21,674 research outputs found
Nernst and Seebeck effect in a graphene nanoribbon
The thermoelectric power, including the Nernst and Seebeck effects, in
graphene nanoribbon is studied. By using the non-equilibrium Green function
combining with the tight-binding Hamiltonian, the Nernst and Seebeck
coefficients are obtained. Due to the electron-hole symmetry, the Nernst
coefficient is an even function of the Fermi energy while the Seebeck
coefficient is an odd function regardless of the magnetic field. In the
presence of a strong magnetic field, the Nernst and Seebeck coefficients are
almost independent of the chirality and width of the nanoribbon, and they show
peaks when the Fermi energy crosses the Landau levels. The height of -th
(excluding ) peak is for the Nernst effect and is
for the Seebeck effect. For the zeroth peak, it is abnormal with height
for the Nernst effect and the peak disappears for the Seebeck effect.
When the magnetic field is turned off, however, the Nernst effect is absent and
only Seebeck effect exists. In this case, the Seebeck coefficient strongly
depends on the chirality of the nanoribbon. The peaks are equidistant for the
nanoribbons with zigzag edge but are irregularly distributed for the armchair
edge. In particular, for the insulating armchair ribbon, the Seebeck
coefficient can be very large near the Dirac point. When the magnetic field
varies from zero to large values, the differences among the Seebeck
coefficients for different chiral ribbons gradually vanish and the nonzero
value of Nernst coefficient appears first near the Dirac point then gradually
extents to the whole energy region.Comment: 8 pages, 7 figure
Non-Adiabatic Spin Transfer Torque in Real Materials
The motion of simple domain walls and of more complex magnetic textures in
the presence of a transport current is described by the
Landau-Lifshitz-Slonczewski (LLS) equations. Predictions of the LLS equations
depend sensitively on the ratio between the dimensionless material parameter
which characterizes non-adiabatic spin-transfer torques and the Gilbert
damping parameter . This ratio has been variously estimated to be close
to 0, close to 1, and large compared to 1. By identifying as the
influence of a transport current on , we derive a concise, explicit and
relatively simple expression which relates to the band structure and
Bloch state lifetimes of a magnetic metal. Using this expression we demonstrate
that intrinsic spin-orbit interactions lead to intra-band contributions to
which are often dominant and can be (i) estimated with some confidence
and (ii) interpreted using the "breathing Fermi surface" model.Comment: 18 pages, 9 figures; submitted to Phys. Rev.
Noise properties of two single electron transistors coupled by a nanomechanical resonator
We analyze the noise properties of two single electron transistors (SETs)
coupled via a shared voltage gate consisting of a nanomechanical resonator.
Working in the regime where the resonator can be treated as a classical system,
we find that the SETs act on the resonator like two independent heat baths. The
coupling to the resonator generates positive correlations in the currents
flowing through each of the SETs as well as between the two currents. In the
regime where the dynamics of the resonator is dominated by the back-action of
the SETs, these positive correlations can lead to parametrically large
enhancements of the low frequency current noise. These noise properties can be
understood in terms of the effects on the SET currents of fluctuations in the
state of a resonator in thermal equilibrium which persist for times of order
the resonator damping time.Comment: Accepted for publication in Phys. Rev.
Calculation of the current noise spectrum in mesoscopic transport: an efficient quantum master equation approach
Based on our recent work on quantum transport [Li et al., Phys. Rev. B 71,
205304 (2005)], where the calculation of transport current by means of quantum
master equation was presented, in this paper we show how an efficient
calculation can be performed for the transport noise spectrum. Compared to the
longstanding classical rate equation or the recently proposed quantum
trajectory method, the approach presented in this paper combines their
respective advantages, i.e., it enables us to tackle both the many-body Coulomb
interactionand quantum coherence on equal footing and under a wide range of
setup circumstances. The practical performance and advantages are illustrated
by a number of examples, where besides the known results and new insights
obtained in a transparent manner, we find that this alternative approach is
much simpler than other well-known full quantum mechanical methods such as the
Landauer-B\"uttiker scattering matrix theory and the nonequilibrium Green's
function technique.Comment: 13 pages, 3 figures, submitted to PR
Weyl corrections to holographic conductivity
For conformal field theories which admit a dual gravitational description in
anti-de Sitter space, electrical transport properties, such as conductivity and
charge diffusion, are determined by the dynamics of a U(1) gauge field in the
bulk and thus obey universality relations at the classical level due to the
uniqueness of the Maxwell action. We analyze corrections to these transport
parameters due to higher-dimension operators in the bulk action, beyond the
leading Maxwell term, of which the most significant involves a coupling to the
bulk Weyl tensor. We show that the ensuing corrections to conductivity and the
diffusion constant break the universal relation with the U(1) central charge
observed at leading order, but are nonetheless subject to interesting bounds
associated with causality in the boundary CFT.Comment: 15 pages, v2: references adde
Thermoelectric and Magnetothermoelectric Transport Measurements of Graphene
The conductance and thermoelectric power (TEP) of graphene is simultaneously
measured using microfabricated heater and thermometer electrodes. The sign of
the TEP changes across the charge neutrality point as the majority carrier
density switches from electron to hole. The gate dependent conductance and TEP
exhibit a quantitative agreement with the semiclassical Mott relation. In the
quantum Hall regime at high magnetic field, quantized thermopower and Nernst
signals are observed and are also in agreement with the generalized Mott
relation, except for strong deviations near the charge neutrality point
Improved position measurement of nano electromechanical systems using cross correlations
We consider position measurements using the cross-correlated output of two
tunnel junction position detectors. Using a fully quantum treatment, we
calculate the equation of motion for the density matrix of the coupled
detector-detector-mechanical oscillator system. After discussing the presence
of a bound on the peak-to-background ratio in a position measurement using a
single detector, we show how one can use detector cross correlations to
overcome this bound. We analyze two different possible experimental
realizations of the cross correlation measurement and show that in both cases
the maximum cross-correlated output is obtained when using twin detectors and
applying equal bias to each tunnel junction. Furthermore, we show how the
double-detector setup can be exploited to drastically reduce the added
displacement noise of the oscillator.Comment: 9 pages, 1 figure; v2: new Sec.
Analytic Solutions to the Constraint Equation for a Force-Free Magnetosphere around a Kerr Black Hole
The Blandford-Znajek constraint equation for a stationary, axisymmetric
black-hole force-free magnetosphere is cast in a 3+1 absolute space and time
formulation, following Komissarov (2004). We derive an analytic solution for
fields and currents to the constraint equation in the far-field limit that
satisfies the Znajek condition at the event horizon. This solution generalizes
the Blandford-Znajek monopole solution for a slowly rotating black hole to
black holes with arbitrary angular momentum. Energy and angular momentum
extraction through this solution occurs mostly along the equatorial plane. We
also present a nonphysical, reverse jet-like solution.Comment: 6 pages, accepted for publication in Ap
Effect of Edge Roughness on Electronic Transport in Graphene Nanoribbon Channel Metal Oxide Semiconductor Field-Effect Transistors
Results of quantum mechanical simulations of the influence of edge disorder
on transport in graphene nanoribbon metal oxide semiconductor field-effect
transistors (MOSFETs) are reported. The addition of edge disorder significantly
reduces ON-state currents and increases OFF-state currents, and introduces wide
variability across devices. These effects decrease as ribbon widths increase
and as edges become smoother. However the bandgap decreases with increasing
width, thereby increasing the band-to-band tunneling mediated subthreshold
leakage current even with perfect nanoribbons. These results suggest that
without atomically precise edge control during fabrication, MOSFET performance
gains through use of graphene will be difficult to achieve.Comment: 8 pages, 5 figure
The Influence of Formulation, Buffering, pH and Divalent Cations on the Activity of Endothall on Hydrilla.
Endothall has been used as an aquatic herbicide for more
than 40 years and provides very effective weed control of
many weeds. Early research regarding the mechanism-of-action
of endothall contradicts the symptomology normally associated
with the product. Recent studies suggest endothall
is a respiratory toxin but the mechanism-of-action remains
unknown. To further elucidate the activity of endothall, several
endothall formulations were evaluated for their effects
on ion leakage, oxygen consumption and photosynthetic oxygen
evolution from hydrilla shoot tips. The influence of pH,
buffering and divalent cations was also evaluated. (PDF contains 6 pages.
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