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

Evidence for String Substructure

We argue that the behavior of string theory at high temperature and high longitudinal boosts, combined with the emergence of p-branes as necessary ingredients in various string dualities, point to a possible reformulation of strings, as well as p-branes, as composites of bits. We review the string-bit models, and suggest generalizations to incorporate p-branes.Comment: Latex file, 21 pages, 11 postscript figure

Pathological changes in seals in Swedish waters: the relation to environmental pollution

This thesis concerns the disease situation for the three seal species that inhabit the Swedish coastal waters; the grey seal (Halichoerus grypus), the ringed seal (Phoca hispida botnica) and the harbour seal (Phoca vitulina). A severe decline of the populations of Baltic grey and ringed seals took place during the second half of the 1960s. It was suggested to be caused by the contamination by industrial chemicals, above all organochlorines such as PCB and DDT. High concentrations of these substances were found in the Baltic biota. The author has performed necropsy or examination of organ samples from animals, which were found dead on shore, by caught at fishery or killed by hunting during 1977-2002. Multiple chronic organ lesions were found most prominent in the female reproductive organs (uterine stenoses and occlusions), intestines (colonic ulcers) and adrenals (cortical hyperplasia). Severe lesions were present also in the skeleton, integument and kidneys. The character and distribution of the lesions was regular and the disease picture tentatively was named the Baltic Seal Disease Complex (BSDC). The changes in the female reproductive organs indicate that reproductive failure is an important factor behind the decline of the Baltic seal populations. Adrenocortical hyperplasia was a regular and striking component of the BSDC. It is a common feature of prolonged stress in animals and man. The animals in this study have suffered from severe inflammatory processes in connection with more or less advanced malnutrition due to hampered ingestion and digestion of food. This is in the author’s opinion the most probable explanation of the adrenal changes. Inflammatory changes were most prominent in the intestines with deep ulcerations, in several cases leading to perforation of the intestinal wall. Bacteriological investigation revealed opportunistic or pathogenic micro-organisms but a common bacterial aetiology could not be suggested. The severity and wide dispersion of the lesions are interpreted as signs of a defective immune response. Minor lesions in the ileocaeco-colonic region caused by hookworms are regarded as the primary event of the ulcerous processes facilitating the establishment of secondary bacterial infections. Harbour seals showed less developed pathological changes but instead were victims of two Distemper epizootics with high mortality (c60%), during 1988 and 2002. During the 14- year-period after 1988 the Swedish harbour seal population gradually attained to the preepizootic size; a fast recover compared with the situation in Baltic grey and ringed seal populations suffering from the BSDC problems. A decrease in the prevalence of the lesions of the BSDC has been demonstrated concurrent with a decreased contamination of the Baltic biota towards the end of the 1900s. This is a strong indication of the role of pollutants as the main factor behind the BSDC. Other factors may also be involved, however, as indicated by the observation that the prevalence of intestinal ulcers still is high in Baltic grey seals

Stability Conditions and Branes at Singularities

I use Bridgeland's definition of a stability condition on a triangulated category to investigate the stability of D-branes on Calabi-Yau cones given by the canonical line bundle over a del Pezzo surface. In this context, I prove the existence of the decay of a D3-brane into a set of fractional branes. This is an important aspect of the derivation of quiver gauge theories from branes at singularities via the technique of equivalences of categories. Some important technical aspects of this equivalence are discussed. I also prove that the representations corresponding to skyscraper sheaves supported off the zero section are simple.Comment: 22 pages, uses utarticle.cls, dcpic.sty, v2: published versio

Stable Non-BPS Dyons in N=2 SYM

As a novel application of string junctions, we provide evidence for the existence of stable non-BPS dyons with magnetic charge greater than 1 in (the semiclassical regime of) N=2 SU(2) Super-Yang-Mills theory. In addition, we find a new curve of marginal stability. Moduli space is therefore divided into four regions, each containing a different stable particle spectrum.Comment: 12 pages LaTex, 5 figures; Added comments in both section 3 and the conclusions regarding the applicability of the string web picture in the field theory limi

The Size of a Polymer of String-Bits: A Numerical Investigation

In string-bit models, string is described as a polymer of point-like constituents. We attempt to use string-bit ideas to investigate how the size of string is affected by string interactions in a non-perturbative context. Lacking adequate methods to deal with the full complications of bit rearrangement interactions, we study instead a simplified analog model with only ``direct'' potential interactions among the bits. We use the variational principle in an approximate calculation of the mean-square size of a polymer as a function of the number of constituents/bits for various interaction strengths g in three specific models.Comment: 14 pages, LaTeX, 9 postscript figure

Strong inner inverses in endomorphism rings of vector spaces

For $V$ a vector space over a field, or more generally, over a division ring, it is well-known that every $x\in\mathrm{End}(V)$ has an inner inverse, i.e., an element $y\in\mathrm{End}(V)$ satisfying $xyx=x.$ We show here that a large class of such $x$ have inner inverses $y$ that satisfy with $x$ an infinite family of additional monoid relations, making the monoid generated by $x$ and $y$ what is known as an inverse monoid (definition recalled). We obtain consequences of these relations, and related results. P. Nielsen and J. \v{S}ter, in a paper to appear, show that a much larger class of elements $x$ of rings $R,$ including all elements of von Neumann regular rings, have inner inverses satisfying arbitrarily large finite subsets of the abovementioned set of relations. But we show by example that the endomorphism ring of any infinite-dimensional vector space contains elements having no inner inverse that simultaneously satisfies all those relations. A tangential result proved is a condition on an endomap $x$ of a set $S$ that is necessary and sufficient for $x$ to belong to an inverse submonoid of the monoid of all endomaps of $S.$Comment: 18pp. The main change from the preceding version is the discussion of three questions posed by the referee, two on p.10, starting on line 6, and one starting at the top of p.16. There are also many small revisions of wording et