The Coulomb gap and low energy statistics for Coulomb glasses
We study the statistics of local energy minima in the configuration space of two-dimensional lattice Coulomb glasses with site disorder and the behavior of the Coulomb gap depending on the strength of random site energies. At intermediate disorder, i.e., when the typical strength of the disorder is of the same order as the nearest-neighbor Coulomb energy, the high energy tail of the distribution of the local minima is exponential. We furthermore analyze the structure of the local minima and show that most sites of the system have the same occupation numbers in all of these states. The density of states (DOS) shows a transition from the crystalline state at zero disorder (with a hard gap) to an intermediate, probably glassy state with a Coulomb gap. We analyze this Coulomb gap in some detail and show that the DOS deviates slightly from the traditional linear behavior in 2D. For finite systems these intermediate Coulomb gap states disappear for large disorder strengths and only a random localized state in which all electrons are in the minima of the random potential exists. Dedication: This paper is dedicated to Thomas Nattermann, our dearest friend, brilliant colleague, and outstanding teacher.
Effect of increasing disorder on domains of the 2d Coulomb glass
We have studied a two dimensional lattice model of Coulomb glass for a wide range of disorders at T∼0. The system was first annealed using Monte Carlo simulation. Further minimization of the total energy of the system was done using an algorithm developed by Baranovskii et al, followed by cluster flipping to obtain the pseudo-ground states. We have shown that the energy required to create a domain of linear size L in d dimensions is proportional to Ld−1. Using Imry–Ma arguments given for random field Ising model, one gets critical dimension dc⩾2 for Coulomb glass. The investigation of domains in the transition region shows a discontinuity in staggered magnetization which is an indication of a first-order type transition from charge-ordered phase to disordered phase. The structure and nature of random field fluctuations of the second largest domain in Coulomb glass are inconsistent with the assumptions of Imry and Ma, as was also reported for random field Ising model. The study of domains showed that in the transition region there were mostly two large domains, and that as disorder was increased the two large domains remained, but a large number of small domains also opened up. We have also studied the properties of the second largest domain as a function of disorder. We furthermore analysed the effect of disorder on the density of states, and showed a transition from hard gap at low disorders to a soft gap at higher disorders. At W=2, we have analysed the soft gap in detail, and found that the density of states deviates slightly (δ≈1.293±0.027) from the linear behaviour in two dimensions. Analysis of local minima show that the pseudo-ground states have similar structure.
In search of optimum chest radiography techniques.
A nation-wide study of chest radiography in Sweden had previously revealed a large variation in the physical and technical factors involved, in particular the radiation dose. In this study the image quality could not be assessed unambiguously. The aim of the present study was to try to establish a correlation between visual grading of radiographs and physical and technical factors in order to find the optimum chest X-ray system. The physical and technical performance of 24 chest units was evaluated. Radiographs were taken of an anthropomorphic chest phantom supplied with test structures simulating various pathologies. Image quality was assessed by visual grading analysis of the radiographs. The physical and technical parameters of the units rated best were used to exemplify good radiographic practice. The results were in agreement with the recommendations issued by the Commission of the European Communities (CEC). It was also shown that low radiation dose is compatible with high-quality radiographic imaging of the chest.