92 research outputs found
Experimental Study of the Intrinsic and Extrinsic Transport Properties of Graphite and Multigraphene Samples
This work deals with the intrinsic and extrinsic properties of the graphene
layers inside the graphite structure, in particular the influence of defects
and interfaces. We discuss the evidence for ballistic transport found in
mesoscopic graphite samples and the possibility to obtain the intrinsic carrier
density of graphite, without the need of free parameters or arbitrary
assumptions. The influence of internal interfaces on the transport properties
of bulk graphite is described in detail. We show that in specially prepared
multigraphene samples the transport properties show clear signs for the
existence of granular superconductivity within the graphite interfaces. We
argue that the superconducting-insulator or metal-insulator transition (MIT)
reported in the literature for bulk graphite is not intrinsic of the graphite
structure but it is due to the influence of these interfaces. Current-Voltage
characteristics curves reveal Josephson-like behavior at the interfaces with
superconducting critical temperatures above 150K.Comment: 26 pages, 15 figures. To be published in "Graphene, Book 2" by
Intech, Open Access Publisher 2011, ISBN: 979-953-307-180-
Ballistic transport at room temperature in micrometer size multigraphene
The intrinsic values of the carriers mobility and density of the graphene
layers inside graphite, the well known structure built on these layers in the
Bernal stacking configuration, are not well known mainly because most of the
research was done in rather bulk samples where lattice defects hide their
intrinsic values. By measuring the electrical resistance through
microfabricated constrictions in micrometer small graphite flakes of a few tens
of nanometers thickness we studied the ballistic behavior of the carriers. We
found that the carriers' mean free path is micrometer large with a mobility
cm/Vs and a carrier density cm per graphene layer at room temperature. These distinctive
transport and ballistic properties have important implications for
understanding the values obtained in single graphene and in graphite as well as
for implementing this last in nanoelectronic devices.Comment: 6 pages, 6 figure
On the low-field Hall coefficient of graphite
We have measured the temperature and magnetic field dependence of the Hall
coefficient () in three, several micrometer long multigraphene
samples of thickness between to ~nm in the temperature range
0.1 to 200~K and up to 0.2~T field. The temperature dependence of the
longitudinal resistance of two of the samples indicates the contribution from
embedded interfaces running parallel to the graphene layers. At low enough
temperatures and fields is positive in all samples, showing a
crossover to negative values at high enough fields and/or temperatures in
samples with interfaces contribution. The overall results are compatible with
the reported superconducting behavior of embedded interfaces in the graphite
structure and indicate that the negative low magnetic field Hall coefficient is
not intrinsic of the ideal graphite structure.Comment: 10 pages with 7 figures, to be published in AIP Advances (2014
Uncompensated magnetization and exchange-bias field in LaSrMnO/YMnO bilayers: The influence of the ferromagnetic layer
We studied the magnetic behavior of bilayers of multiferroic and nominally
antiferromagnetic o-YMnO (375~nm thick) and ferromagnetic
LaSrMnO and LaCaMnO (nm), in particular the vertical magnetization shift and exchange
bias field for different thickness and magnetic dilution of the
ferromagnetic layer at different temperatures and cooling fields. We have found
very large shifts equivalent to up to 100\% of the saturation value of
the o-YMO layer alone. The overall behavior indicates that the properties of
the ferromagnetic layer contribute substantially to the shift and that
this does not correlate straightforwardly with the measured exchange bias field
.Comment: 10 figures, 8 page
Length dependence of the resistance in graphite: Influence of ballistic transport
Using a linear array of voltage electrodes with a separation of several
micrometers on a nm thick and 30 m long multigraphene sample we show
that the measured resistance does not follow the usual length dependence
according to Ohm's law. The deviations can be quantitatively explained taking
into account Sharvin-Knudsen formula for ballistic transport. This allows us to
obtain without free parameters the mean free path of the carriers in the sample
at different temperatures. In agreement with recently reported values obtained
with a different experimental method, we obtain that the carrier mean free path
is of the order of m with a mobility cmVs. The results indicate that the usual Ohm's law is not
adequate to calculate the absolute resistivity of mesoscopic graphite samples.Comment: 5 pages, 5 figures, in press in Journal of Applied Physics (2012
Evidence for semiconducting behavior with a narrow band gap of Bernal graphite
We have studied the resistivity of a large number of highly oriented graphite
samples with areas ranging from several mm to a few m and
thickness from nm to several tens of micrometers. The measured
resistance can be explained by the parallel contribution of semiconducting
graphene layers with low carrier density cm and the one from
metallic-like internal interfaces. The results indicate that ideal graphite
with Bernal stacking structure is a narrow-gap semiconductor with an energy gap
meV.Comment: 14 pages, 4 Figures, to be published in New Journal of Physics (in
press, 2012
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