Presence Of Magnetic Field Parallel Research Articles

Weak Localization in p-Type Heterostructures in the Presence of Parallel Magnetic Field

Theory of weak localization is developed for two-dimensional holes in the presence of in-plane magnetic field. The Zeeman splitting even in the hole momentum results in the spin-dependent phase changing the quantum interference. The negative correction to the conductivity is shown to decrease by a factor of two by the in-plane magnetic field. The positive magnetoconductivity in a classically weak perpendicular field caused by the weak localization is calculated for both quadratic and quartic in momentum Zeeman hole splittings. Calculations show that the conductivity corrections are very close to each other in these two cases of low and high hole density.

Charged rho superconductor in the presence of magnetic field and rotation

In this work, we mainly explore the possibility of charged rho (rho ^pm ) superconductor in the presence of parallel magnetic field and rotation within three-flavor Nambu–Jona-Lasino model. By following similar schemes as in the previous studies of charged pion (pi ^pm ) superfluid, the rho ^pm superconductor is found to be favored for both choices of Schwinger phase in Minkowski and curved spaces. Due to the stability of the internal spin structure, charged rho begins to condensate at a smaller threshold of angular velocity than charged pion for the given large magnetic fields. Even the axial vector meson condensation is checked – the conclusion is that rho ^pm superconductor is the robust ground state at strong magnetic field and fast rotation, which actually sustains to very large angular velocity.

Self-organised fractional quantisation in a hole quantum wire

We have investigated hole transport in quantum wires formed by electrostatic confinement in strained germanium two-dimensional layers. The ballistic conductance characteristics show the regular staircase of quantum levels with plateaux at n2e2/h, where n is an integer, e is the fundamental unit of charge and h is Planck’s constant. However as the carrier concentration is reduced, the quantised levels show a behaviour that is indicative of the formation of a zig-zag structure and new quantised plateaux appear at low temperatures. In units of 2e2/h the new quantised levels correspond to values of n = 1/4 reducing to 1/8 in the presence of a strong parallel magnetic field which lifts the spin degeneracy but does not quantise the wavefunction. A further plateau is observed corresponding to n = 1/32 which does not change in the presence of a parallel magnetic field. These values indicate that the system is behaving as if charge was fractionalised with values e/2 and e/4, possible mechanisms are discussed.

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom.

Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth doses (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field. Two cylindrical electromagnets were used to produce a 0.207T magnetic field parallel to the central axis of a 6MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth dose in polystyrene was simulated using DOSXYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros. The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2mm γ acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to be approximately 10% (relative to no field Dmax ) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (>0.13T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these dose increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison. Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately calculate the PDDs with these surface and buildup dose modifications in a homogeneous phantom.

Emission of acoustic and optical phonons by hot electrons in a two-dimensional electron system in parallel magnetic fields.

In this paper, we present a detailed theoretical study on acoustic- and optical-phonon emission by hot electrons in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As parabolic quantum wells in parallel magnetic fields. We consider the situation where the phonon generation is detected by phonon emission experiments. The dependence of phonon emission on the magnetic field, excitation power, and phonon emission angle has been studied through calculating the electron energy loss rate in this configuration. The main results obtained from the present theoretical study are the following. (i) In low-, intermediate-, and high-energy excitations, phonon generation is mainly through electron-phonon interaction via piezoelectric, deformation-potential, and polar-optical coupling, respectively. (ii) The strongest acoustic-phonon emission can be observed around a magnetic field at which an electronic subband becomes depopulated, and the strongest LO-phonon emission occurs around a magnetic field where the energy spacing between two subbands equals the LO-phonon energy (electrophonon resonance effect). (iii) Phonon generation in the presence of parallel magnetic fields has a different angular distribution from that at zero and perpendicular magnetic fields. \textcopyright{} 1996 The American Physical Society.

Landau assisted vertical transport in MQW p-i-n GaAs/AlGaAs diodes

We report studies of sequential resonant tunnelling in GaAs/AlGaAs multi-quantum well p-i-n structures in the presence of parallel magnetic fields up to 15 T. New features are observed which are attributed to inter-Landau level resonances involving a change of Landau index of +1 and +2. In one of the samples the tunnelling resonances saturate and shift with the magnetic field. This result may be related to the similar effects observed in coupled two-dimensional electron gases at high magnetic fields.

Transverse magnetoresistance of GaAs/AlxGa1-xAs heterojunctions in the presence of parallel magnetic fields.

We have calculated the resistivity of a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction in the presence of both an in-plane magnetic field and a weak perpendicular component using a semiclassical Boltzmann transport theory. These calculations take into account fully the distortion of the Fermi contour which is induced by the parallel magnetic field. The scattering of electrons is assumed to be due to remote ionized impurities. A positive magnetoresistance is found as a function of the perpendicular component, in good qualitative agreement with experimental observations. The main source of this effect is the strong variation of the electronic scattering rate around the Fermi contour which is associated with the variation in the mean distance of the electronic states from the remote impurities. The magnitude of the positive magnetoresistance is strongly correlated with the residual acceptor impurity density in the GaAs layer. The carrier lifetime anisotropy also leads to an observable anisotropy in the resistivity with respect to the angle between the current and the direction of the in-plane magnetic field. \textcopyright{} 1996 The American Physical Society.

Effects of carrier mass differences on the I–V characteristics of resonant interband tunneling structures in the presence of parallel magnetic field

We investigate the effect of differences between the quantum well and the electrode carrier effective masses on the current–voltage characteristics of double-barrier interband tunneling structures under an applied magnetic field parallel to the current. The system is described by a two-band tight-binding Hamiltonian that incorporates electron and light-hole interaction and the current is calculated using the Keldysh nonequilibrium Green’s-function diagrammatic technique. The formalism is applied to InAs/AlSb/GaSb double-barrier structures considering either the InAs or the GaSb as the quantum well. The behavior of the I–V characteristics as a function of the magnetic-field strength is strongly influenced by the mass differences and opposite curvatures of the conduction and the valence bands. The features we obtain are quite different for InAs and GaSb. They are both also different from the more traditional magneto-tunneling results for GaAs/AlGaAs, where the electrodes and the quantum well electronic masses are equal. The behavior of the I–V characteristics with the applied magnetic field is in good qualitative agreement with available experimental data.

Conductivity of inversion layers on InSb in parallel magnetic fields

The d.c. conductivity of electron inversion layers in metal-oxide-semiconductor (MOS) structures on InSb is studied at low source-drain electric fields in the presence of parallel magnetic fields. The conductivity strongly increases as a function of magnetic field strength, both, in the transverse ( j⊥ B>) and longitudinal ( j‖ B) configuration when the directions of current and magnetic field are perpendicular or parallel to each other. The observations are qualitatively described by transport theory for hybrid electric-magnetic levels.

A Model for Layered Superconductors and First Order Transition in the Presence of Magnetic Field Parallel to Layers

A model for layered superconductors is proposed and the possibility of the first order transition caused by the transformation of flux structure in the presence of the magnetic field parallel to the layers is pointed out by means of the boson theory. Flux structure is calculated for a simple case.