Continuum variational and diffusion quantum Monte Carlo calculations

This topical review describes the methodology of continuum variational and diffusion quantum Monte Carlo calculations. These stochastic methods are based on many-body wave functions and are capable of achieving very high accuracy. The algorithms are intrinsically parallel and well-suited to petascale computers, and the computational cost scales as a polynomial of the number of particles. A guide to the systems and topics which have been investigated using these methods is given. The bulk of the article is devoted to an overview of the basic quantum Monte Carlo methods, the forms and optimisation of wave functions, performing calculations within periodic boundary conditions, using pseudopotentials, excited-state calculations, sources of calculational inaccuracy, and calculating energy differences and forces

The Inverse Amplitude Method and Adler Zeros

The Inverse Amplitude Method is a powerful unitarization technique to enlarge the energy applicability region of Effective Lagrangians. It has been widely used to describe resonances from Chiral Perturbation Theory as well as for the Strongly Interacting Symmetry Breaking Sector. In this work we show how it can be slightly modified to account also for the sub-threshold region, incorporating correctly the Adler zeros required by chiral symmetry and eliminating spurious poles. These improvements produce negligible effects on the physical region.Comment: 17 pages, 4 figure

Quantum Monte Carlo study of a positron in an electron gas

Quantum Monte Carlo calculations of the relaxation energy, pair-correlation function, and annihilating-pair momentum density are presented for a positron immersed in a homogeneous electron gas. We find smaller relaxation energies and contact pair-correlation functions in the important low-density regime than predicted by earlier studies. Our annihilating-pair momentum densities have almost zero weight above the Fermi momentum due to the cancellation of electron-electron and electron-positron correlation effects

Energetics of intrinsic point defects in ZrSiO4_4

Using first principles calculations we have studied the formation energies, electron and hole affinities, and electronic levels of intrinsic point defects in zircon. The atomic structures of charged interstitials, vacancies, Frenkel pairs and anti-site defects are obtained. The limit of high concentration of point defects, relevant for the use of this material in nuclear waste immobilization, was studied with a variable lattice relaxation that can simulate the swelling induced by radiation damage. The limit of low concentration of defects is simulated with larger cells and fixed lattice parameters. Using known band offset values at the interface of zircon with silicon, we analyze the foreseeable effect of the defects on the electronic properties of zircon used as gate in metal-oxide-semiconductor devices.Comment: preprint 16 pages, 4 figures, and 5 table

Efficient excitation of cavity resonances of subwavelength metallic gratings

One dimensional rectangular metallic gratings enable enhanced transmission of light for specific resonance frequencies. Two kinds of modes participating to enhanced transmission have already been demonstrated : (i) waveguide modes and (ii) surface plasmon polaritons (SPP). Since the original paper of Hessel and Oliner \cite{hessel} pointing out the existence of (i), no progress was made in their understanding. We present here a carefull analysis, and show that the coupling between the light and such resonances can be tremendously improved using an {\it evanescent} wave. This leads to enhanced localisation of light in cavities, yielding, in particular, to a very selective light transmission through these gratings.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let

Nature of the f_0(600) from its N_c dependence at two loops in unitarized Chiral Perturbation Theory

By using unitarized two-loop Chiral Perturbation Theory partial waves to describe pion-pion scattering we find that the dominant component of the lightest scalar meson does not follow the q-qbar dependence on the number of colors that, in contrast, is obeyed by the lightest vectors. The method suggests that a subdominant q-qbar component of the f_0(600) possibly originates around 1 GeV.Comment: 4 pages, 1 Figure. To appear in Phys. Rev. Let

El yacimiento chatelperroniense al aire libre de Aranbaltza (Barrika, Euskadi)

Se presentan los materiales procedentes de un nuevo yacimiento al aire libre localizado en la costa de Bizkaia. Éste fue localizado como resultado de unas obras de saneamiento por lo que los materiales recuperados están desprovistos de un contexto arqueológico preciso. Aun así un análisis tecnológico y tipológico ha permitido valorar la unidad del conjunto proponiendo la adscripción del grueso del material al Chatelperroniense. Este hallazgo abre nuevas posibilidades para el conocimiento sobre este periodo en la región Cantábrica ya que supondría la primera evidencia de un hábitat al aire libre semejante a los excavados en torno a Bayona

Intrinsic point defects and volume swelling in ZrSiO4 under irradiation

The effects of high concentration of point defects in crystalline ZrSiO4 as originated by exposure to radiation, have been simulated using first principles density functional calculations. Structural relaxation and vibrational studies were performed for a catalogue of intrinsic point defects, with different charge states and concentrations. The experimental evidence of a large anisotropic volume swelling in natural and artificially irradiated samples is used to select the subset of defects that give similar lattice swelling for the concentrations studied, namely interstitials of O and Si, and the anti-site Zr(Si), Calculated vibrational spectra for the interstitials show additional evidence for the presence of high concentrations of some of these defects in irradiated zircon.Comment: 9 pages, 7 (color) figure

Breakdown of the mean-field approximation in a wealth distribution model

One of the key socioeconomic phenomena to explain is the distribution of wealth. Bouchaud and M\'ezard have proposed an interesting model of economy [Bouchaud and M\'ezard (2000)] based on trade and investments of agents. In the mean-field approximation, the model produces a stationary wealth distribution with a power-law tail. In this paper we examine characteristic time scales of the model and show that for any finite number of agents, the validity of the mean-field result is time-limited and the model in fact has no stationary wealth distribution. Further analysis suggests that for heterogeneous agents, the limitations are even stronger. We conclude with general implications of the presented results.Comment: 11 pages, 3 figure