External Transverse Field Research Articles

Saddle-splay elasticity of nematic structures confined to a cylindrical capillary.

The stability of nematic structures within a cylindrical capillary whose wall exhibits a homeotropic boundary condition is studied. The structures are obtained numerically from Euler-Lagrange equations resulting from the minimization of the Frank free energy functional. Stability diagrams are presented showing dependence on elastic properties, surface anchoring, and external transversal field strength. Emphasis is given to the effects of the saddle-splay elastic constant (${\mathit{K}}_{24}$), which plays an important role in the weak anchoring regime. A new structure---the planar polar structure with two line defects---is predicted. It is shown that it is stable in a finite interval of the external field strength in the strong anchoring regime.

Micromagnetics of small unshielded MR elements

A two-dimensional micromagnetic simulation is carried out for small unshielded magnetoresistive (MR) elements. The response of the magnetization to uniform cycling of a transverse external field is studied. For an isolated element without any bias field, the magnetic response is hysteretic, resulting in noisy MR output. For longitudinal patterned exchange bias, the pinning width has to exceed a critical value to achieve a noise-free response. Increasing the edge biasing angle beyond 45° induces undesirable multidomain activities. Soft adjacent layer (SAL) biasing is investigated by using a dual-layer model. Various exchange biasing schemes of the end regions of the MR and SAL layers have been numerically implemented, and it is seen that symmetric exchange coupling of the edge regions of both layers yields a desirable MR response.

Theoretical evaluation of longitudinal a.c. losses in large-capacity superconducting cables for [formula omitted] Hz use

When an a.c. transport current and/or an a.c. magnetic field are applied to multiply twisted superconducting cables, a magnetic field along strands composed of homogeneous and spatially periodic components is induced in addition to the field perpendicular to the strands. The time variation of this longitudinal field along the strands also brings about hysteresis and coupling-current losses. Theoretical expressions for the additional losses are derived both in the self-field configuration of the cable and in the external transverse field configuration. In the self-field configuration, the hysteresis loss due to the homogeneous component is dominant, which can be sufficiently decreased by the cabling procedure and optimization of the twist pitch in the strand. In the external transverse field, on the other hand, the development of high current density superconductors may be necessary for decreasing the a.c. cables in addition to optimization of the cable structure.

Dynamics of single-plaquette Josephson-junction arrays.

We have studied, both analytically and numerically, the response of a single-plaquette square array to a combined ac and dc current. The array consists of 2\ifmmode\times\else\texttimes\fi{}2 superconductors coupled by overdamped Josephson junctions. Our study shows that even a single-plaquette array exhibits all the essential features found in the large arrays; in zero magnetic field, or with a current in the diagonal direction, the array does not show any fractional Shapiro step. On the other hand, in a transverse external field except with diagonal application of a current, the dc I-V characteristics exhibit very rich structure, i.e., a step at every rational number of 2e〈V〉/h\ensuremath{\nu}. The step widths \ensuremath{\Delta}${\mathit{I}}_{\mathit{n}}$ calculated for n=1/2,1/3, 1) / 4 , 1/5, and 1/6 with varying filling factor f show varied characteristic features.

交流用大容量超伝導導体における縦磁界交流損失の理論的評価

When an a.c. transport current and/or an a.c. magnetic field are applied to multiply-twisted superconducting cables, magnetic field along strands composed of homogeneous and spatially periodic ones is induced in addition to the field perpendicular to the strands. The time variation of this longitudinal field along the strands also brings about the hysteresis and coupling-current losses. Theoretical expressions for the additional losses are derived both in the self-field configuration of the cable and in the external-transverse-field one. In the self-field configuration, the hysteresis loss due to the homogeneous component is dominant, which can be sufficiently decreased by the cabling procedure and the optimization of twist pitch in the strand. In the external transverse field, on the other hand, the development of high-current-density superconductors may be necessary for decreasing the losses in the a.c. cables in addition to optimization of the cable structure.

A muon spin rotation study of hydrogen in YBa2Cu3O y

Superconducting YBa2Cu3Oy samples were doped with hydrogen and investigated by the muon spin rotation technique. For hydrogen concentrations above a thresholdx, which depends on the oxygen stoichiometry, we find a well defined precession signal in zero external field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples however, the depolarization rate σ(T→0) as measured in a transverse external field depends strongly on the hydrogen content. Our data are consistent with the assumption that hydrogen acts as an electron donor, filling the hole states in YBa2Cu3Oy.

Determination of plasma shift in a stellarator from magnetic measurements**Translated by A. Watson and S.A. Edminster, English Translation Section, Division of Languages, IAEA, Vienna.

The poloidal magnetic field created by plasma currents in an ordinary stellarator is calculated. An analytical calculation is performed for a plasma column of ‘average’ circular cross-section in the large-aspect-ratio approximation. The problem of determining the position of the column from poloidal magnetic field and flux measurements is investigated. A formula is obtained which links the plasma shift relative to the geometric axis of the stellarator with measurable quantities, and a simple system of magnetic measurements is proposed. The analysis is performed for a finite-beta plasma with allowance for longitudinal current and the external transverse field.

μ + sites and local moments in Van-Vleck paramagnet: PrNi5

Multi-frequencyμ+ SR spectra were observed in PrNi5 at 23 K in an external transverse field of 3010 G. Three types of interstitial sites are identified as the most likely muon stopping positions. A substantial amount of the magnetic moment sampled by the μ is found to be localized on the Ni sites.

Calculations on the current density and the voltage-current relation under a.c. conditions of filaments

Technical applications of multifilamentary wires indicate that filaments are used in complex magnetic fields (a combination of non-parallel a.c./d.c. transverse and rotating fields) carrying an a.c./d.c. transport current of various frequency. Furthermore, due to technical manufacturing processes the filaments are heavily distorted. Therefore, a numerical model is developed to compute the current density of a filament of arbitrary shape in any external transverse field carrying an a.c./d.c. transport current. The great flexibility of the model is shown in several examples.

Binary magnetic lattice gas in transverse field

The response of a magnetic lattice gas of molecules of variable size (single-particle or a pair in this model) to the presence of external magnetic field is considered. The properties of magnetically anisotropic liquid phase are examined, especially the spontaneous appearance of a two-sublattice structure above a threshold valueГ0 of the external magnetic transverse field. The mechanism governing the concentration of single and complex particles in such a magnetic mixture is discussed with respect to the changes of the temperature, external magnetic fields and interactions (magnetic and non-magnetic).

Potential Distribution and Interaction between two Parallel Dielectric Cylinders in a Static Transverse Field. II. Limiting Case of Contacting Cylinders

AbstractThe potential profile and the forces of interaction between two dielectric cylinders of radii R1, and R2, dielectric constants ϵ1, and ϵ2, and acted on by a transverse external field, is evaluated in the limiting case of the two bodies in contact. The results for a static field carry over for alternating fields, provided that dielectric losses do not contribute significantly.

Reaction-field calculation for the quantum Potts model

The reaction-field approximation is applied to the d-dimensional q-state quantum Potts model in an external transverse field. With the exception of d<or approximately=3, the tricritical value qc(d), below which the system has a second-order phase transition, decreases with increasing external field strength. From the ground state of the quantum d-dimensional model, an improved value of qc is obtained for the classical model in d+1 dimensions. A direct explanation is also obtained for the expected tricritical point in finite-thickness slabs of cubic ferromagnets (like DyAl2) with a field in the (111) direction.

Hyperfine field calculations: Search for muon stopping sites in Fe3O4

Muon Spin Rotation (μSR) results for magnetite (Fe3O4) are analyzed and discussed. At room temperature, a μSR signal is observed due to the presence of an internal magnetic field (Bint) at the muon site. External transverse field measurements show that Bint is parallel to the magnetic spin direction, the direction in zero applied field. Calculations of the hyperfine field to pinpoint muon stopping sites in magnetite show that the local field contains supertransfer (covalent) and dipolar field contributions. The implanted muons appear to stop at sites structurally similar to those reported for hematite (α-Fe3O4), where muon-oxygen bond formation was strongly indicated.

Detonation driven induction generators with parallel and antiparallel external and induced magnetic fields

Based on Maxwell’s equations, the electric current and voltage pulses induced in explosion driven induction generators (Rm =μ0σvL≫1) with plane electrodes and homogeneous external magnetic field B0 are calculated analytically for two plasma shock flows, (i) the jet flow with shock speed v(t)=v0, t≥0, and (ii) the plane detonation flow with shock speed v(t)=(2/3)(E0/ρ0)1/3t−1/3, t≥0. The external load circuit with resistance R0 and inductance L0 is connected either (I) to the downstream ends or (II) to the flow entrance ends of the electrodes so that the magnetic self-field B̃(t) of the generated current I(t) is (I) parallel and (II) antiparallel to the transverse external field B0. In both generator models I and II, the jet flow produces large current and voltage pulses, whereas the detonation flow generates considerably smaller and shorter current and voltage pulses. In all cases, the magnetic self-field B̃(t) of the generated current I(t) is of the order-of-magnitude of B0, but the generator with electrode wiring I (positive superposition of B̃ and B0) produces considerably more power and energy than the generator with electrode wiring II (negative superposition of B̃ and B0).

Models for ?+ depolarization in spin glasses reflecting dynamical processes

The muon spin depolarization function is calculated for different dynamical processes. Muon diffusion is shown to cause motional narrowing at high temperatures (T≫Tf=freezing temperature of the spin glass) as well as at low temperatures (T<Tf), where the static line is essentially lorentzian due to the randomness of the spin distribution of the dilute system. Different models for μ+ depolarization reflecting the freezing process of the spin glass system above Tf consider single correlation times as well as wide distributions of correlation times. It will be shown that the μSR depolarization investigated in transverse external fields does not enable us to distinguish between single correlation times and wide correlation time distributions. Therefore, an interpretation of the spin dynamics requires a comparison of TF-μSR with results of different measuring methods.

Losses in composite superconductors at a high level of magnetic field excitation: Part 2

Abstract At relatively high amplitudes and variation rates of a transverse external field, the saturated zone may occupy a considerable part or the entire space of the composite. Differential equations have been derived which enable one to calculate the time dependence of the magnetic moment of screening currents in a round composite conductor under these conditions. This dependence helps, the calculation of losses for different external field variation signals.

Losses in composite superconductors at high levels of magnetic field excitation: part 1

Abstract At relatively high amplitudes and variation rates of a transverse external field, the saturated zone may occupy a considerable part or the entire space of the composite. Differential equations have been derived which enable one to calculate the time dependence of the magnetic moment of screening currents in a round composite conductor under these conditions. This dependence helps, the calculation of losses for different external field variation signals.

Dispersion relation and inelastic light scattering of antiferromagnetic polaritons

In the long-wavelength region, magnons and photons mix to form modes which may be called magnetic polaritons. Using the Bloch equations of motion and classical electromagnetic theory the authors have calculated the linear response of antiferromagnets to a transverse external RF field. The poles of the response function give the dispersion relations of the polaritons. With a strong exchange field they derive an approximate explicit form of the dispersion relation. With the aid of the fluctuation dissipation theorem the correlation functions which govern inelastic light scattering are derived from the response functions. Expressions are given for the scattering cross section and the frequency-dependent linewidth. The results are illustrated with calculations for MnF2.

Theory of inelastic light scattering off antiferromagnetic polaritons

Abstract We calculate the classical linear response functions of antiferromagnets in transverse external fields. The poles give the dispersion relations of the polaritons. We deduce the correlation functions which govern Raman scattering and find the cross section. The dispersion curve and cross section are plotted for MnF2.

Effect of transverse magnetic field on axial magnetic surfaces of a three-turn stellarator perturbed by an equilibrium plasma

The question of maintaining plasma equilibrium in a three-turn stellarator by means of an external transverse field H⊥ is investigated in an ideal magnetohydrodynamic model as a first approximation with respect to β, where β is the ratio of the gas-kinetic pressure of the plasma to the magnetic pressure. An “island” structure of the paraxial magnetic surfaces is observed under conditions of precise compensation and slight undercompensation of the homogeneous component of the plasma magnetic field by the field H⊥. The optimum conditions are found for controlling the position of the magnetic axis with an external transverse field. Relations are obtained for estimating the maximum equilibrium pressure of the plasma in a three turn stellarator in the presence of an external transverse magnetic field.