Presence Of External Field Research Articles

The modified XXZ Heisenberg chain, conformal invariance and the surface exponents of c<1 systems

The authors present the operator content of the XXZ chain in the presence of external fields. The finite-size scaling spectra can be expressed in terms of irreps of shifted U(1) Kac-Moody algebras, the shift depending on the external fields. Using an explicit Feigin-Fuchs construction (1982), they project out from these spectra the operator content of c<1 systems with free, fixed and mixed boundary conditions. They also show how this projection mechanism works for a finite number of sites.

Statistical properties of a multisegmental model of a polymer chain consisting of quasi-elastic segments with fixed average length in the presence of external fields or molecular self-consistent liquid crystalline field (of the mayersuape type)

A dynamic model of a polymer chain is proposed consisting of segments with fixed length for which the authors consider the statistical properties in the oriented state in external mechanical or electric fields and in the self-ordered liquid crystalline phase. The model describes long macromolecules with thermodynamic flexural rigidity or macromolecules with a constant valent angle for an arbitrary degree of ordering. Expressions are obtained for the free energy of the chain in the oriented state, the degree of order and the size of the chain along and perpendicular to the direction of ordering and the characteristics of the phase transition to the liquid crystalline state in the self-consistent field approximation are examined. A comparison is made with the results of the Rusakov and Shliomis theory for a chain of undeformed segments.

Resonance tunneling in a periodic time-dependent external field.

Resonance tunneling in the presence of external field W(x,t)=W(x)cos(\ensuremath{\omega}t) is investigated within the framework of the classical perturbation theory. This approach is applied to a quantum-well structure in a superposition of a dc and sufficiently small ac (laser) field. Rigorous expressions for the shape of the tunneling spectra are obtained in a number of physically important cases. An ac field of the frequency \ensuremath{\omega} large compared to the resonance width \ensuremath{\Gamma} is shown to split the resonance peak into a family of well-separated peaks with spacing between them equal to \ensuremath{\omega}. In the limit of weak, slowly varying fields the form of the resonance peak is determined by the mean value of the time-dependent potential in the well only.

Mixing of the photon with low-mass particles.

Photons can mix with low-mass bosons in the presence of external electromagnetic fields if these particles---not necessarily of spin 1---couple by a two-photon vertex. Important examples are the hypothetical axion (spin 0) and graviton (spin 2). We develop a formalism which is adapted to study the evolution of a photon (axion, graviton) beam in the presence of external fields. We apply our results to discuss the possibility of detecting axions by a measurement of the magnetically induced birefringence of the vacuum. We also discuss photon-axion (graviton) transitions in pulsar magnetic fields. The QED-induced nonlinearity of Maxwell's equations causes magnetic birefringence effects which are much stronger than the axion-induced effects in the range of axion parameters allowed by astrophysical constraints. Also, this QED effect induces an index of refraction for photons in vacuum which is so large near pulsars that photon-axion (graviton) transitions are strongly suppressed. However, this QED effect can be canceled by plasma refractive effects, leading to degeneracy between photons and axions so that resonant transitions can occur in analogy with the Mikheyev-Smirnov-Wolfenstein effect. The adiabatic condition can be met only in spatially extended systems, possibly in the magnetosphere of magnetic white dwarfs. Our conclusions differ substantially from several recent discussions of various aspects of these mixing phenomena.

Killing vectors, constraints and conserved charges in Kaluza-Klein theories

The traditional Abelian and non-Abelian Kaluza-Klein ansatze are analysed from a geometrical point of view using a Bochner-Yano type theorem for geodesic Killing vector fields. It is shown how the presence of Killing vectors in the ansatz gives rise to constraints on the Yang-Mills fields and the curvature. The conserved Komar quantities associated with these Killing vectors are computed and compared with the corresponding ADM charges. A crucial difference between them in the presence of external fields is pointed out.

Determination of density matrices of H( n = 3) atoms formed in charge transfer between protons and helium atoms

Fast hydrogen atoms are formed in charge transfer reactions between incident protons (60 keV) and helium atoms. The formed excited hydrogen atoms are allowed to decay in the presence of externally applied electric fields which dramatically change the intensity and polarization of the emitted radiation. Balmer-alpha radiation emitted from a 7 mm long section of the proton beam is measured as a function of applied electric field strength using a precision polarimeter. Detailed analysis of the experiment in which the time evolution of the hydrogen atoms in the presence of external fields is calculated allows the extraction of the full, axially symmetric density matrix of the H( n = 3) atoms at the time of their production.

Treatment of the relaxed F center in KCl with the recursion method.

Ham's vibronic model for the relaxed excited state of the F center in KCl is investigated with a new approach based on the recursion method. We have calculated the energy and the wave function of the ground vibronic state and the lowest excited state, the dichroism in light emission in the presence of external fields (magnetic field, electric field, uniaxial stress), and the temperature dependence of the polarization induced by the electric field and by the uniaxial stress. Ham's vibronic model is found to account satisfactorily for the above properties. The perspectives of the recursion procedure for studying coupled electron-phonon systems are discussed.

Revisiting sine-Gordon kink dynamics in the presence of external fields

In this paper we discuss Dash's results on the dynamical behavior of a perturbed sine-Gordon (sG) equation. He intends to recover a newtonian behavior for an sG kink by means of a particular choice of initial scattering states a priori different (in what regards physical conclusions) from the one set by Fernandez et al. Following Dash by introducing the characteristic time t 0 when the phonons vanish, we show that we obtain a non-newtonian dynamics. By this we mean that there is no newtonian relationship connecting the applied force to the soliton velocity. We suggest, indeed, that Dash made an erroneous expansion leading to the discrepancy with Fernandez's conclusions.

On the interaction of spin and torsion

The geometry of Cartan connections and their relation to the theory of spontaneous symmetry breaking is discussed. In S. Sternberg and T. Ungar [ Hadronic J. 1 (1978), 33–36] Cartan connections are used to derive equations of motion for classical particles in the presence of external fields. Examined here is the case of a massive chargeless particle whose spin interacts with the curvature and torsion of a gravitational field. These equations are solved for the case of a constant vector torsion on Minkowski space, and the solution for a vector torsion wave is indicated.

Sine-Gordon kink dynamics in the presence of external fields

The main cause of the prevalent confusion over the nature of sine-Gordon solitons is found to be the particular choice of initial scattering states. With the help of different initial conditions the dynamics of the sG kink is obtained using the linear perturbation technique. This justifies the newtonian particle behaviour in the appropriate limit.

Magnetic properties ofMnF2andCoF2determined by implanted positive muons. I. Localization studies

In order to define the characteristics of implanted positive muons as probes of the magnetic properties of materials, simple and well-known antiferromagnetic crystals have been studied: ${\mathrm{CoF}}_{2}$ and ${\mathrm{MnF}}_{2}$. The present paper aims to establish the localization of implanted muons in the temperature region of most interest, i.e., near the phase transitions and in the ordered phase. In the companion paper the measured magnetic features of the crystals are presented and compared with the available data from NMR, neutron scattering, and magnetic susceptibility measurements. The localization of muons has been inferred from the symmetry and magnitude of the frequency shifts and splittings of the muon-spin-rotation signal in the presence of external fields both above and below ${T}_{N}$. A unique site of localization is evident in ${\mathrm{CoF}}_{2}$, which has octahedral symmetry, where the muon experiences a nearly pure dipolar field generated by the Co spins: the local field due to the spontaneous magnetization at low temperatures is 2280 G and corresponds to the value calculated from simple dipole sums. In ${\mathrm{MnF}}_{2}$, some uncertainty still remains between the two octahedral- and tetrahedral-symmetry sites.

On the one-electron theory of crystalline solids in a homogeneous electric field

Assuming that the one-electron states of a perfect crystalline solid are known, an approach for the calculation of the one-electron states in the presence of external fields and/or other deviations from the periodic potential of the perfect crystal is suggested. The treatment is based on the Wannier representation and the use of a method for solving some operator non-polynomial differential equations. In the approximation of the one-band Wannier equation an exact solution of the problem for electron states of the crystals in a homogeneous external electric field is given. The results obtained in the tight binding approximation for cubic crystals show that the electron motion along the field is finite and the degree of its finiteness for a given electric field strength is greater, the smaller the width of the initial energy band considered. In the one-band approximation Considered the electron energy spectrum has the character of the Wannier-Stark ladder. It is also shown that an influence of transverse motion on the character of the finite motion along the field appears when a non-additivity in the initial energy band function with respect to the energies of motions parallel and perpendicular to the field direction is present.

Liquid-crystalline ordering in solutions of rigid macromolecules in the presence of external fields

A study was made of the effect of external fields of dipole and quadrupole type on liquid-crystalline ordering in solutions of very rigid macromolecules (rigid rods) and freely joined semiflexible-chain macromolecules (sequences of rigid rods hinged into long chains). Corresponding phase diagrams were plotted showing variables of field-concentration. A fairly strong orientation field of both types suppresses liquid-crystalline phase transition, which explains the existence of a critical point on the phase diagrams. At the same time the application of fairly strong “disorientation” field of quadrupole type results in the transformation of a first order liquid-crystalline phase transition into a second order transition.

Low-temperature ordered states of dysprosium, terbium, and holmium aluminum garnets

The low-temperature ordered states of dysprosium, terbium, and holmium aluminum garnets have been investigated by the Luttinger-Tisza method, taking rigorously into account the dipole-dipole interaction. It is found that for each of these cases, in the absence of an external magnetic field, the lowest-energy configuration corresponds to antiferromagnetic ordering with three pairs of sublattices, in agreement with neutron scattering experiments. In the presence of external field along the [001] direction, the lowest-energy configuration turns out to be that of ferromagnetic chains in the $x$ and $y$ directions, with the nearest ferromagnetic chains pointing in the opposite directions, the same as that proposed by Landau and Keen. When the external magnetic field is along the [110] direction, there are two configurations which correspond to lowest energy; the individual chains are ferromagnetic but not all ferromagnetic chains are parallel, there being chains which point in opposite directions.

The Boltzmann-equation description of transport in superconductors

Abstract The purpose of the present review article is to give an outline of a quasiclassical approach to the description of transport in superconductors. The Boltzmann equation for normal excitation in a superconductor is formulated and its various properties are discussed and a full set of equations to describe condensate dynamics in the presence of external fields is given. Applications of this approach are demonstrated with a number of examples.

Phenomenology of neutron oscillations

We have recently argued that gauge models with spontaneously broken local B − L symmetry can lead to large ΔB = 2 nucleon transition amplitudes for processes such as n− n oscillations and n + p → pions. We discuss the phenomenology of neutron oscillations both in vacuum and in the presence of external fields. These considerations indicate that neutron oscillation experiments can be much more sensitive to the ΔB = 2 amplitude than tests of nucleon instability.

Quantum interference and the classical limit

A relationship between the quantum phase shift due to an external field in an (infinitesimal) two-beam interference experiment and the classical transverse acceleration, previously given by the author, is generalized covariantly and the phase shift associated with the longitudinal acceleration is deduced. This relation implies that the mass of a spinless elementary particle is a constant of motion, even in the presence of external fields in a curved space-time. Also according to this relation, the infinitesimal Aharonov-Bohm effect is equivalent to the Lorentz force law. The case of a charged particle moving in the electromagnetic field due to electric and magnetic charges obeying Dirac's quantization condition is treated and a prescription is given for constructing its quantum mechanical wave function directly from the classical equation of motion, using path integrals, without requiring a knowledge of the Lagrangian or Hamiltonian of the interaction. Generalization to an arbitrary gauge field is briefly considered and a theorem is proven which implies that the holonomy transformations (path-ordered operators that parallel transport around closed curves) at an arbitrary space-time point contain all the gauge-invariant information of a gravitational or gauge field connection.

Magneto-elastic coupling by dipolar interactions : Spin orientation in MnO

MnO is a low temperature antiferromagnet in which a spin orientation anisotropy within the plane of easy magnetization has been observed experimentally. The microscopic origin of this anisotropy has not yet been elucidated. As a consequence there is a certain uncertainty in the determination of the magnetic structure and in the interpretation of the magnon spectrum. We show here that magneto-elastic coupling by dipolar interactions leads to well defined anisotropy axis which are determined to be the [1, 1̄, 0] directions modulo 2π/3. We have calculated the values of coupling coefficients which are derivatives of the components of the dipolar tensor with respect to the components of the strain tensor. Moreover using the mechanism proposed in this paper it should be possible to interpret correctly the magnetic properties of MnO in presence of external fields (magnetic field and mechanical stresses).

Ising Model on the Bethe Lattice with Competing Interactions

A system composed of Ising spins on the Bethe lattice having the antiferromagnetic nearest neighbour interactions and the ferromagnetic infinite range interactions independent of distance is treated rigorously in the presence of external field. The interesting phase diagram is obtained, which is compared with the result by the mean field approximation.

Mössbauer effect studies of spin-spin relaxation in single crystals of Fe(NO 3) 3·9H 2O at 4.2 K

The spin-spin relaxation of Fe 3+ ions in Fe(NO 3) 3·9H 2O single crystals has been studied at 4.2 K in the presence of external fields up to 80 kG. It is found that the spin-spin relaxation frequency varies substantially when the field direction is changed. Furthermore, the relaxation frequency increases with increasing applied field. Both effects are in agreement with the spin-spin relaxation theory of Bloembergen et al.