212 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
Verification of the Thomson-Onsager reciprocity relation for spin caloritronics
We investigate the Thomson-Onsager relation between the spin-dependent
Seebeck and spin-dependent Peltier effect. To maintain identical device and
measurement conditions we measure both effects in a single
NiFe/Cu/NiFe nanopillar spin valve device subjected
to either an electrical or a thermal bias. In the low bias regime, we observe
similar spin signals as well as background responses, as required by the
Onsager reciprocity relation. However, at large biases, deviation from
reciprocity occurs due to dominant nonlinear contribution of the temperature
dependent transport coefficients. By systematic modeling of these nonlinear
thermoelectric effects and measuring higher order thermoelectric responses for
different applied biases, we identify the transition between the two regimes as
the point at which Joule heating start to dominate over Peltier heating. Our
results signify the importance of local equilibrium for the validity of this
phenomenological reciprocity relation.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
Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves
We have experimentally determined the spin-dependent Seebeck coefficient of
permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices.
The devices were specifically designed to completely separate heat related
effects from charge related effects. A pure heat current through the nanopillar
spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic
layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck
coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where
S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and
spin-down electrons. By using a three-dimensional finite-element model (3D-FEM)
based on spin-dependent thermoelectric theory, whose input material parameters
were measured in separate devices, we were able to accurately determine a
spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5
microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a
Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where
S_{F} is the Seebeck coefficient of the ferromagnet. The results are in
agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent
Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve
structures
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
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