1,574 research outputs found
Conductance fluctuations in diffusive rings: Berry phase effects and criteria for adiabaticity
We study Berry phase effects on conductance properties of diffusive
mesoscopic conductors, which are caused by an electron spin moving through an
orientationally inhomogeneous magnetic field. Extending previous work, we start
with an exact, i.e. not assuming adiabaticity, calculation of the universal
conductance fluctuations in a diffusive ring within the weak localization
regime, based on a differential equation which we derive for the diffuson in
the presence of Zeeman coupling to a magnetic field texture. We calculate the
field strength required for adiabaticity and show that this strength is reduced
by the diffusive motion. We demonstrate that not only the phases but also the
amplitudes of the h/2e Aharonov-Bohm oscillations are strongly affected by the
Berry phase. In particular, we show that these amplitudes are completely
suppressed at certain magic tilt angles of the external fields, and thereby
provide a useful criterion for experimental searches. We also discuss Berry
phase-like effects resulting from spin-orbit interaction in diffusive
conductors and derive exact formulas for both magnetoconductance and
conductance fluctuations. We discuss the power spectra of the
magnetoconductance and the conductance fluctuations for inhomogeneous magnetic
fields and for spin-orbit interaction.Comment: 18 pages, 13 figures; minor revisions. To appear in Phys. Rev.
Wheat experimental results
The development of wheat relative to frost susceptibility, 87NA10. Optimum flowering times of wheat, 87NA11 and 87WH8. Grain growth and development of a historical set of wheats
Factors affecting frost damage to wheat in Western Australia
It may at first seem strange that precious research funds are being channelled into a project concerned with frost damage in a country where high temperatures and moisture stress limit the growth of plants for a large portion of the year. But cereal crops are only grown in the temperate zones of the continent during winter and spring when they may be exposed to low diurnal temperatures. In many areas cold damage is irregular and rare,however it limits yields not only by causing actual damage but also by restricting the most effective period for flowering. For muck of the Australian wheat crop excessive risk of frost damage sets the earliest date for flowering and the beginning of grain filling at a time when other conditions are at their most favourable for carbohydrate assimilation. The latest date for flowering is determined by high temperatures and low water availability towards the end of the grain filling phase. An optimum flowering time and maximised yield in the long term are achieved when a compromise between the effects of frost and drought is reached
Berry phase and persistent current in disordered mesoscopic rings
A novel quantum interference effect in disordered quasi-one-dimensional rings
in the inhomogeneous magnetic field is reported. We calculate the canonical
disorder averaged persistent current using the diagrammatic perturbation
theory. It is shown that within the adiabatic regime the average current
oscillates as a function of the geometric flux which is related to the Berry
phase and the period becomes half the value of the case of a single
one-dimensional ring. We also discuss the magnetic dephasing effect on the
averaged current.Comment: 6 pages, RevTeX, 2 figures. To appear in Phys. Rev. B Rapid
Communications Vol.60 No.12 (1999
Resonant spin polarization and spin current in a two-dimensional electron gas
We study the spin polarization and its associated spin-Hall current due to
EDSR in disordered two-dimensional electron systems. We show that the disorder
induced damping of the resonant spin polarization can be strongly reduced by an
optimal field configuration that exploits the interference between Rashba and
Dresselhaus spin-orbit interaction. This leads to a striking enhancement of the
spin susceptibility while the spin-Hall current vanishes at the same time. We
give an interpretation of the spin current in geometrical terms which are
associated with the trajectories the polarization describes in spin space.Comment: (5 pages), updated references, corrected typo
Coulomb scattering cross-section in a 2D electron gas and production of entangled electrons
We calculate the Coulomb scattering amplitude for two electrons injected with
opposite momenta in an interacting 2DEG. We include the effect of the Fermi
liquid background by solving the 2D Bethe-Salpeter equation for the
two-particle Green function vertex, in the ladder and random phase
approximations. This result is used to discuss the feasibility of producing
spin EPR pairs in a 2DEG by collecting electrons emerging from collisions at a
pi/2 scattering angle, where only the entangled spin-singlets avoid the
destructive interference resulting from quantum indistinguishability.
Furthermore, we study the effective 2D electron-electron interaction due to the
exchange of virtual acoustic and optical phonons, and compare it to the Coulomb
interaction. Finally, we show that the 2D Kohn-Luttinger pairing instability
for the scattering electrons is negligible in a GaAs 2DEG.Comment: 19 pages, 10 figure
Composite Majorana Fermion Wavefunctions in Nanowires
We consider Majorana fermions (MFs) in quasi-one-dimensional nanowire systems
containing normal and superconducting sections where the topological phase
based on Rashba spin orbit interaction can be tuned by magnetic fields. We
derive explicit analytic solutions of the MF wavefunction in the weak and
strong spin orbit interaction regimes. We find that the wavefunction for one
single MF is a composite object formed by superpositions of different MF
wavefunctions which have nearly disjoint supports in momentum space. These
contributions are coming from the extrema of the spectrum, one centered around
zero momentum and the other around the two Fermi points. As a result, the
various MF wavefunctions have different localization lengths in real space and
interference among them leads to pronounced oscillations of the MF probability
density. For a transparent normal-superconducting junction we find that in the
topological phase the MF leaks out from the superconducting into the normal
section of the wire and is delocalized over the entire normal section, in
agreement with recent numerical results by Chevallier et al. (arXiv:1203.2643)
Berry phase and adiabaticity of a spin diffusing in a non-uniform magnetic field
An electron spin moving adiabatically in a strong, spatially non-uniform
magnetic field accumulates a geometric phase or Berry phase, which might be
observable as a conductance oscillation in a mesoscopic ring. Two contradicting
theories exist for how strong the magnetic field should be to ensure
adiabaticity if the motion is diffusive. To resolve this controversy, we study
the effect of a non-uniform magnetic field on the spin polarization and on the
weak-localization effect. The diffusion equation for the Cooperon is solved
exactly. Adiabaticity requires that the spin-precession time is short compared
to the elastic scattering time - it is not sufficient that it is short compared
to the diffusion time around the ring. This strong condition severely
complicates the experimental observation.Comment: 16 pages REVTEX, including 3 figure
- …