2,298 research outputs found
Effects of interaction on an adiabatic quantum electron pump
We study the effects of inter-electron interactions on the charge pumped
through an adiabatic quantum electron pump. The pumping is through a system of
barriers, whose heights are deformed adiabatically. (Weak) interaction effects
are introduced through a renormalisation group flow of the scattering matrices
and the pumped charge is shown to {\it always} approach a quantised value at
low temperatures or long length scales. The maximum value of the pumped charge
is set by the number of barriers and is given by . The
correlation between the transmission and the charge pumped is studied by seeing
how much of the transmission is enclosed by the pumping contour. The (integer)
value of the pumped charge at low temperatures is determined by the number of
transmission maxima enclosed by the pumping contour. The dissipation at finite
temperatures leading to the non-quantised values of the pumped charge scales as
a power law with the temperature (), or with
the system size (), where is a
measure of the interactions and vanishes at . For a double
barrier system, our result agrees with the quantisation of pumped charge seen
in Luttinger liquids.Comment: 9 pages, 9 figures, better quality figures available on request from
author
Andreev Probe of Persistent Current States in Superconducting Quantum Circuits
Using the extraordinary sensitivity of Andreev interferometers to the
superconducting phase difference associated with currents, we measure the
persistent current quantum states in superconducting loops interrupted by
Josephson junctions. Straightforward electrical resistance measurements of the
interferometers give continuous read-out of the states, allowing us to
construct the energy spectrum of the quantum circuit. The probe is estimated to
be more precise and faster than previous methods, and can measure the local
phase difference in a wide range of superconducting circuits.Comment: Changes made in light of referees comments; to appear in PR
Hamiltonian Frenet-Serret dynamics
The Hamiltonian formulation of the dynamics of a relativistic particle
described by a higher-derivative action that depends both on the first and the
second Frenet-Serret curvatures is considered from a geometrical perspective.
We demonstrate how reparametrization covariant dynamical variables and their
projections onto the Frenet-Serret frame can be exploited to provide not only a
significant simplification of but also novel insights into the canonical
analysis. The constraint algebra and the Hamiltonian equations of motion are
written down and a geometrical interpretation is provided for the canonical
variables.Comment: Latex file, 14 pages, no figures. Revised version to appear in Class.
Quant. Gra
Uniqueness of de Sitter space
All inextendible null geodesics in four dimensional de Sitter space dS^4 are
complete and globally achronal. This achronality is related to the fact that
all observer horizons in dS^4 are eternal, i.e. extend from future infinity
scri^+ all the way back to past infinity scri^-. We show that the property of
having a null line (inextendible achronal null geodesic) that extends from
scri^- to scri^+ characterizes dS^4 among all globally hyperbolic and
asymptotically de Sitter spacetimes satisfying the vacuum Einstein equations
with positive cosmological constant. This result is then further extended to
allow for a class of matter models that includes perfect fluids.Comment: 22 pages, 2 figure
Contact lines for fluid surface adhesion
When a fluid surface adheres to a substrate, the location of the contact line
adjusts in order to minimize the overall energy. This adhesion balance implies
boundary conditions which depend on the characteristic surface deformation
energies. We develop a general geometrical framework within which these
conditions can be systematically derived. We treat both adhesion to a rigid
substrate as well as adhesion between two fluid surfaces, and illustrate our
general results for several important Hamiltonians involving both curvature and
curvature gradients. Some of these have previously been studied using very
different techniques, others are to our knowledge new. What becomes clear in
our approach is that, except for capillary phenomena, these boundary conditions
are not the manifestation of a local force balance, even if the concept of
surface stress is properly generalized. Hamiltonians containing higher order
surface derivatives are not just sensitive to boundary translations but also
notice changes in slope or even curvature. Both the necessity and the
functional form of the corresponding additional contributions follow readily
from our treatment.Comment: 8 pages, 2 figures, LaTeX, RevTeX styl
Covariant coarse-graining of inhomogeneous dust flow in General Relativity
A new definition of coarse-grained quantities describing the dust flow in
General Relativity is proposed. It assigns the coarse--grained expansion, shear
and vorticity to finite-size comoving domains of fluid in a covariant,
coordinate-independent manner. The coarse--grained quantities are all
quasi-local functionals, depending only on the geometry of the boundary of the
considered domain. They can be thought of as relativistic generalizations of
simple volume averages of local quantities in a flat space. The procedure is
based on the isometric embedding theorem for S^2 surfaces and thus requires the
boundary of the domain in question to have spherical topology and positive
scalar curvature. We prove that in the limit of infinitesimally small volume
the proposed quantities reproduce the local expansion, shear and vorticity. In
case of irrotational flow we derive the time evolution for the coarse-grained
quantities and show that its structure is very similar to the evolution
equation for their local counterparts. Additional terms appearing in it may
serve as a measure of the backreacton of small-scale inhomogeneities of the
flow on the large-scale motion of the fluid inside the domain and therefore the
result may be interesting in the context of the cosmological backreaction
problem. We also consider the application of the proposed coarse-graining
procedure to a number of known exact solutions of Einstein equations with dust
and show that it yields reasonable results.Comment: 17 pages, 5 figures. Version accepted in Classical and Quantum
Gravity
Spin Structures on Kleinian Manifolds
We derive the topological obstruction to spin-Klein cobordism. This result
has implications for signature change in general relativity, and for the
superstring.Comment: 8 page
Role of a parallel magnetic field in two dimensional disordered clusters containing a few correlated electrons
An ensemble of 2d disordered clusters with a few electrons is studied as a
function of the Coulomb energy to kinetic energy ratio r_s. Between the Fermi
system (small r_s) and the Wigner molecule (large r_s), an interaction induced
delocalization of the ground state takes place which is suppressed when the
spins are aligned by a parallel magnetic field. Our results confirm the
existence of an intermediate regime where the Wigner antiferromagnetism
defavors the Stoner ferromagnetism and where the enhancement of the Lande g
factor observed in dilute electron systems is reproduced.Comment: 4 pages, 3 figure
Hamiltonians for Reduced Gravity
A generalised canonical formulation of gravity is devised for foliations of
spacetime with codimension . The new formalism retains n-dimensional
covariance and is especially suited to 2+2 decompositions of spacetime. It is
also possible to use the generalised formalism to obtain boundary contributions
to the 3+1 Hamiltonian.Comment: 18 pages, revtex, 3 postscript figures include
Linguistic incompetence: giving an account of researching multilingually
This paper considers the place of linguistic competence and incompetence in the context of researching multilingually. It offers a critique of the concept of competence and explores the performative dimensions of multilingual research and its narration, through the philosophy of Judith Butler, and in particular her study Giving an account of oneself. It explores aspects of risk, justice, narrative limit and a morality of multilingualism in emergent multilingual research frameworks. These theoretical dimensions are explored through consideration of âlinguistically incompetentâ ethnographic work with refugees and asylum seekers, in contexts of hospitality and in life long learning research in the Gaza Strip, and of early attempts to learn new languages. The paper offers a prospect of a relational approach to researching multilingually and affirms the vulnerability at the heart of linguistic hospitality
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