579 research outputs found
Strain-induced bi-thermoelectricity in tapered carbon nanotubes
We show that carbon-based nanostructured materials are a novel testbed for controlling thermoelectricity and have the potential to underpin the development of new cost-effective environmentally-friendly thermoelectric materials. In single-molecule junctions, it is known that transport resonances associated with the discrete molecular levels play a key role in the thermoelectric performance, but such resonances have not been exploited in carbon nanotubes (CNTs). Here we study junctions formed from tapered CNTs and demonstrate that such structures possess transport resonances near the Fermi level, whose energetic location can be varied by applying strain, resulting in an ability to tune the sign of their Seebeck coefficient. These results reveal that tapered CNTs form a new class of bi-thermoelectric materials, exhibiting both positive and negative thermopower. This ability to change the sign of the Seebeck coefficient allows the thermovoltage in carbon-based thermoelectric devices to be boosted by placing CNTs with alternating-sign Seebeck coefficients in tandem
Giant thermoemf in multiterminal superconductor/normal metal mesoscopic structures
We considered a mesoscopic superconductor/normal metal (S/N) structure in
which the N reservoirs are maintained at different temperatures. It is shown
that in the absence of current between the N reservoirs a voltage difference
arises between the superconducting and normal conductors. The voltage
oscillates with increasing phase difference between the
superconductors, and its magnitude does not depend on the small parameter
Comment: Resubmited, some changes to Text and Figure
Tachyon Dynamics and the Effective Action Approximation
Recently effective actions have been extensively used to describe tachyon
condensation in string theory. While the various effective actions which have
appeared in the literature have very similar properties for static
configurations, they differ for time-dependent tachyons. In this paper we
discuss general properties of non-linear effective Lagrangians which are first
order in derivatives. In particular we show that some observed properties, such
as asymptotically vanishing pressure, are rather generic features, although the
quantative features differ. On the other hand we argue that certain features of
marginal tachyon profiles are beyond the reach of any first order Lagrangian
description. We also point out that an effective action, proposed earlier,
captures the dynamics of tachyons well.Comment: References added and confusing reference clarifie
On open superstring partition function in inhomogeneous rolling tachyon background
We consider open superstring partition function Z on the disc in
time-dependent tachyon background T= f(x_i) e^{m x_0} where the profile
function f depends on spatial coordinates. We compute Z to second order in
derivatives of f and compare the result with some previously suggested
effective actions depending only on the first derivatives of the tachyon field.
We also compute the target-space stress-energy tensor in this background and
demonstrate its conservation in the ``on-shell'' case of the linear profile f=
f_0 + q_i x_i corresponding to a marginal perturbation. We comment on the role
of the rolling tachyon with linear spatial profile in the decay of an unstable
D-brane.Comment: 32p, latex, v2: introduction revised to expand and correct comments
on the paper by Kutasov and Niarchos, v3: corrected typos and formulas in
section 4, final version to appear in JHE
Tachyon Kinks in Boundary String Field Theory
We study tachyon kinks with and without electromagnetic fields in the context
of boundary string field theory. For the case of pure tachyon only an array of
kink-antikink is obtained. In the presence of electromagnetic coupling, all
possible static codimension-one soliton solutions such as array of
kink-antikink, single topological BPS kink, bounce, half kink, as well as
nonBPS topological kink are found, and their properties including the
interpretation as branes are analyzed in detail. Spectrum of the obtained kinks
coincides with that of Dirac-Born-Infeld type effective theory.Comment: LaTex, 29 pages, 17 Figure
Non-BPS D-brane Near NS5-branes
We use tachyon field theory effective action to study the dynamics of a
non-BPS Dp-brane propagating in the vicinity of k NS5-branes. For the time
dependent tachyon condensation we will concentrate on the case of the large
tachyon and the case when a non-BPS D-brane is close to NS5-branes. For spatial
dependent tachyon condensation we will argue that the problem reduces to the
study of the motion of an array of D(p-1)-branes and D(p-1)-antibranes in the
vicinity of k NS5-branes.Comment: 21 page
Intersection rules, dynamics and symmetries
We consider theories containing gravity, at most one dilaton and form field
strengths. We show that the existence of particular BPS solutions of
intersecting extremal closed branes select the theories, which upon dimensional
reduction to three dimensions possess a simple simply laced Lie group symmetry
G. Furthermore these theories can be fully reconstructed from the dynamics of
such branes and of their openings. Amongst such theories are the effective
actions of the bosonic sector of M-theory and of the bosonic string. The BPS
intersecting brane solutions form representations of a subgroup of the group of
Weyl reflections and outer automorphisms of the triple Kac-Moody extension G+++
of the G algebra, which cannot be embedded in the overextended Kac-Moody
subalgebra G++ characterising the cosmological Kasner solutions.Comment: Latex 30 pages, 3 figure
Evidence of two-electron tunneling interference in Nb/InAs junctions
The impact of junction transparency in driving phase-coherent charge transfer
across diffusive semiconductor-superconductor junctions is demonstrated. We
present conductivity data for a set of Nb-InAs junctions differing only in
interface transparency. Our experimental findings are analyzed within the
quasi-classical Green-function approach and unambiguously show the physical
processes giving rise to the observed excess zero-bias conductivity.Comment: 10 pages (RevTex), 4 figures (PostScript), accepted for pubblication
in Physical Review
Abelian and nonabelian vector field effective actions from string field theory
The leading terms in the tree-level effective action for the massless fields
of the bosonic open string are calculated by integrating out all massive fields
in Witten's cubic string field theory. In both the abelian and nonabelian
theories, field redefinitions make it possible to express the effective action
in terms of the conventional field strength. The resulting actions reproduce
the leading terms in the abelian and nonabelian Born-Infeld theories, and
include (covariant) derivative corrections.Comment: 49 pages, 1 eps figur
Geometrical Tachyon Kinks and NS5 Branes
We further investigate the 5 ring background using the tachyon map.
Mapping the radion fields to the rolling tachyon helps to explain the motion of
a probe -brane in this background. It turns out that the radion field
becomes tachyonic when the brane is confined to one dimensional motion inside
the ring. We find explicit solutions for the geometrical tachyon field that
describe stable kink solutions which are similar to those of the open string
tachyon. Interestingly in the case of the geometric tachyon, the dynamics is
controlled by a cosine potential. In addition, we couple a constant electric
field to the probe-brane, but find that the only stable kink solutions occur
when there is zero electric field or a critical field value. We also
investigate the behaviour of Non-BPS branes in this background, and find that
the end state of any probe brane is that of tachyonic matter 'trapped' around
the interior of the ring. We conclude by considering compactification of the
ring solution in one of the transverse directions.Comment: Latex, 24 pages, 1 eps fig; clarifying comments added to Section 2;
typos correcte
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