100 research outputs found
Structural and electronic properties of grain boundaries in graphite: Planes of periodically distributed point defects
We report on scanning tunneling microscopy and spectroscopy of grain
boundaries in highly oriented pyrolytic graphite. Grain boundaries showed a
periodic structure and an enhanced charge density compared to the bare graphite
surface. Two possible periodic structures have been observed along grain
boundaries. A geometrical model producing periodically distributed point
defects on the basal plane of graphite has been proposed to explain the
structure of grain boundaries. Scanning tunneling spectroscopy on grain
boundaries revealed two strong localized states at -0.3 V and 0.4 V.Comment: 5 pages, 5 figure
Giant inelastic tunneling in epitaxial graphene mediated by localized states
Local electronic structures of nanometer-sized patches of epitaxial graphene
and its interface layer with SiC(0001) have been studied by atomically resolved
scanning tunneling microscopy and spectroscopy. Localized states belonging to
the interface layer of a graphene/SiC system show to have an essential
influence on the electronic structure of graphene. Giant enhancement of
inelastic tunneling, reaching 50% of the total tunneling current, has been
observed at the localized states on a nanometer-sized graphene monolayer
surrounded by defects.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.
Comparison of the magneto-Peltier and magneto-Seebeck effects in magnetic tunnel junctions
Understanding heat generation and transport processes in a magnetic tunnel
junction (MTJ) is a significant step towards improving its application in
current memory devices. Recent work has experimentally demonstrated the
magneto-Seebeck effect in MTJs, where the Seebeck coefficient of the junction
varies as the magnetic configuration changes from a parallel (P) to an
anti-parallel (AP) configuration. Here we report the study on its
as-yet-unexplored reciprocal effect, the magneto-Peltier effect, where the heat
flow carried by the tunneling electrons is altered by changing the magnetic
configuration of the MTJ. The magneto-Peltier signal that reflects the change
in the temperature difference across the junction between the P and AP
configurations scales linearly with the applied current in the small bias but
is greatly enhanced in the large bias regime, due to higher-order Joule heating
mechanisms. By carefully extracting the linear response which reflects the
magneto-Peltier effect, and comparing it with the magneto-Seebeck measurements
performed on the same device, we observe results consistent with Onsager
reciprocity. We estimate a magneto-Peltier coefficient of 13.4 mV in the linear
regime using a three-dimensional thermoelectric model. Our result opens up the
possibility of programmable thermoelectric devices based on the Peltier effect
in MTJs
High sensitive quasi freestanding epitaxial graphene gassensor on 6H-SiC
We have measured the electrical response to NO, N, NH and CO for
epitaxial graphene and quasi freestanding epitaxial graphene on 6H-SiC
substrates. Quasi freestanding epitaxial graphene shows a 6 fold increase in
NO2 sensitivity compared to epitaxial graphene. Both samples show a sensitivity
better than the experimentally limited 1 ppb. The strong increase in
sensitivity of quasi freestanding epitaxial graphene can be explained by a
Fermi-energy close to the Dirac Point leading to a strongly surface doping
dependent sample resistance. Both sensors show a negligible sensitivity to
N, NH and CO
Electronic States of Graphene Grain Boundaries
We introduce a model for amorphous grain boundaries in graphene, and find
that stable structures can exist along the boundary that are responsible for
local density of states enhancements both at zero and finite (~0.5 eV)
energies. Such zero energy peaks in particular were identified in STS
measurements [J. \v{C}ervenka, M. I. Katsnelson, and C. F. J. Flipse, Nature
Physics 5, 840 (2009)], but are not present in the simplest pentagon-heptagon
dislocation array model [O. V. Yazyev and S. G. Louie, Physical Review B 81,
195420 (2010)]. We consider the low energy continuum theory of arrays of
dislocations in graphene and show that it predicts localized zero energy
states. Since the continuum theory is based on an idealized lattice scale
physics it is a priori not literally applicable. However, we identify stable
dislocation cores, different from the pentagon-heptagon pairs, that do carry
zero energy states. These might be responsible for the enhanced magnetism seen
experimentally at graphite grain boundaries.Comment: 10 pages, 4 figures, submitted to Physical Review
Lowest order in inelastic tunneling approximation: Efficient scheme for simulation of inelastic electron tunneling data
We have developed an efficient and accurate formalism which allows the simulation at the ab initio level of inelastic electron tunneling spectroscopy data under a scanning tunneling microscope setup. It exploits fully the tunneling regime by carrying out the structural optimization and vibrational mode calculations for surface and tip independently. The most relevant interactions in the inelastic current are identified as the inelastic tunneling terms, which are taken into account up to lowest order, while all other inelastic contributions are neglected. As long as the system is under tunneling regime conditions and there is no physisorbed molecule on the surface or tip apex, this lowest order in inelastic tunneling (LOIT) approach reduces drastically the computational cost compared to related approaches while maintaining a good accuracy. Adopting the wide-band limit for both tip and surface further reduces calculation times significantly, and is shown to give similar results to when the full energy dependence of the Green's functions is taken into account. The LOIT is applied to the Cu(111)+CO system probed by a clean and a CO contaminated tip to find good agreement with previous works. Different parameters involved in the calculations such as basis sets, k sampling, tip-sample distance, or temperature, among others, are discussed in detail. © 2013 American Physical Society.J.C. acknowledges financial support from the Spanish Ministry of Innovation and Science under Contract No.MAT2010-18432.Peer Reviewe
Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond(111)/Tungsten Carbide Interface
A comprehensive nanotribological study of a hydrogen-terminated diamond(111)/tungsten carbide interface has been performed using ultrahigh vacuum atomic force microscopy. Both contact conductance, which is proportional to contact area, and friction have been measured as a function of applied load. We demonstrate for the first time that the load dependence of the contact area in UHV for this extremely hard single asperity contact is described by the Derjaguin-Müller-Toporov continuum mechanics model. Furthermore, the frictional force is found to be directly proportional to the contact area
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