565 research outputs found
Graphene Spin Valve Devices
Graphene - a single atomic layer of graphite - is a recently-found
two-dimensional form of carbon, which exhibits high crystal quality and
ballistic electron transport at room temperature. Soft magnetic NiFe electrodes
have been used to inject polarized spins into graphene and a 10% change in
resistance has been observed as the electrodes switch from the parallel to the
antiparallel state. This coupled with the fact that a field effect electrode
can modulate the conductivity of these graphene films makes them exciting
potential candidates for spin electronic devices.Comment: 3 pages, 6 figure
On resonant scatterers as a factor limiting carrier mobility in graphene
We show that graphene deposited on a substrate has a non-negligible density
of atomic scale defects. This is evidenced by a previously unnoticed D peak in
the Raman spectra with intensity of about 1% with respect to the G peak. We
evaluated the effect of such impurities on electron transport by mimicking them
with hydrogen adsorbates and measuring the induced changes in both mobility and
Raman intensity. If the intervalley scatterers responsible for the D peak are
monovalent, their concentration is sufficient to account for the limited
mobilities achievable in graphene on a substrate.Comment: version 2: several comments are taken into account and refs adde
A self-consistent theory for graphene transport
We demonstrate theoretically that most of the observed transport properties
of graphene sheets at zero magnetic field can be explained by scattering from
charged impurities. We find that, contrary to common perception, these
properties are not universal but depend on the concentration of charged
impurities . For dirty samples (), the value of the minimum
conductivity at low carrier density is indeed in agreement with early
experiments, with weak dependence on impurity concentration. For cleaner
samples, we predict that the minimum conductivity depends strongly on , increasing to for . A clear strategy to improve graphene mobility is to eliminate
charged impurities or use a substrate with a larger dielectric constant.Comment: To be published in Proc. Natl. Acad. Sci. US
Unconventional quantum Hall effect and Berry’s phase 2pi in bilayer graphene.
There are known two distinct types of the integer quantum Hall effect. One is the conventional quantum Hall effect, characteristic of two-dimensional semiconductor systems, and the other is its relativistic counterpart recently observed in graphene, where charge carriers mimic Dirac fermions characterized by Berry’s phase pi, which results in a shifted positions of Hall plateaus. Here we report a third type of the integer quantum Hall effect. Charge carriers in bilayer graphene have a parabolic energy spectrum but are chiral and exhibit Berry’s phase 2pi affecting their quantum dynamics. The Landau quantization of these fermions results in plateaus in Hall conductivity at standard integer positions but the last (zero-level) plateau is missing. The zero-level anomaly is accompanied by metallic conductivity in the limit of low concentrations and high magnetic fields, in stark contrast to the conventional, insulating behavior in this regime. The revealed chiral fermions have no known analogues and present an intriguing case for quantum-mechanical studies
Strong Suppression of Electrical Noise in Bilayer Graphene Nano Devices
Low-frequency 1/f noise is ubiquitous, and dominates the signal-to-noise
performance in nanodevices. Here we investigate the noise characteristics of
single-layer and bilayer graphene nano-devices, and uncover an unexpected 1/f
noise behavior for bilayer devices. Graphene is a single layer of graphite,
where carbon atoms form a 2D honeycomb lattice. Despite the similar
composition, bilayer graphene (two graphene monolayers stacked in the natural
graphite order) is a distinct 2D system with a different band structure and
electrical properties. In graphene monolayers, the 1/f noise is found to follow
Hooge's empirical relation with a noise parameter comparable to that of bulk
semiconductors. However, this 1/f noise is strongly suppressed in bilayer
graphene devices, and exhibits an unusual dependence on the carrier density,
different from most other materials. The unexpected noise behavior in graphene
bilayers is associated with its unique band structure that varies with the
charge distribution among the two layers, resulting in an effective screening
of potential fluctuations due to external impurity charges. The findings here
point to exciting opportunities for graphene bilayers in low-noise
applications
Thickness Estimation of Epitaxial Graphene on SiC using Attenuation of Substrate Raman Intensity
A simple, non-invasive method using Raman spectroscopy for the estimation of
the thickness of graphene layers grown epitaxially on silicon carbide (SiC) is
presented, enabling simultaneous determination of thickness, grain size and
disorder using the spectra. The attenuation of the substrate Raman signal due
to the graphene overlayer is found to be dependent on the graphene film
thickness deduced from X-ray photoelectron spectroscopy and transmission
electron microscopy of the surfaces. We explain this dependence using an
absorbing overlayer model. This method can be used for mapping graphene
thickness over a region and is capable of estimating thickness of multilayer
graphene films beyond that possible by XPS and Auger electron spectroscopy
(AES).Comment: 14 pages, 9 figure
Editorial: The Mammary Stroma in Normal Development and Function
The mammary gland can no longer be simply viewed as an organ composed of epithelial cells within a passive stromal microenvironment. Many lines of evidence have evolved to reinforce the notion that mammary epithelial cell growth, differentiation, lactation and progression to cancer involves bidirectional interactions between the epithelial population and its surrounding stroma. Within this stroma are numerous systems that are all capable of modulating epithelial function. In this context, the mammary stroma is not simply a depot of adipose tissue in which mammary epithelial cells undertake a unique growth and differentiation process, although adipocytes can impart numerous modulatory signals to epithelial cells, and vice versa. Rather, the stromal environment constitutes and supports a critical vasculature that supplies nutrients and endocrine cues, a lymphatic system that not only removes metabolites but also provides an intimate interface with the immune system, and an extracellular matrix scaffold in which epithelial cells grow, differentiate and regress. Ultimately all of these components play a critical role in directing the epithelial phenotype during normal mammary gland growth and function. An increasing appreciation for these different systems demands a view of mammary epithelial cells in a much different light, and further necessitates the development of model systems that incorporate and integrate increasing complexity
Impurity-assisted tunneling in graphene
The electric conductance of a strip of undoped graphene increases in the
presence of a disorder potential, which is smooth on atomic scales. The
phenomenon is attributed to impurity-assisted resonant tunneling of massless
Dirac fermions. Employing the transfer matrix approach we demonstrate the
resonant character of the conductivity enhancement in the presence of a single
impurity. We also calculate the two-terminal conductivity for the model with
one-dimensional fluctuations of disorder potential by a mapping onto a problem
of Anderson localization.Comment: 6 pages, 3 figures, final version, typos corrected, references adde
High On/Off Ratios in Bilayer Graphene Field Effect Transistors Realized by Surface Dopants
The unique property of bilayer graphene to show a band gap tunable by
external electrical fields enables a variety of different device concepts with
novel functionalities for electronic, optoelectronic and sensor applications.
So far the operation of bilayer graphene based field effect transistors
requires two individual gates to vary the channel's conductance and to create a
band gap. In this paper we report on a method to increase the on/off ratio in
single gated bilayer graphene field effect transistors by adsorbate doping. The
adsorbate dopants on the upper side of the graphene establish a displacement
field perpendicular to the graphene surface breaking the inversion symmetry of
the two graphene layers. Low temperature measurements indicate, that the
increased on/off ratio is caused by the opening of a mobility gap. Beside field
effect transistors the presented approach can also be employed for other
bilayer graphene based devices like photodetectors for THz to infrared
radiation, chemical sensors and in more sophisticated structures such as
antidot- or superlattices where an artificial potential landscape has to be
created.Comment: 4 pages, 4 figure
Models of electron transport in single layer graphene
The main features of the conductivity of doped single layer graphene are
analyzed, and models for different scattering mechanisms are presented.Comment: 15 pages. Submitted to the Proceedings of the ULTI symposium on
Quantum Phenomena and Devices at Low Temperatures, Espoo, Finland, to be
published in the Journ. of Low. Temp. Phy
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