Large External Magnetic Field Research Articles

EXACT VECTORIAL LAW FOR AXISYMMETRIC MAGNETOHYDRODYNAMICS TURBULENCE

Three-dimensional incompressible magnetohydrodynamics turbulence is investigated under the assumptions of homogeneity and axisymmetry. We demonstrate that previous works of Chandrasekhar may be improved significantly by using a different formalism for the representation of two-point correlation tensors. From this axisymmetric kinematics, the equations ` al a von K´´ an–Howarth are derived from which an exact relation is found in terms of measurable correlations. The relation is then analyzed in the particular case of a medium permeated by an imposed magnetic field B0. We make the ansatz that the development of anisotropy implies an algebraic relation between the axial and the radial components of the separation vector r and we derive an exact vectorial law which is parameterized by the intensity of anisotropy. The critical balance proposed by Goldreich & Sridhar is used to fix this parameter and to obtain a unique exact expression; the particular limits of correlations transverse and parallel to B0 are given for which simple expressions are found. Predictions for the energy spectra are also proposed by a straightforward dimensional analysis of the exact law; it gives a stronger theoretical background to the heuristic spectra previously proposed in the context of the critical balance. We also discuss the wave turbulence limit of an asymptotically large external magnetic field which appears as a natural limit of the vectorial relation. A new interpretation of the anisotropic solar wind observations is eventually discussed.

Iron speciation in natural hyperacid water investigated by Mössbauer spectroscopy

We have demonstrated the usefulness of the archetypical solid state-technique of Mössbauer spectroscopy to non-invasive studies of the redox and coordination chemistry of iron in a natural hyperacid solution from Iron Mountain, CA. Suitable fast cooling conditions were used to prepare a glass from the natural solution. Measurements of spectra at low temperatures (6 and 15 K) and in large external magnetic fields (6 T) of the glassy, frozen solution revealed a behaviour characteristic of slow paramagnetic relaxation of ferric ions. Analyses of the spectral components are in good agreement with dominance of hexaaquairon(III) ions.

Magnetization of the QCD vacuum at large fields

The response of the QCD vacuum to very large static external magnetic fields (q B >> Lambda_QCD^2) is studied. In this regime, the magnetization of the QCD vacuum is naturally described via perturbative QCD. Combining pQCD and the Schwinger proper time formalism, we calculate the magnetization of the QCD vacuum due to a strong magnetic field at leading order (one-loop) to be proportional to B log B. We show that the leading perturbative correction (two-loop) vanishes.

Room-temperature simultaneously enhanced magnetization and electric polarization in BiFeO3 ceramic synthesized by magnetic annealing

Multiferroic BiFeO3 ceramics were synthesized by high temperature magnetic annealing using nanosized precursor powders prepared through microwave combustion. Simultaneously enhanced magnetization and electric polarization were observed at room temperature in the sample annealed under an external magnetic field of 10 T. These enhanced properties might be originated from a spin structure severely modulated or the low temperature magnetic phase driven up to room temperature and above by the large external annealing magnetic fields. These results demonstrate that the strong magnetic annealing method is an alternative way to synthesize high performance BiFeO3 materials.

Derivation of a gyrokinetic model. Existence and uniqueness of specific stationary solution

A finite Larmor radius approximation is derived from the classical Vlasov equation, in the limit of large (and uniform) external magnetic field. We also provide an heuristic derivation of the electroneutrality equation in the finite Larmor radius setting. Existence and uniqueness of a solution is proven in the stationary frame for solutions depending only on the direction parallel to the magnetic field and factorizing in the velocity variables.

Effect of the External Magnetic Field on the Resonance Scattering in Cuprate Superconductors

Within the kinetic energy driven superconducting mechanism, the effect of a uniform external magnetic field on the resonance scattering in cuprate superconductors in the superconducting state is studied. It is shown that the commensurate resonance scattering at zero external magnetic field is induced into the incommensurate resonance scattering by applying a large enough external magnetic field. The part of the spin excitation dispersion seems to be an hourglass-like dispersion, which breaks down at the heavily low energy regime.

Faraday Cup Measurements of the Energy Spectrum of Laser-Accelerated Protons

Several experiments at the Nevada Terawatt Facility (NTF) study the interaction of laser-created plasmas with large external magnetic fields. Examples include a ldquosolar windrdquo experiment that studies the development of a shock when an ablation plasma flow interacts with a strong magnetic field, and an isochoric heating experiment, in which the effect of a confining external magnetic field on target heating will be investigated. The plasmas can be created with one of our two multiterawatt laser systems, namely, Tomcat (up to 10 TW) and Leopard (up to 100 TW). Analyzing the experiments requires a thorough understanding of the initial conditions of the laser plasma. The measurements performed with Faraday cups yield the hot-electron temperature, which is characteristic for electrons that directly absorb the laser energy. Energy spectrum measurements have been carried out with Tomcat pulses (1057 nm) with up to 4-J pulse energy and 4 ps/1 ps duration. For an optimized focal spot size, a hot-electron temperature of more than 2.6 keV was measured, which is consistent with a laser intensity between 1015 and 1016 W/cm2.

A topological Dirac insulator in a quantum spin Hall phase

When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields. Bulk Bi(1-x)Sb(x) single crystals are predicted to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic surface states that are higher-dimensional analogues of the edge states that characterize a quantum spin Hall insulator. In addition to its interesting boundary states, the bulk of Bi(1-x)Sb(x) is predicted to exhibit three-dimensional Dirac particles, another topic of heightened current interest following the new findings in two-dimensional graphene and charge quantum Hall fractionalization observed in pure bismuth. However, despite numerous transport and magnetic measurements on the Bi(1-x)Sb(x) family since the 1960s, no direct evidence of either topological Hall states or bulk Dirac particles has been found. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy (IPEM-ARPES), we report the direct observation of massive Dirac particles in the bulk of Bi(0.9)Sb(0.1), locate the Kramers points at the sample's boundary and provide a comprehensive mapping of the Dirac insulator's gapless surface electron bands. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the 'topological metal'. They also suggest that this material has potential application in developing next-generation quantum computing devices that may incorporate 'light-like' bulk carriers and spin-textured surface currents.

Perpendicular Head Structures to Improve the Corner Erasure Robustness

Data erasure can arise when a hard disk drive is exposed to a large external magnetic field. This has been raised as a particular concern for drives employing perpendicular recording. Flux tends to concentrate between the head and the soft-underlayer of the disk, especially around the edges and corners of the head where the data erasure is often most pronounced. In this paper, we use 3-D finite-element modeling to investigate the effects of head geometry on the fields that cause data erasure. An in-plane external field causes concentration of magnetic flux at the edges of the return pole (RP) and the upper and lower shields of the reader (USL/LSL). We found increasing the length (downtrack) of these structures is effective in reducing the erasure fields. Aligning the edges of the RP, USL, and LSL is also shown to be a practicable and effective way to reducing the erasure fields. These erasure fields are aggravated when the write element is active. The fields that arise during writing must be taken into consideration when assessing data erasure

Electron Fractionalization in Two-Dimensional Graphenelike Structures

Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. In this Letter, we show that fractionally charged topological excitations exist on graphenelike structures, where quasiparticles are described by two flavors of Dirac fermions and time-reversal symmetry is respected. The topological zero modes are mathematically similar to fractional vortices in p-wave superconductors. They correspond to a twist in the phase in the mass of the Dirac fermions, akin to cosmic strings in particle physics.

Quenching of the Nonlocal Electron Heat Transport by Large External Magnetic Fields in a Laser-Produced Plasma Measured with Imaging Thomson Scattering

We present a direct measurement of the quenching of nonlocal heat transport in a laser-produced plasma by applying large external magnetic fields (>10 T). The temporally resolved Thomson-scattering measurements of the electron temperature profile show that the heat front propagation transverse to a high-power laser beam is slowed resulting in extremely strong local heating. We find agreement with hydrodynamic modeling when including a magnetic field model that self-consistently evolves the fields in the plasma.

Photon propagation in magnetic and electric fields with scalar/pseudoscalar couplings: A new look

We consider the minimal coupling of two photons to neutral scalar and pseudoscalar fields, as, for instance, in the case of the Higgs boson and axion, respectively. In this framework, we analyze the photon dispersion relations in the presence of static and homogeneous external magnetic and electric fields, by taking into account the contribution of the imaginary part of the scalar/pseudoscalar self-energy. We show that this contribution cannot be neglected when it is of the same order as the photon-scalar/pseudoscalar mixing term. In addition to the usual lightlike photon propagation mode, with a refraction index $n>1$, a massive mode with mass of the order of the coupled boson mass can be induced, provided that the external field is above a particular critical value. Depending on the values of the external field, photon energy, and mass of the scalar/pseudoscalar particle, the scalar/pseudoscalar width could induce a sizable rate of photon splitting in two photons due to a strong resonant phenomenon. This effect has no practical laboratory applications for the Higgs physics due to the very large critical external magnetic or electric fields involved, for a photon energy of the order of a TeV. However, it can have relevant consequences in the axion physics or in any other scenario where light neutral scalar/pseudoscalar fields have minimal coupling with two photons.

Double-helix current drive revisited

The conditions required for efficient current drive in a weakly resistive plasma column, subject to a double helix traveling magnetic field, are examined in detail by using a simple non-linear Ohm's law for the plasma. In agreement with previous numerical work on the subject by Bertram [5], it is shown that field penetration is indeed strongly limited when large external bias longitudinal magnetic fields are used. However, there is a range of small external bias fields that allow for significant penetration when reasonable driving fields are used. This should be of interest for sustaining reversed-field pinches and toroidal screw pinches.

Barocaloric effect: The use of pressure for magnetic cooling in Ce3Pd20Ge6

The barocaloric effect found in many rare-earth compounds opens the way to novel magnetic refrigeration techniques which are rather based on the application of external pressure than on the application of large external magnetic fields as needed for cooling by the magnetocaloric effect. Here we report on the first observation of this effect in a heavy-fermion system, namely the Kondo lattice compound Ce3Pd20Ge6. Upon isothermal application of uniaxial pressure the compound lowers the magnetic entropy, subsequent adiabatic release of the pressure leads to a cooling by as much as −0.75 K per 0.3 GPa pressure release at 4.4 K. Based on entropy considerations the observed cooling effect can be explained by an increase of the Kondo temperature and by a structurally induced splitting of the Γ8 ground-state quartet upon uniaxial pressure.

Quantum field dynamics in a uniform magnetic field: Description using fields in oblique phase space

We present a simple field transformation which changes the field arguments from the ordinary position-space coordinates to the oblique phase-space coordinates that are linear in position and momentum variables. This is useful in studying quantum field dynamics in the presence of external uniform magnetic field: here, the field transformation serves to separate the dynamics within the given Landau level from that between different Landau levels. We apply this formalism to both nonrelativistic and relativistic field theories. In the large external magnetic field our formalism provides an efficient method for constructing the relevant lower-dimensional effective field theories with the field degrees defined only on the lowest Landau level.

Soliton-limited superflow in 3He-A between parallel plates

We have studied theoretically the flow of superfluid 3 He-A in parallel-plate geometry. The equilibrium order-parameter texture is calculated numerically in two spatial dimensions consisting of the coordinates along the flow direction and perpendicular to the plates. The calculations have been done using the hydrostatic theory in the Ginzburg-Landau region and assuming a large external magnetic field perpendicular to the plane of calculation. We have studied a uniform texture and a dipole-unlocked splay soliton as initial configurations. In the former case we find the Freedericksz transition and a helical instability with increasing flow. In the latter case we find instability in the soliton. This instability is closely related to the critical velocity in the presence of a vortex sheet. Also, the transverse NMR frequency shift at the soliton has been calculated.

The Finite Larmor Radius Approximation

The presence of a large external magnetic field in a plasma introduces an additional time-scale which is very constraining for the numerical simulation. Hence it is very useful to introduce averaged models which remove this time-scale. However, depending on other parameters of the plasma, different starting models for the asymptotic analysis may be considered. We introduce here a generic framework for our analysis which fits many of the possible regimes and apply it in particular to justify the finite Larmor radius approximation both in the linear case and in the nonlinear case in the plane transverse to the magnetic field.

LONG TIME BEHAVIOR OF THE TWO-DIMENSIONAL VLASOV EQUATION WITH A STRONG EXTERNAL MAGNETIC FIELD

When charged particles are submitted to a large external magnetic field, their movement in first approximation occurs along the magnetic field lines and obeys a one-dimensional Vlasov equation along these field lines. However, when observing the particles on a sufficiently long time scale, a drift phenomenon perpendicular to the magnetic field lines superposes to this first movement. In this paper, we present a rigorous asymptotic analysis of the two-dimensional Vlasov equation when the magnetic field tends to infinity, the observation time scale increases accordingly. Techniques based on the two-scale convergence and the introduction of a second problem enable us to find an equation verified by the weak limit of the distribution function.

Geometry dependence of magnetization vortices in patterned submicron NiFe elements

When the magnetization is switched in patterned submicron Ni80Fe20 thin-film elements, magnetization vortices, local micromagnetic structures with a core size of 10 nm, are often trapped. This lowers the magnetostatic self-energy of the elements. Trapped vortices can cause anomalous switching during spin reversal between stable magnetization states. To expel trapped vortices, a large external magnetic field, the vortex exit field, is required. The dependence of the vortex exit field on lateral dimensions of the patterned elements is measured. In the limit where the lateral dimensions of the patterned elements are much larger than the vortex core size, smaller structures are more prone to the formation of trapped vortices.

Magnetothermodynamics of the spin- 1 / 2 Kagome antiferromagnet

In this paper, we use a new hybrid method to compute the thermodynamic behavior of the spin- 1 / 2 Kagome antiferromagnet under the influence of a large external magnetic field. We find a T2 low-temperature behavior and a very low sensitivity of the specific heat to a strong external magnetic field. We display clear evidence that this low-temperature magnetothermal effect is associated with the existence of low-lying fluctuating singlets, but also that the whole picture ( T2 behavior of C(v) and the thermally activated spin susceptibility) implies contribution of both nonmagnetic and magnetic excitations. Comparison with experiments is made.