333 research outputs found
Magnetothermal Transport in Spin-Ladder Systems
We study a theoretical model for the magnetothermal conductivity of a
spin-1/2 ladder with low exchange coupling () subject to a strong
magnetic field . Our theory for the thermal transport accounts for the
contribution of spinons coupled to lattice phonon modes in the one-dimensional
lattice. We employ a mapping of the ladder Hamiltonian onto an XXZ spin-chain
in a weaker effective field B_{eff}=B-B_{0}B_{0}=(B_{c1}+B_{c2})/2B{\rm
Br_4(C_5H_{12}N)_2}$ (BPCB).Comment: 14 pages, 4 figure
Nernst Effect as a Signature of Quantum Fluctuations in Quasi-1D Superconductors
We study a model for the transverse thermoelectric response due to quantum
superconducting fluctuations in a two-leg Josephson ladder, subject to a
perpendicular magnetic field B and a transverse temperature gradient. The
off-diagonal Peltier coefficient (\alpha_{xy}) and the Nernst effect are
evaluated as functions of B and the temperature T. The Nernst effect is found
to exhibit a prominent peak close to the superconductor-insulator transition
(SIT), which becomes progressively enhanced at low T. In addition, we derive a
relation to diamagnetic response: \alpha_{xy}= -M/T_0, where M is the
equilibrium magnetization and T_0 a plasma energy in the superconducting legs.Comment: An extended (and hopefully more comprehensible) version of an earlier
postin
Reducing Sexual Risk among Racial/ethnic-minority Ninth Grade Students: Using Intervention Mapping to Modify an Evidenced-based Curriculum
Background: Racial/ethnic-minority 9th graders are at increased risk for teen pregnancy, HIV, and STIs compared to their White peers. Yet, few effective sexual health education programs exist for this population. Purpose: To apply IM Adapt—a systematic theory- and evidence-based approach to program adaptation—to modify an effective middle school sexual health education curriculum, It’s Your Game…Keep It Real! (IYG), for racial/ethnic-minority 9th graders. Methods: Following the six steps of IM Adapt, we conducted a needs assessment to describe the health problems and risk behaviors of the new population; reviewed existing evidence-based programs; assessed the fit of IYG for the new population regarding behavioral outcomes, determinants, change methods, delivery, and implementation; modified materials and activities; planned for implementation and evaluation. Results: Needs assessment findings indicated that IYG targeted relevant health and risk behaviors for racial/ethnic-minority 9th graders but required additional focus on contraceptive use, dating violence prevention, active consent, and access to healthcare services. Behavioral outcomes and matrices of change objectives for IYG were modified accordingly. Theoretical methods and practical applications were identified to address these behavioral outcomes, and new activities developed. Youth provided input on activity modifications. School personnel guided modifications to IYG’s scope and sequence, and delivery. The adapted program, Your Game, Your Life, comprised fifteen 30-minute lessons targeting determinants of sexual behavior and healthy dating relationships. Pilot-test data from 9th graders in two urban high schools indicate promising results. Conclusion: IM Adapt provides a systematic theory- and evidence-based approach for adapting existing evidence-based sexual health education curricula for a new population whilst retaining essential elements that made the original program effective. Youth and school personnel input ensured that the adapted program was age-appropriate, culturally sensitive, and responsive to the needs of the new population. IM Adapt contributes to the limited literature on systematic approaches to program adaptation
A local families index formula for d-bar operators on punctured Riemann surfaces
Using heat kernel methods developed by Vaillant, a local index formula is
obtained for families of d-bar operators on the Teichmuller universal curve of
Riemann surfaces of genus g with n punctures. The formula also holds on the
moduli space M{g,n} in the sense of orbifolds where it can be written in terms
of Mumford-Morita-Miller classes. The degree two part of the formula gives the
curvature of the corresponding determinant line bundle equipped with the
Quillen connection, a result originally obtained by Takhtajan and Zograf.Comment: 47 page
Bounded-Angle Spanning Tree: Modeling Networks with Angular Constraints
We introduce a new structure for a set of points in the plane and an angle
, which is similar in flavor to a bounded-degree MST. We name this
structure -MST. Let be a set of points in the plane and let be an angle. An -ST of is a spanning tree of the
complete Euclidean graph induced by , with the additional property that for
each point , the smallest angle around containing all the edges
adjacent to is at most . An -MST of is then an
-ST of of minimum weight. For , an -ST does
not always exist, and, for , it always exists. In this paper,
we study the problem of computing an -MST for several common values of
.
Motivated by wireless networks, we formulate the problem in terms of
directional antennas. With each point , we associate a wedge of
angle and apex . The goal is to assign an orientation and a radius
to each wedge , such that the resulting graph is connected and its
MST is an -MST. (We draw an edge between and if , , and .) Unsurprisingly, the problem of computing an
-MST is NP-hard, at least for and . We
present constant-factor approximation algorithms for .
One of our major results is a surprising theorem for ,
which, besides being interesting from a geometric point of view, has important
applications. For example, the theorem guarantees that given any set of
points in the plane and any partitioning of the points into triplets,
one can orient the wedges of each triplet {\em independently}, such that the
graph induced by is connected. We apply the theorem to the {\em antenna
conversion} problem
The Role of Vortices in the Mutual Coupling of Superconducting and Normal-Metal Films
I propose a possible explanation to a recently observed ``cross-talk'' effect
in metal-insulator-metal trilayers, indicating a sharp peak near a
superconducting transition in one of the metal films. Coulomb interactions are
excluded as a dominant coupling mechanism, and an alternative is suggested,
based on the local fluctuating electric field induced by mobile vortices in the
superconducting layer. This scenario is compatible with the magnitude of the
peak signal and its shape; most importantly, it addresses the {\it
non-reciprocity} of the effect in exchanging the roles of the films.Comment: 13 pages, RevTe
Quantum Fields in Hyperbolic Space-Times with Finite Spatial Volume
The one-loop effective action for a massive self-interacting scalar field is
investigated in -dimensional ultrastatic space-time ,
being a non-compact hyperbolic manifold with finite volume. Making
use of the Selberg trace formula, the -function related to the small
disturbance operator is constructed. For an arbitrary gravitational coupling,
it is found that has a simple pole at . The one-loop effective
action is analysed by means of proper-time regularisations and the one-loop
divergences are explicitly found. It is pointed out that, in this special case,
also -function regularisation requires a divergent counterterm, which
however is not necessary in the free massless conformal invariant coupling
case. Finite temperature effects are studied and the high-temperature expansion
is presented. A possible application to the problem of the divergences of the
entanglement entropy for a free massless scalar field in a Rindler-like
space-time is briefly discussed.Comment: 13 pages, LaTex. The contribution of hyperbolic elements has been
added. Other minor corrections and reference
Coulomb Drag at the Onset of Anderson Insulators
It is shown that the Coulomb drag between two identical layers in the
Anderson insulting state indicates a striking difference between the Mott and
Efros-Shklovskii (ES) insulators. In the former, the trans-resistance
is monotonically increasing with the localization length ; in the latter,
the presence of a Coulomb gap leads to an opposite result: is enhanced
with a decreasing , with the same exponential factor as the single layer
resistivity. This distinction reflects the relatively pronounced role of
excited density fluctuations in the ES state, implied by the enhancement in the
rate of hopping processes at low frequencies. The magnitude of drag is
estimated for typical experimental parameters in the different cases. It is
concluded that a measurement of drag can be used to distinguish between
interacting and non-interacting insulating state.Comment: 15 pages, revte
Alternating Superconductor--Insulator Transport Characteristics in a Quantum Vortex Chain
Experimental studies of magnetoresistance in thin superconducting strips
subject to a perpendicular magnetic field B exhibit a multitude of transitions,
from superconductor to insulator and vice versa alternately. Motivated by this
observation, we study a theoretical model for the transport properties of a
ladder--like superconducting device close to a superconductor--insulator
transition. In this regime, strong quantum fluctuations dominate the dynamics
of the vortex chain forming along the device. Utilizing a mapping of the vortex
system at low energies to one-dimensional (1D) Fermions at a chemical potential
dictated by B, we find that a quantum phase transition of the Ising type occurs
at critical values of the vortex filling, from a superconducting phase near
integer filling to an insulator near 1/2-filling. The current--voltage (I-V)
characteristics of the weakly disordered device in the presence of a d.c.
current bias I is evaluated, and investigated as a function of B, I, the
temperature T and the disorder strength. In the Ohmic regime (I/e << T), the
resulting magnetoresistance R(B) exhibits oscillations similar to the
experimental observation. More generally, we find that the I-V characteristics
of the system manifests a dramatically distinct behavior in the superconducting
and insulating regimes.Comment: 10 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:1010.066
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