59 research outputs found
Spin-dependent Quantum Interference in Single-Wall Carbon Nanotubes with Ferromagnetic Contacts
We report the experimental observation of spin-induced magnetoresistance in
single-wall carbon nanotubes contacted with high-transparency ferromagnetic
electrodes. In the linear regime the spin-induced magnetoresistance oscillates
with gate voltage in quantitative agreement with calculations based on a
Landauer-Buttiker model for independent electrons. Consistent with this
interpretation, we find evidence for bias-induced oscillation in the
spin-induced magnetoresistance signal on the scale of the level spacing in the
nanotube. At higher bias, the spin-induced magnetoresistance disappears because
of a sharp decrease in the effective spin-polarization injected from the
ferromagnetic electrodes.Comment: Replaced with published versio
Gated Spin Transport through an Individual Single Wall Carbon Nanotube
Hysteretic switching in the magnetoresistance of short-channel,
ferromagnetically contacted individual single wall carbon nanotubes is
observed, providing strong evidence for nanotube spin transport. By varying the
voltage on a capacitively coupled gate, the magnetoresistance can be
reproducibly modified between +10% and -15%. The results are explained in terms
of wave vector matching of the spin polarized electron states at the
ferromagnetic / nanotube interfaces.Comment: 14 pages, 5 figure
Imaging Inter-Edge State Scattering Centers in the Quantum Hall Regime
We use an atomic force microscope tip as a local gate to study the scattering
between edge channels in a 2D electron gas in the quantum Hall regime. The
scattering is dominated by individual, microscopic scattering centers, which we
directly image here for the first time. The tip voltage dependence of the
scattering indicates that tunneling occurs through weak links and localized
states.Comment: 4 pages, 5 figure
Adsorption of Oxygen Molecules on Individual Carbon Single-walled Nanotubes
Our study of the adsorption of oxygen molecules on individual semiconductiong
single-walled carbon nanotubes at ambient conditions reveals that the
adsorption is physisorption, that the resistance without O2 increases by ~two
orders of magnitude as compared to that with O2, and that the sensitive
response is due to the pinning of the Fermi level near the top of the valence
band of the tube resulting from impurity states of O2 appearing above the
valence band.Comment: 16 pages, 4 figure
Separately contacted edge states: A new spectroscopic tool for the investigation of the quantum Hall effect
Using an innovative combination of a quasi-Corbino sample geometry and the
cross-gate technique, we have developed a method that enables us to separately
contact single edge channels in the quantum Hall regime and investigate
equilibration among them. Performing 4-point resistance measurements, we
directly obtain information on the energetic and geometric structure of the
edge region and the equilibration-length for current transport across the
Landau- as well as the spin-gap. Based on an almost free choice in the number
of participating edge channels and their interaction-length a systematic
investigation of the parameter-space becomes possible.Comment: 8 pages, 7 figure
Effect of incoherent scattering on shot noise correlations in the quantum Hall regime
We investigate the effect of incoherent scattering in a Hanbury Brown and
Twiss situation with electrons in edge states of a three-terminal conductor
submitted to a strong perpendicular magnetic field. The modelization of
incoherent scattering is performed by introducing an additional voltage probe
through which the current is kept equal to zero which causes voltage
fluctuations at this probe. It is shown that inelastic scattering can lead in
this framework to positive correlations, whereas correlations remain always
negative for quasi-elastic scattering.Comment: 5 pages latex, 5 eps figure
Individual scatterers as microscopic origin of equilibration between spin- polarized edge channels in the quantum Hall regime
The equilibration length between spin-polarized edge states in the Quantum
Hall regime is measured as a function of a gate voltage applied to an electrode
on top of the edge channels. Reproducible fluctuations in the coupling are
observed and interpreted as a mesoscopic fingerprint of single spin-flip
scatterers which are turned on and off. A model to analyze macroscopic edge
state coupling in terms of individual scatterers is developed, and
characteristic values for these scatterers in our samples are extracted. For
all samples investigated, the distance between spin-flip scatterers lies
between the Drude and the quantum scattering length.Comment: 4 pages, 2 figure
Dynamic nuclear polarization at the edge of a two-dimensional electron gas
We have used gated GaAs/AlGaAs heterostructures to explore nonlinear
transport between spin-resolved Landau level (LL) edge states over a submicron
region of two-dimensional electron gas (2DEG). The current I flowing from one
edge state to the other as a function of the voltage V between them shows
diode-like behavior---a rapid increase in I above a well-defined threshold V_t
under forward bias, and a slower increase in I under reverse bias. In these
measurements, a pronounced influence of a current-induced nuclear spin
polarization on the spin splitting is observed, and supported by a series of
NMR experiments. We conclude that the hyperfine interaction plays an important
role in determining the electronic properties at the edge of a 2DEG.Comment: 8 pages RevTeX, 7 figures (GIF); submitted to Phys. Rev.
Phenomenological Understanding of a Transport Regime with Reflection Symmetry in the Quantum Hall System in a Composite Fermion Picture
In this paper, we present a phenomenological picture based on the composite
fermion theory, in responding to the recent discovery by Shahar et al. of a new
transport regime near the transition from a quantum Hall liquid to a
Hall insulator(ref[8]). In this picture, the seemingly unexpected reflection
symmetry in the longitudinal resistivity can be understood clearly
as due to the symmetry of the gapful excitations which dominate
across the transition, and the abrupt change in at the
transition. The parameter in the linear fit of in ref[8] is
also given a simple physical meaning and the effective mass can be calculated
from , which gives a reasonable value of several electron band mass.
When taking into account the result of network model, the almost invariant Hall
resistivity across the transition is also well-understood.Comment: 4 pages, RevTex, final version to appear in Phys.Rev.
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