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

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

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

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

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

On lattices and their ideal lattices, and posets and their ideal posets

For P a poset or lattice, let Id(P) denote the poset, respectively, lattice, of upward directed downsets in P, including the empty set, and let id(P)=Id(P)-\{\emptyset\}. This note obtains various results to the effect that Id(P) is always, and id(P) often, "essentially larger" than P. In the first vein, we find that a poset P admits no "<"-respecting map (and so in particular, no one-to-one isotone map) from Id(P) into P, and, going the other way, that an upper semilattice S admits no semilattice homomorphism from any subsemilattice of itself onto Id(S). The slightly smaller object id(P) is known to be isomorphic to P if and only if P has ascending chain condition. This result is strengthened to say that the only posets P_0 such that for every natural number n there exists a poset P_n with id^n(P_n)\cong P_0 are those having ascending chain condition. On the other hand, a wide class of cases is noted here where id(P) is embeddable in P. Counterexamples are given to many variants of the results proved.Comment: 8 pages. Copy at http://math.berkeley.edu/~gbergman/papers may be updated more frequently than arXiv copy. After publication, updates, errata, etc. may be noted at that pag

Reletting the Abandoned or Defaulted Public Works Project in New York- To Bid or Not to Bid?

The general requirement that contracts for public works be let pursuant to advertisements for bids to the lowest responsible bidder has long been the law in New York and other jurisdictions. After determining that the mandatory statutory pronouncements apply to a particular contract, there is an entire second level problem of the propriety of bids and the awarding of the contract pursuant thereto. Suppose a contractor has defaulted or abandoned a valid public works contract. Must the public entity now readvertise for bids for the completion of the work? The answer in most instances is no, and this raises the disconcerting specter of a single unsupervised public official having the power to let potentially huge contracts. Of course the size of the relet contract is not the prime consideration. The key point is that the laboriously developed bidding laws can be circumvented in default and abandonment situations, resulting in favortism for the contractor and a bad bargain for the taxpayers. However, the other side of the question is equally vexing. An abandoned or defaulted contract may require immediate continuation or remedial action. If such situations do not fit into the public emergency exception, must the public body expend the time, effort, and expense to analyze the supplies needed or work remaining, formulate proposed contracts, and advertise for bids? This article will analyse this difficulty in the context of the confusing and conflicting status of the legal requirements in New York for the reletting of abandoned or defaulted work