43 research outputs found
Commutative deformations of general relativity: nonlocality, causality, and dark matter
Hopf algebra methods are applied to study Drinfeld twists of
(3+1)-diffeomorphisms and deformed general relativity on \emph{commutative}
manifolds. A classical nonlocality length scale is produced above which
microcausality emerges. Matter fields are utilized to generate self-consistent
Abelian Drinfeld twists in a background independent manner and their continuous
and discrete symmetries are examined. There is negligible experimental effect
on the standard model of particles. While baryonic twist producing matter would
begin to behave acausally for rest masses above TeV, other
possibilities are viable dark matter candidates or a right handed neutrino.
First order deformed Maxwell equations are derived and yield immeasurably small
cosmological dispersion and produce a propagation horizon only for photons at
or above Planck energies. This model incorporates dark matter without any
appeal to extra dimensions, supersymmetry, strings, grand unified theories,
mirror worlds, or modifications of Newtonian dynamics.Comment: 47 pages including references, 0 figures, 0 tables Various
typos/omissions correcte
Scattering Matrix Theory For Nonlinear Transport
We report a scattering matrix theory for dynamic and nonlinear transport in
coherent mesoscopic conductors. In general this theory allows predictions of
low frequency linear dynamic conductance, as well as weakly nonlinear DC
conductance. It satisfies the conditions of gauge invariance and electric
current conservation, and can be put into a form suitable for numerical
computation. Using this theory we examine the third order weakly nonlinear DC
conductance of a tunneling diode
Phase-sensitive quantum effects in Andreev conductance of the SNS system of metals with macroscopic phase breaking length
The dissipative component of electron transport through the doubly connected
SNS Andreev interferometer indium (S)-aluminium (N)-indium (S) has been
studied. Within helium temperature range, the conductance of the individual
sections of the interferometer exhibits phase-sensitive oscillations of
quantum-interference nature. In the non-domain (normal) state of indium
narrowing adjacent to NS interface, the nonresonance oscillations have been
observed, with the period inversely proportional to the area of the
interferometer orifice. In the domain intermediate state of the narrowing, the
magneto-temperature resistive oscillations appeared, with the period determined
by the coherence length in the magnetic field equal to the critical one. The
oscillating component of resonance form has been observed in the conductance of
the macroscopic N-aluminium part of the system. The phase of the oscillations
appears to be shifted by compared to that of nonresonance oscillations.
We offer an explanation in terms of the contribution into Josephson current
from the coherent quasiparticles with energies of order of the Thouless energy.
The behavior of dissipative transport with temperature has been studied in a
clean normal metal in the vicinity of a single point NS contact.Comment: 9 pages, 7 figures, to be published in Low Temp. Phys., v. 29, No.
12, 200
Phase Dependent Thermopower in Andreev Interferometers
We report measurements of the thermopower S of mesoscopic Andreev
interferometers, which are hybrid loops with one arm fabricated from a
superconductor (Al), and one arm from a normal metal (Au). S depends on the
phase of electrons in the interferometer, oscillating as a function of magnetic
flux with a period of one flux quantum (= h/2e). The magnitude of S increases
as the temperature T is lowered, reaching a maximum around T = 0.14 K, and
decreases at lower temperatures. The symmetry of S oscillations with respect to
magnetic flux depends on the topology of the sample.Comment: 4 pages, 4 figure
Diffusive conductors as Andreev interferometers
We present a novel mechanism of phase-dependent electric transport in
diffusive normal metal-superconductor structures. We provide a detailed
theoretical and numerical analysis of recent unexplained experiments
essentially explaining them.Comment: Self extracting file, 7 pages latex and 4 postscript figures. The
paper is also available at http://www.tn.tudelft.nl/tn/thspap.html In this
revision we resolved some printing problems concerning figures 2 and
Phase-dependent magnetoconductance fluctuations in a chaotic Josephson junction
Motivated by recent experiments by Den Hartog et al., we present a
random-matrix theory for the magnetoconductance fluctuations of a chaotic
quantum dot which is coupled by point contacts to two superconductors and one
or two normal metals. There are aperiodic conductance fluctuations as a
function of the magnetic field through the quantum dot and -periodic
fluctuations as a function of the phase difference of the
superconductors. If the coupling to the superconductors is weak compared to the
coupling to the normal metals, the -dependence of the conductance is
harmonic, as observed in the experiment. In the opposite regime, the
conductance becomes a random -periodic function of , in agreement
with the theory of Altshuler and Spivak. The theoretical method employs an
extension of the circular ensemble which can describe the magnetic field
dependence of the scattering matrix.Comment: 4 pages, RevTeX, 3 figure
Density of States in Superconductor - Normal Metal - Superconductor Junctions
We consider the chi_0 dependence of the density of states inside the normal
metal of a superconductor - normal metal - superconductor (SNS) junction.Here
chi_0 is the phase difference of two superconductors of the junction. It is
shown that in the absence of electron-electron interaction the energy
dependence of the density of states has a gap which decreases as chi_0
increases and closes at chi_0= pi. Both the analytical expressions for the
chi_0 dependence of the density of states and the results of numerical
simulations are presented.Comment: 7 pages with 4 included epsf figures, published version with small
change
Weakly Nonlinear AC Response: Theory and Application
We report a microscopic and general theoretical formalism for electrical
response which is appropriate for both DC and AC weakly nonlinear quantum
transport. The formalism emphasizes the electron-electron interaction and
maintains current conservation and gauge invariance. It makes a formal
connection between linear response and scattering matrix theory at the weakly
nonlinear level. We derive the dynamic conductance and predict the
nonlinear-nonequilibrium charge distribution. The definition of a nonlinear
capacitance leads to a remarkable scaling relation which can be measured to
give microscopic information about a conductor
Current partition: Nonequilibrium Green's function Approach
We present a solution to the problem of AC current partition in a multi-probe
mesoscopic conductor within the nonequilibrium Green's function formalism. This
allows the derivation of dynamic conductance which is appropriate for
nonequilibrium situations and which satisfies the current conservation and
gauge invariance requirements. This formalism presents a significant
generalization to previous theory: (i) there is no limit in the frequency, and
(ii) it allows detailed treatments of interactions in the mesoscopic region.
The formalism is applied to calculate dynamic conductance of tunneling
structures with and without assuming wideband limit.Comment: 4 pages, 3 figure