4,872 research outputs found
Band Symmetries and Singularities in Twisted Multilayer Graphene
The electronic spectra of rotationally faulted graphene bilayers are
calculated using a continuum formulation for small fault angles that identifies
two distinct electronic states of the coupled system. The low energy spectra of
one state features a Fermi velocity reduction which ultimately leads to
pairwise annihilation and regeneration of its low energy Dirac nodes. The
physics in the complementary state is controlled by pseudospin selection rules
that prevent a Fermi velocity renormalization and produce second generation
symmetry-protected Dirac singularities in the spectrum. These results are
compared with previous theoretical analyses and with experimental data.Comment: 5 pages, 3 figure
On the Chinese Exchange Rate Regime: an Attempt to Flexibility during 2015
This study will demonstrate, through an econometric and asset allocation approach, if and
how the Chinese exchange rate regime was changing during 2015. Particularly, China to improve its
exchange rate formation system implemented, during July and August 2015, three depreciation as a
step toward a market-oriented exchange rate. This situation, along with the new right of the RMB to
be an international currency in SDR should generate a loss of weight about the USD in the Chinese
basket peg. For this reason, moving from Frankel-Wei’s basic econometric model - but with some
appropriate changes - our objective is to verify if the Chinese monetary policy about the exchange
rate has affected the inner balance of the Chinese basket-peg leading it towards a flexible exchange
rate regime
A note on the Hamiltonian as a polymerisation parameter
In effective models of loop quantum gravity, the onset of quantum effects is
controlled by a so-called polymerisation scale. It is sometimes necessary to
make this scale phase space dependent in order to obtain sensible physics. A
particularly interesting choice recently used to study quantum corrected black
hole spacetimes takes the generator of time translations itself to set the
scale. We review this idea, point out errors in recent treatments, and show how
to fix them in principle.Comment: 7 pages, 2 figures; v2: journal version, minor clarification
Effective Quantum Extended Spacetime of Polymer Schwarzschild Black Hole
The physical interpretation and eventual fate of gravitational singularities
in a theory surpassing classical general relativity are puzzling questions that
have generated a great deal of interest among various quantum gravity
approaches. In the context of loop quantum gravity (LQG), one of the major
candidates for a non-perturbative background-independent quantisation of
general relativity, considerable effort has been devoted to construct effective
models in which these questions can be studied. In these models, classical
singularities are replaced by a "bounce" induced by quantum geometry
corrections. Undesirable features may arise however depending on the details of
the model. In this paper, we focus on Schwarzschild black holes and propose a
new effective quantum theory based on polymerisation of new canonical phase
space variables inspired by those successful in loop quantum cosmology. The
quantum corrected spacetime resulting from the solutions of the effective
dynamics is characterised by infinitely many pairs of trapped and anti-trapped
regions connected via a space-like transition surface replacing the central
singularity. Quantum effects become relevant at a unique mass independent
curvature scale, while they become negligible in the low curvature region near
the horizon. The effective quantum metric describes also the exterior regions
and asymptotically classical Schwarzschild geometry is recovered. We however
find that physically acceptable solutions require us to select a certain subset
of initial conditions, corresponding to a specific mass (de-)amplification
after the bounce. We also sketch the corresponding quantum theory and
explicitly compute the kernel of the Hamiltonian constraint operator.Comment: 50 pages, 10 figures; v2: journal version, minor comment and
references added; v3: minor corrections in section 5.3 to match journal
versio
Fisher Metric, Geometric Entanglement and Spin Networks
Starting from recent results on the geometric formulation of quantum
mechanics, we propose a new information geometric characterization of
entanglement for spin network states in the context of quantum gravity. For the
simple case of a single-link fixed graph (Wilson line), we detail the
construction of a Riemannian Fisher metric tensor and a symplectic structure on
the graph Hilbert space, showing how these encode the whole information about
separability and entanglement. In particular, the Fisher metric defines an
entanglement monotone which provides a notion of distance among states in the
Hilbert space. In the maximally entangled gauge-invariant case, the
entanglement monotone is proportional to a power of the area of the surface
dual to the link thus supporting a connection between entanglement and the
(simplicial) geometric properties of spin network states. We further extend
such analysis to the study of non-local correlations between two non-adjacent
regions of a generic spin network graph characterized by the bipartite
unfolding of an Intertwiner state. Our analysis confirms the interpretation of
spin network bonds as a result of entanglement and to regard the same spin
network graph as an information graph, whose connectivity encodes, both at the
local and non-local level, the quantum correlations among its parts. This gives
a further connection between entanglement and geometry.Comment: 29 pages, 3 figures, revised version accepted for publicatio
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