J. Talamantes

Fluctuations in climate and incidence of coccidioidomycosis in Kern County, California: a review.

Abstract:  Coccidioidomycosis (Valley Fever) is a fungal infection found in the southwestern United States, northern Mexico, and some places in Central and South America. The fungi that cause it (Coccidioides immitis and Coccidioides posadasii) are normally soil dwelling, but, if disturbed, become airborne and infect the host when their spores are inhaled. It is thus natural to surmise that weather conditions, which foster the growth and dispersal of Coccidioides, must have an effect on the number of cases in the endemic areas. This article reviews our attempts to date at quantifying this relationship in Kern County, California (where C. immitis is endemic). We have examined the effect on incidence resulting from precipitation, surface temperature, and wind speed. We have performed our studies by means of a simple linear correlation analysis, and by a generalized autoregressive moving average model. Our first analysis suggests that linear correlations between climatic parameters and incidence are weak; our second analysis indicates that incidence can be predicted largely by considering only the previous history of incidence in the county—the inclusion of climate‐ or weather‐related time sequences improves the model only to a relatively minor extent. Our work therefore suggests that incidence fluctuations (about a seasonally varying background value) are related to biological and/or anthropogenic reasons, and not so much to weather or climate anomalies.

Combining forces--the use of Landsat TM satellite imagery, soil parameter information, and multiplex PCR to detect Coccidioides immitis growth sites in Kern County, California.

Coccidioidomycosis is a fungal disease acquired through the inhalation of spores of Coccidioides spp., which afflicts primarily humans and other mammals. It is endemic to areas in the southwestern United States, including the San Joaquin Valley portion of Kern County, California, our region of interest (ROI). Recently, incidence of coccidioidomycosis, also known as valley fever, has increased significantly, and several factors including climate change have been suggested as possible drivers for this observation. Up to date details about the ecological niche of C. immitis have escaped full characterization. In our project, we chose a three-step approach to investigate this niche: 1) We examined Landsat-5-Thematic-Mapper multispectral images of our ROI by using training pixels at a 750 m×750 m section of Sharktooth Hill, a site confirmed to be a C. immitis growth site, to implement a Maximum Likelihood Classification scheme to map out the locations that could be suitable to support the growth of the pathogen; 2) We used the websoilsurvey database of the US Department of Agriculture to obtain soil parameter data; and 3) We investigated soil samples from 23 sites around Bakersfield, California using a multiplex Polymerase Chain Reaction (PCR) based method to detect the pathogen. Our results indicated that a combination of satellite imagery, soil type information, and multiplex PCR are powerful tools to predict and identify growth sites of C. immitis. This approach can be used as a basis for systematic sampling and investigation of soils to detect Coccidioides spp.

Coulomb interactions in Anderson insulators

Abstract We review the main theoretical aspects concerning Coulomb glasses, that is systems with states localized by disorder and long-range interactions between their particles. The numerical algorithms available for their simulations are explained. We analyse tunnelling experiments and the role of screening by metallic electrodes. We study the mechanism for variable-range hopping conductance in these systems and in particular the role of many-electron correlations. Recent relaxation experiments and the possible glass transitions are reviewed. Finally, we describe different approaches to incorporate quantum effects in the study of Coulomb glasses.

Two electrons in a two-dimensional random potential: Exchange, interaction, and localization

The problem of two electrons in a two-dimensional random potential is addressed numerically. Specifically, the role of the Coulomb interaction between electrons on localization is investigated by writing the Hamiltonian on a localized basis and diagonalizing it exactly. The result of that procedure is discussed in terms of level statistics, the expectation value of the electron-electron separation, and a configuration-space inverse participation number. We argue that, in the interacting problem, a localization-delocalization crossover in real space does not correspond exactly to a Poisson-Wigner crossover in level statistics.

Phononless Hopping in the Coulomb Glass

We studied the effects of coherent hopping on localization in disordered systems of interacting localized electrons. Using a computational approach, we found that, within the parameters of our study, the inter-site Coulomb interactions between electrons enhance delocalization. We suggest that this is because short low-energy few-electron coherent hops are more effective in delocalizing electrons in the presence of those interactions. These transitions are relatively rare in the non-interacting case. As the system size is increased the effect is reduced, although it remains important. The reason seems to be that, for small systems, our approach magnifies coherent hopping due to our choice of boundary conditions.

Relaxation effects in the Coulomb glass

Abstract Results are reported of simulated energy relaxation at zero and at finite temperature, T, of three-dimensional random systems of interacting localized electrons. The work is based on the knowledge of the low energy states of the system, obtainable with methods described elsewhere. Relaxation at T = 0 is done by step-wise descent in energy, using the fastest process available at every step. Finite T relaxation is simulated by applying the master equation. Initial relaxation is rapid, but later slows down dramatically, suggesting glassy behaviour at low T. The longest T-dependent relaxation time is studied. Total relaxation times at T = 0 from different initial states provide some idea about the hierarchical structure in phase space, and reveal an interesting connection with neutral networks.

On the states of localized interacting systems

Compares the relative merits of three methods that can be used to find the low-energy states of localized systems in which the interaction between particles cannot be ignored. The algorithms are: (i) on which builds up the complete system by considering first the states of small subsystems and then combining the subsystems; (ii) a Monte Carlo process at low, but not zero, temperatures; (iii) a process similar to a zero-temperature Monte Carlo simulation. The authors find that the first algorithm works very well in principle, but the number of states that must be kept at every step of the build-up process is so large that it renders the method unfeasible for systems of reasonable size. The second method is the best of the three, but requires very extensive simulations for systems of reasonable size. The third method does not give a good set of low-lying states, but does give a good pseudo-ground state.

R-ε percolation in moderate-field hopping transport

Abstract The problem of the onset of nonlinear hopping conductivity in disordered systems is addressed using the Mott model. Four effects have been proposed which lead to nonlinearities in the current as a function of the applied electric field in moderate fields. In previous (R percolation) work, a model was used which circumvented two of these effects and evaluated the relative importance of the other two. In the present (R-∊ percolation) work, one of the other effects is included, and it is argued that the remaining effect does not play a role in the resulting model. As in the work for R percolation, a technique is used which employs a percolative approach on the scale of a macrobond and an effective-medium theory on a scale larger than a macrobond. The result is that the current is superlinear, in accordance with previous (but less rigorous) results.

Specific heat of the Coulomb glass

Abstract The specific heat c of the Coulomb glass is studied by numerical simulations. Both the lattice model with various strengths of disorder, and the random-position model are considered for the one- to three-dimensional cases. In order to extend the investigations down to very low temperatures where the many-valley structure of the configuration space is of great importance we use a hybrid Metropolis procedure. This algorithm bridges the gap between Metropolis simulation and analytical statistical mechanics. The analysis of the simulation results shows that the correlation length of the relevant processes is rather small, and that multi-particle processes yield an essential contribution to the specific heat in all cases except the one-dimensional random-position model.

The Delocalization Problem of Two Interacting Electrons in a Two‐Dimensional Random Potential

The problem of two electrons in a two-dimensional random potential is addressed numerically. Specifically, the role of the Coulomb interaction between electrons on localization is investigated by writing the Hamiltonian on a localized basis and diagonalizing it exactly. The result of that procedure is discussed in terms of level statistics, and the expectation value of the electron-electron separation.