Weak Transverse Magnetic Field Research Articles

Superconducting current in a thin film of poly(phthalidylidene biphenylene)

We report on the Josephson oscillations observed in a weak transverse magnetic field applied to a superconductor-poly(phthalidylidene biphenylene)-superconductor structure. The weak coupling is provided by point contacts at the polymer-superconductor interface. It is experimentally established that a characteristic scale of the Josephson oscillations depends on the I/Ic ratio, where Ic is the critical current.

Microwave polarization diagnostics of solar current sheets with transverse component of magnetic field

We present analitical solution to the problem of linear coupling of extraordinary (X) and ordinary (O) modes in a plasma current sheet with a transverse component of magnetic field. We indicate conditions of effective X-, O-mode coupling when the polarization features of microwaves having passed through the sheet differ essentially from those calculated in the geometrical optics approximation. We show that the corresponding polarization observations may give valuable data on the inhomogeneity and strength of the main and transverse components of magnetic field in the coupling region inside the sheet. The theory of mode coupling is applied to the calculations of the polarization features of solar microwave emission that turn out to be well interpreted if the proper current sheet stabilized by a weak transverse magnetic field is situated in the solar corona on the pathway of radio waves toward the Earth.

Numerical solutions for the liquid-metal flow in a rotating cylinder with a weak transverse magnetic field

This paper treats the steady, three-dimensional, laminar flow of an electrically conducting liquid in a finite-length, insulating cylinder which rotates at a constant angular velocity about its axis. A steady, uniform, weak, transverse magnetic field produces a small deviation from a rigid-body rotation with the cylinder. This deviation consists of an axisymmetric flow, which is very similar to the classical Ekman flow for the spin-up of a cylinder, and a nonaxisymmetric flow. This paper presents numerical results for the nonaxisymmetric flow and for the ratio of the magnitude of the nonaxisymmetric flow to that of the axisymmetric flow for various values of the Reynolds number.

Periodic spin polarization of small quantum dots

We demonstrate, using spin density functional theory, that a quantum dot in a weak transverse magnetic field undergoes spontaneous spin polarization which fluctuates periodically with field strength B. The fluctuations are related to anti-crossing of developing Landau levels at the Fermi surface. For electron number N even, a crossing of two levels is sufficient to cause an instability to spontaneous polarization. For N odd, a simultaneous crossing of three or more levels is required. Such multiple crossings are characteristic of a circularly parabolic potential. Our calculations show that the realistic confining potential of a dot, calculated self-consistently in 3D, is sufficiently close to parabolicity for the multiple crossings to persist.

Silicon Float Zone Process with a Weak Transverse Magnetic Field and Crystal Rotation

This paper presents numerical calculations of the melt motion for a model of the float zone process with buoyant and thermocapillary convection, with rotation of the crystal and feed rod about their common vertical centerline, and with a uniform, steady, transverse magnetic field. With only buoyant and thermocapillary convection, there are two meridional circulations in which the radial flow adjacent to the crystal‐melt interface is outward near the center and inward near the periphery. Adding crystal rotation or a transverse magnetic field decreases the magnitude of the buoyant and thermocapillary convection, but neither alone changes its pattern. Combining crystal rotation and a transverse magnetic field produces an axial variation in the centrifugal force due to the azimuthal melt velocity which overwhelms the radially inward convection and produces radially outward flow over the entire crystal‐melt interface, and this pattern is desirable for both heat transfer and dopant transport. The magnetic field must be weak enough to produce negligible deviations from axisymmetry in the heat and mass transfer. © 2000 The Electrochemical Society. All rights reserved.

Spin tunneling and phonon-assisted relaxation inMn12-acetate

We present a comprehensive theory of the magnetization relaxation in a Mn12-acetate crystal in the high-temperature regime (T>1 K), which is based on phonon-assisted spin tunneling induced by quartic magnetic anisotropy and weak transverse magnetic fields. The overall relaxation rate as function of the longitudinal magnetic field is calculated and shown to agree well with experimental data including all resonance peaks measured so far. The Lorentzian shape of the resonances, which we obtain via a generalized master equation that includes spin tunneling, is also in good agreement with recent data. We derive a general formula for the tunnel splitting energy of these resonances. We show that fourth-order diagonal terms in the Hamiltonian lead to satellite peaks. A derivation of the effective linewidth of a resonance peak is given and shown to agree well with experimental data. In addition, previously unknown spin-phonon coupling constants are calculated explicitly. The values obtained for these constants and for the sound velocity are also in good agreement with recent data. We show that the spin relaxation in Mn12-acetate takes place via several transition paths of comparable weight. These transition paths are expressed in terms of intermediate relaxation times, which are calculated and which can be tested experimentally.

First-Order Vortex-Lattice Melting Transition inYBa2Cu3O7−δnear the Critical Temperature Detected by Magnetic Torque

High-resolution magnetic-torque studies on an untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal near its critical temperature ${T}_{c}$ reveal that the first-order vortex-lattice melting transition (VLMT) persists at least up to 0.5 K below ${T}_{c}$. The associated sharp discontinuity in magnetization is detectable even at temperatures where the torque signal deviates from mean-field behavior due to fluctuations. The magnetic irreversibility at the VLMT can be suppressed by applying a weak transverse ac magnetic field. This offers the possibility of separating the irreversibility line from the melting line near ${T}_{c}$.

Polarization of spin-1/2 particles in an axisymmetric magnetic field

The present paper shows that the nature of the polarization of charged spin-1/2 particles moving in a uniform magnetic field changes dramatically in a relatively weak transverse axisymmetric magnetic field. The direction along which the spin projection is quantized has a fixed orientation with respect to the axes of a cylindrical coordinate system and can form a substantial angle with the direction of the uniform magnetic field. The presence of spin quantization is proved both by the fact that the commutator of the Hamiltonian operator and the projection of the polarization operator in the direction of quantization is zero and by analyzing the Bargmann-Michel-Telegdi equation for this given case. Finally, the possibilities of detecting this effect and utilizing it are discussed.

A new possible method for decreasing the wavelenght of Gyrotron

The weak transverse shear magnetic field is first proposed to decrease the working wavelength of Gyrotron. Using Lie perturbation method, we give the modified wave particle resonant relation in guiding magnetic field with weak transverse shear magnetic field.

A one-dimensional model of a negative ion source

A theoretical model is presented which uses one-dimensional plasma transport to describe a hydrogen discharge in which negative ions are formed. The model is based on discharges which are confined by a magnetic multipole field on the anode surface (multipole or `bucket' sources) and includes the effects of weak transverse magnetic fields within the plasma to cool the electron temperature in the discharge to a level at which dissociative attachment collisions allow significant fractional densities of ions to form. The transport equations are based on the moments of the Boltzmann equation and these are coupled with the plasma collision processes associated with ionization and negative ion production. The presence of negative ions in the plasma allows the electron transport across the magnetic fields to be anomalously high because each electron spends part of its time as a negative ion which is only weakly affected by the field. The model is tested on two multipole sources which have produced negative current densities up to around and have extensive measurements of the electron and ion densities, electron temperature and plasma potential as functions of position, arc current, voltage and pressure. The model uses the source geometry and arc parameters to calculate the plasma properties and obtains good agreement with experiment in both cases. In the companion paper the model is tested on a third source chamber with different diagnostics.

Observation of muonium atom in solid nitrogen

Muonium (μ +e - or Mu, a light isotope of the H atom) has been directly observed for the first time in solid nitrogen ( 14N 2). In both α-N 2 and β-N 2, muonium is manifest as a long-lived μ +SR spin precession signal at the characteristic Mu frequency in a weak transverse magnetic field. The temperature dependence of this signal'srelaxation rate T 2 −1 indicates that the diffusion of Mu involves quantum tunneling; a theory is presented that qualitatively explains T 2 −1( T) in nitrogen.

Frequency shifts of a two-mode He–Ne/CH4laser stabilized by frequency resonances, under the action of a magnetic field in the active medium

Experimental and theoretical studies were made of the influence of a weak axial transverse magnetic field on the frequency shifts of a two-mode He–Ne/CH4 laser (λ = 3.39 μm) stabilized using narrow saturated dispersion resonances in the radiation frequency. In some cases a magnetic field applied to the laser active medium eliminated the asymmetry of the frequency resonances observed when the centers of the gain profile (ω0( + )) and the absorption line (ω0( – )) did not coincide. At ω0( + ) ≈ ω0( – ), the magnetic field does not influence the profile of the frequency resonances or the laser frequency shifts.

Melt motion in a Czochralski puller with a weak transverse magnetic field

The melt motion in a Czochralski silicon crystal puller with a uniform transverse magnetic field is intrinsically three-dimensional. When the magnetic field is relatively weak, the motion consists of an axisymmetric base solution, plus two perturbations which behave as sin 2θ and cos 2θ, neglecting terms which are proportional to the fourth power of the magnetic field strength. A 700 G magnetic field stabilizes a melt motion which is unsteady without a magnetic field. As the strength of the magnetic field increases, the magnitudes of the axial and radial velocities increase to maxima at some field strength above 1000 G. This contrasts with an axial magnetic field for which the axial and radial velocities always decrease as the field strength increases. We estimate that 1500 G is the maximum field strength for which our weak-field asymptotic solution is valid.

Attenuations and atomic spin precessions of ?-angular correlations for coulomb excited19F nuclei in single-electron ions

Attenuated γ-angular correlations and atomic spin precessions were measured for single-electron fluorine ions recoiling through vacuum in weak transverse magnetic fields The Coulomb excited long-lived19F(5/2+) state at 197 keV was used as probe. The angular correlations observed in the focal coordinate system of the Coulomb excitation process as well as the spin precessions in weak external fields are well explained with simple electron configurations associated with the highly stripped ions.

Experimental tests for detecting spin polarization pick-up of single-electron19F-ions on emergence from magnetized iron

A new method to detect polarization pick-up by ions emerging from thin polarized ferromagnetic layers has been tested. The applied technique consists in the observation of attenuatedγ-angular correlations and atomic spin precessions for single-electron ions recoiling through vacuum in weak transverse magnetic fields. The Coulomb-excited19F(5/2+) state at 197 keV was used as probe. The experimental concept is described and the expected effects are discussed using the perturbed angular correlation formalism for polarized hyperfine interactions.

Atomic spin precessions of fluorine ions recoiling through vacuum in weak transverse magnetic fields

A new experimental method for unveiling the strong hyperfine interactions of multi-electron ion configurations is described. It consists in observing atomic spin precessions of nuclear-excited ions recoiling through vacuum in weak external transverse magnetic fields. Due to the strong hyperfine coupling the nuclear orientation follows the atomic precession, resulting in a rotation of the angular correlation of decayγ-rays. The method is illustrated using the long-lived 197keV19F (5/2+) state as probe. The observed large precessions as well as the strong attenuations of theγ-angular correlations are well accounted for by considering all possible configurations resulting from the coupling ofL-shell electrons in the relevant ionic charge states.

Average charge state dependence in atomic spin precessions of fluorine ions emerging from iron and copper

The technique of perturbedγ-angular correlation has been applied to measure atomic spin precessions of nuclear excited fluorine ions on recoil through vacuum in a weak transverse magnetic field. The ions were allowed to emerge from thin iron or copper backed targets and were stopped after flight through vacuum in a Ag foil. The differing precessions observed are attributed to different average charge states of the ions on emergence from the two target backings.

A proposed new method for detecting polarization pick-up by ions emerging from a ferromagnetic surface

Atomic spin precessions in isolated fluorine ions on recoil through weak transverse magnetic fields have been observed for studying the complex hyperfine interactions of multi-electron configurations. The method is illustrated using the long-lived 197 keV 19F( 5 2 + ) state as probe. The measured precessions were found to vary linearly with the applied magnetic field integrated over the flight path of the recoiling ions. The data are well accounted for, assuming configurations from the coupling of 2s and 2p electrons in the relevant ionic charge states. The application of this method to the detection of polarization pick-up from a ferromagnetic surface is discussed.

Observation of atomic spin precessions of isolated fluorine ions on recoil through weak transverse magnetic fields

A new experimental method for studying the hyperfine interactions of multi-electron configurations in isolated ions is illustrated for fluorine using the long-lived 197 keV19F(5/2+) state as probe. The observed atomic precessions were found to vary linearly with the applied weak magnetic field integrated over the flight path of the recoiling ions. The data are well accounted for, assuming configurations from the coupling of 2s and 2p electrons in the relevant ionic charge states.

Effect of twist on wires made from i n s i t u superconductors

The effect of twist on the transverse field hysteresis of in situ superconducting wires is assumed to result from a lowering of the axial critical current density due to anisotropy of critical current density in the material. In weak transverse magnetic fields the twist raises the hysteresis loss, while in stronger fields the twist lowers the loss. It is shown that the axial critical current density determined from hysteresis or magnetic moment measurements in a twisted wire can be lower than the value obtained from transport measurements.