1,236 research outputs found
Propagating, evanescent, and localized states in carbon nanotube-graphene junctions
We study the electronic structure of the junctions between a single graphene
layer and carbon nanotubes, using a tight-binding model and the continuum
theory based on Dirac fermion fields. The latter provides a unified description
of different lattice structures with curvature, which is always localized at
six heptagonal carbon rings around each junction. When these are evenly spaced,
we find that it is possible to curve the planar lattice into armchair (6n,6n)
as well as zig-zag (6n,0) nanotubes. We show that the junctions fall into two
different classes, regarding the low-energy electronic behavior. One of them,
constituted by the junctions made of the armchair nanotubes and the zig-zag
(6n,0) geometries when n is a multiple of 3, is characterized by the presence
of two quasi-bound states at the Fermi level, which are absent for the rest of
the zig-zag nanotubes. These states, localized at the junction, are shown to
arise from the effective gauge flux induced by the heptagonal carbon rings,
which has a direct reflection in the local density of states around the
junction. Furthermore, we also analyze the band structure of the arrays of
junctions, finding out that they can also be classified into two different
groups according to the low-energy behavior. In this regard, the arrays made of
armchair and (6n,0) nanotubes with n equal to a multiple of 3 are characterized
by the presence of a series of flat bands, whose number grows with the length
of the nanotubes. We show that such flat bands have their origin in the
formation of states confined to the nanotubes in the array. This is explained
in the continuum theory from the possibility of forming standing waves in the
mentioned nanotube geometries, as a superposition of modes with opposite
momenta and the same quantum numbers under the C_6v symmetry of the junction.Comment: 13 pages, 8 figure
Open timelike curves violate Heisenberg's uncertainty principle
Toy models for quantum evolution in the presence of closed timelike curves
(CTCs) have gained attention in the recent literature due to the strange
effects they predict. The circuits that give rise to these effects appear quite
abstract and contrived, as they require non-trivial interactions between the
future and past which lead to infinitely recursive equations. We consider the
special case in which there is no interaction inside the CTC, referred to as an
open timelike curve (OTC), for which the only local effect is to increase the
time elapsed by a clock carried by the system. Remarkably, circuits with access
to OTCs are shown to violate Heisenberg's uncertainty principle, allowing
perfect state discrimination and perfect cloning of coherent states. The model
is extended to wave-packets and smoothly recovers standard quantum mechanics in
an appropriate physical limit. The analogy with general relativistic
time-dilation suggests that OTCs provide a novel alternative to existing
proposals for the behaviour of quantum systems under gravity
Calculating the local-type fNL for slow-roll inflation with a non-vacuum initial state
Single-field slow-roll inflation with a non-vacuum initial state has an
enhanced bispectrum in the local limit. We numerically calculate the local-type
fNL signal in the CMB that would be measured for such models (including the
full transfer function and 2D projection). The nature of the result depends on
several parameters, including the occupation number N_k, the phase angle
\theta_k between the Bogoliubov parameters, and the slow-roll parameter
\epsilon. In the most conservative case, where one takes \theta_k \approx
\eta_0 k (justified by physical reasons discussed within) and \epsilon\lesssim
0.01, we find that 0 < fNL < 1.52 (\epsilon/0.01), which is likely too small to
be detected in the CMB. However, if one is willing to allow a constant value
for the phase angle \theta_k and N_k=O(1), fNL can be much larger and/or
negative (depending on the choice of \theta_k), e.g. fNL \approx 28
(\epsilon/0.01) or -6.4 (\epsilon/0.01); depending on \epsilon, these scenarios
could be detected by Planck or a future satellite. While we show that these
results are not actually a violation of the single-field consistency relation,
they do produce a value for fNL that is considerably larger than that usually
predicted from single-field inflation.Comment: 8 pages, 1 figure. v2: Version accepted for publication in PRD. Added
greatly expanded discussion of the phase angle \theta_k; this allows the
possibility of enhanced fNL, as mentioned in abstract. More explicit
comparisons with earlier wor
Conformal invariance and apparent universality of semiclassical gravity
In a recent work, it has been pointed out that certain observables of the
massless scalar field theory in a static spherically symmetric background
exhibit a universal behavior at large distances. More precisely, it was shown
that, unlike what happens in the case the coupling to the curvature \xi is
generic, for the special cases \xi=0 and \xi = 1/6 the large distance behavior
of the expectation value turns out to be independent of the
internal structure of the gravitational source. Here, we address a higher
dimensional generalization of this result: We first compute the difference
between a black hole and a static spherically symmetric star for the
observables and in the far field limit. Thus, we show
that the conformally invariant massless scalar field theory in a static
spherically symmetric background exhibits such universality phenomenon in D\geq
4 dimensions. Also, using the one-loop effective action, we compute
for a weakly gravitating object. These results lead to the
explicit expression of the expectation value for a
Schwarzschild-Tangherlini black hole in the far field limit. As an application,
we obtain quantum corrections to the gravitational potential in D dimensions,
which for D=4 are shown to agree with the one-loop correction to the graviton
propagator previously found in the literature.Comment: 11 page
Quantum Connectivity of Space-Time and Gravitationally Induced Decorrelation of Entanglement
We discuss an alternative formulation of the problem of quantum optical
fields in a curved space-time using localized operators. We contrast the new
formulation with the standard approach and find observable differences for
entangled states. We propose an experiment in which an entangled pair of
optical pulses are propagated through non-uniform gravitational fields and find
that the new formulation predicts de-correlation of the optical entanglement
under experimentally realistic conditions
Loss of Spin Entanglement For Accelerated Electrons in Electric and Magnetic Fields
Using an open quantum system we calculate the time dependence of the
concurrence between two maximally entangled electron spins with one accelerated
uniformly in the presence of a constant magnetic field and the other at rest
and isolated from fields. We find at high Rindler temperature the proper time
for the entanglement to be extinguished is proportional to the inverse of the
acceleration cubed.Comment: 10 pages, 4 figures, appendix and other discussion added, fixed some
typographical errors and some references were correcte
The Electronic Spectrum of Fullerenes from the Dirac Equation
The electronic spectrum of sheets of graphite (plane honeycomb lattice)
folded into regular polihedra is studied. A continuum limit valid for
sufficiently large molecules and based on a tight binding approximation is
derived. It is found that a Dirac equation describes the flat graphite lattice.
Curving the lattice by insertion of odd numbered rings can be mimicked by
coupling effective gauge fields. In particular the and related
molecules are well described by the Dirac equation on the surface of a sphere
coupled to a color monopole sitting at its center.Comment: 29 pages, 7 figures. IASSNS-HEP-92/5
Algebraic Classification of Weyl Anomalies in Arbitrary Dimensions
Conformally invariant massless field systems involving only dimensionless
parameters are known to describe particle physics at very high energy. In the
presence of an external gravitational field, the conformal symmetry may
generalize to Weyl invariance. However, the latter symmetry no longer survives
after quantization: A Weyl anomaly appears. In this Letter, a purely algebraic
understanding of the universal structure of the Weyl anomalies is presented.
The results hold in arbitrary dimensions and independently of any
regularization scheme.Comment: 4 pages - accepted for publication in Physical Review Letter
Naturalness in Cosmological Initial Conditions
We propose a novel approach to the problem of constraining cosmological
initial conditions. Within the framework of effective field theory, we classify
initial conditions in terms of boundary terms added to the effective action
describing the cosmological evolution below Planckian energies. These boundary
terms can be thought of as spacelike branes which may support extra
instantaneous degrees of freedom and extra operators. Interactions and
renormalization of these boundary terms allow us to apply to the boundary terms
the field-theoretical requirement of naturalness, i.e. stability under
radiative corrections. We apply this requirement to slow-roll inflation with
non-adiabatic initial conditions, and to cyclic cosmology. This allows us to
define in a precise sense when some of these models are fine-tuned. We also
describe how to parametrize in a model-independent way non-Gaussian initial
conditions; we show that in some cases they are both potentially observable and
pass our naturalness requirement.Comment: 35 pages, 8 figure
Not all adiabatic vacua are physical states
Adiabatic vacua are known to be Hadamard states. We show, however that the
energy-momentum tensor of a linear Klein-Gordon field on Robertson-Walker
spaces developes a generic singularity on the initial hypersurface if the
adiabatic vacuum is of order less than four. Therefore, adiabatic vacua are
physically reasonable only if their order is at least four.
A certain non-local large momentum expansion of the mode functions has
recently been suggested to yield the subtraction terms needed to remove the
ultraviolet divergences in the energy-momentum tensor. We find that this scheme
fails to reproduce the trace anomaly and therefore is not equivalent to
adiabatic regularisation.Comment: 13 pages, LaTex2
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