599 research outputs found
Assessing GMM Estimates of the Federal Reserve Reaction Function.
Estimating a forward-looking monetary policy rule by the Generalized Method of Moments (GMM) has become a popular approach since the influential paper by Clarida, Gali, and Gertler (1998). However, an abundant econometric literature underlines the unappealing small-samples properties of GMM estimators. Focusing on the Federal Reserve reaction function, we assess GMM estimates in the context of monetary policy rules. First, we show that three usual alternative GMM estimators yield substantially different results. Then, we compare the GMM estimates with two Maximum-Likelihood (ML) estimates, obtained using a small model of the economy. We use Monte-Carlo simulations to investigate the empirical results. We find that the GMM are biased in small sample, inducing an overestimate of the inflation parameter. The two-step GMM estimates are found to be rather close to the ML\ estimates. By contrast, iterative and continuous-updating GMM procedures produce more biased and more dispersed estimators.Forward-looking model ; monetary policy reaction function ; GMM estimator, FIML estimator ; small-sample properties of an estimator.
The Kondo effect in bosonic spin liquids
In a metal, a magnetic impurity is fully screened by the conduction electrons
at low temperature. In contrast, impurity moments coupled to spin-1 bulk
bosons, such as triplet excitations in paramagnets, are only partially
screened, even at the bulk quantum critical point. We argue that this
difference is not due to the quantum statistics of the host particles but
instead related to the structure of the impurity-host coupling, by
demonstrating that frustrated magnets with bosonic spinon excitations can
display a bosonic version of the Kondo effect. However, the Bose statistics of
the bulk implies distinct behavior, such as a weak-coupling impurity quantum
phase transition, and perfect screening for a range of impurity spin values. We
discuss implications of our results for the compound Cs2CuCl4, as well as
possible extensions to multicomponent bosonic gases.Comment: 4 pages, 3 figures. The weak coupling RG flow was corrected and
expanded in last versio
Critical points and non-Fermi liquids in the underscreened pseudogap Kondo model
Numerical Renormalization Group simulations have shown that the underscreened
spin-1 Kondo impurity model with power-law bath density of states (DOS)
\rho(\w) \propto |\w|^r possesses various intermediate-coupling fixed points,
including a stable non-Fermi liquid phase. In this paper we discuss the
corresponding universal low-energy theories, obtain thermodynamic quantities
and critical exponents by renormalization group analysis together with suitable
-expansions, and compare our results with numerical data. Whereas the
particle-hole symmetric critical point can be controlled at weak coupling using
a simple generalization of the spin-1/2 model, we show that the stable
non-Fermi liquid fixed point must be accessed near strong coupling via a
mapping onto an effective ferromagnetic S_\mr{eff}=1/2 model with singular
bath DOS with exponent r_\mr{eff}=-r<0. In addition, we consider the
particle-hole asymmetric critical fixed point, for which we propose a universal
field theory involving the crossing between doublet and triplet levels.Comment: 10 pages, 8 figures. Minor modifications in updated versio
Influence of spin fluctuations near the Mott transition: a DMFT study
Dynamics of magnetic moments near the Mott metal-insulator transition is
investigated by a combined slave-rotor and Dynamical Mean-Field Theory solution
of the Hubbard model with additional fully-frustrated random Heisenberg
couplings. In the paramagnetic Mott state, the spinon decomposition allows to
generate a Sachdev-Ye spin liquid in place of the collection of independent
local moments that typically occurs in the absence of magnetic correlations.
Cooling down into the spin-liquid phase, the onset of deviations from pure
Curie behavior in the spin susceptibility is found to be correlated to the
temperature scale at which the Mott transition lines experience a marked
bending. We also demonstrate a weakening of the effective exchange energy upon
approaching the Mott boundary from the Heisenberg limit, due to quantum
fluctuations associated to zero and doubly occupied sites.Comment: 6 pages, 3 figures. V3 was largely expande
Magnetotransport in the Kondo model with ferromagnetic exchange interaction
We consider the transport properties in an applied magnetic field of the spin
S=1/2 Kondo model with ferromagnetic exchange coupling to electronic
reservoirs, a description relevant for the strong coupling limit of
underscreened spin S=1 Kondo impurities. Because the ferromagnetic Kondo
interaction is marginally irrelevant, perturbative methods should prove
accurate down to low energies. For the purpose of this study, we use a
combination of Majorana diagrammatic theory with Density Matrix Numerical
Renormalization Group simulations. In the standard case of antiferromagnetic
Kondo exchange, we first show that our technique recovers previously obtained
results for the T-matrix and spin relaxation at weak coupling (above the Kondo
temperature). Considering then the ferromagnetic case, we demonstrate how the
low-energy Kondo anomaly splits for arbitrary small values of the Zeeman
energy, in contrast to fully screened Kondo impurities near the strong coupling
Fermi liquid fixed point, and in agreement with recent experimental findings
for spin S=1 molecular quantum dots.Comment: 14 pages, 13 figures, minor changes in V
Boundary quantum criticality in models of magnetic impurities coupled to bosonic baths
We investigate quantum impurity problems, where a local magnetic moment is
coupled to the spin density of a bosonic environment, leading to bosonic
versions of the standard Kondo and Anderson impurity models. In a physical
situation, these bosonic environments can correspond either to deconfined
spinons in certain classes of Z_2 frustrated antiferromagnets, or to particles
in a multicomponent Bose gase (in which case the spin degree of freedom is
attributed to hyperfine levels). Using renormalization group techniques, we
establish that our impurity models, which feature an exchange interaction
analogous to Kondo impurities in Fermi liquids, allow the flow towards a stable
strong-coupling state. Since the low-energy bosons live around a single point
in momentum space, and there is no Fermi surface, an impurity quantum phase
transition occurs at intermediate coupling, separating screened and unscreened
phases. This behavior is qualitatively different from previously studied
spin-isotropic variants of the spin-boson model, which display stable
intermediate-coupling fixed points and no screening.Comment: 15 pages, 10 fig
Classical percolation fingerprints in the high-temperature regime of the integer quantum Hall effect
We have performed magnetotransport experiments in the high-temperature regime
(up to 50 K) of the integer quantum Hall effect for two-dimensional electron
gases in semiconducting heterostructures. While the magnetic field dependence
of the classical Hall law presents no anomaly at high temperatures, we find a
breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a
crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the
onset of the integer quantum Hall effect at low temperatures. We show that the
high magnetic field regime at B > B_c can be understood in terms of classical
percolative transport in a smooth disordered potential. From the temperature
dependence of the peak longitudinal conductance, we extract scaling exponents
which are in good agreement with the theoretically expected values. We also
prove that inelastic scattering on phonons is responsible for dissipation in a
wide temperature range going from 1 to 50 K at high magnetic fields.Comment: 14 pages + 8 Figure
Universal crossovers and critical dynamics of quantum phase transitions: A renormalization group study of the pseudogap Kondo problem
The pseudogap Kondo problem, describing a magnetic impurity embedded in an
electronic environment with a power-law density of states, displays continuous
quantum phase transitions between free and screened moment phases. In this
paper we employ renormalization group techniques to analytically calculate
universal crossover functions, associated to these transitions, for various
observables. Quantitative agreement with the results of Numerical
Renormalization Group (NRG) simulations is obtained for temperature-dependent
static and zero-temperature dynamic quantities, at and away from criticality.
In the notoriously difficult realm of finite-temperature low-frequency
dynamics, usually inaccessible to both NRG and perturbative methods, we show
that progress can be made by a suitable renormalization procedure in the
framework of the Callan-Symanzik equations. Our general strategy can be
extended to other zero-temperature phase transitions, both in quantum impurity
models and bulk systems.Comment: 19 pages, 18 figures; (v3) version as publishe
A Mammalian Mediator Subunit that Shares Properties with Saccharomyces cerevisiae Mediator Subunit Cse2
The multiprotein Mediator complex is a coactivator required for activation of RNA polymerase II transcription by DNA bound transcription factors. We previously identified and partially purified a mammalian Mediator complex from rat liver nuclei (Brower, C.S., Sato, S., Tomomori-Sato, C., Kamura, T., Pause, A., Stearman, R., Klausner, R.D., Malik, S., Lane, W.S., Sorokina, I., Roeder, R.G., Conaway, J.W., and Conaway, R.C. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 10353-10358). Analysis by tandem mass spectrometry of proteins present in the most highly purified rat Mediator fractions led to the identification of a collection of new mammalian Mediator subunits, as well as several potential Mediator subunits including a previously uncharacterized protein encoded by the FLJ10193open reading frame. In this study, we present direct biochemical evidence that the FLJ10193protein, which we designate Med25, is a bona fide subunit of the mammalian Mediator complex. In addition, we present evidence that Med25 shares structural and functional properties with Saccharomyces cerevisiae Mediator subunit Cse2 and may be a mammalian Cse2 ortholog. Taken together, our findings identify a novel mammalian Mediator subunit and shed new light on the architecture of the mammalian Mediator complex
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