22,562 research outputs found
Majorana Fermions, Exact Mappings between Classical and Topological Orders
Motivated by the duality between site-centered spin and bond-centered spin in
one-dimensional system, which connects two different constructions of fermions
from the same set of Majorana fermions, we show that two-dimensional models
with topological orders can be constructed from certain well-known models with
classical orders characterized by symmetry-breaking. Topology-dependent ground
state degeneracy, vanishing two-point correlation functions, and unpaired
Majorana fermions on boundaries emerge naturally from such construction. The
approach opens a new way to construct and characterize topological orders.Comment: 5 pages, 4 figure
Why concave rather than convex inflaton potential?
The Planck data on cosmic microwave background indicates that the
Starobinsky-type model with concave inflation potential is favored over the
convex-type chaotic inflation. Is there any reason for that? Here we argue that
if our universe began with a Euclidean wormhole, then the Starobinsky-type
inflation is probabilistically favored. It is known that for a more generic
choice of parameters than that originally assumed by Hartle and Hawking, the
Hartle-Hawking wave function is dominated by Euclidean wormholes, which can be
interpreted as the creation of two classical universes from nothing. We show
that only one end of the wormhole can be classicalized for a convex potential,
while both ends can be classicalized for a concave potential. The latter is
therefore more probable.Comment: 9 pages, 3 figure
d-Wave Checkerboard Order in Cuprates
We show that the d-wave ordering in particle-hole channels, dubbed d-wave
checkerboard order, possesses important physics that can sufficiently explain
the scanning tunneling microscopy (STM) results in cuprates. A weak d-wave
checkerboard order can effectively suppress the coherence peak in the
single-particle spectrum while leaving the spectrum along the nodal direction
almost unaffected. Simultaneously, it generates a Fermi arc with little
dispersion around the nodal points at finite temperature that is consistent
with the results of angle-resolved photoemission spectroscopy (ARPES)
experiments in the pseudogap phase. We also show that there is a general
complementary connection between the d-wave checkerboard order and the
pair-density-wave order. Suppressing superconductivity locally or globally
through phase fluctuations should induce both orders in underdoped cuprates and
explain the nodal-antinodal dichotomy observed in ARPES and STM experiments
Performance Improvement of Variable Speed Rotors by Gurney Flaps
Gurney flaps are used for improving the performance of variable speed rotors. An analytical model able to predict helicopter rotor power is first presented, and the flight data of the UH-60A helicopter is used for validation. The predictions of the rotor power are in good agreement with the flight test data, justifying the use of this tool in analyzing helicopter performance. A fixed Gurney flap can enhance the performance of variable speed rotors and expand the corresponding flight envelop, especially near stall and high speed flight. A retractable Gurney flap at 1/rev yields more power savings than a fixed Gurney flap or a retractable one with higher a harmonic prescribed motion. At a speed of 200km/h, the retractable Gurney flap at 1/rev can obtain 3.22% more power reduction at a rotor speed of 85% nominal rotor speed, and this value is 8.37% at a speed of 220km/h. The height corresponding to the minimum power increases slowly in low to medium speed flight, and increases dramatically in high speed flight. With increasing take-off weight (i.e. rotor thrust), the retractable Gurney flap at 1/rev can obtain more rotor power savings
Phantom of the Hartle-Hawking instanton: connecting inflation with dark energy
If the Hartle-Hawking wave function is the correct boundary condition of our
universe, the history of our universe will be well approximated by an
instanton. Although this instanton should be classicalized at infinity, as long
as we are observing a process of each history, we may detect a
non-classicalized part of field combinations. When we apply it to a dark energy
model, this non-classicalized part of fields can be well embedded to a
quintessence and a phantom model, i.e., a quintom model. Because of the
property of complexified instantons, the phantomness will be naturally free
from a big rip singularity. This phantomness does not cause perturbative
instabilities, as it is an effect emergent from the entire wave function. Our
work may thus provide a theoretical basis for the quintom models, whose
equation of state (EoS) can cross the cosmological constant boundary (CCB)
phenomenologically.Comment: 20 pages, 7 figure
Hawking radiation as instantons
There have been various interpretations of Hawking radiation proposed based
on the perturbative approach, and all have confirmed Hawking's original
finding. One major conceptual challenge of Hawking evaporation is the
associated black hole information loss paradox, which remains unresolved. A key
factor to the issue is the end-stage of the black hole evaporation.
Unfortunately by then the evaporation process becomes non-perturbative. Aspired
to provide a tool for the eventual solution to this problem, here we introduce
a new interpretation of Hawking radiation as the tunneling of instantons. We
study instantons of a massless scalar field in Einstein gravity. We consider a
complex-valued instanton that connects an initial pure black hole state to a
black hole with a scalar field that represents the Hawking radiation at future
null infinity, where its action depends only on the areal entropy difference.
By comparing it with several independent approaches to Hawking radiation in the
perturbative limit, we conclude that Hawking radiation may indeed be described
by a family of instantons. Since the instanton approach can describe
non-perturbative processes, we hope that our new interpretation and holistic
method may shed lights on the information loss problem.Comment: 17 pages, 7 figure
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