High field magnetotransport in composite conductors: the effective medium approximation revisited

The self consistent effective medium approximation (SEMA) is used to study three-dimensional random conducting composites under the influence of a strong magnetic field {\bf B}, in the case where all constituents exhibit isotropic response. Asymptotic analysis is used to obtain almost closed form results for the strong field magnetoresistance and Hall resistance in various types of two- and three-constituent isotropic mixtures for the entire range of compositions. Numerical solutions of the SEMA equations are also obtained, in some cases, and compared with those results. In two-constituent free-electron-metal/perfect-insulator mixtures, the magnetoresistance is asymptotically proportional to $|{\bf B}|$ at {\em all concentrations above the percolation threshold}. In three-constituent metal/insulator/superconductor mixtures a line of critical points is found, where the strong field magnetoresistance switches abruptly from saturating to non-saturating dependence on $|{\bf B}|$, at a certain value of the insulator-to-superconductor concentration ratio. This transition appears to be related to the phenomenon of anisotropic percolation.Comment: 16 pages, 3 figure

Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure

A recently developed self-consistent effective medium approximation, for composites with a columnar microstructure, is applied to such a three-constituent mixture of isotropic normal conductor, perfect insulator, and perfect conductor, where a strong magnetic field {\bf B} is present in the plane perpendicular to the columnar axis. When the insulating and perfectly conducting constituents do not percolate in that plane, the microstructure-induced in-plane magnetoresistance is found to saturate for large {\bf B}, if the volume fraction of the perfect conductor $p_S$ is greater than that of the perfect insulator $p_I$. By contrast, if $p_S<p_I$, that magnetoresistance keeps increasing as ${\bf B}^2$ without ever saturating. This abrupt change in the macroscopic response, which occurs when $p_S=p_I$, is a critical point, with the associated critical exponents and scaling behavior that are characteristic of such points. The physical reasons for the singular behavior of the macroscopic response are discussed. A new type of percolation process is apparently involved in this phenomenon.Comment: 4 pages, 1 figur

Eddy currents in a conducting sphere

This report analyzes the eddy current induced in a solid conducting sphere by a sinusoidal current in a circular loop. Analytical expressions for the eddy currents are derived as a power series in the vectorial displacement of the center of the sphere from the axis of the loop. These are used for first order calculations of the power dissipated in the sphere and the force and torque exerted on the sphere by the electromagnetic field of the loop

Grackles

Numbering in the tens of millions of birds, grackle populations in North America can cause a variety of conflicts with people. Grackles eat agricultural crops and livestock feed, damage property, spread pathogens, and collide with aircraft. Their large roosts can be a nuisance in urban and suburban areas. A combination of dispersal techniques, exclusion, and lethal removal may help to reduce grackle damage. Grackles adapt easily to human-dominated environments, and exploit human food and other features of human landscapes. Thus, an integrated damage management approach to grackle damage focuses on reducing and eliminating the damage, rather than simply controlling grackle populations. Three species of grackles are present in North America: the common grackle, the boat-tailed grackle, and the great-tailed grackle. A fourth species, the greater Antillean grackle (Q. niger) is present in Puerto Rico. All are part of the Family Icteridae that includes blackbirds, orioles, cowbirds, meadowlarks, and bobolinks. The population status of all three grackle species is considered common to overabundant

Microscopic colitis : epidemiology, death and associated disorders

Microscopic colitis (MC) is the most recently recognized inflammatory condition of the large intestine. MC is an umbrella term for two disease entities, namely lymphocytic colitis (LC) and collagenous colitis (CC). These subtypes are distinguished by their histopathological presentation and cannot be separated based on clinical observation or symptoms. The most prominent clinical presentation is watery, non-bloody diarrhea. However, a proportion of patients also suffer from abdominal pain, weight loss, fecal incontinence and reduced quality of life. MC primarily affects the elderly and shows a clear female preponderance with some 2/3 of patients being women. Large scale, epidemiological research of the disease has historically been hampered by insufficient awareness. Therefore, using data from Swedish health care registers, this thesis aims to elucidate temporal patterns of MC as well as the association with mortality, cancer and celiac disease (CD). In study I, we examined the validity of having a MC diagnosis recorded in Swedish regional pathology registers. Through manual review of medical charts (n=211), carried out by two independent reviewers, we computed a positive predictive value for MC of 95%. Thus, we concluded that Swedish pathology registers are a reliable source for identifying patients with MC. In study II, we identified every patient with a first-time diagnosis of MC recorded in Swedish pathology registers from 1995-2015. Using this cohort, we examined temporal trends, age distribution and sex differences in MC. As expected, a majority of patients (72%) were female and mean age at diagnosis was 60.2 years. Incidence rates increased appreciably from 1995 to 2012, after which rates have stabilized. The mean age-standardized incidence rate from 2006 to 2015 was 10.5 cases/100,000 person-years, with a female to male incidence rate ratio of 2.4, adjusted for age and calendar period. When analyzing age-specific incidence, incidence rates increased up to 75-79 years after which they declined. Furthermore, we estimated that during a life-time 1 in 115 women and 1 in 286 men are expected to be diagnosed with MC. In study III we examined mortality in patients with MC. This was done using a matched cohort study design where each exposed individual (MC) (n=14,333) was matched according to age, sex, county of residence and year of biopsy to five reference individuals from the general population. During the study period (1990 to 2017) patients with MC had a higher probability of death. However, after adjustment for comorbidities the association vanished. Thus, we concluded that the increased risk of death is attributable to the burden of concomitant disease. Study IV aimed to investigate the association between MC and cancer. Again, this was done using the matched cohort design described above. In total, we identified 11,758 patients diagnosed with MC between 1990 and 2016 with MC that were matched to 50,828 reference individuals. After adjustments for the matching variables and comorbidities (CD and diabetes), we estimated an adjusted hazard ratio (aHR) of 1.08 (95%CI=1.02-1.16) for overall cancer. In secondary analyses, we found a decreased probability of colorectal cancer (aHR, 0.52 (95%CI_0.40-0.66)). The same pattern was observed for gastrointestinal cancers overall (aHR, 0.72(95%CI=0.60-0.85)). In study V we examined the association between CD and MC. Using the same matched cohort study design described above, we identified 45,267 patients with CD and 224,568 reference individuals between 1990 and 2016. 456 patients with CD were diagnosed with MC during the study period compared to 198 reference individuals that developed MC during the same period. These figures correspond to an aHR of 11.5 (95%CI=9.3-13.7). However, as the proportional hazards assumption was violated, the main result should not be interpreted as the probability for a CD patient to develop MC at any instant during the study period compared to the reference population, but rather a mean aHR based on all lengths of follow up. However, as the increased risk remained even after >10 years of follow up, our results indicate that the concomitance of these diagnoses should be considered if symptoms persist or reoccur despite a gluten free diet