62 research outputs found
Nonlinearities in Macroeconomic Tail Risk through the Lens of Big Data Quantile Regressions
Modeling and predicting extreme movements in GDP is notoriously difficult and
the selection of appropriate covariates and/or possible forms of nonlinearities
are key in obtaining precise forecasts. In this paper, our focus is on using
large datasets in quantile regression models to forecast the conditional
distribution of US GDP growth. To capture possible non-linearities, we include
several nonlinear specifications. The resulting models will be huge dimensional
and we thus rely on a set of shrinkage priors. Since Markov Chain Monte Carlo
estimation becomes slow in these dimensions, we rely on fast variational Bayes
approximations to the posterior distribution of the coefficients and the latent
states. We find that our proposed set of models produces precise forecasts.
These gains are especially pronounced in the tails. Using Gaussian processes to
approximate the nonlinear component of the model further improves the good
performance, in particular in the right tail
Mapping Itinerant Electrons around Kondo Impurities
We investigate single Fe and Co atoms buried below a Cu(100) surface using
low temperature scanning tunneling spectroscopy. By mapping the local density
of states of the itinerant electrons at the surface, the Kondo resonance near
the Fermi energy is analyzed. Probing bulk impurities in this well-defined
scattering geometry allows separating the physics of the Kondo system and the
measuring process. The line shape of the Kondo signature shows an oscillatory
behavior as a function of depth of the impurity as well as a function of
lateral distance. The oscillation period along the different directions reveals
that the spectral function of the itinerant electrons is anisotropic.Comment: 5 pages, 4 figures, accepted by Physical Review Letter
Aspects of a phase transition in high-dimensional random geometry
A phase transition in high-dimensional random geometry is analyzed as it
arises in a variety of problems. A prominent example is the feasibility of a
minimax problem that represents the extremal case of a class of financial risk
measures, among them the current regulatory market risk measure Expected
Shortfall. Others include portfolio optimization with a ban on short selling,
the storage capacity of the perceptron, the solvability of a set of linear
equations with random coefficients, and competition for resources in an
ecological system. These examples shed light on various aspects of the
underlying geometric phase transition, create links between problems belonging
to seemingly distant fields and offer the possibility for further
ramifications
Interplay between Kondo effect and Ruderman-Kittel-Kasuya-Yosida interaction
The interplay between the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction
and the Kondo effect is expected to provide the driving force for the emergence
of many phenomena in strongly correlated electron materials. Two magnetic
impurities in a metal are the smallest possible system containing all these
ingredients and define a bottom up approach towards a long term understanding
of concentrated / dense systems. Here we report on the experimental and
theoretical investigation of iron dimers buried below a Cu(100) surface by
means of low temperature scanning tunnelling spectroscopy (STS) combined with
density functional theory (DFT) and numerical renormalization group (NRG)
calculations. The Kondo effect, in particular the width of the Abrikosov-Suhl
resonance, is strongly altered or even suppressed due to magnetic coupling
between the impurities. It oscillates as function of dimer separation revealing
that it is related to the RKKY interaction mediated by the conduction
electrons. Simulations based on density functional theory support this concept
showing the same oscillation period and trends in the coupling strength as
found in the experiment
Estimating Fiscal Multipliers by Combining Statistical Identification with Potentially Endogenous Proxies
Different proxy variables used in fiscal policy SVARs lead to contradicting
conclusions regarding the size of fiscal multipliers. In this paper, we show
that the conflicting results are due to violations of the exogeneity
assumptions, i.e. the commonly used proxies are endogenously related to the
structural shocks. We propose a novel approach to include proxy variables into
a Bayesian non-Gaussian SVAR, tailored to accommodate potentially endogenous
proxy variables. Using our model, we show that increasing government spending
is a more effective tool to stimulate the economy than reducing taxes. We
construct new exogenous proxies that can be used in the traditional proxy VAR
approach resulting in similar estimates compared to our proposed hybrid SVAR
model.Comment: 10 figure
Nature of the volcano transition in the fully disordered Kuramoto model
Randomly coupled phase oscillators may synchronize into disordered patterns
of collective motion. We analyze this transition in a large, fully connected
Kuramoto model with symmetric but otherwise independent random interactions.
Using the dynamical cavity method we reduce the dynamics to a stochastic single
oscillator problem with self-consistent correlation and response functions that
we study analytically and numerically. We clarify the nature of the volcano
transition and elucidate its relation to the existence of an oscillator glass
phase
Long-range Kondo signature of a single magnetic impurity
The Kondo effect, one of the oldest correlation phenomena known in condensed
matter physics, has regained attention due to scanning tunneling spectroscopy
(STS) experiments performed on single magnetic impurities. Despite the
sub-nanometer resolution capability of local probe techniques one of the
fundamental aspects of Kondo physics, its spatial extension, is still subject
to discussion. Up to now all STS studies on single adsorbed atoms have shown
that observable Kondo features rapidly vanish with increasing distance from the
impurity. Here we report on a hitherto unobserved long range Kondo signature
for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We
present a theoretical interpretation of the measured signatures using a
combined approach of band structure and many-body numerical renormalization
group (NRG) calculations. These are in excellent agreement with the rich
spatially and spectroscopically resolved experimental data.Comment: 7 pages, 3 figures + 8 pages supplementary material; Nature Physics
(Jan 2011 - advanced online publication
From the strange death to the odd afterlife of Lutheran England
Research on the relationship between England and Protestant Germany during the sixteenth century has recently experienced a revival. A significant area of concentration for confessional interests among Lutherans a century ago, Anglo-German relations took a backseat in Reformation historiography during the twentieth century, but during the last decade or so a host of scholars in the UK, Germany, and USA have once again turned their attention to the topic. This review article surveys trends in scholarship on Reformation studies in both England and Germany before turning specifically to works considering instances of interaction, co-operation, and adaptation across the confessional and geographic divides. Gathering a considerable array of secondary materials, the article offers an overview of the merits and criticisms of previous analyses and concludes by pointing out a few areas for future inquiry
Forty years of carabid beetle research in Europe - from taxonomy, biology, ecology and population studies to bioindication, habitat assessment and conservation
Volume: 100Start Page: 55End Page: 14
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