Algorithmic problems for free-abelian times free groups

We study direct products of free-abelian and free groups with special emphasis on algorithmic problems. After giving natural extensions of standard notions into that family, we find an explicit expression for an arbitrary endomorphism of \ZZ^m \times F_n. These tools are used to solve several algorithmic and decision problems for \ZZ^m \times F_n : the membership problem, the isomorphism problem, the finite index problem, the subgroup and coset intersection problems, the fixed point problem, and the Whitehead problem.Comment: 38 page

Temperature and doping dependence of normal state spectral properties in a two-orbital model for ferropnictides

Using a second-order perturbative Green's functions approach we determined the normal state single-particle spectral function $A(\vec{k},\omega)$ employing a minimal effective model for iron-based superconductors. The microscopic model, used before to study magnetic fluctuations and superconducting properties, includes the two effective tight-binding bands proposed by S.Raghu et al. [Phys. Rev. B 77, 220503 (R) (2008)], and intra- and inter-orbital local electronic correlations, related to the Fe-3d orbitals. Here, we focus on the study of normal state electronic properties, in particular the temperature and doping dependence of the total density of states, $A(\omega)$, and of $A(\vec{k},\omega)$ in different Brillouin zone regions, and compare them to the existing angle resolved photoemission spectroscopy (ARPES) and previous theoretical results in ferropnictides. We obtain an asymmetric effect of electron and hole doping, quantitative agreement with the experimental chemical potential shifts as a function of doping, as well as spectral weight redistributions near the Fermi level as a function of temperature consistent with the available experimental data. In addition, we predict a non-trivial dependence of the total density of states with the temperature, exhibiting clear renormalization effects by correlations. Interestingly, investigating the origin of this predicted behaviour by analyzing the evolution with temperature of the k-dependent self-energy obtained in our approach, we could identify a number of specific Brillouin zone points, none of them probed by ARPES experiments yet, where the largest non-trivial effects of temperature on the renormalization are present.Comment: Manuscript accepted in Physics Letters A on Feb. 25, 201

Normal state electronic properties of LaO1−x_{1-x}Fx_{x}BiS2_{2} superconductors

A good description of the electronic structure of BiS$_{2}$-based superconductors is essential to understand their phase diagram, normal state and superconducting properties. To describe the first reports of normal state electronic structure features from angle resolved photoemission spectroscopy (ARPES) in LaO$_{1-x}$F$_{x}$BiS$_{2}$, we used a minimal microscopic model to study their low energy properties. It includes the two effective tight-binding bands proposed by Usui et al [Phys.Rev.B 86, 220501(R)(2012)], and we added moderate intra- and inter-orbital electron correlations related to Bi-($p_{Y}$, $p_{X}$) and S-($p_{Y}$, $p_{X}$) orbitals. We calculated the electron Green's functions using their equations of motion, which we decoupled in second-order of perturbations on the correlations. We determined the normal state spectral density function and total density of states for LaO$_{1-x}$F$_{x}$BiS$_{2}$, focusing on the description of the k-dependence, effect of doping, and the prediction of the temperature dependence of spectral properties. Including moderate electron correlations, improves the description of the few experimental ARPES and soft X-ray photoemission data available for LaO$_{1-x}$F$_{x}$BiS$_{2}$. Our analytical approximation enabled us to calculate the spectral density around the conduction band minimum at $\vec{k}_{0}=(0.45\pi,0.45\pi)$, and to predict the temperature dependence of the spectral properties at different BZ points, which might be verified by temperature dependent ARPES.Comment: 9 figures. Manuscript accepted in Physica B: Condensed Matter on Jan. 25, 201

Nanoscopic processes of Current Induced Switching in thin tunnel junctions

In magnetic nanostructures one usually uses a magnetic field to commute between two resistance (R) states. A less common but technologically more interesting alternative to achieve R-switching is to use an electrical current, preferably of low intensity. Such Current Induced Switching (CIS) was recently observed in thin magnetic tunnel junctions, and attributed to electromigration of atoms into/out of the insulator. Here we study the Current Induced Switching, electrical resistance, and magnetoresistance of thin MnIr/CoFe/AlO$_x$/CoFe tunnel junctions. The CIS effect at room temperature amounts to 6.9% R-change between the high and low states and is attributed to nanostructural rearrangements of metallic ions in the electrode/barrier interfaces. After switching to the low R-state some electro-migrated ions return to their initial sites through two different energy channels. A low (high) energy barrier of $\sim$0.13 eV ($\sim$0.85 eV) was estimated. Ionic electromigration then occurs through two microscopic processes associated with different types of ions sites/defects. Measurements under an external magnetic field showed an additional intermediate R-state due to the simultaneous conjugation of the MR (magnetic) and CIS (structural) effects.Comment: 6 pages, 4 figure

The two gap transitions in Ge1−x_{1-x}Snx_x: effect of non-substitutional complex defects

The existence of non-substitutional $\beta$-Sn defects in Ge$_{1-x}$Sn$_{x}$ was confirmed by emission channeling experiments [Decoster et al., Phys. Rev. B 81, 155204 (2010)], which established that although most Sn enters substitutionally ($\alpha$-Sn) in the Ge lattice, a second significant fraction corresponds to the Sn-vacancy defect complex in the split-vacancy configuration ( $\beta$-Sn ), in agreement with our previous theoretical study [Ventura et al., Phys. Rev. B 79, 155202 (2009)]. Here, we present our electronic structure calculation for Ge$_{1-x}$Sn$_{x}$, including substitutional $\alpha$-Sn as well as non-substitutional $\beta$-Sn defects. To include the presence of non-substitutional complex defects in the electronic structure calculation for this multi-orbital alloy problem, we extended the approach for the purely substitutional alloy by Jenkins and Dow [Jenkins and Dow, Phys. Rev. B 36, 7994 (1987)]. We employed an effective substitutional two-site cluster equivalent to the real non-substitutional $\beta$-Sn defect, which was determined by a Green's functions calculation. We then calculated the electronic structure of the effective alloy purely in terms of substitutional defects, embedding the effective substitutional clusters in the lattice. Our results describe the two transitions of the fundamental gap of Ge$_{1-x}$Sn$_{x}$ as a function of the total Sn-concentration: namely from an indirect to a direct gap, first, and the metallization transition at higher $x$. They also highlight the role of $\beta$-Sn in the reduction of the concentration range which corresponds to the direct-gap phase of this alloy, of interest for optoelectronics applications.Comment: 11 pages, 9 Figure

The medicinal use of leeches in Malta

The medical practice based on the Greek doctrine of the four humors considered that disease was due to alterations in the composition of these humors, and therapy was therefore based on attempting to restore the balance. Bleeding was the first resort in the treatment of a large majority of diseases ­ a therapeutic measure that persisted throughout the centuries. Bloodletting generally took the form of using a knife or lancet to open a vein, a procedure referred to as venesection or phlebotomy. A gentler and more desirable form of bleeding was to put a leech on the affected part and to let the animal engorge itself with the bad blood thought to dwell below the point of application. Leeches have been used medically for centuries; in Europe the use of leeches to drain off blood reached its height of popularity in the 19th century. The practice of bloodletting in the Maltese Islands dates at least to about the 2nd century AD. A tomb-slab from the late Roman/Palaeo-Christian period depicts a set of surgical instruments that include two Roman cupping vessels 1. Only 93 kilometers away from Sicily and 290 km from Northern Africa, the Maltese archipelago with its central position in the Mediterranean was an important meeting place for the various cultures that dominated the region. Malta's location at the crossroads of culture is reflected by the medical practices in use throughout the centuries. The practice of venesection in Malta persisted through the ages and during the medieval period (1539), the procedure was being carried out by the barbersurgeons against payment of one unza2. The barber-surgeons or barberotti remained responsible for venesection well into the 19th century and were only removed from the list of medical practitioners in 19213. The gentler form of bloodletting through the use of leeches was also practiced in Malta.peer-reviewe