26 research outputs found
Magnetic field generated resistivity maximum in graphite
In zero magnetic field, B, the electrical resistivity, rho(O,T) of highly oriented pyrolytic (polycrystalline) graphite drops smoothly with decreasing T, becoming constant below 4 K. However, in a fixed applied magnetic field B, the resistivity rho(B,T) goes through a maximum as a function of T, with larger maximum for larger B. The temperature of the maximum increases with B, but saturates to a constant value near 25 K (exact T depends on sample) at high B. In single crystal graphite a maximum in rho(B,T) as a function of T is also present, but has the effects of Landau level quantization superimposed. Several possible explanations for the rho(B,T) maximum are proposed, but a complete explanation awaits detailed calculations involving the energy band structure of graphite, and the particular scattering mechanisms involved
Electronic and Magnetic Properties of Nanographite Ribbons
Electronic and magnetic properties of ribbon-shaped nanographite systems with
zigzag and armchair edges in a magnetic field are investigated by using a tight
binding model. One of the most remarkable features of these systems is the
appearance of edge states, strongly localized near zigzag edges. The edge state
in magnetic field, generating a rational fraction of the magnetic flux (\phi=
p/q) in each hexagonal plaquette of the graphite plane, behaves like a
zero-field edge state with q internal degrees of freedom. The orbital
diamagnetic susceptibility strongly depends on the edge shapes. The reason is
found in the analysis of the ring currents, which are very sensitive to the
lattice topology near the edge. Moreover, the orbital diamagnetic
susceptibility is scaled as a function of the temperature, Fermi energy and
ribbon width. Because the edge states lead to a sharp peak in the density of
states at the Fermi level, the graphite ribbons with zigzag edges show
Curie-like temperature dependence of the Pauli paramagnetic susceptibility.
Hence, it is shown that the crossover from high-temperature diamagnetic to
low-temperature paramagnetic behavior of the magnetic susceptibility of
nanographite ribbons with zigzag edges.Comment: 13 pages including 19 figures, submitted to Physical Rev
Quantum point contact on graphite surface
The conductance through a quantum point contact created by a sharp and hard
metal tip on the graphite surface has features which to our knowledge have not
been encountered so far in metal contacts or in nanowires. In this paper we
first investigate these features which emerge from the strongly directional
bonding and electronic structure of graphite, and provide a theoretical
understanding for the electronic conduction through quantum point contacts. Our
study involves the molecular-dynamics simulations to reveal the variation of
interlayer distances and atomic structure at the proximity of the contact that
evolves by the tip pressing toward the surface. The effects of the elastic
deformation on the electronic structure, state density at the Fermi level, and
crystal potential are analyzed by performing self-consistent-field
pseudopotential calculations within the local-density approximation. It is
found that the metallicity of graphite increases under the uniaxial compressive
strain perpendicular to the basal plane. The quantum point contact is modeled
by a constriction with a realistic potential. The conductance is calculated by
representing the current transporting states in Laue representation, and the
variation of conductance with the evolution of contact is explained by taking
the characteristic features of graphite into account. It is shown that the
sequential puncturing of the layers characterizes the conductance.Comment: LaTeX, 11 pages, 9 figures (included), to be published in Phys. Rev.
B, tentatively scheduled for 15 September 1998 (Volume 58, Number 12
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-
Quantum galvanomagnetic and thermomagnetic effects in graphite to 18.3 teslas /180 kG/ at low temperatures
Quantum galvanomagnetic and thermomagnetic effects in graphite in magnetic fields at low temperature
Atomic theory of the scanning tunneling microscope
Ankara : The Department of Physics and the Institute of Graduate Studies of Bilkent Univ. , 1988.Thesis (Master's) -- Bilkent University, 1988.Includes bibliographical references leaves 93-99.The Scanning Tunneling Microscope is proven to be one of the most
powerful tools for surface structure determination. Present theories are able to
explain the operation of the microscope when the tip is far from the surface. For
the small tip height case the atomic-scale interaction of the tip and the surface
has to be included in the theory. The electronic structure of the combined
system of the tip and the surface is calculated with an Empirical Tight Binding
approach for graphite. It is found that in the vicinity of the tip some Tip
Induced Localized States are formed. These states play an important role in
the tunneling phenomenon. The contribution of these states to the tunneling
current is calculated.Tekman, Ahmet ErkanM.S
Diamagnetism Of Graphite
A calculation of the diamagnetism of graphite including the effects of trigonal warping of the Fermi surfaces has been performed using Fukuyama's formulation of the diamagnetism of Bloch electrons. Inclusion of the trigonal warping increases the diamagnetism by about 13% at low temperatures and reduces it by about 1% at high temperatures. A paramagnetic constant is used to represent all effects not associated with the free carriers. The experimental diamagnetism can be fitted quite well using values of the energy band parameters which give agreement with the de Haas-van Alphen effect, optical absorption, and magnetoreflection experiments. © 1974 The American Physical Society.962467247
Taming the magnetoresistance anomaly in graphite
At low temperatures, graphite presents a magnetoresistance anomaly which
manifests as a transition to a high-resistance state (HRS) above a certain
critical magnetic field . Such HRS is currently attributed
to a c-axis charge-density-wave taking place only when the lowest Landau level
is populated. By controlling the charge carrier concentration of a gated sample
through its charge neutrality level (CNL), we were able to experimentally
modulate the HRS in graphite for the first time. We demonstrate that the HRS is
triggered both when electrons and holes are the majority carriers but is
attenuated near the CNL. Taking screening into account, our results indicate
that the HRS possess a strong in-plane component and can occur below the
quantum limit, being at odds with the current understanding of the phenomenon.
We also report the effect of sample thickness on the HRS
Electronic transport in two dimensional graphene
We provide a broad review of fundamental electronic properties of
two-dimensional graphene with the emphasis on density and temperature dependent
carrier transport in doped or gated graphene structures. A salient feature of
our review is a critical comparison between carrier transport in graphene and
in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells,
inversion layers) so that the unique features of graphene electronic properties
arising from its gap- less, massless, chiral Dirac spectrum are highlighted.
Experiment and theory as well as quantum and semi-classical transport are
discussed in a synergistic manner in order to provide a unified and
comprehensive perspective. Although the emphasis of the review is on those
aspects of graphene transport where reasonable consensus exists in the
literature, open questions are discussed as well. Various physical mechanisms
controlling transport are described in depth including long- range charged
impurity scattering, screening, short-range defect scattering, phonon
scattering, many-body effects, Klein tunneling, minimum conductivity at the
Dirac point, electron-hole puddle formation, p-n junctions, localization,
percolation, quantum-classical crossover, midgap states, quantum Hall effects,
and other phenomena.Comment: Final version as accepted for publication in Reviews of Modern
Physics (in press), 69 pages with 38 figure