All-electron magnetic response with pseudopotentials: NMR chemical shifts

A theory for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials is presented. It is formulated for both finite and infinitely periodic systems and is based on an extension to the Projector Augmented Wave approach of Bloechl [P. E. Bloechl, Phys. Rev. B 50, 17953 (1994)] and the method of Mauri et al [F. Mauri, B.G. Pfrommer, and S.G. Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated for molecules by comparison with a selection of quantum chemical results, and in periodic systems by comparison with plane-wave all-electron results for diamond.Comment: 25 pages, 4 tables, submitted to Physical Review

A Self-Consistent First-Principles Technique Having Linear Scaling

An algorithm for first-principles electronic structure calculations having a computational cost which scales linearly with the system size is presented. Our method exploits the real-space localization of the density matrix, and in this respect it is related to the technique of Li, Nunes and Vanderbilt. The density matrix is expressed in terms of localized support functions, and a matrix of variational parameters, L, having a finite spatial range. The total energy is minimized with respect to both the support functions and the elements of the L matrix. The method is variational, and becomes exact as the ranges of the support functions and the L matrix are increased. We have tested the method on crystalline silicon systems containing up to 216 atoms, and we discuss some of these results.Comment: 12 pages, REVTeX, 2 figure

Characterizing top gated bilayer graphene interaction with its environment by Raman spectroscopy

In this work we study the behavior of the optical phonon modes in bilayer graphene devices by applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (Eg) and infrared (Eu) phonon modes, due to different doping in the two layers. We theoretically analyze our data in terms of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the interaction of bilayer graphene with its surrounding environment

Epilepsia mioclónica juvenil: Pronóstico a largo plazo y retirada de tratamiento

Fundamento. La epilepsia mioclónica juvenil (EMJ) es un síndrome epiléptico clásico que carece de consenso pleno sobre la posibilidad de suspensión de tratamiento con fármacos antiepilépticos (FAE). Método. Estudio observacional, retrospectivo de una serie de pacientes con diagnóstico de EMJ de más de 20 años de evolución, atendiendo a la evolución de aquellos con retirada de FAE. Resultados. Se estudiaron veinte pacientes (edad media 44, 1 años, 55% hombres) con EMJ de 30 años de evolución media y edad media al inicio de 14, 2 años. El tipo de crisis motora más frecuente fue la combinación de mioclónicas y tónico-clónicas (70%); el 60% de los pacientes llevan más de cinco años libres de crisis. A cuatro pacientes (20%) se les suspendieron los FAE, a dos de ellos con edad media 23 años y un tiempo medio libre de crisis de 7, 5 años, que recayeron, y a los otros dos con edad media 39 años y tras 23, 5 años libres de crisis, que llevan dos y nueve años sin crisis en la actualidad. Conclusiones. Existe la posibilidad de suspender FAE en pacientes con EMJ con libertad de crisis mantenida en el tiempo. Sugerimos tener en cuenta la edad de suspensión y la existencia previa de un tiempo prolongado de libertad de crisis. Background. Juvenile myoclonic epilepsy (JME) is a classic epileptic syndrome that lacks consensus on the possibility of suspending treatment with antiepileptic drugs (AD). Method. Retrospective observational study of a series of patients diagnosed with JME with 20 years or more of evolution, focusing on those with withdrawal from AD. Results. The study involved twenty patients (average age 44.1 years, 55% men) with JME of 30 years average evolution and average age at its outset of 14.2 years. The most frequent type of motor crisis was the com-bination of myoclonic and tonic-clonic seizures (70%); 60% of the patients have been free of seizures for more than five years. Four patients (20%) were withdrawn from AD, two of them with an average age of 23 years and an average time free of seizures of 7.5 years, who relapsed, and the other two with an average age of 39 years and following 23.5 years free of seizures, who currently have been without seizures for two and nine years. Conclusions. The possibility of withdrawing AD in patients with JME who have been free of seizures over an extended time seems feasible. We suggest taking into account age at withdrawal and prior existence of a prolonged period of time free of seizures

Single impurity operators at critical wrapping order in the beta-deformed N=4 SYM

We study the spectrum of one single magnon in the superconformal beta-deformed N=4 SYM theory in the planar limit. We compute the anomalous dimensions of one-impurity operators O_{1,L}= tr(phi Z^{L-1}), including wrapping contributions at their critical order L.Comment: LaTeX, feynmf, Metapost, 20 pages, 11 figures, v2: results up to 11 loops completed, appendix on integral calculation extende

Basis Functions for Linear-Scaling First-Principles Calculations

In the framework of a recently reported linear-scaling method for density-functional-pseudopotential calculations, we investigate the use of localized basis functions for such work. We propose a basis set in which each local orbital is represented in terms of an array of `blip functions'' on the points of a grid. We analyze the relation between blip-function basis sets and the plane-wave basis used in standard pseudopotential methods, derive criteria for the approximate equivalence of the two, and describe practical tests of these criteria. Techniques are presented for using blip-function basis sets in linear-scaling calculations, and numerical tests of these techniques are reported for Si crystal using both local and non-local pseudopotentials. We find rapid convergence of the total energy to the values given by standard plane-wave calculations as the radius of the linear-scaling localized orbitals is increased.Comment: revtex file, with two encapsulated postscript figures, uses epsf.sty, submitted to Phys. Rev.

Rotating Strings with Two Unequal Spins in Lunin-Maldacena Background

We study a string motion in the Lunin-Maldacena background, that is, the \beta-deformed AdS_5 \times \tilde{S}^5 background dual to a \beta-deformation of \mathcal{N} = 4 super Yang-Mills theory. For real \beta we construct a rotating and wound string solution which has two unequal spins in \tilde{S}^5. The string energy is expressed in terms of the spins, the winding numbers and the deformation parameter. In the expansion of \lambda/J^2 with the total spin J and the string tension \sqrt{\lambda} we present ``one-loop" and ``two-loop" energy corrections. The ``one-loop" one agrees with the one-loop anomalous dimension of the corresponding gauge-theory scalar operators obtained in hep-th/0503192 from the \beta-deformed Bethe equation as well as the anisotropic Landau-Lifshitz equation.Comment: 13 pages, LaTeX, no figure

The Assertive Brain : Anterior Cingulate Phosphocreatine plus Creatine Levels Correlate With Self-Directedness in Healthy Adolescents

Despite various advances in the study of the neurobiological underpinnings of personality traits, the specific neural correlates associated with character and temperament traits are not yet fully understood. Therefore, this study aims to fill this gap by exploring the biochemical basis of personality, which is explored with the temperament and character inventory (TCI), during brain development in a sample of adolescents. Twenty-six healthy adolescents (aged between 13 and 21 years; 9 males and 18 females) with behavioral and emotional problems underwent a TCI evaluation and a 3T single-voxel proton magnetic resonance spectroscopy (1H MRS) acquisition of the anterior cingulate cortex (ACC). Absolute metabolite levels were estimated using LCModel: significant correlations between metabolite levels and selective TCI scales were identified. Specifically, phosphocreatine plus creatine (PCr+Cre) significantly correlated with self-directedness, positively, and with a self-transcendence (ST), negatively, while glycerophosphocholine plus phosphocholine (GPC+PC) and myo-inositol negatively correlated with ST. To the best of our knowledge, this is the first study reporting associations of brain metabolites with personality traits in adolescents. Therefore, our results represent a step forward for personality neuroscience within the study of biochemical systems and brain structures

Sobre la dificultad diagnóstica de las ausencias en el adulto

Sr. Editor: El relato de las características de una crisis convulsiva por parte de testigos es la base para el diagnóstico, la clasificación y el tratamiento de los síndromes epilépticos, pero es un elemento que en muchas ocasiones no puede estar presente. En la práctica clínica, un tercio de las convulsiones focales con alteraciones cognitivas no son controladas con la medicación antiepiléptica, siendo aún común el error diagnóstico, motivo por el cual la monitorización con vídeo- electroencefalograma (EEG) provee una ayuda inestimable en el momento del diagnóstico, la clasificación y el tratamiento. A continuación, presentamos un caso que expone la dificultad que puede suponer la evaluación de un paciente adulto con «crisis de ausencias» en el cual el tratamiento y un «reset» en la historia clínica puede llevar al diagnóstico correcto y por ende un buen tratamiento. Se trata de un paciente de 45 años de edad referido de otro centro hospitalario, sin antecedentes familiares de relevancia, que desarrolla 15 años antes un cuadro de recurrente «desconexión» del medio con ocasionales automatismos orales de aproximadamente 5 s de duración en los cuales se mantenía la postura..

Towards a Linear-Scaling DFT Technique: The Density Matrix Approach

A recently proposed linear-scaling scheme for density-functional pseudopotential calculations is described in detail. The method is based on a formulation of density functional theory in which the ground state energy is determined by minimization with respect to the density matrix, subject to the condition that the eigenvalues of the latter lie in the range [0,1]. Linear-scaling behavior is achieved by requiring that the density matrix should vanish when the separation of its arguments exceeds a chosen cutoff. The limitation on the eigenvalue range is imposed by the method of Li, Nunes and Vanderbilt. The scheme is implemented by calculating all terms in the energy on a uniform real-space grid, and minimization is performed using the conjugate-gradient method. Tests on a 512-atom Si system show that the total energy converges rapidly as the range of the density matrix is increased. A discussion of the relation between the present method and other linear-scaling methods is given, and some problems that still require solution are indicated.Comment: REVTeX file, 27 pages with 4 uuencoded postscript figure