The reason why doping causes superconductivity in LaFeAsO

The experimental observation of superconductivity in LaFeAsO appearing on doping is analyzed with the group-theoretical approach that evidently led in a foregoing paper (J. Supercond 24:2103, 2011) to an understanding of the cause of both the antiferromagnetic state and the accompanying structural distortion in this material. Doping, like the structural distortions, means also a reduction of the symmetry of the pure perfect crystal. In the present paper we show that this reduction modifies the correlated motion of the electrons in a special narrow half-filled band of LaFeAsO in such a way that these electrons produce a stable superconducting state

The Sources of Aggregate Productivity Growth - U.S. Manufacturing Industries, 1958-1996

The sources of aggregate productivity growth are explored using detailed data for four-digit U.S. manufacturing industries during 1958-96 and a decomposition formula which allows to quantify the contribution of structural change. Labor productivity as well as total factor productivity are considered and the aggregation is performed with either value-added or employment shares. It is shown that structural change generally works in favor of industries with increasing productivity. This effect is particularly strong in the years since 1990, in high-tech industries and in durable goods producing industries. The impact of the computer revolution can be clearly identified.aggregate productivity growth, structural change, manufacturing

Using the Manufacturing Productivity Distribution to Evaluate Growth Theories

Some multi-sector endogenous growth models make strong predictions about productivity differences across sectors in the form of a distribution or density function. In this paper it is demonstrated that this distribution is left-skewed for a wide range of plausible parameter values. This stands in strong contrast to the right-skewed shape of the respective empirical distribution estimated by kernel methods for a measure of relative productivity for more than 450 four-digit U.S. manufacturing industries during 1958-96. This difference is interpreted as evidence in favor of devoting more emphasis on the effects of structural change on the sectoral level in growth models.multisector growth models, manufacturing productivity distribution, skewness

The Global Trends of Total Factor Productivity. Evidence from the Nonparametric Malmquist Index Approach

In this paper the Malmquist index of total factor productivity is applied to a sample of 87 countries observed over the period 1960-90. This index and the method needed to quantify it, the data envelopment analysis, has substantial advantages as compared to traditional growth accounting. Two of these advantages are that it does not rely on questionable equilibrium assumptions to merge multiple inputs into a single index and that the rate of total factor productivity growth can explicitly be decomposed into a measure of efficiency change and the rate of technological progress. Results are reported both in the form of growth rates and measures of relative productivity levels. In each case related labour productivity measures are calculated and the differences to the total factor productivity measures are analysed. Among the topics covered are the productivity slowdown, the Asian Miracle and the bimodality of the distribution of relative producitivity levels.nonparametric productivity measurement, Malmquist index, growth accounting, productivity levels, distribution dynamics

Productivity Dynamics and Structural Change in the U.S. Manufacturing Sector

The paper investigates structural change among the four-digit (SIC) industries of the U.S. manufacturing sector during 1958-96 within a distribution dynamics framework. Focus is on the transition density of the Markov process that characterizes the value added shares of the industries. This transition density is estimated nonparametrically as well as by maximum likelihood, in which case the functional form of the density is derived from a search theoretic model. The nonparametric and the maximum likelihood fits show striking similarities. The relation of structural change to a relative measure of total factor productivity change is tested by an application of quantile regression and is found to be significantly positive throughout.structural change, productivity, manufacturing, quantile regression

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

The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach

As experimentally well established, undoped LaFeAsO is antiferromagnetic below 137K with the magnetic moments lying on the Fe sites. We determine the orthorhombic body-centered group Imma (74) as the space group of the experimentally observed magnetic structure in the undistorted lattice, i.e., in a lattice possessing no structural distortions in addition to the magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic band" with Bloch functions that can be unitarily transformed into optimally localized Wannier functions adapted to the space group Imma. This finding is interpreted in the framework of a nonadiabatic extension of the Heisenberg model of magnetism, the nonadiabatic Heisenberg model. Within this model, however, the magnetic structure with the space group Imma is not stable but can be stabilized by a (slight) distortion of the crystal turning the space group Imma into the space group Pnn2 (34). This group-theoretical result is in accordance with the experimentally observed displacements of the Fe and O atoms in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)]

Entanglement entropies and fermion signs of critical metals

The fermion sign problem is often viewed as a sheer inconvenience that plagues numerical studies of strongly interacting electron systems. Only recently, it has been suggested that fermion signs are fundamental for the universal behavior of critical metallic systems and crucially enhance their degree of quantum entanglement. In this work we explore potential connections between emergent scale invariance of fermion sign structures and scaling properties of bipartite entanglement entropies. Our analysis is based on a wavefunction ansatz that incorporates collective, long-range backflow correlations into fermionic Slater determinants. Such wavefunctions mimic the collapse of a Fermi liquid at a quantum critical point. Their nodal surfaces -- a representation of the fermion sign structure in many-particle configurations space -- show fractal behavior up to a length scale $\xi$ that diverges at a critical backflow strength. We show that the Hausdorff dimension of the fractal nodal surface depends on $\xi$, the number of fermions and the exponent of the backflow. For the same wavefunctions we numerically calculate the second R\'enyi entanglement entropy $S_2$. Our results show a cross-over from volume scaling, $S_2\sim \ell^\theta$ ($\theta=2$ in $d=2$ dimensions), to the characteristic Fermi-liquid behavior $S_2\sim \ell\ln \ell$ on scales larger than $\xi$. We find that volume scaling of the entanglement entropy is a robust feature of critical backflow fermions, independent of the backflow exponent and hence the fractal dimension of the scale invariant sign structure.Comment: 9.5 pages, 10 figure