that currents build up exponentially, leading to preferential heating in these areas. The work described here highlights the importance of understanding the magnetic topology of the coronal magnetic field for the location of wave heating.
Theory and applications of maps on SO(3) in nuclear magnetic resonance
Cho, H.M.
1987-02-01
Theoretical approaches and experimental work in the design of multiple pulse sequences in Nuclear Magnetic Resonance (NMR) are the subjects of this dissertation. Sequences of discrete pulses which reproduce the nominal effect of single pulses, but over substantially broader, narrower, or more selective ranges of transition frequencies, radiofrequency field amplitudes, and spin-spin couplings than the single pulses they replace, are developed and demonstrated. 107 refs., 86 figs., 6 tabs.
Zero modes in magnetic systems: General theory and an efficient computational scheme
NASA Astrophysics Data System (ADS)
Buijnsters, F. J.; Fasolino, A.; Katsnelson, M. I.
2014-05-01
The presence of topological defects in magnetic media often leads to normal modes with zero frequency (zero modes). Such modes are crucial for long-time behavior, describing, for example, the motion of a domain wall as a whole. Conventional numerical methods to calculate the spin-wave spectrum in magnetic media are either inefficient or they fail for systems with zero modes. We present a new efficient computational scheme that reduces the magnetic normal-mode problem to a generalized Hermitian eigenvalue problem also in the presence of zero modes. We apply our scheme to several examples, including two-dimensional domain walls and Skyrmions, and show how the effective masses that determine the dynamics can be calculated directly. These systems highlight the fundamental distinction between the two types of zero modes that can occur in spin systems, which we call special and inertial zero modes. Whereas the inertial modes are generic Goldstone modes related to a broken continuous symmetry, the special modes arise naturally when two broken continuous symmetries coexist. Our method is suitable for both conservative and dissipative systems. For the latter case, we present a perturbative scheme to take into account damping, which can also be used to calculate dynamical susceptibilities.
Relaxation and Saturation in High-Gradient Force-Detected Magnetic Resonance: Theory and Observation
NASA Astrophysics Data System (ADS)
Dougherty, William; Bruland, Kelly; Chao, Shih-Hui; Garbini, Joseph; Jensen, Seth; Sidles, John
1999-05-01
We have observed microwave power-scaling behavior of electron spin resonance in a field gradient of 250,000 Tesla per meter by the technique of magnetic resonance force microscopy (MRFM). Resonance was induced in a dilute sample of DPPH in polystyrene at 77 and 10 K by an amplitude-modulated microwave field in a polarizing field produced by a micrometer-scale SmCo magnet mounted on a force microscope cantilever. Microwave power-dependent spin-gradient forces of 10--10,000 attonewtons were observed, with an electron spin-moment sensitivity comparable to the most sensitive SQUIDs. At low power, data were in good accord with analytical expressions derived from the Bloch equations for high gradients. At high power, saturation effects modify the induced forces, creating a distinct ``knee" in the force-response curve. The values of spin-lattice relaxation times inferred from the phase lag of the cantilever response agree with published values for similar samples obtained by conventional resonance spectroscopy at 77 K, but were shorter than published values at 10 K. MRFM techniques allow the observation of magnetic resonance in external field gradients which can meet or exceed the local intermolecular gradients, providing an interesting experimental and theoretical regime in which to test models of spin diffusion and relaxation.
Magnetic field effects in few-level quantum dots: Theory and application to experiment
NASA Astrophysics Data System (ADS)
Wright, Christopher J.; Galpin, Martin R.; Logan, David E.
2011-09-01
We examine several effects of an applied magnetic field on Anderson-type models for both single- and two-level quantum dots, and we make direct comparison between numerical renormalization group (NRG) calculations and recent conductance measurements. On the theoretical side, the focus is on magnetization, single-particle dynamics, and zero-bias conductance, with emphasis on the universality arising in strongly correlated regimes, including a method to obtain the scaling behavior of field-induced Kondo resonance shifts over a very wide field range. NRG is also used to interpret recent experiments on spin-(1)/(2) and spin-1 quantum dots in a magnetic field, which we argue do not wholly probe universal regimes of behavior, and the calculations are shown to yield good qualitative agreement with essentially all features seen in experiment. The results capture in particular the observed field dependence of the Kondo conductance peak in a spin-(1)/(2) dot, with quantitative deviations from experiment occurring at fields in excess of ˜5T, indicating the eventual inadequacy of using the equilibrium single-particle spectrum to calculate the conductance at finite bias.
NASA Technical Reports Server (NTRS)
Srnka, L. J.; Mendenhall, M. H.
1979-01-01
A model is presented for the global thermoremanent magnetization of spherical lithospheres which cool in the presence of central dipole fields. Reversals and intensity variations of the field are incorporated in this model, which is applicable to bodies whose interiors have remained above the Curie point throughout their evolution. The model demonstrates that even considering Runcorn's (1975) magnetostatics theorems for spherical shells, a nonzero magnetic permeability and a finite cooling rate in the lithosphere permit the acquisition of a sizable global remanent dipole moment, which would be detectable by external measurements after the magnetizing field has disappeared. Preliminary application of this model to Mercury, Venus, and Mars suggests that only the combination of a nonreversing ancient source field with a surface value near 1 Oe plus a sizable concentration (about 1% by volume) of ferromagnetic material in their crusts could produce remanent planetary dipole fields as large as those measured by spacecraft. On the other hand, if ancient reversing dynamos existed in these planets, it is unlikely that large planetary-scale fields like those observed at Mercury could be due to remanence in their crusts, irrespective of their composition.
Lee, H. C.; Jiang, T. F.
2010-11-15
We analytically solve the relativistic equation of motion for an electron in ion plasma channels and calculate the corresponding trajectory as well as the synchrotron radiation. The relativistic effect on a trajectory is strong, i.e., many high-order harmonic terms in the trajectory, when the ratio of the initial transverse velocity (v{sub x0}) to the longitudinal velocity (v{sub z0}) of the electron injected to ion plasma channels is high. Interestingly, these high-order harmonic terms result in a quite broad and intense radiation spectrum, especially at an oblique angle, in contrast to an earlier understanding. As the initial velocity ratio (v{sub x0}:v{sub z0}) decreases, the relativistic effect becomes weak; only the first and second harmonic terms remain in the transverse and longitudinal trajectories, respectively, which coincides with the result of Esarey et al. [Phys. Rev. E 65, 056505 (2002)]. Our formalism also allows the description of electron's trajectory in the presence of an applied magnetic field. Critical magnetic fields for cyclotron motions are figured out and compared with semiclassical results. The cyclotron motion leads to more high-order harmonic terms than the trajectory without magnetic fields and causes an immensely broad spectrum with vastly large radiation amplitude for high initial velocity ratios (v{sub x0}:v{sub z0}). The radiation from hard x-ray to gamma-ray regions can be generated with a broad radiation angle, thus available for applications.
Theory of coherent electron-scale magnetic structures in space plasma turbulence
NASA Astrophysics Data System (ADS)
Jovanović, Dušan; Alexandrova, Olga; Maksimović, Milan
2015-08-01
Recent spacecraft observations in the solar wind and in the Earth’s magnetosheath indicate that the dissipation range of magnetic turbulence probably takes place at electron scales. Here, we derive nonlinear electron magnetohydrodynamic (EMHD) equations for warm plasma, i.e. with the ratio of thermodynamic and magnetic pressures, β ∼ 1. This model describes plasma turbulence under the solar wind and magnetosheath conditions on the electron spatial scales and with the characteristic frequency that does not exceed the electron gyrofrequency. We show that at electron scales and in the presence of a sufficiently large temperature anisotropy {T}{e\\perp }/{T}{e\\parallel }\\gt 1, there exist self-organized, coherent, nonlinear dipole vortex structures associated with obliquely propagating whistler waves. These can be visualized as pairs of counterstreaming helicoidal currents that produce both the compressional and torsional perturbations of the magnetic field. In contrast to the previously known long-range EMHD dipolar vortices in a cold plasma, this novel solution is an evanescent mode, strongly localized in space (with wave numbers {k}\\perp \\gg {k}\\parallel ). It can constitute a building block for the plasma turbulence at short scales and provide a possible scenario of turbulence dissipation at electron scales.
The Circuit Theory Behind Coupled-Mode Magnetic Resonance-Based Wireless Power Transmission.
Kiani, Mehdi; Ghovanloo, Maysam
2012-09-01
Inductive coupling is a viable scheme to wirelessly energize devices with a wide range of power requirements from nanowatts in radio frequency identification tags to milliwatts in implantable microelectronic devices, watts in mobile electronics, and kilowatts in electric cars. Several analytical methods for estimating the power transfer efficiency (PTE) across inductive power transmission links have been devised based on circuit and electromagnetic theories by electrical engineers and physicists, respectively. However, a direct side-by-side comparison between these two approaches is lacking. Here, we have analyzed the PTE of a pair of capacitively loaded inductors via reflected load theory (RLT) and compared it with a method known as coupled-mode theory (CMT). We have also derived PTE equations for multiple capacitively loaded inductors based on both RLT and CMT. We have proven that both methods basically result in the same set of equations in steady state and either method can be applied for short- or midrange coupling conditions. We have verified the accuracy of both methods through measurements, and also analyzed the transient response of a pair of capacitively loaded inductors. Our analysis shows that the CMT is only applicable to coils with high quality factor (Q) and large coupling distance. It simplifies the analysis by reducing the order of the differential equations by half compared to the circuit theory.
The Circuit Theory Behind Coupled-Mode Magnetic Resonance-Based Wireless Power Transmission
Kiani, Mehdi; Ghovanloo, Maysam
2014-01-01
Inductive coupling is a viable scheme to wirelessly energize devices with a wide range of power requirements from nanowatts in radio frequency identification tags to milliwatts in implantable microelectronic devices, watts in mobile electronics, and kilowatts in electric cars. Several analytical methods for estimating the power transfer efficiency (PTE) across inductive power transmission links have been devised based on circuit and electromagnetic theories by electrical engineers and physicists, respectively. However, a direct side-by-side comparison between these two approaches is lacking. Here, we have analyzed the PTE of a pair of capacitively loaded inductors via reflected load theory (RLT) and compared it with a method known as coupled-mode theory (CMT). We have also derived PTE equations for multiple capacitively loaded inductors based on both RLT and CMT. We have proven that both methods basically result in the same set of equations in steady state and either method can be applied for short- or midrange coupling conditions. We have verified the accuracy of both methods through measurements, and also analyzed the transient response of a pair of capacitively loaded inductors. Our analysis shows that the CMT is only applicable to coils with high quality factor (Q) and large coupling distance. It simplifies the analysis by reducing the order of the differential equations by half compared to the circuit theory. PMID:24683368
NASA Astrophysics Data System (ADS)
Yamazaki, K.
2011-12-01
magnetic field following the 11 March 2011 Mw 9.0 Tohoku-oki earthquake are investigated. One-second resolution geomagnetic data at three stations in Japan provided by the Japan Meteorological Agency are used for the investigation. In vector components of the geomagnetic field (i.e. X, Y, and Z), clear variations as large as 5 nT are recorded simultaneous to the seismic wave propagations. However, they are likely apparent variations due to the ground motions, and to extract only the actual signals is quite difficult. A way to avoid this difficulty is to focus only the total intensity of the geomagnetic field because total intensities measured by the proton or Overhauser magnetometers are expected to be robust against sensor vibrations. Variations in the total intensities are smaller than 0.1 nT, meaning that variations in vector components are mainly arising from sensor vibrations. The observed magnitudes of variations in the magnetic field are not larger than those predicted by theory; thus, we are unable to find an evidence of unknown mechanisms converting seismic waves to electromagnetic phenomena.
Hofacker, H.B.
1958-09-23
This patent relates to nmgnets used in a calutron and more particularly to means fur clamping an assembly of magnet coils and coil spacers into tightly assembled relation in a fluid-tight vessel. The magnet comprises windings made up of an assembly of alternate pan-cake type coils and spacers disposed in a fluid-tight vessel. At one end of the tank a plurality of clamping strips are held firmly against the assembly by adjustable bolts extending through the adjacent wall. The foregoing arrangement permits taking up any looseness which may develop in the assembly of coils and spacers.
Mendive-Tapia, Eduardo; Staunton, Julie B
2017-05-12
We describe a disordered local moment theory for long-period magnetic phases and investigate the temperature and magnetic field dependence of the magnetic states in the heavy rare earth elements (HREs), namely, paramagnetic, conical and helical antiferromagnetic (HAFM), fan, and ferromagnetic (FM) states. We obtain a generic HRE magnetic phase diagram which is consequent on the response of the common HRE valence electronic structure to f-electron magnetic moment ordering. The theory directly links the first-order HAFM-FM transition to the loss of Fermi surface nesting, induced by this magnetic ordering, as well as provides a template for analyzing the other phases and exposing where f-electron correlation effects are particularly intricate. Gadolinium, for a range of hexagonal, close-packed lattice constants c and a, is the prototype, described ab initio, and applications to other HREs are made straightforwardly by scaling the effective pair and quartic local moment interactions that emerge naturally from the theory with de Gennes factors and choosing appropriate lanthanide-contracted c and a values.
Control designs for low-loss active magnetic bearings: Theory and implementation
NASA Astrophysics Data System (ADS)
Wilson, Brian Christopher David
Active Magnetic Bearings (AMB) have been proposed for use in Electromechanical Flywheel Batteries. In these devices, kinetic energy is stored in a magnetically levitated flywheel which spins in a vacuum. The AMB eliminates all mechanical losses, however, electrical loss, which is proportional to the square of the magnetic flux, is still significant. For efficient operation, the flux bias, which is typically introduced into the electromagnets to improve the AMB stiffness, must be reduced, preferably to zero. This zero-bias (ZB) mode of operation cripples the classical control techniques which are customarily used and nonlinear control is required. As a compromise between AMB stiffness and efficiency, a new flux bias scheme is proposed called the generalized complementary flux condition (gcfc). A flux-bias dependent trade-off exists between AMB stiffness, power consumption, and power loss. This work theoretically develops and experimentally verifies new low-loss AMB control designs which employ the gcfc condition. Particular attention is paid to the removal of the singularity present in the standard nonlinear control techniques when operating in ZB. Experimental verification is conduced on a 6-DOF AMB reaction wheel. Practical aspects of the gcfc implementation such as flux measurement and flux-bias implementation with voltage mode amplifiers using IR compensation are investigated. Comparisons are made between the gcfc bias technique and the standard constant-flux-sum (cfs) bias method. Under typical operating circumstances, theoretical analysis and experimental data show that the new gcfc bias scheme is more efficient in producing the control flux required for rotor stabilization than the ordinary cfs bias strategy.
Latif, Iqbal A; Hansda, Shekhar; Datta, Sambhu N
2012-08-23
The Schlenk diradical has been known since 1915. After a detailed experimental work by Rajca, its magnetic nature has remained more or less unexplored. We have investigated by quantum chemical calculations the nature of magnetic coupling in 11 substituted Schlenk diradicals. Substitution has been considered at the fifth carbon atom of the meta-phenylene moiety. The UB3LYP method has been used to study 12 diradicals including the original one. The 6-311G(d,p) basis set has been employed for optimization of molecular geometry in both singlet and triplet states for each species. The singlet optimization has led to the optimization of the broken-symmetry structure for 10 species including the unsubstituted one. This development makes it possible to carry out further broken symmetry calculations in two ways. The triplet calculation has been done using 6-311++G(d,p) basis set and the optimized triplet geometry in both procedures. The broken symmetry calculations have used the optimized geometries of either the triplet states or the broken symmetry solutions. The first method leads to the prediction of electron paramagnetic resonance (EPR) compatible magnetic exchange coupling constant (J) in the range 517-617 cm(-1). A direct optimization of the broken symmetry geometry gives rise to a lower estimate of J, in the range of 411-525 cm(-1) and compatible with macroscopic Curie studies. The calculated J for the unsubstituted Schlenk diradical is 512 cm(-1) that can be compared with 455 cm(-1) estimated by Rajca. In both cases, introduction of groups with +M and +I effects (Ingold's notation) decreases the J value from that for the unsubstituted Schlenk diradical while -I and -M groups at the same position increases J. These trends have been explained in terms of Hammett constants, atomic spin densities, and dihedral angles.
NASA Astrophysics Data System (ADS)
Lileev, A. S.
2017-01-01
Amethod for simulating the hysteretic properties and the process of magnetization reversal of uniaxial highly anisotropic ferromagnets is developed with allowance for the presence of magnetostatic interaction between microvolumes of the material. It is based on the phenomenological approach to an elementary act of the process of magnetization reversal of these materials. Computer simulation is used to explain and illustrate the phenomenon of "thermal magnetization," the formation of "channels of reversal magnetization," the interrelation between the degree of crystal texture and the spin reorientation transfer, the coarsening of the domain structure in rapidly hardened powders, and the behavior of permanent magnets in a magnetic system in contact with a magnetically soft material.
Agterberg, D F; Dodgson, Matthew J W
2002-07-01
For multicomponent superconductors, it is known that the presence of symmetry breaking fields can lead to multiple superconducting phase transitions. Motivated by recent small angle neutron scattering experiments on the vortex state of UPt3, the London theory in the vicinity of such phase transitions is determined. It is found that the form of this London theory is in general quite different than that for conventional superconductors. This is due to the existence of a diverging correlation length associated with these phase transitions. One striking consequence is that nontrivial vortex lattices exist arbitrarily close to H(c1). Applications to UPt3, CeIn3, U(1-x)Th(x)Be(13), electron doped cuprate superconductors, Sr(2)RuO(4), and MgCNi(3) are discussed.
NASA Astrophysics Data System (ADS)
Cvetkovic, Vladimir; Vafek, Oskar
2012-02-01
The superconductivity in pnictides arises due to interband scattering between Cooper pairs belonging to hole and particle Fermi surfaces. The amplitude of the scattering, while weak at bare level, is enhanced under RG flow, and competes with the SDW to become the leading instability when the hole and particle FS's are nearly, but not perfectly, nested. This motivates us to construct a GL theory with multiple order parameters from the microscopic action. It allows us to study the two competing orders and their interplay including a possible coexistence. The presence of both attractive and repulsive pairing terms requires a novel approach to the derivation with imaginary terms in the action as one consequence. The construction of the GL theory is a two-step process, with RG flow to an intermediate cut-off scale, determined by the deviations from the nesting, as the first step.
Magnetovolume effect and finite-temperature theory of magnetism in transition metals and alloys
NASA Astrophysics Data System (ADS)
Kakehashi, Y.
1990-01-01
A review of recent developments in the theory of magnetovolume effects based on the Liberman-Pettifor virial theorem is presented. The general expression of the electronic contribution to the thermal expansion is shown to cover a wide range of magnetovolume effects from the insulator to the weak ferromagnets. It consists of the positive term proportional to the specific heat and a term proportional to the temperature derivative of the amplitude of the local moment. By using the single-site spin fluctuation theory (SSF) the Fe-Ni as well as Fe3Pt invar alloys are shown to be understood from this viewpoint. The local environment effects and the electron correlations at finite temperatures improve the difficulties in the SSF.
Magnetovolume effect and finite-temperature theory of magnetism in transition metals and alloys
NASA Astrophysics Data System (ADS)
Kakehashi, Y.
1989-10-01
A review of recent developments in the theory of magnetovolume effects based on the Liberman-Pettifor virial theorem is presented. The general expression of the electronic contribution to the thermal expansion is shown to cover a wide range of magnetovolume effects from the insulator to the weak ferromagnets. It consists of the positive term proportional to the specific heat and a term proportional to the temperature derivative of the amplitude of the local moment. By using the single-site spin fluctuation theory (SSF) the Fe-Ni as well as Fe 3Pt invar alloys are shown to be understood from this viewpoint. The local environment effects and the electron correlations at finite temperatures improve the difficulties in the SSF.
First-principles theory of quantum well resonance in double barrier magnetic tunnel junctions.
Wang, Yan; Lu, Zhong-Yi; Zhang, X-G; Han, X F
2006-08-25
Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.
Anharmonic and Standing Dynamo Waves: Theory and Observation of Stellar Magnetic Activity
2005-04-29
ACRONYM(S) MWI Mount Wilson Institute P.O. Box 1909 11. SPONSOR/MONITOR’S REPORT Atlanta, GA 30301-1909I NUMBER(S) N/A 12. DISTRIBUTION/AVAILABILITY...From studies of time series of Ca II H and K emission fluxes that go back more than 35 years and have been accumulated for such stars at the Mount...the inference of surface magnetism by proxy from the fluxes of the emission cores at the centers of the singly-ionized calcium H (396.8 nm) and K
NASA Astrophysics Data System (ADS)
Egedal, J.; Le, A.; Daughton, W.; Wetherton, B.; Cassak, P. A.; Chen, L.-J.; Lavraud, B.; Torbert, R. B.; Dorelli, J.; Gershman, D. J.; Avanov, L. A.
2016-10-01
Supported by a kinetic simulation, we derive an exclusion energy parameter EX providing a lower kinetic energy bound for an electron to cross from one inflow region to the other during magnetic reconnection. As by a Maxwell demon, only high-energy electrons are permitted to cross the inner reconnection region, setting the electron distribution function observed along the low-density side separatrix during asymmetric reconnection. The analytic model accounts for the two distinct flavors of crescent-shaped electron distributions observed by spacecraft in a thin boundary layer along the low-density separatrix.
A new method of Curie depth evaluation from magnetic data: Theory
NASA Technical Reports Server (NTRS)
Won, I. J. (Principal Investigator)
1981-01-01
An approach to estimating the Curie point isotherm uses the classical Gauss method inverting a system of nonlinear equations. The method, slightly modified by a differential correction technique, directly inverts filtered Magsat data to calculate the crustal structure above the Curie depth, which is modeled as a magnetized layer of varying thickness and susceptibility. Since the depth below the layer is assumed to be nonmagnetic, the bottom of the layer is interpreted as the Curie depth. The method, once fully developed, tested, and compared with previous work by others, is to be applied to a portion of the eastern U.S. when sufficient Magsat data are accumulated for the region.
Weakly relativistic quantum kinetic theory for electrostatic wave modes in magnetized plasmas
Hussain, Azhar; Stefan, Martin; Brodin, Gert
2014-03-15
We have derived the electrostatic dispersion relation in a magnetized plasma using a recently developed quantum kinetic model based on the Dirac equation. The model contains weakly relativistic spin effects such as Thomas precession, the polarization currents associated with the spin and the spin-orbit coupling. It turns out that for strictly electrostatic perturbations the non-relativistic spin effects vanish, and the modification of the classical dispersion relation is solely associated with the relativistic terms. Several new wave modes appear due the electron spin effects, and an example for astrophysical plasmas are given.
Cuny, Jérôme; Sykina, Kateryna; Fontaine, Bruno; Le Pollès, Laurent; Pickard, Chris J; Gautier, Régis
2011-11-21
Solid-state (95)Mo nuclear magnetic resonance (NMR) properties of molybdenum hexacarbonyl have been computed using density functional theory (DFT) based methods. Both quadrupolar coupling and chemical shift parameters were evaluated and compared with parameters of high precision determined using single-crystal (95)Mo NMR experiments. Within a molecular approach, the effects of major computational parameters, i.e. basis set, exchange-correlation functional, treatment of relativity, have been evaluated. Except for the isotropic parameter of both chemical shift and chemical shielding, computed NMR parameters are more sensitive to geometrical variations than computational details. Relativistic effects do not play a crucial part in the calculations of such parameters for the 4d transition metal, in particular isotropic chemical shift. Periodic DFT calculations were tackled to measure the influence of neighbouring molecules on the crystal structure. These effects have to be taken into account to compute accurate solid-state (95)Mo NMR parameters even for such an inorganic molecular compound.
Fermi-liquid-based theory for the in-plane magnetic anisotropy in untwinned high-Tc superconductors.
Eremin, I; Manske, D
2005-02-18
Using a generalized RPA-type theory we calculate the in-plane anisotropy of the magnetic excitations in hole-doped high-Tc superconductors. Extending our earlier Fermi-liquid-based studies on the resonance peak by inclusion of orthorhombicity we still find two-dimensional spin excitations, however, being strongly anisotropic. This reflects the underlying anisotropy of the hopping matrix elements and of the resultant superconducting gap function. We compare our calculations with new experimental data on fully untwinned YBa2Cu3O6.85 and find good agreement. Our results are in contrast to earlier interpretations on the in-plane anisotropy in terms of stripes [H. Mook, Nature (London) 404, 729 (2000)], but reveal a conventional solution to this important problem.
SivaRanjan, Uppala; Ramachandran, Ramesh
2014-02-07
A quantum-mechanical model integrating the concepts of reduced density matrix and effective Hamiltonians is proposed to explain the multi-spin effects observed in rotational resonance (R{sup 2}) nuclear magnetic resonance (NMR) experiments. Employing this approach, the spin system of interest is described in a reduced subspace inclusive of its coupling to the surroundings. Through suitable model systems, the utility of our theory is demonstrated and verified with simulations emerging from both analytic and numerical methods. The analytic results presented in this article provide an accurate description/interpretation of R{sup 2} experimental results and could serve as a test-bed for distinguishing coherent/incoherent effects in solid-state NMR.
GEMS (Gravity Electro-Magnetism Strong) SU(5) Theory and The Prediction of Exchange Boson Masses
NASA Astrophysics Data System (ADS)
Brandenburg, John
2012-10-01
The GEMS SU(5) [1] theory includes short range Nuclear Forces in the GEM unification theory [2], where the importance of the square root of the proton-electron mass ratio: σ = 42.8503 was found. The creation of mass by a Higgs field coupling must, by the Equivalence Principle, be viewed in the context of General Relativity. This is done here using Kaluza-Klein theory in a Feynman-Hawkings path integral formalism. GEM theory, quantum concepts of virtual particles, and ZPF (Zero Point Fluctuation) allow understanding of the Strong Force and Weak forces as the extension of electrodynamics in the quantum limit. The Strong and Weak forces are found to be associated with EM models of the electron and proton as finite sized structures respectively. Higher order Mie resonances off the EM ``mass at a distance'' structures associated with the electron, proton and fifth dimension generate the quanta with masses of the pion mπ = 2 me /α 140.0 MeV and Z boson: mZ = 2σ mp = 80.4 GeV. The ηc meson mη = 2980 GeV is identified with the 5^th dimension compactification force mediated by the Radion field. Another particle associated with this mass inducing field is the ``Radion'' or Higgs scattering quanta off the fifth dimension with a mass σmη 128.6 GeV which is the Higgs Boson. A GEMS SU(5) Georgi-Glashow model, is proposed, where the unification energy is now the Planck energy.[0pt] [1] Brandenburg, J.E. (2012)., STAIF II Conference Albuquerque NM[0pt] [2] Brandenburg, J.E. (2007). IEEE Transactions On Plasma Science, Vol. 35, No. 4., p845.
NASA Technical Reports Server (NTRS)
Barnes, A.
1979-01-01
An exact solution of the kinetic and electromagnetic equations for a large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma is presented. The solution gives simple relations among the magnetic-field strength, density, stress tensor, and plasma velocity, all of which are measurable in the interplanetary plasma. These relations are independent of the electron and ion velocity distributions, subject to certain restrictions on 'high-velocity tails.' The magnetic field of the wave is linearly polarized. The wave steepens to form a shock much as the analogous waves of MHD theory do.
Wilkin, N.K.; Moore, M.A. )
1993-08-01
We have recently studied the magnetization and specific heat for two- and three-dimensional superconductors, starting from Landau-Ginzburg theory. In this paper we compare the results of this study with available experimental data. All our results have been calculated for a strong magnetic field along the [ital c] axis in the region near the [ital H][sub [ital c]2] line where the lowest-Landau-level approximation is valid. We show that the theoretical two-dimensional magnetization compares very well with that measured for the high-[ital T][sub [ital c
Afshar, Mahdi; Sargolzaei, Mohsen
2013-11-15
We have demonstrated electronic structure and magnetic properties of Cu{sub 3}, Ag{sub 3} and Au{sub 3} trimers using a full potential local orbital method in the framework of relativistic density functional theory. We have also shown that the non-relativistic generalized gradient approximation for the exchange-correlation energy functional gives reliable magnetic properties in coinage metal trimers compared to experiment. In addition we have indicated that the spin-orbit coupling changes the structure and magnetic properties of gold trimer while the structure and magnetic properties of copper and silver trimers are marginally affected. A significant orbital moment of 0.21μ{sub B} was found for most stable geometry of the gold trimer whereas orbital magnetism is almost quenched in the copper and silver trimers.
Afshar, Mahdi; Sargolzaei, Mohsen
2013-11-15
We have demonstrated electronic structure and magnetic properties of Cu{sub 3}, Ag{sub 3} and Au{sub 3} trimers using a full potential local orbital method in the framework of relativistic density functional theory. We have also shown that the non-relativistic generalized gradient approximation for the exchange-correlation energy functional gives reliable magnetic properties in coinage metal trimers compared to experiment. In addition we have indicated that the spin-orbit coupling changes the structure and magnetic properties of gold trimer while the structure and magnetic properties of copper and silver trimers are marginally affected. A significant orbital moment of 0.21μ{sub B} was found for most stable geometry of the gold trimer whereas orbital magnetism is almost quenched in the copper and silver trimers.
NASA Astrophysics Data System (ADS)
Arefi, Mohammad; Zenkour, Ashraf M.
2016-11-01
In this paper, based on the sinusoidal shear deformation plate theory, equations of motion for a sandwich nanoplate containing a nano core and two integrated piezo-magnetic face-sheets are derived. The piezo-magnetic face-sheets are subjected to three dimensional electric and magnetic potentials. Nonlocal piezo-magneto-elastic relations are derived in a thermal environment. Hamilton’s principle is used to derive seven equations of motion in terms of three deformation components of mid-surface, two shear components and electric and magnetic potentials. Natural frequencies of the sandwich nanoplate are derived in terms of nonlocal parameter. After finding solutions to the governing equations of motion, the effect of important parameters of the nanoplate are investigated on the mechanical, electrical and magnetic components of the nanoplate. Based on the present study, with increasing applied electric potential, dimensionless deflection is decreased and maximum electric and magnetic potentials are increased. Furthermore, with increasing applied magnetic potential, deflection is increased and maximum electric and magnetic potentials are decreased significantly. The numerical results of this problem indicate that one can control deformation or stress in the nano structure by changing the applied electric and magnetic potentials.
Theory of Transition-Metal Substitutions in High-Temperature Rare Earth-Cobalt Magnets
NASA Astrophysics Data System (ADS)
Kashyap, A.; Skomski, R.; Sabiryanov, R.; Jaswal, S. S.; Sellmyer, D. J.
2003-03-01
The effect of transition-metal substitutions in 1:5 and 2:17 intermetallics for high-temperature applications is investigated by first-principle and model calculations. Self consistent calculations of magnetic moments and exchange-coupling constants are performed for a variety of Y(Co_1-xT_x)5 and Y_2(Co_1-xT_x)17 compounds (T = Cu, Ti and Zr) using a local density functional approach. The substitutional atoms carry only a small moment and reduce the interatomic exchange of the transition-metal sublattice. However, the strength of the effect depends on the site where the substitution occurs. Furthermore, in all cases the intrasublattice exchange exhibits some anisotropy. A transparent example is pure YCo_5, where the interatomic transition-metal exchange in the rare-earth transition-metal planes is only about 155 K, as compared to the corresponding interplane exchange of 757 K. Model calculations show that the exchange reduction has a drastic effect on the finite-temperature anisotropy of the phases, because it also influences the net rare-earth transition-metal intersublattice exchange. The reduced interatomic exchange can therefore be regarded as the atomic origin of the anomalous temperature dependence of the coercivity of advanced Sm-Co based high-temperature magnets. This research is supported by AFOSR, DOE, NSF, MRSEC(DMR-0213808), and CMRA.
Electronic and magnetic properties of iron doped zirconia: Theory and experiment
Debernardi, A. Sangalli, D.; Lamperti, A.; Cianci, E.; Lupo, P.; Casoli, F.; Albertini, F.; Nasi, L.
2014-05-07
We systematically investigated, both theoretically and experimentally, Zr{sub 1−x}Fe{sub x}O{sub 2−y} ranging from diluted (x ≈ 0.05) up to large (x ≈ 0.25) Fe concentration. By atomic layer deposition, we grew thin films of high-κ zirconia in cubic phase with Fe uniformly distributed in the film, as proven by time of flight secondary ion mass spectrometry and transmission electron microscopy measurements. Iron is in Fe{sup 3+} oxidation state suggesting the formation of oxygen vacancies with y concentration close to x/2. By ab-initio simulations, we studied the phase diagram relating the stability of monoclinic vs. tetragonal phase as a function of Fe doping and film thickness: the critical thickness at which the pure zirconia is stabilized in the tetragonal phase is estimated ranging from 2 to 6 nm according to film morphology. Preliminary results by X-ray magnetic circular dichroism and alternating gradient force magnetometry are discussed in comparison to ab initio data enlightening the role of oxygen vacancies in the magnetic properties of the system.
Magnetic Compton scattering study of the Co2FeGa Heusler alloy: Experiment and theory
NASA Astrophysics Data System (ADS)
Deb, Aniruddha; Itou, M.; Sakurai, Y.; Hiraoka, N.; Sakai, N.
2001-02-01
The spin density in Co2FeGa Heusler alloy has been measured in a magnetic Compton scattering experiment using 274-keV circularly polarized synchrotron radiation at the high energy inelastic scattering beamline (BL08W) at SPring-8, Japan. A detailed band-structure calculation including hyperfine field study was performed utilizing the generalized gradient corrected full-potential linear augmented plane-wave (FLAPW-GGA) method. The magnetic Compton profiles for the [100], [110], and [111] principal directions, reported here, show anisotropy in the momentum density which is in good agreement with the FLAPW-GGA results based on ferromagnetic ground state. The conduction electrons were found to have a negative spin polarization of 0.60μB, which is at variance with the prediction of a positive moment from the recent neutron data. In the calculation, 3d spin moment at the Co and Fe site was found to be 1.20μB and 2.66μB, and their respective contribution in the eg and t2g sub-bands are in excellent agreement with the earlier reported neutron-diffraction measurements. It is also seen from our calculated results that the Co and Fe moment are mainly eg in character.
Electronic and magnetic properties of iron doped zirconia: Theory and experiment
NASA Astrophysics Data System (ADS)
Debernardi, A.; Sangalli, D.; Lamperti, A.; Cianci, E.; Lupo, P.; Casoli, F.; Albertini, F.; Nasi, L.; Ciprian, R.; Torelli, P.
2014-05-01
We systematically investigated, both theoretically and experimentally, Zr1-xFexO2-y ranging from diluted (x ≈ 0.05) up to large (x ≈ 0.25) Fe concentration. By atomic layer deposition, we grew thin films of high-κ zirconia in cubic phase with Fe uniformly distributed in the film, as proven by time of flight secondary ion mass spectrometry and transmission electron microscopy measurements. Iron is in Fe3+ oxidation state suggesting the formation of oxygen vacancies with y concentration close to x/2. By ab-initio simulations, we studied the phase diagram relating the stability of monoclinic vs. tetragonal phase as a function of Fe doping and film thickness: the critical thickness at which the pure zirconia is stabilized in the tetragonal phase is estimated ranging from 2 to 6 nm according to film morphology. Preliminary results by X-ray magnetic circular dichroism and alternating gradient force magnetometry are discussed in comparison to ab initio data enlightening the role of oxygen vacancies in the magnetic properties of the system.
Structural and magnetic properties of TlTF3 (T=Fe, Co and Ni) by hybrid functional theory
NASA Astrophysics Data System (ADS)
Zeb, Raham; Ali, Zahid; Ahmad, Iftikhar; Khan, Imad
2015-08-01
DFT studies are performed to investigate the structural, mechanical and magneto-electronic properties of the TlTF3 (T=Fe, Co and Ni) perovskites for the first time using GGA, GGA+U and hybrid density functional theory (HF). Our calculations show that HF give better results than GGA and GGA+U and more consistent with the experiments. The comparison of the lattice constants calculated by HF with experiments shows a maximum underestimation less than 0.2%. The chemical bonding between different ions in these compounds is explained on the bases of electronic clouds, which reveals that in TlFeF3, Fe has more ionic character with F than the rest. The mechanical properties explain the hardness of these compounds and show that TlFeF3 is more ductile. Spin-dependent electronic band profiles show that TlFeF3 and TlCoF3 are metallic, whereas TlNiF3 is pseudo direct wide bandgap semiconductor. The stable magnetic phase optimizations and the calculated magnetic susceptibility confirm that TlFeF3 and TlNiF3 are ferromagnetic whereas TlCoF3 is anti-ferromagnetic material.
Xie, L; Wang, P; Pan, X Q
2014-08-01
The novel discovery of electron vortices carrying quantized orbital angular momentum motivated intensive research of their basic properties as well as applications, e.g. structural characterization of magnetic materials. In this paper, the fundamental interactions of electron vortices within infinitely long atomic-column-like electromagnetic fields are studied based on the relativistically corrected Pauli-Schrödinger equation and the perturbation theory. The relative strengths of three fundamental interactions, i.e. the electron-electric potential interaction, the electron-magnetic potential/field interaction and the spin-orbit coupling are discussed. The results suggest that the perturbation energies of the last two interactions are in an order of 10(3)-10(4) smaller than that of the first one for electron vortices. In addition, it is also found that the strengths of these interactions are strongly dependant on the spatial distributions of the electromagnetic field as well as the electron vortices. Copyright © 2014 Elsevier Ltd. All rights reserved.
Nekrasov, Anatoly K.; Shadmehri, Mohsen E-mail: mshadmehri@thphys.nuim.i
2010-12-01
We develop a general theory of buoyancy instabilities in the electron-ion plasma with the electron heat flux based not upon magnetohydrodynamic (MHD) equations, but using a multicomponent plasma approach in which the momentum equation is solved for each species. We investigate the geometry in which the background magnetic field is perpendicular to the gravity and stratification. General expressions for the perturbed velocities are given without any simplifications. Collisions between electrons and ions are taken into account in the momentum equations in a general form, permitting us to consider both weakly and strongly collisional objects. However, the electron heat flux is assumed to be directed along the magnetic field, which implies a weakly collisional case. Using simplifications justified for an investigation of buoyancy instabilities with electron thermal flux, we derive simple dispersion relations for both collisionless and collisional cases for arbitrary directions of the wave vector. Our dispersion relations considerably differ from that obtained in the MHD framework and conditions of instability are similar to Schwarzschild's criterion. This difference is connected with simplified assumptions used in the MHD analysis of buoyancy instabilities and with the role of the longitudinal electric field perturbation which is not captured by the ideal MHD equations. The results obtained can be applied to clusters of galaxies and other astrophysical objects.
Wong, P.S.; Sastre, A.
1995-10-01
The goal of this study was to obtain data that could be used to evaluate the applicability of ``resonance`` theories of biological effects in residential settings. The authors first describe a measurement system which allows the study of ac and dc magnetic fields simultaneously in space and in time. Sample measurements were taken near two power lines, two objects and in two residential homes. The results show that the earth`s (dc) magnetic field was unaffected near power lines. The compass orientation of the power line influenced the relative values of the ac components parallel and perpendicular to the dc field. The electric heating system greatly affected the ac field levels in the home, causing the levels to increase from less than 1 mG to a maximum of 7.5 mG during heating. The magnitudes of the dc field in the two homes varied from about 380 to 650 mG, with the larger variations near metallic or magnetic objects such as the refrigerator or a metallic air duct. The earth`s field was elevated above its natural level within a distance of 8 feet from a subcompact passenger car, e.g., the level changed from about 540 to 1,100 mG beside the headlight. A steel chair changed the earth`s field by up to 60 mG within a distance of one foot. These results suggest that some of the narrow ``resonances`` described in laboratory studies may be difficult to observe against the variations in do field amplitude and direction resulting from the presence of everyday metallic objects.
Tkach, M.; Seti, Ju.; Voitsekhivska, O.; Fartushynsky, R.
2009-12-14
The quasi-stationary electron states are studied in the three-barrier resonance-tunnel structure which is the basic element of coherent quantum cascade lasers. In the models of rectangular and delta-barrier potentials there is established theory of evolution and collapse of double resonance complexes in a symmetric resonance-tunnel structure. The induced conductivity of nano-system is calculated within the both models. It is shown that the negative induced conductivity of three-barrier resonance-tunnel structure in delta-barrier model is dozens times smaller than more realistic magnitudes obtained within the rectangular potentials model.
Mikhailenko, V. V. Mikhailenko, V. S.; Lee, Hae June
2015-10-15
The developed kinetic theory for the stability of a magnetic-field-aligned (parallel) shear flow with inhomogeneous ion temperature [Mikhailenko et al., Phys. Plasmas 21, 072117 (2014)] predicted that a kinetic instability arises from the coupled reinforcing action of the flow velocity shear and ion temperature gradient in the cases where comparable ion and electron temperatures exist. In the present paper, the nonlinear theory was developed for the instability caused by the combined effects of ion-temperature-gradient and shear-flow (ITG–SF). The level of the electrostatic turbulence is determined for the saturation state of the instability on the basis of the nonlinear dispersion equation, which accounts for a nonlinear scattering of ions by the developed turbulence in a sheared flow. The renormalized quasilinear equation for the ion distribution function, which accounts for the turbulent scattering of ions by ITG–SF driven turbulence, was derived and employed for the estimation of the turbulent ion viscosity, the anomalous ion thermal conductivity, and anomalous ion heating rate at the saturation state of the instability.
NASA Astrophysics Data System (ADS)
Petitjean, Cyril; Luc, David; Waintal, Xavier
2012-09-01
Spins transverse to the magnetization of a ferromagnet only survive over a short distance. We develop a drift-diffusion approach that captures the main features of transverse spin effects in systems with arbitrary spin textures (e.g., vortices and domain walls) and generalizes the Valet-Fert theory. In addition to the standard characteristic lengths (mean free path for majority and minority electrons, and spin diffusion length), the theory introduces two length scales, the transverse spin coherence length ℓ⊥ and the (Larmor) spin precession length ℓL. We show how ℓL and ℓ⊥ can be extracted from ab initio calculations or measured with giant magnetoresistance experiments. In long (adiabatic) domain walls, we provide an analytic formula that expresses the so-called “nonadiabatic” (or fieldlike) torque in terms of these length scales. However, this nonadiabatic torque is no longer a simple material parameter but depends on the actual spin texture: in thin (<10nm) domain walls, we observe very significant deviations from the adiabatic limit.
Hamoud Al-Tamimi, Mohammed Sabbih; Sulong, Ghazali; Shuaib, Ibrahim Lutfi
2015-07-01
Resection of brain tumors is a tricky task in surgery due to its direct influence on the patients' survival rate. Determining the tumor resection extent for its complete information via-à-vis volume and dimensions in pre- and post-operative Magnetic Resonance Images (MRI) requires accurate estimation and comparison. The active contour segmentation technique is used to segment brain tumors on pre-operative MR images using self-developed software. Tumor volume is acquired from its contours via alpha shape theory. The graphical user interface is developed for rendering, visualizing and estimating the volume of a brain tumor. Internet Brain Segmentation Repository dataset (IBSR) is employed to analyze and determine the repeatability and reproducibility of tumor volume. Accuracy of the method is validated by comparing the estimated volume using the proposed method with that of gold-standard. Segmentation by active contour technique is found to be capable of detecting the brain tumor boundaries. Furthermore, the volume description and visualization enable an interactive examination of tumor tissue and its surrounding. Admirable features of our results demonstrate that alpha shape theory in comparison to other existing standard methods is superior for precise volumetric measurement of tumor.
Theory of Magnetic Response and Hall Effect in Bulk Rashba System
NASA Astrophysics Data System (ADS)
Ando, Tsuneya; Suzuura, Hidekatsu
2017-01-01
The magnetic susceptibility and transport quantities are calculated in a three-dimensional system with a giant Rashba spin-orbit interaction for scatterers with short-range potential in a self-consistent Born approximation. The susceptibility exhibits a sharp peak toward the diamagnetic direction at the band crossing, being broadened depending on disorder. No special feature appears in the diagonal conductivity in both in-plane and vertical directions. The orbital Hall conductivity due to cyclotron motion is considerably reduced from -(nec)-1 in the low-energy region and exhibits a small step-like jump at the band crossing, becoming almost the same as -(nec)-1, where n is the carrier concentration. The Hall conductivity due to the spin-Zeeman energy has a sharp peak at the band crossing, but is significantly reduced due to inter-band scattering.
Quasi-optical theory of microwave plasma heating in open magnetic trap
NASA Astrophysics Data System (ADS)
Shalashov, A. G.; Balakin, A. A.; Gospodchikov, E. D.; Khusainov, T. A.
2016-11-01
Microwave heating of a high-temperature plasma confined in a large-scale open magnetic trap, including all important wave effects like diffraction, absorption, dispersion, and wave beam aberrations, is described for the first time within the first-principle technique based on consistent Maxwell's equations. With this purpose, the quasi-optical approach is generalized over weakly inhomogeneous gyrotrotropic media with resonant absorption and spatial dispersion, and a new form of the integral quasi-optical equation is proposed. An effective numerical technique for this equation's solution is developed and realized in a new code QOOT, which is verified with the simulations of realistic electron cyclotron heating scenarios at the Gas Dynamic Trap at the Budker Institute of Nuclear Physics (Novosibirsk, Russia).
NASA Astrophysics Data System (ADS)
Wang, Xiao; Lian, Jie; Li, Ping; Xu, XiJin; Li, MengMeng
2017-01-01
The Fresnel equations are solved to analyze the reflection and propagation properties of the ordinary and extraordinary light of the optically anisotropic magnetic film. Using the boundary and propagation matrix, the longitudinal magneto-optical Kerr rotation expression is derived. After that, simulations are performed on optically anisotropic and isotropic Co/SiO2 film. Results show that for Co material in the thin-film limit, the anisotropic Co can provide larger max rotations than the isotropic Co in the visible region. This is because that the refractive index discrepancy of optically anisotropic Co film reduces the Fresnel reflective coefficient rpp, which improves the Kerr rotation. This makes the optically anisotropic Co film more effective in magneto optical sensor design and device fabrication.
NASA Technical Reports Server (NTRS)
Sornchamni, T.; Jovanovic, G. N.; Reed, B. P.; Atwater, J. E.; Akse, J. R.; Wheeler, R. R.
2004-01-01
The conversion of solid waste into useful resources in support of long duration manned missions in space presents serious technological challenges. Several technologies, including supercritical water oxidation, microwave powered combustion and fluidized bed incineration, have been tested for the conversion of solid waste. However, none of these technologies are compatible with microgravity or hypogravity operating conditions. In this paper, we present the gradient magnetically assisted fluidized bed (G-MAFB) as a promising operating platform for fluidized bed operations in the space environment. Our experimental and theoretical work has resulted in both the development of a theoretical model based on fundamental principles for the design of the G-MAFB, and also the practical implementation of the G-MAFB in the filtration and destruction of solid biomass waste particles from liquid streams. c2004 COSPAR. Published by Elsevier Ltd. All rights reserved.
NASA Technical Reports Server (NTRS)
Sornchamni, T.; Jovanovic, G. N.; Reed, B. P.; Atwater, J. E.; Akse, J. R.; Wheeler, R. R.
2004-01-01
The conversion of solid waste into useful resources in support of long duration manned missions in space presents serious technological challenges. Several technologies, including supercritical water oxidation, microwave powered combustion and fluidized bed incineration, have been tested for the conversion of solid waste. However, none of these technologies are compatible with microgravity or hypogravity operating conditions. In this paper, we present the gradient magnetically assisted fluidized bed (G-MAFB) as a promising operating platform for fluidized bed operations in the space environment. Our experimental and theoretical work has resulted in both the development of a theoretical model based on fundamental principles for the design of the G-MAFB, and also the practical implementation of the G-MAFB in the filtration and destruction of solid biomass waste particles from liquid streams. c2004 COSPAR. Published by Elsevier Ltd. All rights reserved.
Theory for Magnetism and Triplet Superconductivity in LiFeAs
NASA Astrophysics Data System (ADS)
Daghofer, Maria; Brydon, Philip; Timm, Carsten; van den Brink, Jeroen
2011-03-01
Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet p -wave superconductivity. P.M.R. Brydon, M. Daghofer, C. Timm, and J. van den Brink, arXiv:1009.3104 Support from the DFG under the priority program 1458 and the Emmy-Noether program.
Gerber, Bernhard L; Raman, Subha V; Nayak, Krishna; Epstein, Frederick H; Ferreira, Pedro; Axel, Leon; Kraitchman, Dara L
2008-04-28
In less than two decades, first-pass perfusion cardiovascular magnetic resonance (CMR) has undergone a wide range of changes with the development and availability of improved hardware, software, and contrast agents, in concert with a better understanding of the mechanisms of contrast enhancement. The following review provides a perspective of the historical development of first-pass CMR, the developments in pulse sequence design and contrast agents, the relevant animal models used in early preclinical studies, the mechanism of artifacts, the differences between 1.5T and 3T scanning, and the relevant clinical applications and protocols. This comprehensive overview includes a summary of the past clinical performance of first-pass perfusion CMR and current clinical applications using state-of-the-art methodologies.
Gerber, Bernhard L; Raman, Subha V; Nayak, Krishna; Epstein, Frederick H; Ferreira, Pedro; Axel, Leon; Kraitchman, Dara L
2008-01-01
In less than two decades, first-pass perfusion cardiovascular magnetic resonance (CMR) has undergone a wide range of changes with the development and availability of improved hardware, software, and contrast agents, in concert with a better understanding of the mechanisms of contrast enhancement. The following review provides a perspective of the historical development of first-pass CMR, the developments in pulse sequence design and contrast agents, the relevant animal models used in early preclinical studies, the mechanism of artifacts, the differences between 1.5T and 3T scanning, and the relevant clinical applications and protocols. This comprehensive overview includes a summary of the past clinical performance of first-pass perfusion CMR and current clinical applications using state-of-the-art methodologies. PMID:18442372
Sornchamni, T; Jovanovic, G N; Reed, B P; Atwater, J E; Akse, J R; Wheeler, R R
2004-01-01
The conversion of solid waste into useful resources in support of long duration manned missions in space presents serious technological challenges. Several technologies, including supercritical water oxidation, microwave powered combustion and fluidized bed incineration, have been tested for the conversion of solid waste. However, none of these technologies are compatible with microgravity or hypogravity operating conditions. In this paper, we present the gradient magnetically assisted fluidized bed (G-MAFB) as a promising operating platform for fluidized bed operations in the space environment. Our experimental and theoretical work has resulted in both the development of a theoretical model based on fundamental principles for the design of the G-MAFB, and also the practical implementation of the G-MAFB in the filtration and destruction of solid biomass waste particles from liquid streams. c2004 COSPAR. Published by Elsevier Ltd. All rights reserved.
Theory of bosons in two-leg ladders with large magnetic fields
NASA Astrophysics Data System (ADS)
Wei, Ran; Mueller, Erich J.
2014-06-01
We calculate the ground state of a Bose gas trapped on a two-leg ladder where Raman-induced hopping mimics the effect of a large magnetic field. In the mean-field limit, where there are large numbers of particles per site, this maps onto a uniformly frustrated two-leg ladder classical spin model. The net particle current always vanishes in the ground state, but generically there is a finite "chiral current," corresponding to equal and opposite flow on the two legs. We vary the strength of the hopping across the rungs of the ladder and the interaction between the bosons. We find the following three phases. (1) A "saturated chiral current phase" (SCCP), where the density is uniform and the chiral current is simply related to the strength of the magnetic field. In this state the only broken symmetry is the U(1) condensate phase. (2) A "biased ladder phase" (BLP), where the density is higher on one leg than the other. The fluid velocity is higher on the lower density leg, so the net current is zero. In addition to the U(1) condensate phase, this has a broken Z2 reflection symmetry. (3) A "modulated density phase" (MDP), where the atomic density is modulated along the ladder. In addition to the U(1) condensate phase, this has a second broken U(1) symmetry corresponding to translations of the density wave. We further study the fluctuations of the condensate in the BLP, finding a roton-maxon-like excitation spectrum. Decreasing the hopping along the rungs softens the spectrum. As the energy of the "roton" reaches to zero, the BLP becomes unstable. We describe the experimental signatures of these phases, including the response to changing the frequency of the Raman transition.
Theory and modelling of quench in cable-in-conduit superconducting magnets
Shajii, A.
1994-04-01
A new simple, self consistent theoretical model is presented that describes the phenomena of quench propagation in Cable-In-Conduit superconducting magnets. The model (Quencher) circumvents many of the difficulties associated with obtaining numerical solutions in more general existing models. Specifically, a factor of 30-50 is gained in CPU time over the general, explicit time dependent codes used to study typical quench events. The corresponding numerical implementation of the new model is described and the numerical results are shown to agree very well with those of the more general models, as well as with experimental data. Further, well justified approximations lead to the MacQuench model that is shown to be very accurate and considerably more efficient than the Quencher model. The MacQuench code is suitable for performing quench studies on a personal computer, requiring only several minutes of CPU time. In order to perform parametric studies on new conductor designs it is required to utilize a model such as MacQuench because of the high computational efficiency of this model. Finally, a set of analytic solutions for the problem of quench propagation in Cable-In-Conduit Conductors is presented. These analytic solutions represent the first such results that remain valid for the long time scales of interest during a quench process. The assumptions and the resulting simplifications that lead to the analytic solutions are discussed, and the regimes of validity of the various approximations are specified. The predictions of the analytic results are shown to be in very good agreement with numerical as well as experimental results. Important analytic scaling relations are verified by such comparisons, and the consequences of some of these scalings on currently designed superconducting magnets are discussed.
Ionospheric effects of the March 1990 magnetic storm: Comparison of theory and measurement
Richards, P.G.; Torr, D.G.; Buonsanto, M.J.; Sipler, D.P.
1994-12-01
This paper presents a comparison of the measured and modeled inospheric response to magnetic storms at Millstone Hill and Arecibo during March 16-23, 1990. Magnetic activity was low until midday UT on day 18 when Kp reached 6, days 19 and 20 were quiet, but a large storm occurred around midnight UT on day 20 (Kp=7) and it was moderately disturbed (Kp=4) for the remainder of the study period. At Millstone Hill, the daytime peak electron density (NmF{sub 2}) showed only a modest 30% decrease in response to the first storm and recovered to prestorm values before the onset of the second storm. The model reproduces the daytime peak electron density well for this period. However, the severe storm on March 20 caused a factor of 4 depletion in electron density, while the model densities were not greatly affected. The inclusion of vibrationally excited nitrogen (N{sub 2}) in the model was unable to account for the observed large electron density depletions afterward March 20. The storm did not appear to affect the overall magnitude of the electron density at Arecibo very much, but did cause unusual wavelike structure in the peak density and peak height following the storm. The model reproduces the daytime NmF{sub 2} very well for Arecibo, but after sunset the model densities decay too rapidly. This study indicates that successful modeling of severe ionospheric storms will require better definition of the storm time inputs, especially of the neutral atmosphere. 26 refs., 10 figs.
NASA Technical Reports Server (NTRS)
Hesse, Michael; Birn, Joachim; Schindler, Karl
1990-01-01
A self-consistent two-fluid theory that includes the magnetic field and shear patterns is developed to model stationary electrostatic structures with field-aligned potential drops. Shear flow is also included in the theory since this seems to be a prominent feature of the structures of interest. In addition, Ohmic dissipation, a Hall term, and pressure gradients in a generalized Ohm's law, modified for cases without quasi-neutrality, are included. In the analytic theory, the electrostatic force is balanced by field-aligned pressure gradients (i.e., thermal effects in the direction of the magnetic field) and by pressure gradients and magnetic stresses in the perpendicular direction. Within this theory, simple examples of applications are presented to demonstrate the kind of solutions resulting from the model. The results show how the effects of charge separation and shear in the magnetic field and the velocity can be combined to form self-consistent structures such as are found to exist above the aurora, suggested also in association with solar flares.
The large-scale properties of simulated cosmological magnetic fields
NASA Astrophysics Data System (ADS)
Marinacci, Federico; Vogelsberger, Mark; Mocz, Philip; Pakmor, Rüdiger
2015-11-01
We perform uniformly sampled large-scale cosmological simulations including magnetic fields with the moving mesh code AREPO. We run two sets of MHD simulations: one including adiabatic gas physics only; the other featuring the fiducial feedback model of the Illustris simulation. In the adiabatic case, the magnetic field amplification follows the B ∝ ρ2/3 scaling derived from `flux-freezing' arguments, with the seed field strength providing an overall normalization factor. At high baryon overdensities the amplification is enhanced by shear flows and turbulence. Feedback physics and the inclusion of radiative cooling change this picture dramatically. In haloes, gas collapses to much larger densities and the magnetic field is amplified strongly and to the same maximum intensity irrespective of the initial seed field of which any memory is lost. At lower densities a dependence on the seed field strength and orientation, which in principle can be used to constrain models of cosmic magnetogenesis, is still present. Inside the most massive haloes magnetic fields reach values of ˜ 10-100 μG, in agreement with galaxy cluster observations. The topology of the field is tangled and gives rise to rotation measure signals in reasonable agreement with the observations. However, the rotation measure signal declines too rapidly towards larger radii as compared to observational data.
Meinert, Markus; Schmalhorst, Jan-Michael; Klewe, Christoph; Reiss, Guenter; Arenholz, Elke; Boehnert, Tim; Nielsch, Kornelius
2011-10-01
Epitaxial thin films of the half-metallic X{sub a} compound Mn{sub 2}CoGa (Hg{sub 2}CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of 1.95(5) {mu}{sub B} per unit cell were obtained. The L{sub 3,2} x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.
Analytical theory of BAE gap modification due to a magnetic island
NASA Astrophysics Data System (ADS)
Cook, Carson; Hegna, Chris; Spong, Donald
2014-10-01
The Beta-induced Alfven Eigenmode (BAE) gap is a break in the frequencies of the shear Alfven continuum. This gap is important because a discrete Alfven eigenmode can exist within the gap frequency range and will not be affected by continuum damping. In order for the BAE gap to appear, finite beta and curvature effects must be present. Under these conditions, there is a coupling between the equation for shear Alfven waves involving inertia and bending energy terms and the sound wave equation. The presence of a magnetic island has been shown to cause an upshift in the BAE gap frequency. In the absence of an island the minimum of the continuum frequencies is located at the resonant rational surface; the island moves the location of the minimum to the island separatrix as a result of the coupling between helical mode numbers. The physical mechanism for this shift will be described employing analytical modeling. The shear Alfven spectrum is obtained globally through analytical methods, inside and outside the separatrix, for the first time. A WKB approximation is used in this analysis, and good agreement is found with previous numerical results. Research supported by the U.S. DOE under Grants DE-FG02-99ER54546 and DE-SC0006103.
Investigation of Linear Stability Theory for Wavy Interface in Magnetic Pulse Welding
NASA Astrophysics Data System (ADS)
Nassiri, Ali; Chini, Gregory; Kinsey, Brad
2012-11-01
Magnetic Pulse Welding (MPW) is a solid state, high strain-rate joining process in which a weld of dissimilar or similar materials can be created via high-speed oblique impact of two workpieces. MPW is a lap welding method: the two workpieces are placed in a roughly parallel configuration with a small gap between them to achieve high impact velocity and pressure. Intriguingly, experiments routinely show the emergence of a distinctive wavy pattern, with a well defined amplitude and wavelength of approximately 20 and 70 micrometers, respectively, at the interface between the two welded materials. The mechanism underlying this wavy pattern is still not well understand. Some researchers have proposed that the interfacial waves are formed in a process akin to Kelvin-Helmholtz instability, with relative shear movement of the flyer and base plates providing an energy source for the vortical pattern. Here, we employ a linear stability analysis to investigate whether the wavy pattern could be the signature of a shear-driven high strain-rate instability of a perfectly plastic solid material. Preliminary results confirm that an instability giving rise to a wavy interfacial pattern is possible.
Covariant Spectator Theory of np scattering: Deuteron magnetic moment and form factors
Gross, Franz L.
2014-06-01
The deuteron magnetic moment is calculated using two model wave functions obtained from 2007 high precision fits to $np$ scattering data. Included in the calculation are a new class of isoscalar $np$ interaction currents which are automatically generated by the nuclear force model used in these fits. After normalizing the wave functions, nearly identical predictions are obtained: model WJC-1, with larger relativistic P-state components, gives 0.863(2), while model WJC-2 with very small $P$-state components gives 0.864(2) These are about 1\\% larger than the measured value of the moment, 0.857 n.m., giving a new prediction for the size of the $\\rho\\pi\\gamma$ exchange, and other purely transverse interaction currents that are largely unconstrained by the nuclear dynamics. The physical significance of these results is discussed, and general formulae for the deuteron form factors, expressed in terms of deuteron wave functions and a new class of interaction current wave functions, are given.
Application of Fermi scattering theory to a magnetically scanned electron linear accelerator.
Sandison, G A; Huda, W
1988-01-01
This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root-mean-square spatial Gaussian spread sigma (z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source-surface distance (SSD) for a 7-MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.
Application of Fermi scattering theory to a magnetically scanned electron linear accelerator
Sandison, G.A.; Huda, W.
1988-07-01
This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root-mean-square spatial Gaussian spread sigma (z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source-surface distance (SSD) for a 7-MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.
NASA Astrophysics Data System (ADS)
Pastor, G. M.; Dorantes-Dávila, J.
2016-06-01
A spin-rotational-invariant approach to the spin-fluctuation theory of itinerant-electron magnetism is proposed and evaluated in the framework of a d -band model Hamiltonian including intra-atomic exchange interactions J and the coupling to a local magnetic field B . Using a vector-field Hubbard-Stratonovich transformation, we obtain a static approximation to the density matrix operator from which the equilibrium properties are directly derived. The method is applied to a single-site model taking Fe as a representative example. Exact and approximate analytical results are given for the local magnetic moments, their longitudinal and transversal components, the field-induced magnetizations, entropy, and heat capacity. Goals and limitations of various approximations are discussed as a function of J ,B , and temperature. The quantum-mechanical origin of some important drawbacks found in previous vector-field static approaches is identified. The significant improvements achieved with the static density operator are demonstrated.
Theory of solid-state photo-CIDNP in the earth's magnetic field.
Jeschke, Gunnar; Anger, Ben C; Bode, Bela E; Matysik, Jörg
2011-09-08
To date, solid-state photo-CIDNP experiments have been performed only using magic angle spinning NMR in a high-field regime, which is not associated with physiologically relevant spin dynamics. Here, we predict that nuclear spin polarization up to 10%, almost 9 orders of magnitude larger than thermal equilibrium polarization, can arise in cyclic photoreactions at the earth field due to a coherent three-spin mixing mechanism in the S-T(-) or S-T(+) manifold. The effect is maximal at a distance of about 30 Å between the two radicals, which nearly coincides with the separation between the donor and secondary acceptor in natural photosynthetic reaction centers. Analytical expressions are given for a simple limiting case. Numerical computations for photosynthetic reaction centers show that many nuclei in the chromophores and their vicinity are likely to become polarized. The theory predicts that only modest hyperfine couplings of a few hundred kilohertz are required to generate polarization of more than 1% for radical-radical distances between 20 and 50 Å, that is, for a large number of radical pairs in electron-transfer proteins.
NASA Astrophysics Data System (ADS)
Varini, Nicola; Ceresoli, Davide; Martin-Samos, Layla; Girotto, Ivan; Cavazzoni, Carlo
2013-08-01
One of the most promising techniques used for studying the electronic properties of materials is based on Density Functional Theory (DFT) approach and its extensions. DFT has been widely applied in traditional solid state physics problems where periodicity and symmetry play a crucial role in reducing the computational workload. With growing compute power capability and the development of improved DFT methods, the range of potential applications is now including other scientific areas such as Chemistry and Biology. However, cross disciplinary combinations of traditional Solid-State Physics, Chemistry and Biology drastically improve the system complexity while reducing the degree of periodicity and symmetry. Large simulation cells containing of hundreds or even thousands of atoms are needed to model these kind of physical systems. The treatment of those systems still remains a computational challenge even with modern supercomputers. In this paper we describe our work to improve the scalability of Quantum ESPRESSO (Giannozzi et al., 2009 [3]) for treating very large cells and huge numbers of electrons. To this end we have introduced an extra level of parallelism, over electronic bands, in three kernels for solving computationally expensive problems: the Sternheimer equation solver (Nuclear Magnetic Resonance, package QE-GIPAW), the Fock operator builder (electronic ground-state, package PWscf) and most of the Car-Parrinello routines (Car-Parrinello dynamics, package CP). Final benchmarks show our success in computing the Nuclear Magnetic Response (NMR) chemical shift of a large biological assembly, the electronic structure of defected amorphous silica with hybrid exchange-correlation functionals and the equilibrium atomic structure of height Porphyrins anchored to a Carbon Nanotube, on many thousands of CPU cores.
Beyond Néel's theories: thermal demagnetization of narrow-band partial thermoremanent magnetizations
NASA Astrophysics Data System (ADS)
Dunlop, David J.; Özdemir, Özden
2001-10-01
Partial thermoremanent magnetization (pTRM) was imparted over a narrow temperature interval, T=370-350°C, to a suite of crushed and annealed natural magnetite samples, ranging in grain size from ≈1 to 125-150 μm and in domain state from small pseudo-single-domain (PSD) to multidomain (MD). In this way, effectively a single blocking temperature, TB, of pTRM was activated. Stepwise thermal demagnetization did not erase the pTRMs sharply at TB, as for single-domain (SD) grains. Demagnetization began well below 350°C and continued above 370°C, with a median unblocking temperature, TUB, close to 360°C. The largest grains deviated most from SD behavior. Their pTRM demagnetized over the entire interval from room temperature to the Curie point, in accordance with predictions for MD grains. In terms of the unblocking temperature distribution f( TUB) or slope d M/d T of the thermal demagnetization curve, SD grains have a sharp spectrum, TUB= TB; MD grains were observed to have a broad, roughly symmetrical spectrum centered on TB; and intermediate size grains in the PSD range had non-Gaussian spectra, combining a central peak near TB with broad tails above and below TB. In this respect, PSD grains display a superposition of SD and MD remanences, not a blend between the two. Practical implications of these observations are that Thellier's law of reciprocity ( TUB= TB) will be increasingly violated as grain size increases in the PSD range. The low- TUB part of f( TUB) produces anomalously large demagnetization of NRM in low-temperature heating steps of Thellier-type paleointensity determinations and a sagging shape of the Arai plot. The high- TUB part of f( TUB) results in undemagnetized remanence at and above TB in thermal demagnetization. Among pre-treatments designed to make remanence more SD-like in subsequent thermal cleaning, alternating field (AF) pre-cleaning sharpened f( TUB) more effectively than low-temperature demagnetization for the 20 μm sample.
NASA Astrophysics Data System (ADS)
Borkar, M. S.; Ameen, A.
2015-01-01
In this paper, Bianchi type VI0 magnetized anisotropic dark energy models with constant deceleration parameter have been studied by solving the Rosen's field equations in Bimetric theory of gravitation. The models corresponding to power law expansion and exponential law expansion have been evaluated and studied their nature geometrically and physically. It is seen that there is real visible matter (baryonic matter) suddenly appeared only for small interval of time 0.7 ≤ t < 0.7598 and for the remaining whole range of time t, there is dark energy matter in the universe. Our investigations are supported to the observational fact that the usual matter described by known particle theory is about 4% and the dark energy cause the accelerating expansion of the universe and several high precision observational experiments, especially the Wilkinson Microwave Anisotropic Probe (WMAP) satellite experiment (see [C. L. Bennett et al., Astrophys. J. Suppl. Ser. 148 (2003) 1; WMAP Collab. (D. N. Spergel et al.), Astrophys. J. Suppl. Ser. 148 (2003) 175; D. N. Spergel et al., Astrophys. J. Suppl. 170 (2007) 377; WMAP Collab. (E. Komastu et al.), Astrophys. J. Suppl. 180 (2009) 330; WMAP Collab. (G. Hinshaw et al.), Astrophys. J. Suppl. 208 (2013) 19; Plank Collab. (P. A. R. Ade), arXiv:1303.5076; arXiv:1303.5082]) conclude that the dark energy occupies near about 73% of the energy of the universe and dark matter is about 23%. In exponential law of expansion, our model is fully occupied by real visible matter and there is no chance of dark energy and dark matter.
Benjamin A. Frandsen; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J.; Staunton, Julie B.; Billinge, Simon J. L.
2016-05-11
Here, we present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ~1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. Furthermore, the Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.
Benjamin A. Frandsen; Brunelli, Michela; Page, Katharine; Uemura, Yasutomo J.; Staunton, Julie B.; Billinge, Simon J. L.
2016-05-11
Here, we present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ~1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. Furthermore, the Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.
Tscherbul, T V; Dalgarno, A
2010-11-14
An efficient method is presented for rigorous quantum calculations of atom-molecule and molecule-molecule collisions in a magnetic field. The method is based on the expansion of the wave function of the collision complex in basis functions with well-defined total angular momentum in the body-fixed coordinate frame. We outline the general theory of the method for collisions of diatomic molecules in the (2)Σ and (3)Σ electronic states with structureless atoms and with unlike (2)Σ and (3)Σ molecules. The cross sections for elastic scattering and Zeeman relaxation in low-temperature collisions of CaH((2)Σ(+)) and NH((3)Σ(-)) molecules with (3)He atoms converge quickly with respect to the number of total angular momentum states included in the basis set, leading to a dramatic (>10-fold) enhancement in computational efficiency compared to the previously used methods [A. Volpi and J. L. Bohn, Phys. Rev. A 65, 052712 (2002); R. V. Krems and A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)]. Our approach is thus well suited for theoretical studies of strongly anisotropic molecular collisions in the presence of external electromagnetic fields.
Dodziuk, Helena; Szymański, Sławomir; Jaźwiński, Jarosław; Marchwiany, Maciej E; Hopf, Henning
2010-09-30
Strained cyclophanes with small (-CH(2)-)(n) bridges connecting two benzene rings are interesting objects of basic research, mostly because of the nonplanarity of the rings and of interference of π-electrons of the latter. For title [3.3]paracyclophane, in solutions occurring in two interconverting cis and trans conformers, the published nuclear magnetic resonance (NMR) data are incomplete and involve its partially deuterated isotopomers. In this paper, variable-temperature NMR studies of its perprotio isotopomer combined with DFT quantum chemical calculations provide a complete characterization of the solution structure, NMR parameters, and interconversion of the cis and trans isomers of the title compound. Using advanced methods of spectral analysis, total quantitative interpretation of its proton NMR spectra in both the static and dynamic regimes is conducted. In particular, not only the geminal but also all of the vicinal J(HH) values for the bridge protons are determined, and for the first time, complete Arrhenius data for the interconversion process are reported. The experimental proton and carbon chemical shifts and the (n)J(HH), (1)J(CH), and (1)J(CC) coupling constants are satisfactorily reproduced theoretically by the values obtained from the density functional theory calculations.
Walsh, Aron; Wei, S.-H.; Yan Yanfa; Al-Jassim, M. M.; Turner, John A.; Woodhouse, Michael; Parkinson, B. A.
2007-10-15
A systematic study of nine binary and ternary spinel oxides formed from Co, Al, and Fe is presented by means of density functional theory. Analysis of the structural, magnetic, and electronic properties through the series of materials is carried out. Preference for the octahedral spinel sites are found in the order Fe
NASA Astrophysics Data System (ADS)
Aboud, Essam; El-Masry, Nabil; Qaddah, Atef; Alqahtani, Faisal; Moufti, Mohammed R. H.
2015-06-01
The Rahat volcanic field represents one of the widely distributed Cenozoic volcanic fields across the western regions of the Arabian Peninsula. Its human significance stems from the fact that its northern fringes, where the historical eruption of 1256 A.D. took place, are very close to the holy city of Al-Madinah Al-Monawarah. In the present work, we analyzed aeromagnetic data from the northern part of Rahat volcanic field as well as carried out a ground gravity survey. A joint interpretation and inversion of gravity and magnetic data were used to estimate the thickness of the lava flows, delineate the subsurface structures of the study area, and estimate the depth to basement using various geophysical methods, such as Tilt Derivative, Euler Deconvolution and 2D modeling inversion. Results indicated that the thickness of the lava flows in the study area ranges between 100 m (above Sea Level) at the eastern and western boundaries of Rahat Volcanic field and getting deeper at the middle as 300-500 m. It also showed that, major structural trend is in the NW direction (Red Sea trend) with some minor trends in EW direction.
Magnetic fields from phase transitions
NASA Astrophysics Data System (ADS)
Hindmarsh, Mark; Everett, Allen
1998-11-01
The generation of primordial magnetic fields from cosmological phase transitions is discussed, paying particular attention to the electroweak transition and to the various definitions of the ``average'' field that have been put forward. It is emphasized that only the volume average has dynamical significance as a seed for galactic dynamos. On rather general grounds of causality and energy conservation, it is shown that, in the absence of MHD effects that transfer power in the magnetic field from small to large scales, processes occurring at the electroweak transition cannot generate fields stronger than 10-20 G on a scale of 0.5 Mpc. However, it is implausible that this upper bound could ever be reached, as it would require all the energy in the Universe to be turned into a magnetic field coherent at the horizon scale. Non-linear MHD effects seem therefore to be necessary if the electroweak transition is to create a primordial seed field.
NASA Astrophysics Data System (ADS)
Rieder, Michael; Teyssier, Romain
2016-04-01
The origin and evolution of magnetic fields in the Universe is still an open question. Their observations in galaxies suggest strong magnetic fields already at high redshift as well as at present time. However, neither primordial magnetic fields nor battery processes can account for such high field strengths, which implies the presence of a dynamo process with rapid growth rates in high-redshift galaxies and subsequent maintenance against decay. We investigate the particular role played by feedback mechanisms in creating strong fluid turbulence, allowing for a magnetic dynamo to emerge. Performing magnetohydrodynamic simulations of isolated cooling gas haloes, we compare the magnetic field evolution for various initial field topologies and various stellar feedback mechanisms. We find that feedback can indeed drive strong gas turbulence and dynamo action. We see typical properties of Kolmogorov turbulence with a k-5/3 kinetic energy spectrum, as well as a small-scale dynamo, with a k3/2 magnetic energy spectrum predicted by Kazantsev dynamo theory. We also investigate simulations with a final quiescent phase. As turbulence decreases, the galactic fountain settles into a thin, rotationally supported disc. The magnetic field develops a large-scale well-ordered structure with even symmetry, which is in good agreement with magnetic field observations of nearby spirals. Our findings suggest that weak initial seed fields were first amplified by a small-scale dynamo during a violent feedback-dominated early phase in the galaxy formation history, followed by a more quiescent evolution, where the fields have slowly decayed or were maintained via large-scale dynamo action.
First-principles theory of electronic structure and magnetism of Cr nano-islands on Pd(1 1 1)
NASA Astrophysics Data System (ADS)
Carvalho de Melo Rodrigues, Debora; Pereiro, Manuel; Bergman, Anders; Eriksson, Olle; Burlamaqui Klautau, Angela
2017-01-01
We report on the electronic structure, magnetic moments and exchange interactions of one- and two-dimensional Cr clusters on a Pd(1 1 1) substrate, using a real-space method based on density functional theory in the local spin density approximation. We find in general that for the investigated clusters, the magnetic moments are sizeable and almost entirely of spin-character. We demonstrate that the interactions in general are dominated by nearest-neighbor antiferromagnetic Heisenberg form, which implies that Cr on Pd(1 1 1) forms an ideal model system, in which clusters of almost any shape and size can be investigated from a Heisenberg Hamiltonian, using a nearest-neighbor exchange model. We have also found that complex magnetic structures can be realized for linear chains of Cr, due to a competition between exchange interaction and a weaker Dzyaloshinskii-Moriya interaction.
NASA Astrophysics Data System (ADS)
Zhang, Lei; Chen, Zhida
2001-09-01
Magnetic exchange interaction for the μ-hydroxo bridged vanadium(IV) dimers is investigated based on calculations of density functional theory combined with the broken-symmetry approach. It is found that there is an exponential correlation between the V-O(hydroxo)-V angle, the V-O(hydroxo) distance and the exchange coupling constants J. Meanwhile, the calculated results reveal that the deprotonation of the bridging hydroxo ligand causes a sharp increase of the exchange coupling interaction, but the magnetic coupling constant J is insensitive to the deprotonation of the bridging aquo ligand. Moreover, simplifying each 1,2-bridging squarate ligand with two -OCH 2 groups almost does not influence magnetic exchange behavior between the two vanadium(IV) ions, and the principle of the shortest superexchange pathway is available for the multiplicity of the bridging ligand in the μ-hydroxo bridged vanadium(IV) dimers.
First-principles theory of electronic structure and magnetism of Cr nano-islands on Pd(1 1 1).
de Melo Rodrigues, Debora Carvalho; Pereiro, Manuel; Bergman, Anders; Eriksson, Olle; Klautau, Angela Burlamaqui
2017-01-18
We report on the electronic structure, magnetic moments and exchange interactions of one- and two-dimensional Cr clusters on a Pd(1 1 1) substrate, using a real-space method based on density functional theory in the local spin density approximation. We find in general that for the investigated clusters, the magnetic moments are sizeable and almost entirely of spin-character. We demonstrate that the interactions in general are dominated by nearest-neighbor antiferromagnetic Heisenberg form, which implies that Cr on Pd(1 1 1) forms an ideal model system, in which clusters of almost any shape and size can be investigated from a Heisenberg Hamiltonian, using a nearest-neighbor exchange model. We have also found that complex magnetic structures can be realized for linear chains of Cr, due to a competition between exchange interaction and a weaker Dzyaloshinskii-Moriya interaction.
Mouesca, Jean-Marie
2014-01-01
The goal of this "how to" chapter is to present in a way as simple and practical as possible some of the concepts, key issues, and practices behind the so-called broken symmetry (BS) state which is widely used within the density functional theory (DFT) (for a very nice but thoughtful introduction to DFT (without equations!), read Perdew et al. (J Chem Theory Comput 5:902-908, 2009)) community to compute energetic as well as spectroscopic properties pertaining to (poly-)radicals, bioinorganic clusters (especially those containing transition metal ions), etc. Such properties encompass exchange coupling constants J (molecular magnetism) but also (among other things) g-tensors and hyperfine coupling tensors A (from electron paramagnetic resonance), isomer shifts δ and quadrupolar tensors ΔE Q (from Mössbauer), etc.Hopefully, this chapter will appeal to those DFT practitioners who would like to understand the basics behind the BS state and help them "demystify" some of the issues involved with them. More technical issues will only be alluded to, and appropriate references will be given for those interested to go beyond this mere introduction. This chapter is however not a review of the field. Consequently, it will be primarily based on my own experience. The goal here (in the spirit of a "how to" chapter) is to accompany the readers' thoughts in a progressive way along increasingly complex issues rather than encumbering the same thoughts with too complicate mathematical details (the few derivations which are given will therefore be explicit). Moreover, I will emphasize in this chapter the interplay between the computation of BS states on the one hand, and the derivation of phenomenological models on the other hand, whose parameters can be supplied from appropriate BS states. Finally, this chapter is dedicated to Louis Noodleman (Scripps Research Institute, CA, USA), pioneer (Noodleman, J Chem Phys 74:5737-5743, 1981; Noodleman, Chem Phys 109:131-143, 1986) and
Wang, Jun; Duan, Hongyan; Lin, Xiong; Aguilar, Victor; Mosqueda, Aaron; Zhao, Guo-meng
2012-01-01
Magnetic hysteresis loops were measured for ferrimagnetic iron chalcogenide Fe3Se4 nanoparticles in the whole temperature range below the Curie temperature TC (315 K). The coercivity of the material is huge, reaching about 40 kOe at 10 K. The magnetic anisotropy constant K was determined from the magnetic hysteresis loop using the law of approach to saturation. The deduced anisotropy constant at 10 K is 5.22×106 erg/cm3, which is over one order of magnitude larger than that of Fe3O4. We also demonstrated that the experimental magnetic hysteresis loop is in good agreement with the theoretical curve calculated by Stoner and Wohlfarth for a noninteracting randomly oriented uniaxial single-domain particle system. Moreover, we show that K is proportional to the cube of the saturation magnetization Ms, which confirms earlier theoretical models for uniaxial magnets. PMID:23258940
Ertaş, Mehmet; Deviren, Bayram; Keskin, Mustafa
2012-11-01
Nonequilibrium magnetic properties in a two-dimensional kinetic mixed spin-2 and spin-5/2 Ising system in the presence of a time-varying (sinusoidal) magnetic field are studied within the effective-field theory (EFT) with correlations. The time evolution of the system is described by using Glauber-type stochastic dynamics. The dynamic EFT equations are derived by employing the Glauber transition rates for two interpenetrating square lattices. We investigate the time dependence of the magnetizations for different interaction parameter values in order to find the phases in the system. We also study the thermal behavior of the dynamic magnetizations, the hysteresis loop area, and dynamic correlation. The dynamic phase diagrams are presented in the reduced magnetic field amplitude and reduced temperature plane and we observe that the system exhibits dynamic tricritical and reentrant behaviors. Moreover, the system also displays a double critical end point (B), a zero-temperature critical point (Z), a critical end point (E), and a triple point (TP). We also performed a comparison with the mean-field prediction in order to point out the effects of correlations and found that some of the dynamic first-order phase lines, which are artifacts of the mean-field approach, disappeared.
Bhunia, Snehasis; Vyas, Nidhi; Sahu, Chandan; Ojha, Animesh K
2014-11-01
Structural, electronic, and magnetic properties of ScN (N=2-14) clusters have been investigated using density functional theory (DFT) calculations. Different spin states isomer for each cluster size has been optimized with symmetry relaxation. The structural stability, dissociation energy, binding energy, spin stability, vertical ionization energy, electron affinity, chemical hardness, and size dependent magnetic moment per atom are calculated for the energetically most stable spin isomer for each size. The structural stability for a specific size cluster has been explained in terms of atomic shell closing effect, close packed symmetric structure, and chemical bonding. Spin stability of each cluster size is determined by calculating the value of spin gaps. The maximum value for second-order energy difference is observed for the clusters of size N = 2, 6, 11, and 13, which implies that these clusters are relatively more stable. The magnetic moment per atom corresponding to lowest energy structure has also been calculated. The magnetic moment per atom corresponding to lowest energy structures has been calculated. The calculated values of magnetic moment per atom vary in an oscillatory fashion with cluster size. The calculated results are compared with the available experimental data.
NASA Astrophysics Data System (ADS)
Yokoyama, T.; Tanaka, Y.; Golubov, A. A.; Inoue, J.; Asano, Y.
2006-01-01
Charge transport in the diffusive normal metal/insulator/s-wave superconductor junctions is studied in the presence of the magnetic impurity for various situations, where we have used the Usadel equation with Nazarov's generalized boundary condition. It is revealed that the magnetic impurity scattering suppresses the proximity effect. Wide variety of the line shapes of the tunneling conductance is obtained. Only for high transparent junction the normalized conductance around zero voltage can be enhanced by the magnetic impurity scattering.
NASA Astrophysics Data System (ADS)
Boscaino, V.; Cipriani, G.; Di Dio, V.; Corpora, M.; Curto, D.; Franzitta, V.; Trapanese, M.
2017-05-01
An experimental study on the effect of permanent magnet tolerances on the performances of a Tubular Linear Ferrite Motor is presented in this paper. The performances that have been investigated are: cogging force, end effect cogging force and generated thrust. It is demonstrated that: 1) the statistical variability of the magnets introduces harmonics in the spectrum of the cogging force; 2) the value of the end effect cogging force is directly linked to the values of then remanence field of the external magnets placed on the slider; 3) the generated thrust and its statistical distribution depend on the remanence field of the magnets placed on the translator.
Zhang, J.; Peebles, W. A.; Crocker, N. A.; Carter, T. A.; Doyle, E. J.; Rhodes, T. L.; Wang, G.; Zeng, L.; Hyatt, A. W.
2013-10-15
Mueller-Stokes theory can be used to calculate the polarization evolution of an electromagnetic (EM) wave as it propagates through a magnetized plasma. Historically, the theory has been used to interpret polarimeter signals from systems operating on fusion plasmas. These interpretations have mostly employed approximations of Mueller-Stokes theory in regimes where either the Faraday rotation (FR) or the Cotton-Mouton (CM) effect is dominant. The current paper presents the first systematic comparison of polarimeter measurements with the predictions of full Mueller-Stokes theory where conditions transition smoothly from a FR-dominant (i.e., weak CM effect) plasma to one where the CM effect plays a significant role. A synthetic diagnostic code, based on Mueller-Stokes theory accurately reproduces the trends evident in the experimentally measured polarimeter phase over this entire operating range, thereby validating Mueller-Stokes theory. The synthetic diagnostic code is then used to investigate the influence of the CM effect on polarimetry measurements. As expected, the measurements are well approximated by the FR effect when the CM effect is predicted to be weak. However, the code shows that as the CM effect increases, it can compete with the FR effect in rotating the polarization of the EM-wave. This results in a reduced polarimeter response to the FR effect, just as observed in the experiment. The code also shows if sufficiently large, the CM effect can even reverse the handedness of a wave launched with circular polarization. This helps to understand the surprising experimental observations that the sensitivity to the FR effect can be nearly eliminated at high enough B{sub T} (2.0 T). The results also suggest that the CM effect on the plasma midplane can be exploited to potentially measure magnetic shear in tokamak plasmas. These results establish increased confidence in the use of such a synthetic diagnostic code to guide future polarimetry design and interpret
NASA Astrophysics Data System (ADS)
Zhou, Yanru; Zhao, Yuxiang; Tian, Hui; Zhang, Dengwei; Huang, Tengchao; Miao, Lijun; Shu, Xiaowu; Che, Shuangliang; Liu, Cheng
2016-12-01
In an axial magnetic field (AMF), which is vertical to the plane of the fiber coil, a polarization-maintaining fiber optic gyro (PM-FOG) appears as an axial magnetic error. This error is linearly related to the intensity of an AMF, the radius of the fiber coil, and the light wavelength, and also influenced by the distribution of fiber twist. When a PM-FOG is manufactured completely, this error only appears a linear correlation with the AMF. A real-time compensation model is established to eliminate the error, and the experimental results show that the axial magnetic error of the PM-FOG is decreased from 5.83 to 0.09 deg/h in 12G AMF with 18-dB suppression.
Hennion, Sophie; Delbeuck, Xavier; Koelkebeck, Katja; Brion, Marine; Tyvaert, Louise; Plomhause, Lucie; Derambure, Philippe; Lopes, Renaud; Szurhaj, William
2016-12-01
Although patients with mesial temporal lobe epilepsy (mTLE) are known to have theory of mind (ToM) impairments, the latter's neural functional bases have yet to be explored. We used functional magnetic resonance imaging (fMRI) to gain insights into the neural dysfunction associated with ToM impairments in patients with mTLE. Twenty-five patients (12 and 13 with right and left mTLE, respectively) and 25 healthy controls performed the "animated shapes" task during fMRI. This complex ToM task requires both explicit reasoning about mental states and implicit processing of information on biological motion and action. The animated shapes evoke both ToM and non-ToM interaction perception, and the corresponding neural activation patterns were compared. Behavioral performance (i.e. categorization of the interactions) was also recorded. Relative to healthy controls, both patients with right and left mTLE were impaired in categorizing ToM interactions. The fMRI results showed that both patients with right and left mTLE had less intense neural activation (relative to controls) in regions involved in the implicit component of ToM processes (i.e. the fusiform gyrus in patients with right mTLE and the supplementary motor area in patients with left mTLE). In patients with right mTLE, we also observed more intense activation (relative to controls) in regions involved in the explicit component of ToM processes (i.e. the dorsal medial prefrontal cortex); age at onset of epilepsy also mediated activation in regions involved in the explicit component (i.e. the ventral medial prefrontal cortex and the temporoparietal junction). Patients with left mTLE displayed greater activation of the contralateral mesial regions (relative to controls); we speculate that this may correspond to the deployment of a compensatory mechanism. This study provides insights into the disturbances of the implicit/explicit ToM neural network in patients with mTLE. These impairments in the ToM neural network
Reta, Daniel; Moreira, Ibério de P R; Illas, Francesc
2016-07-12
In the most general case of three electrons in three symmetry unrelated centers with Ŝ1 = Ŝ2 = Ŝ3 = 1/2 localized magnetic moments, the low energy spectrum consists of one quartet (Q) and two doublet (D1, D2) pure spin states. The energy splitting between these spin states can be described with the well-known Heisenberg-Dirac-Van Vleck (HDVV) model spin Hamiltonian, and their corresponding energy expressions are expressed in terms of the three different two-body magnetic coupling constants J12, J23, and J13. However, the values of all three magnetic coupling constants cannot be extracted using the calculated energy of the three spin-adapted states since only two linearly independent energy differences between pure spin states exist. This problem has been recently investigated by Reta et al. (J. Chem. Theory Comput. 2015, 11, 3650), resulting in an alternative proposal to the original Noodleman's broken symmetry mapping approach. In the present work, this proposal is validated by means of ab initio effective Hamiltonian theory, which allows a direct extraction of all three J values from the one-to-one correspondence between the matrix elements of both effective and HDVV Hamiltonian. The effective Hamiltonian matrix representation has been constructed from configuration interaction wave functions for the three spin states obtained for two model systems showing a different degree of delocalization of the unpaired electrons. These encompass a trinuclear Cu(II) complex and a π-conjugated purely organic triradical.
NASA Astrophysics Data System (ADS)
Zhen, Yaxin; Zhou, Lin
2017-03-01
Based on nonlocal strain gradient theory, wave propagation in fluid-conveying viscoelastic single-walled carbon nanotubes (SWCNTs) is studied in this paper. With consideration of thermal effect and surface effect, wave equation is derived for fluid-conveying viscoelastic SWCNTs under longitudinal magnetic field utilizing Euler-Bernoulli beam theory. The closed-form expressions are derived for the frequency and phase velocity of the wave motion. The influences of fluid flow velocity, structural damping coefficient, temperature change, magnetic flux and surface effect are discussed in detail. SWCNTs’ viscoelasticity reduces the wave frequency of the system and the influence gets remarkable with the increase of wave number. The fluid in SWCNTs decreases the frequency of wave propagation to a certain extent. The frequency (phase velocity) gets larger due to the existence of surface effect, especially when the diameters of SWCNTs and the wave number decrease. The wave frequency increases with the increase of the longitudinal magnetic field, while decreases with the increase of the temperature change. The results may be helpful for better understanding the potential applications of SWCNTs in nanotechnology.
Numerical simulations of quiet Sun magnetic fields seeded by the Biermann battery
NASA Astrophysics Data System (ADS)
Khomenko, E.; Vitas, N.; Collados, M.; de Vicente, A.
2017-08-01
The magnetic fields of the quiet Sun cover at any time more than 90% of its surface and their magnetic energy budget is crucial to explain the thermal structure of the solar atmosphere. One of the possible origins of these fields is the action of the local dynamo in the upper convection zone of the Sun. Existing simulations of the local solar dynamo require an initial seed field and sufficiently high spatial resolution in order to achieve the amplification of the seed field to the observed values in the quiet Sun. Here we report an alternative model of seeding based on the action of the Bierman battery effect. This effect generates a magnetic field due to the local imbalances in electron pressure in the partially ionized solar plasma. We show that the battery effect self-consistently creates from zero an initial seed field of a strength of the order of micro G, and together with dynamo amplification allows the generation of quiet Sun magnetic fields of a similar strength to those from solar observations.
NASA Astrophysics Data System (ADS)
Paduan-Filho, A.; Oliveira, N. F.; Bindilatti, V.; Foner, S.; Shapira, Y.
2003-12-01
A theory for the equilibrium low-temperature magnetization M of a diluted Heisenberg antiferromagnetic chain is presented. Only the nearest-neighbor (NN) exchange interaction is included, and the distribution of the magnetic ions is assumed to be random. Values of the magnetic fields Bi at the magnetization steps (MST’s) from finite chains with two to five spins (pairs, triplets, quartets, and quintets) are given for chains composed of spins S=5/2. The magnitudes of these MST’s as a function of the fraction, x, of cations that are magnetic are given for any S. An expression for the apparent saturation value of M is derived. The magnetization curve, M versus B, is calculated using the exact contributions of finite chains with one to five spins, and the “rise and ramp approximation” for longer chains. An expression for the low-temperature saturation magnetic field Bs(n) of a finite chain with n spins is given. Some nonequilibrium effects that occur in a rapidly changing B are also considered. Some of these result from the absence of thermal equilibrium within the sample itself, whereas others are caused by the absence of thermal equilibrium between the sample and its environment (e.g., liquid-helium bath). Specific nonequilibrium models based on earlier treatments of the phonon bottleneck, and of spin flips associated with cross relaxation and with level crossings (anticrossings), are discussed. Magnetization data on powders of TMMC diluted with cadmium [i.e., (CH3)4NMnxCd1-xCl3, with 0.16⩽x⩽0.50] were measured at 0.55 K in 18-T superconducting magnets. The field B1 at the first MST from pairs is used to determine the NN exchange constant J. This J/kB changes from -5.9 K to -6.5 K as x increases from 0.16 to 0.50. The magnetization curves obtained in the superconducting magnets are compared with simulations based on the equilibrium theory. A reasonably good agreement is found. Data for the differential susceptibility, dM/dB, were taken in pulsed magnetic
Small-scale dynamo at low magnetic Prandtl numbers.
Schober, Jennifer; Schleicher, Dominik; Bovino, Stefano; Klessen, Ralf S
2012-12-01
The present-day Universe is highly magnetized, even though the first magnetic seed fields were most probably extremely weak. To explain the growth of the magnetic field strength over many orders of magnitude, fast amplification processes need to operate. The most efficient mechanism known today is the small-scale dynamo, which converts turbulent kinetic energy into magnetic energy leading to an exponential growth of the magnetic field. The efficiency of the dynamo depends on the type of turbulence indicated by the slope of the turbulence spectrum v(ℓ)∝ℓ^{ϑ}, where v(ℓ) is the eddy velocity at a scale ℓ. We explore turbulent spectra ranging from incompressible Kolmogorov turbulence with ϑ=1/3 to highly compressible Burgers turbulence with ϑ=1/2. In this work, we analyze the properties of the small-scale dynamo for low magnetic Prandtl numbers Pm, which denotes the ratio of the magnetic Reynolds number, Rm, to the hydrodynamical one, Re. We solve the Kazantsev equation, which describes the evolution of the small-scale magnetic field, using the WKB approximation. In the limit of low magnetic Prandtl numbers, the growth rate is proportional to Rm^{(1-ϑ)/(1+ϑ)}. We furthermore discuss the critical magnetic Reynolds number Rm_{crit}, which is required for small-scale dynamo action. The value of Rm_{crit} is roughly 100 for Kolmogorov turbulence and 2700 for Burgers. Furthermore, we discuss that Rm_{crit} provides a stronger constraint in the limit of low Pm than it does for large Pm. We conclude that the small-scale dynamo can operate in the regime of low magnetic Prandtl numbers if the magnetic Reynolds number is large enough. Thus, the magnetic field amplification on small scales can take place in a broad range of physical environments and amplify week magnetic seed fields on short time scales.
Small-scale dynamo at low magnetic Prandtl numbers
NASA Astrophysics Data System (ADS)
Schober, Jennifer; Schleicher, Dominik; Bovino, Stefano; Klessen, Ralf S.
2012-12-01
The present-day Universe is highly magnetized, even though the first magnetic seed fields were most probably extremely weak. To explain the growth of the magnetic field strength over many orders of magnitude, fast amplification processes need to operate. The most efficient mechanism known today is the small-scale dynamo, which converts turbulent kinetic energy into magnetic energy leading to an exponential growth of the magnetic field. The efficiency of the dynamo depends on the type of turbulence indicated by the slope of the turbulence spectrum v(ℓ)∝ℓϑ, where v(ℓ) is the eddy velocity at a scale ℓ. We explore turbulent spectra ranging from incompressible Kolmogorov turbulence with ϑ=1/3 to highly compressible Burgers turbulence with ϑ=1/2. In this work, we analyze the properties of the small-scale dynamo for low magnetic Prandtl numbers Pm, which denotes the ratio of the magnetic Reynolds number, Rm, to the hydrodynamical one, Re. We solve the Kazantsev equation, which describes the evolution of the small-scale magnetic field, using the WKB approximation. In the limit of low magnetic Prandtl numbers, the growth rate is proportional to Rm(1-ϑ)/(1+ϑ). We furthermore discuss the critical magnetic Reynolds number Rmcrit, which is required for small-scale dynamo action. The value of Rmcrit is roughly 100 for Kolmogorov turbulence and 2700 for Burgers. Furthermore, we discuss that Rmcrit provides a stronger constraint in the limit of low Pm than it does for large Pm. We conclude that the small-scale dynamo can operate in the regime of low magnetic Prandtl numbers if the magnetic Reynolds number is large enough. Thus, the magnetic field amplification on small scales can take place in a broad range of physical environments and amplify week magnetic seed fields on short time scales.
NASA Astrophysics Data System (ADS)
Jault, Dominique
2013-04-01
Understanding the main magnetic field variations has been hindered by the discrepancy between the periods (from months to years) of the simplest linear wave phenomena and the relatively long time intervals (10 to 100 years) over which magnetic field changes can be confidently monitored. A theoretical description of short-period waves within the Earth's fluid core is at hand. Quasi-geostrophic inertial waves (akin to Rossby waves in the atmosphere) are slightly modified in the presence of magnetic fields and torsional oscillations consist of differential motion between coaxial rigid cylindrical annuli. Torsional oscillations are sensitive to the whole magnetic field that they shear in the course of their propagation. From their modelling, we have thus gained an estimate for the magnetic field strength in the core interior. There is now ongoing work to extend the theoretical framework to longer times. Furthermore, data collected from the Swarm constellation of three satellites to be launched this year by ESA will permit to better separate the internal and external magnetic signals. We may thus dream to detect quasi-geostrophic inertial waves. As the spectral ranges of theoretical models and observations begin to overlap, we can now go beyond the understanding of the magnetic field variations as the juxtaposition of partial models, arranged as a set of nested Matryoshka dolls. This talk will give illustrations for this statement, among which the question of induction in the lower mantle.
Guilet, Jerome; Foglizzo, Thierry
2010-03-01
The effect of a magnetic field on the linear phase of the advective-acoustic instability is investigated as a first step toward a magnetohydrodynamic (MHD) theory of the stationary accretion shock instability taking place during stellar core collapse. We study a toy model where the flow behind a planar stationary accretion shock is adiabatically decelerated by an external potential. Two magnetic field geometries are considered: parallel or perpendicular to the shock. The entropy-vorticity wave, which is simply advected in the unmagnetized limit, separates into five different waves: the entropy perturbations are advected, while the vorticity can propagate along the field lines through two Alfven waves and two slow magnetosonic waves. The two cycles existing in the unmagnetized limit, advective-acoustic and purely acoustic, are replaced by up to six distinct MHD cycles. The phase differences among the cycles play an important role in determining the total cycle efficiency and hence the growth rate. Oscillations in the growth rate as a function of the magnetic field strength are due to this varying phase shift. A vertical magnetic field hardly affects the cycle efficiency in the regime of super-Alfvenic accretion that is considered. In contrast, we find that a horizontal magnetic field strongly increases the efficiencies of the vorticity cycles that bend the field lines, resulting in a significant increase of the growth rate if the different cycles are in phase. These magnetic effects are significant for large-scale modes if the Alfven velocity is a sizable fraction of the flow velocity.
Bast, Radovan; Saue, Trond; Henriksson, Johan; Norman, Patrick
2009-01-14
The quadratic response function has been derived and implemented at the adiabatic four-component Kohn-Sham density functional theory level with inclusion of noncollinear spin magnetization and gradient corrections in the exchange-correlation functional-a work that is an extension of our previous report where magnetization dependencies in the exchange-correlation functional were ignored [J. Henriksson, T. Saue, and P. Norman, J. Chem. Phys. 128, 024105 (2008)]. The electric-field induced second-harmonic generation experiments on CF(3)Cl and CF(3)Br are addressed by a determination of beta(-2omega;omega, omega) for a wavelength of 694.3 nm, and the same property is also determined for CF(3)I. The relativistic effects on the static hyperpolarizability for the series of molecules amount to 1%, 5%, and 9%, respectively. At the experimental wavelength, the contributions to beta due to the magnetization dependence in the exchange-correlation functional are negligible for CF(3)Cl and CF(3)Br and small for CF(3)I. The noticeable effect of magnetization in the latter case is attributed to a near two-photon resonance with the excited state 1 (3)E (nonrelativistic notation). It is emphasized, however, that the effect of magnetization on beta for CF(3)I is negligible both in comparison to the total relativistic correction as well as to the effects of electron correlation. It is concluded that, in calculations of hyperpolarizabilities under nonresonant conditions, the magnetization dependence in the exchange-correlation functional may be ignored.
Burrola-Gándara, L. A. Santillan-Rodriguez, C. R.; Rivera-Gomez, F. J.; Saenz-Hernandez, R. J.; Botello-Zubiate, M. E.; Matutes-Aquino, J. A.
2015-05-07
Magnetocaloric materials with second order phase transition near the Curie temperature can be described by critical phenomena theory. In this theory, scaling, universality, and renormalization are key concepts from which several phase transition order criteria are derived. In this work, the rescaled universal curve, Banerjee and mean field theory criteria were used to make a comparison for several magnetocaloric materials including pure Gd, SmCo{sub 1.8}Fe{sub 0.2}, MnFeP{sub 0.46}As{sub 0.54}, and La{sub 0.7}Ca{sub 0.15}Sr{sub 0.15}MnO{sub 3}. Pure Gd, SmCo{sub 1.8}Fe{sub 0.2}, and La{sub 0.7}Ca{sub 0.15}Sr{sub 0.15}MnO{sub 3} present a collapse of the rescaled magnetic entropy change curves into a universal curve, which indicates a second order phase transition; applying Banerjee criterion to H/σ vs σ{sup 2} Arrot plots and the mean field theory relation |ΔS{sub M}| ∝ (μ{sub 0}H/T{sub c}){sup 2/3} for the same materials also determines a second order phase transition. However, in the MnFeP{sub 0.46}As{sub 0.54} sample, the Banerjee criterion applied to the H/σ vs σ{sup 2} Arrot plot indicates a first order magnetic phase transition, while the mean field theory prediction for a second order phase transition, |ΔS{sub M}| ∝ (μ{sub 0}H/T{sub c}){sup 2/3}, describes a second order behavior. Also, a mixture of first and second order behavior was indicated by the rescaled universal curve criterion. The diverse results obtained for each criterion in MnFeP{sub 0.46}As{sub 0.54} are apparently related to the magnetoelastic effect and to the simultaneous presence of weak and strong magnetism in Fe (3f) and Mn (3g) alternate atomic layers, respectively. The simultaneous application of the universal curve, the Banerjee and the mean field theory criteria has allowed a better understanding about the nature of the order of the phase transitions in different magnetocaloric materials.
NASA Astrophysics Data System (ADS)
Burrola-Gándara, L. A.; Santillan-Rodriguez, C. R.; Rivera-Gomez, F. J.; Saenz-Hernandez, R. J.; Botello-Zubiate, M. E.; Matutes-Aquino, J. A.
2015-05-01
Magnetocaloric materials with second order phase transition near the Curie temperature can be described by critical phenomena theory. In this theory, scaling, universality, and renormalization are key concepts from which several phase transition order criteria are derived. In this work, the rescaled universal curve, Banerjee and mean field theory criteria were used to make a comparison for several magnetocaloric materials including pure Gd, SmCo1.8Fe0.2, MnFeP0.46As0.54, and La0.7Ca0.15Sr0.15MnO3. Pure Gd, SmCo1.8Fe0.2, and La0.7Ca0.15Sr0.15MnO3 present a collapse of the rescaled magnetic entropy change curves into a universal curve, which indicates a second order phase transition; applying Banerjee criterion to H/σ vs σ2 Arrot plots and the mean field theory relation |ΔSM| ∝ (μ0H/Tc)2/3 for the same materials also determines a second order phase transition. However, in the MnFeP0.46As0.54 sample, the Banerjee criterion applied to the H/σ vs σ2 Arrot plot indicates a first order magnetic phase transition, while the mean field theory prediction for a second order phase transition, |ΔSM| ∝ (μ0H/Tc)2/3, describes a second order behavior. Also, a mixture of first and second order behavior was indicated by the rescaled universal curve criterion. The diverse results obtained for each criterion in MnFeP0.46As0.54 are apparently related to the magnetoelastic effect and to the simultaneous presence of weak and strong magnetism in Fe (3f) and Mn (3g) alternate atomic layers, respectively. The simultaneous application of the universal curve, the Banerjee and the mean field theory criteria has allowed a better understanding about the nature of the order of the phase transitions in different magnetocaloric materials.
NASA Astrophysics Data System (ADS)
Livens, G. H.
2016-10-01
Preface; 1. The electric field; 2. Dielectric theory; 3. Electric currents; 4. The magnetic field; 5. The dynamics of the magnetic field; 6. Maxwell's electromagnetic theory; 7. Electromagnetic oscillations and waves; 8. The electrodynamics of moving media; Appendix 1. On the mechanism of magnetic induction; Appendix 2. On the mechanism of metallic conduction; Index.
NASA Astrophysics Data System (ADS)
Zhuravlev, Vladimir; Duan, Wenye; Maniv, Tsofar
2017-01-01
A self-consistent Bogoliubov-de Gennes theory of the vortex lattice state in a 2D strong type-II superconductor at high magnetic fields reveals a novel quantum mixed state around the semiclassical Hc 2, characterized by a well-defined Landau-Bloch band structure in the quasiparticle spectrum and suppressed order-parameter amplitude, which sharply crossover into the well-known semiclassical (Helfand-Werthamer) results upon decreasing magnetic field. Application to the 2D superconducting state observed recently on the surface of the topological insulator Sb2Te3 accounts well for the experimental data, revealing a strong type-II superconductor, with unusually low carrier density and very small cyclotron mass, which can be realized only in the strong coupling superconductor limit.
NASA Astrophysics Data System (ADS)
Krykunov, Mykhaylo; Seth, Michael; Ziegler, Tom; Autschbach, Jochen
2007-12-01
A time-dependent density functional theory (TDDFT) formalism with damping for the calculation of the magnetic optical rotatory dispersion and magnetic circular dichroism (MCD) from the complex Verdet constant is presented. For a justification of such an approach, we have derived the TDDFT analog of the sum-over-states formula for the Verdet constant. The results of the MCD calculations by this method for ethylene, furan, thiophene, selenophene, tellurophene, and pyrrole are in good agreement with our previous theoretical sum-over-states MCD spectra. For the π →π* transition of propene, we have obtained a positive Faraday B term. It is located between the two negative B terms. This finding is in agreement with experiment in the range of 6-8eV.
ERIC Educational Resources Information Center
Lowman, Charles E.
A guide to the technology of magnetic recorders used in such fields as audio recording, broadcast and closed-circuit television, instrumentation recording, and computer data systems is presented. Included are discussions of applications, advantages, and limitations of magnetic recording, its basic principles and theory of operation, and its…
NASA Astrophysics Data System (ADS)
Wickramasinghe, Dayal T.; Tout, Christopher A.; Ferrario, Lilia
2014-01-01
Observations of magnetic A, B and O stars show that the poloidal magnetic flux per unit mass Φp/M appears to have an upper bound of approximately 10-6.5 G cm2 g-1. A similar upper bound to the total flux per unit mass is found for the magnetic white dwarfs even though the highest magnetic field strengths at their surfaces are much larger. For magnetic A and B stars, there also appears to be a well-defined lower bound below which the incidence of magnetism declines rapidly. According to recent hypotheses, both groups of stars may result from merging stars and owe their strong magnetism to fields generated by a dynamo mechanism as they merge. We postulate a simple dynamo that generates magnetic field from differential rotation. We limit the growth of magnetic fields by the requirement that the poloidal field stabilizes the toroidal and vice versa. While magnetic torques dissipate the differential rotation, toroidal field is generated from poloidal by an Ω dynamo. We further suppose that mechanisms that lead to the decay of toroidal field lead to the generation of poloidal. Both poloidal and toroidal fields reach a stable configuration which is independent of the size of small initial seed fields but proportional to the initial differential rotation. We pose the hypothesis that strongly magnetic stars form from the merging of two stellar objects. The highest fields are generated when the merge introduces differential rotation that amounts to critical break-up velocity within the condensed object. Calibration of a simplistic dynamo model with the observed maximum flux per unit mass for main-sequence stars and white dwarfs indicates that about 1.5 × 10-4 of the decaying toroidal flux must appear as poloidal. The highest fields in single white dwarfs are generated when two degenerate cores merge inside a common envelope or when two white dwarfs merge by gravitational-radiation angular momentum loss. Magnetars are the most magnetic neutron stars. Though these are
NASA Astrophysics Data System (ADS)
Yu, Rong; Miclea, Corneliu F.; Weickert, Franziska; Movshovich, Roman; Paduan-Filho, Armando; Zapf, Vivien S.; Roscilde, Tommaso
2012-10-01
In this paper we investigate the quantum phase transition from magnetic Bose Glass to magnetic Bose-Einstein condensation induced by a magnetic field in NiCl2·4SC(NH2)2 (dichloro-tetrakis-thiourea-nickel, or DTN), doped with Br (Br-DTN) or site diluted. Quantum Monte Carlo simulations for the quantum phase transition of the model Hamiltonian for Br-DTN, as well as for site-diluted DTN, are consistent with conventional scaling at the quantum critical point and with a critical exponent z verifying the prediction z=d; moreover the correlation length exponent is found to be ν=0.75(10), and the order parameter exponent to be β=0.95(10). We investigate the low-temperature thermodynamics at the quantum critical field of Br-DTN both numerically and experimentally, and extract the power-law behavior of the magnetization and of the specific heat. Our results for the exponents of the power laws, as well as previous results for the scaling of the critical temperature to magnetic ordering with the applied field, are incompatible with the conventional crossover-scaling Ansatz proposed by Fisher [Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.40.546 40, 546 (1989)]. However they can all be reconciled within a phenomenological Ansatz in the presence of a dangerously irrelevant operator.
NASA Astrophysics Data System (ADS)
Vázquez-Quesada, A.; Franke, T.; Ellero, M.
2017-03-01
In this work, an analytical model for the behavior of superparamagnetic chains under the effect of a rotating magnetic field is presented. It is postulated that the relevant mechanisms for describing the shape and breakup of the chains into smaller fragments are the induced dipole-dipole magnetic force on the external beads, their translational and rotational drag forces, and the tangential lubrication between particles. Under this assumption, the characteristic S-shape of the chain can be qualitatively understood. Furthermore, based on a straight chain approximation, a novel analytical expression for the critical frequency for the chain breakup is obtained. In order to validate the model, the analytical expressions are compared with full three-dimensional smoothed particle hydrodynamics simulations of magnetic beads showing excellent agreement. Comparison with previous theoretical results and experimental data is also reported.
Halasyamani, Shiv; Fennie, Craig
2016-11-03
We have focused on the synthesis, characterization, and ab initio theory on multi-functional mixed-metal fluorides. With funding from the DOE, we have successfully synthesized and characterized a variety of mixed metal fluoride materials.
Véron, A; Sugimura, A; Luckhurst, G R; Martins, A F
2012-11-01
This work describes an investigation of the static (or quasistatic) nuclear magnetic resonance (NMR) response in a nematic liquid crystal confined between two planar conducting plates and subject to a magnetic field and an electric field produced by a difference of voltage applied on the plates. Deuterium NMR spectroscopy of 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) under these conditions has revealed a voltage dependent inhomogeneous director distribution for a particular narrow range of voltages and for a fixed magnetic field (that of the spectrometer). In the ideal setup the two plates are assumed to be rigorously parallel, so that a difference of voltage applied on the plates leads to a constant electric field normal to them. When the magnetic field is parallel to the plates (orthogonal geometry) there exists a threshold value of the electric field for which the effect of both fields exactly compensate; moreover, for stronger electric field the director aligns with the electric field while for weaker electric field the director aligns with the magnetic field. If there is a lack of parallelism between the two plates, the electric field becomes inhomogeneous so that it may be larger than the threshold value in some region of the sample and smaller in the remaining part of the sample. In that case the director will adopt essentially two orientations within the sample, namely, parallel or perpendicular to the magnetic field, and the position of the frontier between the two domains depends on the voltage. This feature is clearly shown by deuterium NMR spectra that exhibit a transfer of intensity between two quadrupolar doublets with increase in the applied voltage. The coexistence of two director populations occurs for a range of voltages that depends on the degree of nonparallelism; accordingly, an estimation of this range by NMR yields an experimental estimation of the lack of parallelism. A tiny tilt of the magnetic field (nonorthogonal geometry) entrains a
NASA Astrophysics Data System (ADS)
Kato, Seikou; Kondo, Kei-Ichi; Shibata, Akihiro
2015-02-01
In the S U (2 ) Yang-Mills theory on the four-dimensional Euclidean lattice, we confirm the gauge-independent "Abelian" dominance (or the restricted field dominance) and gauge-independent magnetic-monopole dominance in the string tension of the linear potential extracted from the Wilson loop in the fundamental representation. The dual Meissner effect is observed by demonstrating the squeezing of the chromoelectric field flux connecting a pair of a quark and an antiquark. In addition, the circular magnetic-monopole current is induced around the chromoelectric flux. The type of the dual superconductivity is also determined by fitting the result with the dual Ginzburg-Landau model. Thus, the dual superconductor picture for quark confinement is supported in a gauge-independent manner. These results are obtained based on a reformulation of the lattice Yang-Mills theory based on the change of variables à la Cho-Duan-Ge-Faddeev-Niemi combined with a non-Abelian Stokes theorem for the Wilson loop operator. We give a new procedure (called the reduction) for obtaining the color direction field that plays the central role in this reformulation.
Ahuatzin, Giovanna; Flores-Mendieta, Ruben; Hernandez-Ruiz, Maria A.; Hofmann, Christoph P.
2014-02-01
The magnetic and transition magnetic moments of the ground-state baryons are computed in heavy baryon chiral perturbation theory in the large-N{sub c} limit, where N{sub c} is the number of colors. SU(3) symmetry breaking is systematically studied twofold: On the one hand, one-loop nonanalytic corrections of orders m{sub q}{sup 1/2} and m{sub q} ln m{sub q} are included, with contributions of baryon intermediate states from both flavor octet and flavor decuplet multiplets, assuming degeneracy between baryon states within a given flavor multiplet but nondegeneracy between baryons of different multiplets. On the other hand, perturbative SU(3) symmetry breaking is also analyzed by including all relevant leading-order operators that explicitly break SU(3) at linear order. The resultant expressions are compared with the available experimental data and with other determinations in the context of conventional heavy baryon chiral perturbation theory for three flavors of light quarks and at the physical value N{sub c}=3. The agreement reached is quite impressive.
Fast diffusion of magnetic field in turbulence and origin of cosmic magnetism
NASA Astrophysics Data System (ADS)
Cho, Jungyeon
2013-02-01
Turbulence is believed to play important roles in the origin of cosmic magnetism. While it is well known that turbulence can efficiently amplify a uniform or spatially homogeneous seed magnetic field, it is not clear whether or not we can draw a similar conclusion for a localized seed magnetic field. The main uncertainty is the rate of magnetic field diffusion on scales larger than the outer scale of turbulence. To measure the diffusion rate of magnetic field on those large scales, we perform a numerical simulation in which the outer scale of turbulence is much smaller than the size of the system. We numerically compare diffusion of a localized seed magnetic field and a localized passive scalar. We find that diffusion of the magnetic field can be much faster than that of the passive scalar and that turbulence can efficiently amplify the localized seed magnetic field. Based on the simulation result, we construct a model for fast diffusion of magnetic field. Our model suggests that a localized seed magnetic field can fill the whole system in ˜Lsys/L times the large-eddy turnover time and that growth of the magnetic field stops in ˜max(15,Lsys/L) times the large-eddy turnover time, where Lsys is the size of the system and L is the driving scale. Our finding implies that, regardless of the shape of the seed field, fast magnetization is possible in turbulent systems, such as large-scale structure of the Universe or galaxies.
NASA Astrophysics Data System (ADS)
Rai, D. P.; Sandeep; Shankar, A.; Pradhan Sakhya, Anup; Sinha, T. P.; Khenata, R.; Ghimire, M. P.; Thapa, R. K.
2016-07-01
The electronic and magnetic properties of Heusler compounds X2YZ and XYZ (X = Co, Ni, Pt, Fe; Y = Mn, Cr, Vi; Z = Al, Sb, Ga) are investigated by using the density functional theory with generalized gradient approximation (GGA), GGA plus U (LSDA+U), and modified Becke-Johnson (mBJ) exchange potential. It is found that the half-metallic gaps are generally widened reasonably by LSDA+U and mBJ as compared to the conventional GGA. For the Co-based Heusler compounds the inclusion of U in GGA leads to a larger minority band gap while it is destroyed for Fe2VAl and NiMnSb. The magnetic properties of Co2VSi and Co2VSn are well defined within LSDA+U and mBJ with an exact integer value of magnetic moment. The band gaps of Fe2VAl and CoMnSb given by mBJ are in good agreement with the available experimental data of x-ray absorption spectroscopy. Except for the reasonably larger band gap, the mBJ band structure is almost same as that of GGA but is remarkably different from that of LSDA+U.
NASA Astrophysics Data System (ADS)
Xu, Duo; Zhao, Jingxiang; Wang, Xuanzhang
2013-04-01
Recent studies have suggested that graphene can serve as an excellent support material for the synthesis of advanced metal nanoparticle-graphene electrocatalysts. Compared with single-metal systems, rational design of bimetallic nanostructures with various compositions can provide more attractive opportunities to enhance their functionalities because of the novel electronic and magnetic properties. In this study, we have studied the adsorption of a series of bimetallic Fe n Pt m clusters ( n + m ≤ 4) on defective graphene with monovacancy by performing density functional theory calculations. Particular attention is paid to addressing the structural stability and exploring the effects of Fe n Pt m clusters anchoring on the electronic and magnetic properties of defective graphene. The results reveal that all studied Fe n Pt m clusters can be stably adsorbed on defective graphene, with large binding energies ranging from 6.44 (for Fe2Pt2) to 7.94 eV (for Fe2Pt). Moreover, the functionalized defective graphenes exhibit semiconducting or half-metallic nature, which is dependent on the values of n and m. Meanwhile, most of decorated defective graphenes exhibit nonzero magnetic moments, contributed mainly by the adsorbed clusters. In addition, these composites of Fe n Pt m /graphenes possess high chemical reactivity toward O2. The elongation of the O-O bonds of the adsorbed O2 molecules indicates that they can be used as oxidative catalysts.
ERIC Educational Resources Information Center
Rapp, Alexander M.; Leube, Dirk T.; Erb, Michael; Grodd, Wolfgang; Kircher, Tilo T. J.
2007-01-01
We investigated processing of metaphoric sentences using event-related functional magnetic resonance imaging (fMRI). Seventeen healthy subjects (6 female, 11 male) read 60 novel short German sentence pairs with either metaphoric or literal meaning and performed two different tasks: judging the metaphoric content and judging whether the sentence…
Kinetic theory of the positive column of a low-pressure discharge in a transverse magnetic field
Londer, Ya. I.; Ul'yanov, K. N.
2011-10-15
The influence of a transverse magnetic field on the characteristics of the positive column of a planar low-pressure discharge is studied theoretically. The motion of magnetized electrons is described in the framework of a continuous-medium model, while the ion motion in the ambipolar electric field is described by means of a kinetic equation. Using mathematical transformations, the problem is reduced to a secondorder ordinary differential equation, from which the spatial distribution of the potential is found in an analytic form. The spatial distributions of the plasma density, mean plasma velocity, and electric potential are calculated, the ion velocity distribution function at the plasma boundary is found, and the electron energy as a function of the magnetic field is determined. It is shown that, as the magnetic field rises, the electron energy increases, the distributions of the plasma density and mean plasma velocity become asymmetric, the maximum of the plasma density is displaced in the direction of the Ampere force, and the ion flux in this direction becomes substantially larger than the counter-directed ion flux.
NASA Astrophysics Data System (ADS)
Wang, Xiaoli; Cao, Jian; Ulmer, M. P.; Graham, M. E.; Vaynman, S.; Savoie, J.; Bellavia, B.
2012-10-01
This paper will present the procedure of measuring the deformation of the magnetostrictive bimorph specimens under an applied external magnetic field, and the theoretical and numerical analysis of the deformation. The magnetically smart material (MSM) KelvinAllTM and Terfenol-D is deposited on the nickel or glass substrates. The profiles of thin-film specimens were measured under an external magnetic field with White Light Interferometry. Using the theoretical calculation, the magnetostrictive property was evaluated for the coated Ni sample and glass sample. Employing the numerical approach, the influence of the magnetostrictive film on the deformation of the sample was simulated and compared with experimental results. The coated Ni specimen exhibited larger deformation than the coated glass specimen when the specimen is immersed in a 0.16 T magnetic field. In our experiments, the residual stress calculated in the thin film of the bimorph is acceptable and could be decreased by changing the parameters in the specimen preparation process. The experimental results in this paper was employed as the preliminary step to realize the future application of the magnetostrictive thin film bimorph to the adaptive X-ray mirror, and the theoretical and numerical approach was used to predict the influence of the magnetostrictive film on the larger mirror surface deformation.
ERIC Educational Resources Information Center
Rapp, Alexander M.; Leube, Dirk T.; Erb, Michael; Grodd, Wolfgang; Kircher, Tilo T. J.
2007-01-01
We investigated processing of metaphoric sentences using event-related functional magnetic resonance imaging (fMRI). Seventeen healthy subjects (6 female, 11 male) read 60 novel short German sentence pairs with either metaphoric or literal meaning and performed two different tasks: judging the metaphoric content and judging whether the sentence…
NASA Astrophysics Data System (ADS)
Gußmann, Alexander
2017-03-01
The existence of the classical black hole solutions of the Einstein-Yang-Mills-Higgs equations with non-Abelian Yang-Mills-Higgs hair implies that not all classical stationary magnetically charged black holes can be uniquely described by their asymptotic characteristics. In fact, in a certain domain of parameters, there exist different spherically-symmetric, non-rotating and asymptotically-flat classical black hole solutions of the Einstein-Yang-Mills-Higgs equations which have the same ADM mass and the same magnetic charge but significantly different geometries in the near-horizon regions. (These are black hole solutions which are described by a Reissner-Nordström metric on the one hand and the black hole solutions with non-Abelian Yang-Mills-Higgs hair which are described by a metric which is not of Reissner-Nordström form on the other hand). One can experimentally distinguish such black holes with the same asymptotic characteristics but different near-horizon geometries classically by probing the near-horizon regions of the black holes. We argue that one way to probe the near-horizon region of a black hole which allows one to distinguish magnetically charged black holes with the same asymptotic characteristics but different near-horizon geometries is by classical scattering of waves. Using the example of a minimally-coupled massless probe scalar field scattered by magnetically charged black holes which can be obtained as solutions of the Einstein-Yang-Mills-Higgs equations with a Higgs triplet and gauge group SU(2) in the limit of an infinite Higgs self-coupling constant we show how, in this case, the scattering cross sections differ for the magnetically charged black holes with different near-horizon geometries but the same asymptotic characteristics. We find in particular that the characteristic glory peaks in the cross sections are located at different scattering angles.
Polarity reversals and tilt of the Earth's magnetic dipole
NASA Technical Reports Server (NTRS)
Dolginov, A. Z.
1993-01-01
There is evidence that the terrestrial magnetic field is connected with the Earth's mantle: (1) there are magnetic anomalies that do not take part in the westward drift of the main field, but are fixed with respect to the mantle; (2) the geomagnetic pole position flips in a particular way by preferred meridional paths during a reversal; and (3) magnetic polarity reversals are correlated with the activations of geological processes. These facts may be explained if we take into account that a significant horizontal temperature gradient can exist in the top levels of the liquid core because of the different thermoconductivity of the different areas of the core-mantle boundary. These temperature inhomogeneities can penetrate the core because fluxes along the core boundary (the thermal wind) can be strongly suppressed by a small redistribution of the chemical composition in the top of the core. The nonparallel gradients of the temperature, density, and composition on the top of the core create a curled electric field that produces a current and a magnetic field. This seed-field can be amplified by motions in the core. The resulting field does not forget the seed-field distribution and in this way the field on the Earth surface (that can be created only in regions with high conductivity, i.e. in the core) is connected with the core-mantle boundary. Contrary to the usual approach to the dynamo problem, we will take into account that the seed field of thermoelectric origin is acting not only at some initial moment of time but permanently.
NASA Astrophysics Data System (ADS)
Wang, Zaijun; Ren, Zhongzhou; Dong, Tiekuang; Xu, Chang
2014-08-01
The ground-state spins and parities of the odd-A phosphorus isotopes 25-47P are studied with the relativistic mean-field (RMF) model and relativistic elastic magnetic electron-scattering theory (REMES). Results of the RMF model with the NL-SH, TM2, and NL3 parameters show that the 2s1/2 and 1d3/2 proton level inversion may occur for the neutron-rich isotopes 37-47P, and, consequently, the possible spin-parity values of 37-47P may be 3/2+, which, except for P47, differs from those given by the NUBASE2012 nuclear data table by Audi et al. Calculations of the elastic magnetic electron scattering of 37-47P with the single valence proton in the 2s1/2 and 1d3/2 state show that the form factors have significant differences. The results imply that elastic magnetic electron scattering can be a possible way to study the 2s1/2 and 1d3/2 level inversion and the spin-parity values of 37-47P. The results can also provide new tests as to what extent the RMF model, along with its various parameter sets, is valid for describing the nuclear structures. In addition, the contributions of the upper and lower components of the Dirac four-spinors to the form factors and the isotopic shifts of the magnetic form factors are discussed.
NASA Astrophysics Data System (ADS)
Wu, Wei
2013-05-01
The electronic structure and magnetic properties of LiFeAs and FeSe have been studied using hybrid-exchange density functional theory. The total energies for a unit cell in LiFeAs and FeSe with different spin states including non-magnetic and spin-2 are calculated. The spin-2 configuration has the lower energy for both LiFeAs and FeSe. The computed anti-ferromagnetic exchange interactions between spins on the nearest (next nearest) neighbouring Fe atoms in LiFeAs and FeSe are approximately 14 (17) meV and 6 (13) meV, respectively. The total energies of the checkerboard and stripe-type anti-ferromagnetic ordering for LiFeAs and FeSe are compared, yielding that for LiFeAs the checkerboard is lower whereas for FeSe the stripe-type is lower. However, owing to the fact that the exchange interaction of the next nearest neighbour is larger than that of the nearest one, which means that the collinear ordering might be the ground state. These results are in agreement with the previous theoretical calculations and experiments. Especially the calculations for LiFeAs indicate a co-existence of conducting d-bands at the Fermi surface and d-orbital magnetism far below the Fermi surface. The theoretical results presented here might be useful for the experimentalists working on the electronic structure and magnetism of iron-based superconductors.
Bénard, M; Berry, J F; Cotton, F A; Gaudin, C; López, X; Murillo, C A; Rohmer, M-M
2006-05-15
The ground-state electronic structure of the trinuclear complex Cu3(dpa)4Cl2 (1), where dpa is the anion of di(2-pyridyl)amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M3(dpa)4Cl2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag3(dpa)4Cl2 compound. Both coinage metal compounds display three singly occupied x2-y2-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3(dpa)4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu(II) atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1+ displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N4 moieties via four ligand pathways converging through the x2-y2 orbital of the central metal. The silver homologues of 1 and 1+ display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.
Stanley, R.K. )
1995-01-01
In a previous paper (Kirkwood and Stanley, 1992), the authors have outlined the total magnetic flux (TMF) method for the inspection of elongated ferromagnetic product, such as tubing, drill pipe, sucker rods, wire rope, and piping in refineries and chemical plants. This paper is presented to provide the scientific background to this nondestructive evaluation (NDE) method, which has not previously been presented, and to provide a list of references in which the technique is known to be used. Because the method detects wall loss in ferromagnetic steel parts, and is often used in conjunction with other NDE methods such as magnetic flux leakage (MFL), it represents yet another technique for rapid volumetric scanning of such parts.
Analytical theory of the shear Alfvén continuum in the presence of a magnetic island
Cook, C. R. Hegna, C. C.
2015-04-15
The effect of a magnetic island chain on the shear Alfvén continuum is calculated analytically. Using a WKB approximation of the linearized ideal MHD equations, the island is shown to cause an upshift in the continuum accumulation point frequency. This minimum of the frequency spectrum is shifted from the rational surface to the island separatrix. The structure of the eigenmodes is also presented.
NASA Astrophysics Data System (ADS)
Coppi, B.
2007-11-01
Differentially rotating structures in the prevalent field of a central object have been shown to develop a ``crystal'' magnetic structure resulting from toroidal internal currents and leading to the formation of density ring sequencesootnotetextB. Coppi and F. Rousseau, Ap. J. 641, 458 (2006) rather than disks. Poloidal current densities with appropriate symmetries are found to be connected with angular momentum transport processes represented by an effective viscosity. Jets are suggested to consist of a series of stable ``smoke- rings'' ejected vertically in opposite directions from the central region of the considered ring sequence. A small inward flow velocity is shown to induce a spiral pattern in the magnetic field lines on a selected family of magnetic surfaces. The accretion theoryootnotetextB. Coppi, Nuc. Fus. 42, 1 (2002) of the spontaneous rotation phenomenon in toroidal laboratory plasmas relies on the ejection of angular momentum toward the surrounding material wall, by collisional ballooning modes excited at the edge, whose phase velocity depends on collisionality. The resulting recoil gives rise to the rotation of the main body of the plasma column as other plasma modes (called VTG) provide the needed inward transport of angular momentum. *Sponsored in part by the US D.O.E.
Cao, Jinjia; Gong, Xueyu; Xiang, Dong; Huang, Qianhong; Wang, Aike; Yu, Jun
2016-01-15
The enhanced transport of passing energetic ions (PEIs) in presence of the resonant interactions with a rotating magnetic island is investigated within the drift kinetic framework. When the island rotation plays a role in the resonant interaction, we find that the velocities of PEIs satisfy a constraint relation of resonant flux surface in phase space. The resonant flux surfaces overlap with the magnetic flux surfaces in real space. A new transport channel responsible for the PEIs moving across the magnetic flux surfaces, i.e., continuously overlapping, is found. Two kinds of radial motions can be induced by the surface overlapping: one arises from the coupling between the resonance and the collision with the background plasma and the other from not completely overlapping of the two surfaces. The two radial motions and the symmetry-breaking induced radial motion constitute the total radial motion. When the pitch-angle scattering rate is very weak, the surface-shear induced transport is dominant. Only a small increase in the collision rate can significantly influence the total transport.
NASA Astrophysics Data System (ADS)
Waltz, R. E.; Ferraro, N. M.
2015-04-01
The linear response profiles for the 3D perturbed magnetic fields, currents, ion velocities, plasma density, pressures, and electric potential from low-n external resonant magnetic field perturbations (RMPs) are obtained from the collisional two-fluid M3D-C1 code [N. M. Ferraro and S. C. Jardin, J. Comput. Phys. 228, 7742 (2009)]. A newly developed post-processing RMPtran code computes the resulting quasilinear E×B and magnetic (J×B) radial transport flows with respect to the unperturbed flux surfaces in all channels. RMPtran simulations focus on ion (center of mass) particle and transient non-ambipolar current flows, as well as the toroidal angular momentum flow. The paper attempts to delineate the RMP transport mechanisms that might be responsible for the RMP density pump-out seen in DIII-D [M. A. Mahdavi and J. L. Luxon, Fusion Sci. Technol. 48, 2 (2005)]. Experimentally, the starting high toroidal rotation does not brake to a significantly lower rotation after the pump-out suggesting that convective and E×B transport mechanisms dominate. The direct J×B torque from the transient non-ambipolar radial current expected to accelerate plasma rotation is shown to cancel much of the Maxwell stress J×B torque expected to brake the plasma rotation. The dominant E×B Reynolds stress accelerates rotation at the top of the pedestal while braking rotation further down the pedestal.
Magnetic fields in spiral galaxies
NASA Astrophysics Data System (ADS)
Chiba, Masashi
The magnetic-field characteristics in spiral galaxies are investigated, with emphasis on the Milky Way. The dynamo theory is considered, and axisymmetric spiral (ASS) and bisymmetric spiral (BSS) magnetic fields are analyzed. Toroidal and poloidal magnetic fields are discussed.
Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves
NASA Astrophysics Data System (ADS)
Gregori, G.; Ravasio, A.; Murphy, C. D.; Schaar, K.; Baird, A.; Bell, A. R.; Benuzzi-Mounaix, A.; Bingham, R.; Constantin, C.; Drake, R. P.; Edwards, M.; Everson, E. T.; Gregory, C. D.; Kuramitsu, Y.; Lau, W.; Mithen, J.; Niemann, C.; Park, H.-S.; Remington, B. A.; Reville, B.; Robinson, A. P. L.; Ryutov, D. D.; Sakawa, Y.; Yang, S.; Woolsey, N. C.; Koenig, M.; Miniati, F.
2012-01-01
The standard model for the origin of galactic magnetic fields is through the amplification of seed fields via dynamo or turbulent processes to the level consistent with present observations. Although other mechanisms may also operate, currents from misaligned pressure and temperature gradients (the Biermann battery process) inevitably accompany the formation of galaxies in the absence of a primordial field. Driven by geometrical asymmetries in shocks associated with the collapse of protogalactic structures, the Biermann battery is believed to generate tiny seed fields to a level of about 10-21 gauss (refs 7, 8). With the advent of high-power laser systems in the past two decades, a new area of research has opened in which, using simple scaling relations, astrophysical environments can effectively be reproduced in the laboratory. Here we report the results of an experiment that produced seed magnetic fields by the Biermann battery effect. We show that these results can be scaled to the intergalactic medium, where turbulence, acting on timescales of around 700 million years, can amplify the seed fields sufficiently to affect galaxy evolution.
NASA Astrophysics Data System (ADS)
Scudder, J. D.; Mozer, F. S.
2005-05-01
Abrupt, intense bipolar and unipolar electric spikes with E > 100mV/m surveyed over 3 years of Polar data (Mozer et al 2005) have been organized to answer the limited question: can they be involved in the local demagnetization of thermal electrons? We determine a lower bound on the electric strength sufficient to cause non-gyrotropic effects on the electron pressure tensor of the form E>E*=B{we}/{8c Im}, where B is the ambient magnetic field strength, we=√2kTe/me, c is the speed of light, and Im is an electron velocity space weighted average displacement along the electric field while transiting the layer (assumed localized with a scale Δ x=a ρe, where ρe is the electron thermal gyroradius). The variation of Im as a function of a for equal mean energy Maxwellian and more typical κ distributions seen in the Earth's magnetosphere provides strong evidence that the surveyed electric field spikes are generally smaller than E* (assuming ≍ 1), although 23% (n=57) exceed E* . Only 11% (n=6) of the bipolar class exceed E* ; the frequency of occurrence distribution for the bipolar class of spikes is peaked at 0.1E*. The unipolar occurrence is flat below E*, but has a significant 26% subgroup (n=51) that exceed E* . While E* does not depend on the ambient density, the occurrence distribution of all demagnetizing events is well organized by the ratio ℜ=λDe/ρe=&Omegace/ωpe, residing almost exclusively in the regime ℜ <1. Spikes with E < E* generally occur with ℜ >1 . All the electrostatic spike events surveyed occur in the regime 10-8≤βe≤3×10-2. The demagnetizing events of either class occupy the more restricted low beta regime 10-3≤βe≤3×10-2. Because these demagnetizing events occur in βe ≪ 1 they would not, however, be considered unmagnetized at current channels as thin as the electron skin depth, de, since for such current channels ρe ≡ βe-1/2de ≪ de. As a group the subset of unipolar events with E > E* are consistently understood as
NASA Technical Reports Server (NTRS)
Lerche, I.
1981-01-01
An analysis is conducted regarding the properties of cylindrically symmetric self-similar blast waves propagating away from a line source into a medium whose density and magnetic field (with components in both the phi and z directions) both vary as r to the -(omega) power (with omega less than 1) ahead of the blast wave. The main results of the analysis can be divided into two classes, related to a zero azimuthal field and a zero longitudinal field. In the case of the zero longitudinal field it is found that there are no physically acceptable solutions with continuous postshock variations of flow speed and gas density.
NASA Technical Reports Server (NTRS)
Lerche, I.
1981-01-01
An analysis is conducted regarding the properties of cylindrically symmetric self-similar blast waves propagating away from a line source into a medium whose density and magnetic field (with components in both the phi and z directions) both vary as r to the -(omega) power (with omega less than 1) ahead of the blast wave. The main results of the analysis can be divided into two classes, related to a zero azimuthal field and a zero longitudinal field. In the case of the zero longitudinal field it is found that there are no physically acceptable solutions with continuous postshock variations of flow speed and gas density.
NASA Astrophysics Data System (ADS)
Ćaǧlar, Tolga; Berker, A. Nihat
2015-12-01
Hard-spin mean-field theory has recently been applied to Ising magnets, correctly yielding the absence and presence of an interface roughening transition respectively in d =2 and d =3 dimensions and producing the ordering-roughening phase diagram for isotropic and anisotropic systems. The approach has now been extended to the effects of quenched random pinning centers and missing bonds on the interface of isotropic and anisotropic Ising models in d =3 . We find that these frozen impurities cause domain boundary roughening that exhibits consecutive thresholding transitions as a function of interaction anisotropy. For both missing-bond and pinning-center impurities, for moderately large values of the anisotropy, the systems saturate to the "solid-on-solid" limit, exhibiting a single universal curve for the domain boundary width as a function of impurity concentration.
NASA Astrophysics Data System (ADS)
Caglar, Tolga; Berker, A. Nihat
Hard-spin mean-field theory has recently been applied to Ising magnets, correctly yielding the absence and presence of an interface roughening transition respectively in d = 2 and d = 3 dimensions and producing the ordering-roughening phase diagram for isotropic and anisotropic systems. The approach has now been extended to the effects of quenched random pinning centers and missing bonds on the interface of isotropic and anisotropic Ising models in d = 3. We find that these frozen impurities cause domain boundary roughening that exhibits consecutive thresholding transitions as a function of interaction anisotropy. For both missing-bond and pinning-center impurities, for moderately large values of the anisotropy, the systems saturate to the ''solid-on-solid'' limit, exhibiting a single universal curve for the domain boundary width as a function of impurity concentration.