1,271 research outputs found
Strings, T-duality breaking, and nonlocality without the shortest distance
T-duality of string theory suggests nonlocality manifested as the shortest
possible distance. As an alternative, we suggest a nonlocal formulation of
string theory that breaks T-duality at the fundamental level and does not
require the shortest possible distance. Instead, the string has an objective
shape in spacetime at all length scales, but different parts of the string
interact in a nonlocal Bohmian manner.Comment: 7 pages, revised, to appear in Eur. Phys. J.
Boson-fermion unification, superstrings, and Bohmian mechanics
Bosonic and fermionic particle currents can be introduced in a more unified
way, with the cost of introducing a preferred spacetime foliation. Such a
unified treatment of bosons and fermions naturally emerges from an analogous
superstring current, showing that the preferred spacetime foliation appears
only at the level of effective field theory, not at the fundamental superstring
level. The existence of the preferred spacetime foliation allows an objective
definition of particles associated with quantum field theory in curved
spacetime. Such an objective definition of particles makes the Bohmian
interpretation of particle quantum mechanics more appealing. The superstring
current allows a consistent Bohmian interpretation of superstrings themselves,
including a Bohmian description of string creation and destruction in terms of
string splitting. The Bohmian equations of motion and the corresponding
probabilistic predictions are fully relativistic covariant and do not depend on
the preferred foliation.Comment: 30 pages, 1 figure, revised, to appear in Found. Phy
Quantum Transparency of Anderson Insulator Junctions: Statistics of Transmission Eigenvalues, Shot Noise, and Proximity Conductance
We investigate quantum transport through strongly disordered barriers, made
of a material with exceptionally high resistivity that behaves as an Anderson
insulator or a ``bad metal'' in the bulk, by analyzing the distribution of
Landauer transmission eigenvalues for a junction where such barrier is attached
to two clean metallic leads. We find that scaling of the transmission
eigenvalue distribution with the junction thickness (starting from the single
interface limit) always predicts a non-zero probability to find high
transmission channels even in relatively thick barriers. Using this
distribution, we compute the zero frequency shot noise power (as well as its
sample-to-sample fluctuations) and demonstrate how it provides a single number
characterization of non-trivial transmission properties of different types of
disordered barriers. The appearance of open conducting channels, whose
transmission eigenvalue is close to one, and corresponding violent mesoscopic
fluctuations of transport quantities explain at least some of the peculiar
zero-bias anomalies in the Anderson-insulator/superconductor junctions observed
in recent experiments [Phys. Rev. B {\bf 61}, 13037 (2000)]. Our findings are
also relevant for the understanding of the role of defects that can undermine
quality of thin tunnel barriers made of conventional band-insulators.Comment: 9 pages, 8 color EPS figures; one additional figure on mesoscopic
fluctuations of Fano facto
Comment on "Classical interventions in quantum systems II. Relativistic invariance"
In a recent paper [Phys. Rev. A 61, 022117 (2000)], quant-ph/9906034, A.
Peres argued that quantum mechanics is consistent with special relativity by
proposing that the operators that describe time evolution do not need to
transform covariantly, although the measurable quantities need to transform
covariantly. We discuss the weaknesses of this proposal.Comment: 4 pages, to appear in Phys. Rev.
Optimizing the speed of a Josephson junction
We review the application of dynamical mean-field theory to Josephson
junctions and study how to maximize the characteristic voltage IcRn which
determines the width of a rapid single flux quantum pulse, and thereby the
operating speed in digital electronics. We study a wide class of junctions
ranging from SNS, SCmS (where Cm stands for correlated metal), SINIS (where the
insulating layer is formed from a screened dipole layer), and SNSNS structures.
Our review is focused on a survey of the physical results; the formalism has
been developed elsewhere.Comment: (36 pages, 15 figures, to appear in Int. J. Mod. Phys. B
Quantum mechanics: Myths and facts
A common understanding of quantum mechanics (QM) among students and practical
users is often plagued by a number of "myths", that is, widely accepted claims
on which there is not really a general consensus among experts in foundations
of QM. These myths include wave-particle duality, time-energy uncertainty
relation, fundamental randomness, the absence of measurement-independent
reality, locality of QM, nonlocality of QM, the existence of well-defined
relativistic QM, the claims that quantum field theory (QFT) solves the problems
of relativistic QM or that QFT is a theory of particles, as well as myths on
black-hole entropy. The fact is that the existence of various theoretical and
interpretational ambiguities underlying these myths does not yet allow us to
accept them as proven facts. I review the main arguments and counterarguments
lying behind these myths and conclude that QM is still a
not-yet-completely-understood theory open to further fundamental research.Comment: 51 pages, pedagogic review, revised, new references, to appear in
Found. Phy
Shot Noise Probing of Magnetic Ordering in Zigzag Graphene Nanoribbons
The nonequilibrium time-dependent fluctuations of charge current have
recently emerged as a sensitive experimental tool to probe ballistic transport
through evanescent wave functions introduced into clean wide and short graphene
strips by the attached metallic electrodes. We demonstrate that such
"pseudo-diffusive" shot noise can be substantially modified in zigzag graphene
nanoribbon (ZGNR) due to the topology of its edges responsible for localized
states that facilitate ferromagnetic ordering along the edge when Coulomb
interaction is taken into account. Thus, the shot noise enhancement of
unpolarized, and even more sensitively of spin-polarized, charge currents
injected into ZGNR will act as an all-electrical and edge-sensitive probe of
such low-dimensional magnetism.Comment: 5 pages, 3 color figures; references update
Typical medium theory of Anderson localization: A local order parameter approach to strong disorder effects
We present a self-consistent theory of Anderson localization that yields a
simple algorithm to obtain \emph{typical local density of states} as an order
parameter, thereby reproducing the essential features of a phase-diagram of
localization-delocalization quantum phase transition in the standard lattice
models of disordered electron problem. Due to the local character of our
theory, it can easily be combined with dynamical mean-field approaches to
strongly correlated electrons, thus opening an attractive avenue for a genuine
{\em non-perturbative} treatment of the interplay of strong interactions and
strong disorder.Comment: 7 pages, 4 EPS figures, revised version to appear in Europhysics
Letter
Probability in relativistic Bohmian mechanics of particles and strings
Even though the Bohmian trajectories given by integral curves of the
conserved Klein-Gordon current may involve motions backwards in time, the
natural relativistic probability density of particle positions is well-defined.
The Bohmian theory predicts subtle deviations from the statistical predictions
of more conventional formulations of quantum theory, but it seems that no
present experiment rules this theory out. The generalization to the case of
many particles or strings is straightforward, provided that a preferred
foliation of spacetime is given.Comment: 12 pages, version accepted for publication in Found. Phy
Relativistic contraction and related effects in noninertial frames
Although there is no relative motion among different points on a rotating
disc, each point belongs to a different noninertial frame. This fact, not
recognized in previous approaches to the Ehrenfest paradox and related
problems, is exploited to give a correct treatment of a rotating ring and a
rotating disc. Tensile stresses are recovered, but, contrary to the prediction
of the standard approach, it is found that an observer on the rim of the disc
will see equal lengths of other differently moving objects as an inertial
observer whose instantaneous position and velocity are equal to that of the
observer on the rim. The rate of clocks at various positions, as seen by
various observers, is also discussed. Some results are generalized for
observers arbitrarily moving in a flat or a curved spacetime. The generally
accepted formula for the space line element in a non-time-orthogonal frame is
found inappropriate in some cases. Use of Fermi coordinates leads to the result
that for any observer the velocity of light is isotropic and is equal to ,
providing that it is measured by propagating a light beam in a small
neighborhood of the observer.Comment: 15 pages, significantly revised version, title changed, to appear in
Phys. Rev.
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