On the stability of hole crystals in layered cuprates

Recent STM measurements have revealed the existence of periodic charge modulations at the surface of certain cuprate superconductors. Here we show that the observed patterns are compatible with the formation of a three-dimensional crystal of doped holes, with space correlations extending between different Cu-O layers. This puts severe constraints on the dynamical stability of the crystallised hole structure, resulting in a close relationship between the periodicity of the electronic modulation and the interlayer distance.Comment: completed reference list, fig. 3 corrected; accepted for publication in Eur. Phys. J. B, Rapid Note

On the Choice of Tool Material in Friction Stir Welding of Titanium Alloys

Friction Stir Welding (FSW) is a solid state welding process patented in 1991 by TWI; initially adopted to weld aluminum alloys, is now being successfully used also for magnesium alloys, copper and steels. The wide diffusion the process is having is due to the possibility to weld both materials traditionally considered difficult to be welded or "unweldable" by traditional fusion welding processes due to peculiar thermal and chemical material properties, and complex geometries as sandwich structures and straightening panels. Recently, research is focusing on titanium alloys thanks to the high interest that such materials are getting from the industry due to the extremely high strength-weight ratio together with good corrosion resistance properties. At the moment, the main limit to the industrial applicability of FSW to titanium alloys is the tool life, as ultra wear and deformation resistant materials must be used. In this paper a, experimental study of the tool life in FSW of titanium alloys sheets at the varying of the main process parameters is performed. Numerical simulation provided important information for the fixture design and analysis of results. Tungsten and Rhenium alloy W25Re tools are found to be the most reliable among the ones considered

Bose-Fermi mixtures in the molecular limit

We consider a Bose-Fermi mixture in the molecular limit of the attractive interaction between fermions and bosons. For a boson density smaller or equal to the fermion density, we show analytically how a T-matrix approach for the constituent bosons and fermions recovers the expected physical limit of a Fermi-Fermi mixture of molecules and atoms. In this limit, we derive simple expressions for the self-energies, the momentum distribution function, and the chemical potentials. By extending these equations to a trapped system, we determine how to tailor the experimental parameters of a Bose-Fermi mixture in order to enhance the 'indirect Pauli exclusion effect' on the boson momentum distribution function. For the homogeneous system, we present finally a Diffusion Monte Carlo simulation which confirms the occurrence of such a peculiar effect.Comment: 13 pages, 7 figures; final versio

Signatures of polaronic charge ordering in optical and dc conductivity using dynamical mean field theory

We apply dynamical mean field theory to study a prototypical model that describes charge ordering in the presence of both electron-lattice interactions and intersite electrostatic repulsion between electrons. We calculate the optical and d.c. conductivity, and derive approximate formulas valid in the limiting electron-lattice coupling regimes. In the weak coupling regime, we recover the usual behavior of charge density waves, characterized by a transfer of spectral weight due to the opening of a gap in the excitation spectrum. In the opposite limit of very strong electron-lattice coupling, instead, the charge ordering transition is signaled by a global enhancement of the optical absorption, with no appreciable spectral weight transfer. Such behavior is related to the progressive suppression of thermally activated charge defects taking place below the critical temperature. At intermediate values of the coupling within the polaronic regime, a complex behavior is obtained where both mechanisms of transfer and enhancement of spectral weight coexist.Comment: 1 figure added, illustrating the optical sum rul

Quantum Monte Carlo Study of a Resonant Bose-Fermi Mixture

We study a resonant Bose-Fermi mixture at zero temperature by using the fixed-node diffusion Monte Carlo method. We explore the system from weak to strong boson-fermion interaction, for different concentrations of the bosons relative to the fermion component. We focus on the case where the boson density $n_B$ is smaller than the fermion density $n_F$, for which a first-order quantum phase transition is found from a state with condensed bosons immersed in a Fermi sea, to a Fermi-Fermi mixture of composite fermions and unpaired fermions. We obtain the equation of state and the phase diagram, and we find that the region of phase separation shrinks to zero for vanishing $n_B$.Comment: 5 pages, 3 figures, published versio

On the interface polaron formation in organic field-effect transistors

A model describing the low density carrier state in an organic single crystal FET with high-$\kappa$ gate dielectrics is studied. The interplay between charge carrier coupling with inter-molecular vibrations in the bulk of the organic material and the long-range interaction induced at the interface with a polar dielectric is investigated. This interplay is responsible for the stabilization of a polaronic state with an internal structure extending on few lattice sites, at much lower coupling strengths than expected from the polar interaction alone. This effect could give rise to polaron self-trapping in high-$\kappa$ organic FET's without invoking unphysically large values of the carrier interface interaction.Comment: 9 pages, 9 figure

Towards a consistent eddy-covariance processing: An intercomparison of EddyPro and TK3

A comparison of two popular eddy-covariance software packages is presented, namely, EddyPro and TK3. Two approximately 1-month long test data sets were processed, representing typical instrumental setups (i.e., CSAT3/LI-7500 above grassland and Solent R3/LI-6262 above a forest). The resulting fluxes and quality flags were compared. Achieving a satisfying agreement and understanding residual discrepancies required several iterations and interventions of different nature, spanning from simple software reconfiguration to actual code manipulations. In this paper, we document our comparison exercise and show that the two software packages can provide utterly satisfying agreement when properly configured. Our main aim, however, is to stress the complexity of performing a rigorous comparison of eddy-covariance software. We show that discriminating actual discrepancies in the results from inconsistencies in the software configuration requires deep knowledge of both software packages and of the eddy-covariance method. In some instances, it may be even beyond the possibility of the investigator who does not have access to and full knowledge of the source code. Being the developers of EddyPro and TK3, we could discuss the comparison at all levels of details and this proved necessary to achieve a full understanding. As a result, we suggest that researchers are more likely to get comparable results when using EddyPro (v5.1.1) and TK3 (v3.11) – at least with the setting presented in this paper – than they are when using any other pair of EC software which did not undergo a similar cross-validation. As a further consequence, we also suggest that, to the aim of assuring consistency and comparability of centralized flux databases, and for a confident use of eddy fluxes in synthesis studies on the regional, continental and global scale, researchers only rely on software that have been extensively validated in documented intercomparisons

2D-3D registration of CT vertebra volume to fluoroscopy projection: A calibration model assessment (doi:10.1155/2010/806094)

This study extends a previous research concerning intervertebral motion registration by means of 2D dynamic fluoroscopy to obtain a more comprehensive 3D description of vertebral kinematics. The problem of estimating the 3D rigid pose of a CT volume of a vertebra from its 2D X-ray fluoroscopy projection is addressed. 2D-3D registration is obtained maximising a measure of similarity between Digitally Reconstructed Radiographs (obtained from the CT volume) and real fluoroscopic projection. X-ray energy correction was performed. To assess the method a calibration model was realised a sheep dry vertebra was rigidly fixed to a frame of reference including metallic markers. Accurate measurement of 3D orientation was obtained via single-camera calibration of the markers and held as true 3D vertebra position; then, vertebra 3D pose was estimated and results compared. Error analysis revealed accuracy of the order of 0.1 degree for the rotation angles of about 1?mm for displacements parallel to the fluoroscopic plane, and of order of 10?mm for the orthogonal displacement.<br/