Wire In Magnetic Field Research Articles

The influence of BaSnO3 artificial pinning centres on the resistive transition of 2G high-temperature superconductor wire in magnetic field

In this work, we present a study of the resistive transition in magnetic field of 2G high-temperature superconductor wire samples with BaSnO3 (BSO) artificial pinning centres (APCs) in a GdBa2Cu3O7−x (GdBCO) superconductor. The GdBCO layer was fabricated by pulsed laser deposition, using production equipment to obtain samples with varied concentrations of APC: 0, 6, 12 and 18 mol% of BSO. Resistive transition curves were measured in the magnetic field range from 0 to 9 T, and magnetic field orientations from B∣∣c (θ = 0°) to B∣∣ab (θ = 90°) with an angle step of 15°. We observed an irreversibility temperature peak at the B∣∣ab orientation. We determined the angular regions where correlated pinning centres dominated and found them to expand with the increase of BSO concentration. The activation energy (Ua) obtained from the lg(ρ/ρ0) against the 1/T plots was almost constant in the whole angular range, with a small peak at B∣∣ab. The activation energy as well as the irreversibility temperature decreased with the increasing doping level, but the Ua(θ) curves looked similar for all samples with APCs. The logarithmic resistivity plot of the undoped sample looked substantially different from those of the APC samples, with a kink and two different slopes corresponding to two regions with different Ua. We discuss that this difference is related to the different pinning landscapes in samples with and without APCs.

Fuzzy sliding mode control of flexible spinning beam using a wireless piezoelectric stack actuator

Spinning structures play an outstanding role in aerospace engineering. The vibration caused by eccentric forces or external excitations would damage the structures. Vibration suppression of such spinning structures is necessary, but challenging because of the arrangement of actuators. A new wireless piezoelectric stack actuator spinning in a varying magnetic field is introduced in this paper to avoid the wires winding with large control force. The control voltage on the actuator is generated by the motion of the wires in magnetic field. Besides, fuzzy sliding mode control with universal fuzzy sets based on the wireless actuator is investigated. Numerical simulation demonstrates that the tip displacement response of the spinning beam reduces over 98% compared with that open loop, while the magnitude of the peak in frequency domain decreases from 0.0976dB to 0.0383dB. It proved that the proposed actuator is feasible and the fuzzy sliding mode controller could suppress the vibration of spinning beam effectively with a strong robustness to noise.

Quantum wires in magnetic field: a comparative study of the Hartree–Fock and the spindensity functional approaches

We present a detailed comparison of the self-consistent calculations based on theHartree–Fock and the spin density functional theory for a split-gate quantum wire inthe IQH regime. We demonstrate that both approaches provide qualitatively(and, in most cases, quantitatively) similar results for the spin-resolved electrondensity, spin polarization, spatial spin separation at the edges and the effectiveg factor. Both approaches produce the same values of the magnetic fields corresponding tothe successive subband depopulation and qualitatively similar evolution of themagnetosubbands. Quantitatively, however, the HF and the DFT subbands are different(even though the corresponding total electron densities are practically the same). Incontrast to the HF approach, the DFT calculations predict much larger spatial spinseparation near the wire edge for the low magnetic fields (when the compressible strips forspinless electrons are not formed yet). In the opposite limit of the large fields, theHartree–Fock and the DFT approaches give very similar values for the spatial spinseparation.

Limiting current mechanisms of Bi2223 wires in magnetic fields

Controlled overpressure (CT-OP) processing eliminates pores and heals cracks, whichdensifies the Bi2223 filaments and increases the critical current of Ag-sheathed Bi2223 wire.High current capacity wires with critical current values around 210 A at 77 K and self-fieldhave been achieved. The enhancement of the current capacity seems to be due to reducingthe weak links between Bi2223 grains and improving the flux pinning. In order tounderstand the mechanisms for transport critical current in Bi2223 wires, the dependenceof the critical current on magnetic field and temperature has been investigated. The criticalcurrent is measured as a function of the perpendicular and parallel magnetic fields, up to12 T, at temperatures ranging from 4.2 to 90 K. These results indicate that higher criticalcurrent is associated with highly textured and well connected Bi2223 grains. Tofurther improve a critical current of Bi2223 wires, it is necessary to decrease themisalignment angle and reduce the fraction of Bi2212 and secondary phases.

Failure of geometric electromagnetism in the adiabatic vector Kepler problem

The magnetic moment of a particle orbiting a straight current-carrying wire may precess rapidly enough in the wire's magnetic field to justify an adiabatic approximation, eliminating the rapid time dependence of the magnetic moment and leaving only the particle position as a slow degree of freedom. To zeroth order in the adiabatic expansion, the orbits of the particle in the plane perpendicular to the wire are Keplerian ellipses. Higher-order postadiabatic corrections make the orbits precess, but recent analysis of this ``vector Kepler problem'' has shown that the effective Hamiltonian incorporating a postadiabatic scalar potential (``geometric electromagnetism'') fails to predict the precession correctly, while a heuristic alternative succeeds. In this paper we resolve the apparent failure of the postadiabatic approximation, by pointing out that the correct second-order analysis produces a third Hamiltonian, in which geometric electromagnetism is supplemented by a tensor potential. The heuristic Hamiltonian of Schmiedmayer and Scrinzi is then shown to be a canonical transformation of the correct adiabatic Hamiltonian, to second order. The transformation has the important advantage of removing a ${1/r}^{3}$ singularity which is an artifact of the adiabatic approximation.

Tuned transition from a quantum well to a quantum wire investigated by magnetophonon resonance

This article describes experimental investigations of the transition from a two-dimensional electron gas to a quasi-one-dimensional quantum wire by the magnetophonon effect, tuned by Schottky gates in GaAs/AlGaAs. We present a model of the depletion region linked to the gates. The measurements allow the band-edge effective mass to be determined from two to one dimension between 89.5 and 348 K. We performed the measurements with a single, isolated quantum wire in magnetic fields up to 17 T.

Collective and single-particle excitations in coupled quantum wires in magnetic fields

The full spectra of magnetoplasmons and single-particle excitations are obtained of coupled one-dimensional electron gases in parallel semiconductor quantum wires with tunneling. We show the effects of the interwire Coulomb interaction and the tunneling, as well as the magnetic-field-induced localization on the elementary excitations in symmetric and asymmetric coulped quantum wire structures. The interacton and resonance between the plasmon and the intersubband single-particle excitations are found in magnetic fields.

T-shaped quantum wires in magnetic fields: Weakly confined magnetoexcitons beyond the diamagnetic limit

Optical excitations of magnetoexcitons in T-shaped wires are calculated and compared with experiment. We find the single-particle states for electrons and holes confined to a wire in a magnetic field and use these states as a basis for calculations of magnetoexciton states. We accurately reproduce the field dependence of the exciton states and explain the small, field-induced, energy shifts that are observed for these states. The shifts are small because the T-junction provides weak confinement, rather than strong quantum confinement. Diamagnetic shifts calculated from perturbation theory fail to describe the experimental results. We determine when perturbation theory is valid for these nanostructures and which gauge should be used to give the diamagnetic shift that best reproduces the field dependence at low fields.

Studies on the third-harmonic generation of double-layered quantum wires in magnetic fields

The third-harmonic generations of two vertically stacked quantum wires in magnetic fields are investigated. The analytic formula for the third-harmonic generation of double-layered quantum wires is derived by means of density matrix treatment. The numerical results are presented for GaAs/Al x Ga1−x As double quantum wires. Finally, the calculated third-harmonic generations are plotted versus the magnetic field B, the photon energy hω and the interlayer distance D.

Studies on the electro-optic effects of double-layered quantum wires in magnetic fields

The electro-optic effects of two vertically stacked quantum wires in magnetic fields are investigated, and the analytic forms of the electro-optic effects of this system are derived by means of density-matrix treatment. The numerical results are presented for GaAs/Al x Ga 1− x As double quantum wires. Finally, the electro-optic effects versus the magnetic field B, the photon energy ℏ ω and the interlayer distance D are plotted.

Transport properties of double quantum wires in magnetic field

Transport properties of the long double quantum wires, expected to exhibit the effect of strong mobility enhancement in magnetic field, are investigated theoretically. We treat the problem of elastic scattering on the basis of obtained analytical expression for the exact T matrix of scattering by a single defect. It is shown that under conditions of the effect above, the Born approximation for scattering rate is not applicable and the higher order corrections, corresponding to multiple scattering of electron by the same defect, play dominating role in the momentum dissipation of carriers.

Collective intersubband spin-density excitations in a quantum wire in a magnetic field

We study an interacting electron gas parabolically confined to a quantum wire in a perpendicular magnetic field. We consider the case of more than one subband occupied in the ground state. The intersubband spin-density excitations are calculated within the Hartree-Fock random-phase approximation. Similarly to the case in the absence of a magnetic field, vertex corrections to the electron spin polarizability are important, leading to collective spin-density excitations red-shifted with respect to the Hartree-Fock single-particle energies. In a finite magnetic field, the triplet of spin-density excitations splits. The splitting is at its maximum if the chemical potential is in between the bottom of a spin-up subband and the bottom of a spin-down subband, when the g-factor is greatly enhanced due to the exchange interaction.

Electron scattering by a ring-shaped barrier in magnetic fields

We investigate the transport phenomena through a region containing a ring-shaped barrier in a quasi-one-dimensional quantum wire in magnetic fields. The calculated magnetoconductance curve shows a periodic dip structure, which is superimposed upon by another quasi-periodic dip structure. The current distributions for resonant states and the magnetoconductance features are well explained on the basis of the magnetic field dependence of the eigenvalue in the two-dimensional system.

Impurity states in coupled quantum wires in magnetic field

The spectrum of impurity states in a system of two parallel quantum wires coupled by tunneling in an external magnetic field is investigated theoretically. The problem of poles of the T matrix of scattering is reduced to an algebraic equation. A detailed analysis of its roots corresponding to the bound states and resonances of scattering depending on the parameters of the system and the magnitude of the magnetic field is presented. We show that in a magnetic field the interwire resonances appear in the spectrum for both attractive and repulsive potentials of impurity.

Many-body effects in the quasi-one-dimensional magnetoplasma

We compare measured and calculated luminescence spectra of quantum wires in normal magnetic fields. The experiments have been performed on modulated barrier ${\mathrm{In}}_{0.13}$${\mathrm{Ga}}_{0.87}$As/GaAs quantum wires in magnetic fields up to B=10.5 T. In the regime of high magnetic fields in which the cyclotron energy ${\mathrm{\ensuremath{\omega}}}_{\mathrm{c}}$ exceeds the lateral intersubband energy \ensuremath{\Omega} the carriers show the behavior of a fully quantized system. The experimental magnetoluminescence spectra for different excitation intensities are in excellent agreement with calculated spectra. The calculations contain not only the influence of the strong magnetic field, but also the many-body effects on a Hartree-Fock level in terms of state filling, band-gap renormalization, and excitonic correlations with up to four lateral subbands. A magnetic-field-dependent momentum cutoff is introduced, which ensures that electrons and holes are not pushed out of the quantum wire under the influence of the Lorentz force. By fitting the calculated to the measured spectra we determine the density and the temperature in the one-dimensional magnetoplasma. In contrast to the field-free case (B=0) the renormalization of one subband is mainly determined by the occupation of the other subbands in the high-field regime because the excitons on one subband form, to a good approximation, an ideal gas. Its formation becomes possible because the symmetry under continuous rotations in the electron and hole isospin space that is broken by the lateral confinement is to a good approximation restored by high magnetic fields, which suppress the motion of the free carriers along the wire.

Internal electron-electron interactions in one-dimensional systems detected by Raman spectroscopy.

We have investigated experimentally and theoretically electronic Raman excitations in GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wires in magnetic fields. In both the experiments and the calculations we find strong anticrossings of the observed confined magnetoplasmon modes in the vicinity of 2${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$, where ${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$ is the cyclotron-resonance frequency. The observed splittings and anticrossings arise from internal Coulomb interaction in the one-dimensional electron systems and is found to depend sensitively not only on the wave vector but also on the shape of the confining potential and the mode index of the confined plasmon mode. \textcopyright{} 1996 The American Physical Society.

Self-consistent results in quantum wires in magnetic fields: Temperature effects

Results of a self-consistent calculation are reported on the energy level structure of quantum wires formed by a split gate at GaAs/AlGaAs heterostructures in magnetic fields at different temperatures. The electron density associated with partially filled Landau level oscillates spatially to minimize the Coulomb energy and has a characteristic double-step variation near the edge at low temperatures.

Overall critical current density of chevrel wires in magnetic fields up to 24 tesla

Wires of Pb0.6Sn0.4Mo6S8 with a protective sheet of Niobium have been carefully drawn and the critical current density Jc has been measured on small coils in magnetic fields up to 24 tesla. At 1.9K, the overall JCO, exceeds 100 A/mm2 at 20 tesla and 60 A/mm2 at 24 tesla. These large values highlight the good sintering between all the grains. However, the field dependence of JC still indicates that the effective upper critical field Bc2 is too low especially at 4.2K (31.5 tesla instead of 50 tesla).

A study of vibrating wires for viscometry in high magnetic fields

We report on a systematic study of the temperature and magnetic field dependence of the intrinsic damping and the resonance frequency of vibrating wires in magnetic fields up to 11.5T and at temperatures 1 mK<T<250 mK. The aim is to test the applicability of various materials for viscometry in spin polarized3He−4He solutions. The materials investigated are NbTi in the superconducting and normal state, the alloys Manganin and PtW, a gold plated glass wire, and three Ag wires where one has been exposed to a heat treatment. For all samples the impact of the magnetic field on the mechanical proporties of the wires is much larger than any temperature dependent effects.

A probe for measuring magnetic field profiles inside superconductors from 4.2 K to TC in high magnetic fields

A probe has been designed and built for measuring the magnetization critical current density Jc(B, T) and magnetic field profiles inside superconducting samples as a function of temperature in large DC magnetic fields. Magnetic field profiles provide a unique opportunity to study the spatial variation of Jc within a superconducting sample. In these measurements, a small AC field is superimposed upon a much larger DC field. This is the first flux penetration probe capable of making measurements as a function of temperature, in the range 2-30 K (accurate to 100 mK), which combines high AC fields of up to 100 mT (at 19.7 Hz) with very large DC fields of up to 17 T. The probe can be used to study short sections of superconducting cables, wires, tapes, thin films or bulk samples. Its capabilities are demonstrated by presenting variable-temperature data obtained from a commercial multifilamentary NbTi wire in magnetic fields up to 10 T.