911 research outputs found
Superlattice properties of carbon nanotubes in a transverse electric field
Electron motion in a (n,1) carbon nanotube is shown to correspond to a de
Broglie wave propagating along a helical line on the nanotube wall. This
helical motion leads to periodicity of the electron potential energy in the
presence of an electric field normal to the nanotube axis. The period of this
potential is proportional to the nanotube radius and is greater than the
interatomic distance in the nanotube. As a result, the behavior of an electron
in a (n,1) nanotube subject to a transverse electric field is similar to that
in a semiconductor superlattice. In particular, Bragg scattering of electrons
from the long-range periodic potential results in the opening of gaps in the
energy spectrum of the nanotube. Modification of the bandstructure is shown to
be significant for experimentally attainable electric fields, which raises the
possibility of applying this effect to novel nanoelectronic devices.Comment: 7 pages, 3 figure
Three-Dimensionally Confined Optical Modes in Quantum Well Microtube Ring Resonators
We report on microtube ring resonators with quantum wells embedded as an
optically active material. Optical modes are observed over a broad energy
range. Their properties strongly depend on the exact geometry of the microtube
along its axis. In particular we observe (i) preferential emission of light on
the inside edge of the microtube and (ii) confinement of light also in
direction of the tube axis by an axially varying geometry which is explained in
an expanded waveguide model.Comment: 5 pages, 4 figure
Maximum Metallic Conductivity in Si-MOS Structures
We found that the conductivity of the two-dimensional electron system in
Si-MOS structures is limited to a maximum value, G_{max}, as either density
increases or temperature decreases. This value G_{max} is weakly disorder
dependent and ranging from 100 to 140 e^2/h for samples whose mobilities differ
by a factor of 4.Comment: 3 pages, 3 ps-figs, RevTex, new dat
Quantum Dot as Spin Filter and Spin Memory
We consider a quantum dot in the Coulomb blockade regime weakly coupled to
current leads and show that in the presence of a magnetic field the dot acts as
an efficient spin-filter (at the single-spin level) which produces a
spin-polarized current. Conversely, if the leads are fully spin-polarized the
up or down state of the spin on the dot results in a large sequential or small
cotunneling current, and thus, together with ESR techniques, the setup can be
operated as a single-spin memory.Comment: 4 pages, 3 figures, REVTe
Cylindrical Two-Dimensional Electron Gas in a Transverse Magnetic Field
We compute the single-particle states of a two-dimensional electron gas
confined to the surface of a cylinder immersed in a magnetic field. The
envelope-function equation has been solved exactly for both an homogeneous and
a periodically modulated magnetic field perpendicular to the cylinder axis. The
nature and energy dispersion of the quantum states reflects the interplay
between different lengthscales, namely, the cylinder diameter, the magnetic
length, and, possibly, the wavelength of the field modulation. We show that a
transverse homogeneous magnetic field drives carrier states from a quasi-2D
(cylindrical) regime to a quasi-1D regime where carriers form channels along
the cylinder surface. Furthermore, a magnetic field which is periodically
modulated along the cylinder axis may confine the carriers to tunnel-coupled
stripes, rings or dots on the cylinder surface, depending on the ratio between
the the field periodicity and the cylinder radius. Results in different regimes
are traced to either incipient Landau levels formation or Aharonov-Bohm
behaviour.Comment: 23 pages, 14 figure
Structural and magnetic properties of an InGaAs/FeSi superlattice in cylindrical geometry
The structure and the magnetic properties of an InGaAs/Fe3Si superlattice in
a cylindrical geometry are investigated by electron microscopy techniques,
x-ray diffraction and magnetometry. To form a radial superlattice, a
pseudomorphic InGaAs/Fe3As bilayer has been released from its substrate
self-forming into a rolled-up microtube. Oxide-free interfaces as well as areas
of crystalline bonding are observed and an overall lattice mismatch between
succeeding layers is determined. The cylindrical symmetry of the final radial
superlattice shows a significant effect on the magnetization behavior of the
rolled-up layers
Spin Transport in Two Dimensional Hopping Systems
A two dimensional hopping system with Rashba spin-orbit interaction is
considered. Our main interest is concerned with the evolution of the spin
degree of freedom of the electrons. We derive the rate equations governing the
evolution of the charge density and spin polarization of this system in the
Markovian limit in one-particle approximation. If only two-site hopping events
are taken into account, the evolution of the charge density and of the spin
polarization is found to be decoupled. A critical electric field is found,
above which oscillations are superimposed on the temporal decay of the total
polarization. A coupling between charge density and spin polarization occurs on
the level of three-site hopping events. The coupling terms are identified as
the anomalous Hall effect and the recently proposed spin Hall effect. Thus, an
unpolarized charge current through a sheet of finite width leads to a
transversal spin accumulation in our model system.Comment: 15 pages, 3 figure
Longitudinal spin transport in diluted magnetic semiconductor superlattices: the effect of the giant Zeeman splitting
Longitudinal spin transport in diluted magnetic semiconductor superlattices
is investigated theoretically. The longitudinal magnetoconductivity (MC) in
such systems exhibits an oscillating behavior as function of an external
magnetic field. In the weak magnetic field region the giant Zeeman splitting
plays a dominant role which leads to a large negative magnetoconductivity. In
the strong magnetic field region the MC exhibits deep dips with increasing
magnetic field. The oscillating behavior is attributed to the interplay between
the discrete Landau levels and the Fermi surface. The decrease of the MC at low
magnetic field is caused by the exchange interaction between the electron
in the conduction band and the magnetic ions.Comment: 6 pages, 9 figures, submitted to Phys. Rev.
Weak anisotropy and disorder dependence of the in-plane magnetoresistance in high mobility (100) Si-inversion layers
We report studies of the magnetoresistance (MR) in a two-dimensional electron
system in (100) Si-inversion layers, for perpendicular and parallel
orientations of the current with respect to the magnetic field in the 2D-plane.
The magnetoresistance is almost isotropic; this result does not support the
suggestion of the orbital origin of the MR in Si-inversion layer. In the
hopping regime, however, the MR contains a weak anisotropic component that is
non-monotonic in magnetic field. We found that the field, at which the MR
saturates, for different samples varies by a factor of two, being lower or
higher than the field of complete spin polarization of free carriers.
Therefore, the saturation of the MR can not be identified with the spin
polarization of free carriers.Comment: 4 pages, 4 figures; New data adde
Permanent current from non-commutative spin algebra
We show that a spontaneous electric current is induced in a nano-scale
conducting ring just by putting three ferromagnets. The current is a direct
consequence of the non-commutativity of the spin algebra, and is proportional
to the non-coplanarity (chirality) of the magnetization vectors. The
spontaneous current gives a natural explanation to the chirality-driven
anomalous Hall effect.Comment: 7 pages, 4 figures on separate pag
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