6,809 research outputs found
Generic Conditions for Forecast Dominance
Recent studies have analyzed whether one forecast method dominates another
under a class of consistent scoring functions. While the existing literature
focuses on empirical tests of forecast dominance, little is known about the
theoretical conditions under which one forecast dominates another. To address
this question, we derive a new characterization of dominance among forecasts of
the mean functional. We present various scenarios under which dominance occurs.
Unlike existing results, our results allow for the case that the forecasts'
underlying information sets are not nested, and allow for uncalibrated
forecasts that suffer, e.g., from model misspecification or parameter
estimation error. We illustrate the empirical relevance of our results via data
examples from finance and economics
Bose and Mott Glass Phases in Dimerized Quantum Antiferromagnets
We examine the effects of disorder on dimerized quantum antiferromagnets in a
magnetic field, using the mapping to a lattice gas of hard-core bosons with
finite-range interactions. Combining a strong-coupling expansion, the replica
method, and a one-loop renormalization group analysis, we investigate the
nature of the glass phases formed. We find that away from the tips of the Mott
lobes, the transition is from a Mott insulator to a compressible Bose glass,
however the compressibility at the tips is strongly suppressed. We identify
this finding with the presence of a rare Mott glass phase not previously
described by any analytic theory for this model and demonstrate that the
inclusion of replica symmetry breaking is vital to correctly describe the
glassy phases. This result suggests that the formation of Bose and Mott glass
phases is not simply a weak localization phenomenon but is indicative of much
richer physics. We discuss our results in the context of both ultracold atomic
gases and spin-dimer materials.Comment: 10 pages (including supplementary material), 3 figure
Entropy of unimodular Lattice Triangulations
Triangulations are important objects of study in combinatorics, finite
element simulations and quantum gravity, where its entropy is crucial for many
physical properties. Due to their inherent complex topological structure even
the number of possible triangulations is unknown for large systems. We present
a novel algorithm for an approximate enumeration which is based on calculations
of the density of states using the Wang-Landau flat histogram sampling. For
triangulations on two-dimensional integer lattices we achive excellent
agreement with known exact numbers of small triangulations as well as an
improvement of analytical calculated asymptotics. The entropy density is
consistent with rigorous upper and lower bounds. The presented
numerical scheme can easily be applied to other counting and optimization
problems.Comment: 6 pages, 7 figure
Theoretical investigation of the magnetic structure in YBa_2Cu_3O_6
As experimentally well established, YBa_2Cu_3O_6 is an antiferromagnet with
the magnetic moments lying on the Cu sites. Starting from this experimental
result and the assumption, that nearest-neighbor Cu atoms within a layer have
exactly antiparallel magnetic moments, the orientation of the magnetic moments
has been determined within a nonadiabatic extension of the Heisenberg model of
magnetism, called nonadiabatic Heisenberg model. Within this group-theoretical
model there exist four stable magnetic structures in YBa_2Cu_3O_6, two of them
are obviously identical with the high- and low-temperature structure
established experimentally. However, not all the magnetic moments which appear
to be antiparallel in neutron-scattering experiments are exactly antiparallel
within this group-theoretical model. Furthermore, within this model the
magnetic moments are not exactly perpendicular to the orthorhombic c axis
Flight of a heavy particle nonlinearly coupled to a quantum bath
Fluctuation and dissipation are by-products of coupling to the `environment.'
The Caldeira-Leggett model, a successful paradigm of quantum Brownian motion,
views the environment as a collection of harmonic oscillators linearly coupled
to the system. However, symmetry considerations may forbid a linear coupling,
e.g. for a neutral particle in quantum electrodynamics. We argue that nonlinear
couplings can lead to a fundamentally different behavior. Specifically, we
consider a heavy particle quadratically coupled to quantum fluctuations of the
bath. In one dimension the particle undergoes anomalous diffusion, unfolding as
a power-law distribution in space, reminiscent of L\'evy flights. We suggest
condensed matter analogs where similar effects may arise.Comment: Introduction expanded. Appendices adde
One-Nucleon Effective Generators of the Poincare Group derived from a Field Theory: Mass Renormalization
We start from a Lagrangian describing scalar "nucleons" and mesons which
interact through a simple vertex. Okubo's method of unitary transformation is
used to describe a single nucleon dressed by its meson cloud. We find an
expression for the physical mass of the nucleon being correct up to second
order in the coupling constant. It is then verified that this result is the
same as the corresponding expression found by Feynman techniques. Finally we
also express the three boost operators in terms of the physical nucleon mass.
Doing so we find expressions for all the ten generators of Poincar\'e
transformations for the system of one single dressed nucleon.Comment: 19 pages, no figure
Forward-Backward Asymmetry in
The Forward-backward asymmetry in the angular distribution of is
studied in the process . The
possibility of observing CP violation through the asymmetries in these two
processes is examined.Comment: 5 pages, latex formatte
Electronic spin-triplet nematic with a twist
We analyze a model of itinerant electrons interacting through a quadrupole
density-density repulsion in three dimensions. At the mean field level, the
interaction drives a continuous Pomeranchuk instability towards -wave,
spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation
and spatial rotational symmetries. This order results in spin antisymmetric,
elliptical deformations of the Fermi surfaces of up and down spins. We show
that the effects of quantum fluctuations are similar to those in metallic
ferromagnets, rendering the nematic transition first-order at low temperatures.
Using the fermionic quantum order-by-disorder approach to self-consistently
calculate fluctuations around possible modulated states, we show that the
first-order transition is pre-empted by the formation of a nematic state that
is intertwined with a helical modulation in spin space. Such a state is closely
related to -wave bond density wave order in square-lattice systems.
Moreover, we show that it may coexist with a modulated, -wave
superconducting state.Comment: 15 pages, 9 figure
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