2,221 research outputs found
Einstein's Equations in the Presence of Signature Change
We discuss Einstein's field equations in the presence of signature change
using variational methods, obtaining a generalization of the Lanczos equation
relating the distributional term in the stress tensor to the discontinuity of
the extrinsic curvature. In particular, there is no distributional term in the
stress tensor, and hence no surface layer, precisely when the extrinsic
curvature is continuous, in agreement with the standard result for constant
signature.Comment: REVTeX, 8 pages; to appear in JM
Time-symmetric boundary conditions and quantum foundations
Despite the widely-held premise that initial boundary conditions (BCs)
corresponding to measurements/interactions can fully specify a physical
subsystem, a literal reading of Hamilton's principle would imply that both
initial and final BCs are required (or more generally, a BC on a closed
hypersurface in spacetime). Such a time-symmetric perspective of BCs, as
applied to classical fields, leads to interesting parallels with quantum
theory. This paper will map out some of the consequences of this
counter-intuitive premise, as applied to covariant classical fields. The most
notable result is the contextuality of fields constrained in this manner,
naturally bypassing the usual arguments against so-called "realistic"
interpretations of quantum phenomena.Comment: 16 pages. With 0906.5409, companion paper to pirsa.org/09060031. v2:
Minor edit, updates to reference
Doped high-Tc cuprate superconductors elucidated in the light of zeros and poles of electronic Green's function
We study electronic structure of hole- and electron-doped Mott insulators in
the two-dimensional Hubbard model to reach a unified picture for the normal
state of cuprate high-Tc superconductors. By using a cluster extension of the
dynamical mean-field theory, we demonstrate that structure of coexisting zeros
and poles of the single-particle Green's function holds the key to understand
Mott physics in the underdoped region. We show evidence for the emergence of
non-Fermi-liquid phase caused by the topological quantum phase transition of
Fermi surface by analyzing low-energy charge dynamics. The spectra calculated
in a wide range of energy and momentum reproduce various anomalous properties
observed in experiments for the high-Tc cuprates. Our results reveal that the
pseudogap in hole-doped cuprates has a d-wave-like structure only below the
Fermi level, while it retains non-d-wave structure with a fully opened gap
above the Fermi energy even in the nodal direction due to a zero surface
extending over the entire Brillouin zone. In addition to the non-d-wave
pseudogap, the present comprehensive identifications of the spectral asymmetry
as to the Fermi energy, the Fermi arc, and the back-bending behavior of the
dispersion, waterfall, and low-energy kink, in agreement with the experimental
anomalies of the cuprates, do not support that these originate from (the
precursors of) symmetry breakings such as the preformed pairing and the
d-density wave fluctuations, but support that they are direct consequences of
the proximity to the Mott insulator. Several possible experiments are further
proposed to prove or disprove our zero mechanism.Comment: 17 pages, 15 figure
The Weyl-Lanczos Equations and the Lanczos Wave Equation in 4 Dimensions as Systems in Involution
Using the work by Bampi and Caviglia, we write the Weyl-Lanczos equations as
an exterior differential system. Using Janet-Riquier theory, we compute the
Cartan characters for all spacetimes with a diagonal metric and for the plane
wave spacetime since all spacetimes have a plane wave limit. We write the
Lanczos wave equation as an exterior differential system and, with assistance
from Janet-Riquier theory, we find that it forms a system in involution. This
result can be derived from the scalar wave equation itself. We compute its
Cartan characters and compare them with those of the Weyl-Lanczos equations.Comment: 18 pages, latex, no figures, references correcte
Cylindrical thin-shell wormholes and energy conditions
We prove the impossibility of cylindrical thin-shell wormholes supported by
matter satisfying the energy conditions everywhere, under reasonable
assumptions about the asymptotic behaviour of the - in general different -
metrics at each side of the throat. In particular, we reproduce for singular
sources previous results corresponding to flat and conical asymptotics, and
extend them to a more general asymptotic behaviour. Besides, we establish
necessary conditions for the possibility of non exotic cylindrical thin-shell
wormholes.Comment: 9 pages; slightly improved version of the article accepted in Int. J.
Mod. Phys.
Brane-worlds in T-dual Bulks
We consider brane-world models with a Schwarzschild-AdS black hole bulk. In
the particular case of a flat black hole horizon geometry, we study the
behaviour of the brane cosmological equations when T-duality transformations
act on the bulk. We find that the scale factor is inverted and that either the
Friedmann equation or the energy conservation equation are unchanged. However,
these become both invariant if we include a tension in the brane action. In
this case, the T-duality in the bulk is completely equivalent to the scale
factor duality on the brane.Comment: 11 pages, Latex, (v2) minor corrections, references adde
Double Time Window Targeting Technique: Real time DMRG dynamics in the PPP model
We present a generalized adaptive time-dependent density matrix
renormalization group (DMRG) scheme, called the {\it double time window
targeting} (DTWT) technique, which gives accurate results with nominal
computational resources, within reasonable computational time. This procedure
originates from the amalgamation of the features of pace keeping DMRG
algorithm, first proposed by Luo {\it et. al}, [Phys.Rev. Lett. {\bf 91},
049701 (2003)], and the time-step targeting (TST) algorithm by Feiguin and
White [Phys. Rev. B {\bf 72}, 020404 (2005)]. Using the DTWT technique, we
study the phenomena of spin-charge separation in conjugated polymers (materials
for molecular electronics and spintronics), which have long-range
electron-electron interactions and belong to the class of strongly correlated
low-dimensional many-body systems. The issue of real time dynamics within the
Pariser-Parr-Pople (PPP) model which includes long-range electron correlations
has not been addressed in the literature so far. The present study on PPP
chains has revealed that, (i) long-range electron correlations enable both the
charge and spin degree of freedom of the electron, to propagate faster in the
PPP model compared to Hubbard model, (ii) for standard parameters of the PPP
model as applied to conjugated polymers, the charge velocity is almost twice
that of the spin velocity and, (iii) the simplistic interpretation of
long-range correlations by merely renormalizing the {\it U} value of the
Hubbard model fails to explain the dynamics of doped holes/electrons in the PPP
model.Comment: Final (published) version; 39 pages, 13 figures, 1 table; 2 new
references adde
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