Quantum-Mechanically Induced Asymmetry in the Phase Diagrams of Spin-Glass Systems

The spin-1/2 quantum Heisenberg model is studied in all spatial dimensions d by renormalization-group theory. Strongly asymmetric phase diagrams in temperature and antiferromagnetic bond probability p are obtained in dimensions d \geq 3. The asymmetry at high temperatures approaching the pure ferromagnetic and antiferromagnetic systems disappears as d is increased. However, the asymmetry at low but finite temperatures remains in all dimensions, with the antiferromagnetic phase receding to the ferromagnetic phase. A finite-temperature second-order phase boundary directly between the ferromagnetic and antiferromagnetic phases occurs in d \geq 6, resulting in a new multicritical point at its meeting with the boundaries to the paramagnetic phase. In d=3,4,5, a paramagnetic phase reaching zero temperature intervenes asymmetrically between the ferromagnetic and reentrant antiferromagnetic phases. There is no spin-glass phase in any dimension.Comment: Added discussion of second-order transitions between ordered phases, driven by quenched disorder. 4 pages, 1 figure, 3 tables. Published versio

A Carrot-and-Stick Approach to Environmental Improvement: Marrying Agri-Environmental Payments and Water Quality Regulations

Agri-environmental programs, such as the Environmental Quality Incentives Program, provide payments to livestock and crop producers to generate broadly defined environmental benefits and to help them comply with federal water quality regulations, such as those that require manure nutrients generated on large animal feeding operations to be spread on cropland at no greater than agronomic rates. We couch these policy options in terms of agri-environmental "carrots" and regulatory "sticks," respectively. The U.S. agricultural sector is likely to respond to these policies in a variety of ways. Simulation analysis suggests that meeting nutrient standards would result in decreased levels of animal production, increased prices for livestock and poultry products, increased levels of crop production, and water quality improvements. However, estimated impacts are not homogeneous across regions. In regions with relatively less cropland per ton of manure produced, the impacts of these policies are more pronounced.Environmental Economics and Policy,

WHEN THE !%$? HITS THE LAND: IMPLICATIONS FOR US AGRICULTURE AND ENVIRONMENT WHEN LAND APPLICATION OF MANURE IS CONSTRAINED

Confined animal production in the U.S. and its associated discharge of manure nutrients into area waters is considered a leading contributor to current water quality impairments. A common option to mitigate these impairments is to limit land application of manure. This paper evaluates the implications of alternative land application constraints for U.S. agriculture and the environment at the regional and sector level. The results suggest that when these constraints are particularly binding, due to minimal acceptance of manure as a substitute for commercial fertilizer, potentially large and unanticipated changes in returns to agricultural production and water quality may occur. Furthermore, we find that some of the cost of meeting the land application constraints will be passed on to consumers through higher prices and to a portion of rural economies through lower production rates and labor expenditures.Environmental Economics and Policy, Livestock Production/Industries,

Frustrated Further-Neighbor Antiferromagnetic and Electron-Hopping Interactions in the d=3 tJ Model: Finite-Temperature Global Phase Diagrams from Renormalization-Group Theory

The renormalization-group theory of the d=3 tJ model is extended to further-neighbor antiferromagnetic or electron-hopping interactions, including the ranges of frustration. The global phase diagram of each model is calculated for the entire ranges of temperatures, electron densities, and further/first-neighbor interaction strength ratios. In addition to the \tau_{tJ} phase seen in earlier studies of the nearest-neighbor d=3 tJ model, the \tau_{Hb} phase seen before in the d=3 Hubbard model appears both near and away from half-filling. These distinct \tau phases potentially correspond to different (BEC-like and BCS-like) superconducting phases.Comment: Improved figures, added discussions, added references. Published version. 12 pages, 5 figures, 6 table

The Evolution of Buyout Pricing and Financial Structure

This paper presents evidence on systematic changes in the pricing and financial structure of 124 large management buyouts completed between 1980 and 1989. We find that over tine (1) prices increased relative to current cash flows with no accompanying decrease in risk or increase in projected future cash flows; (2) required bank principal repayments accelerated, leading to sharply lower ratios of cash flow to total debt obligations; (3) private subordinated debt was replaced by public debt while the use of strip-financing techniques declined; and (4) management teams invested a smaller fraction of their net worth in post-buyout equity. These patterns of buyout prices and structures suggest that based on ex ante data, one could have expected lower returns and more frequent financial distress in later buyouts. Preliminary post-buyout evidence is consistent with this interpretation.

Wavefunction Statistics using Scar States

We describe the statistics of chaotic wavefunctions near periodic orbits using a basis of states which optimise the effect of scarring. These states reflect the underlying structure of stable and unstable manifolds in phase space and provide a natural means of characterising scarring effects in individual wavefunctions as well as their collective statistical properties. In particular, these states may be used to find scarring in regions of the spectrum normally associated with antiscarring and suggest a characterisation of templates for scarred wavefunctions which vary over the spectrum. The results are applied to quantum maps and billiard systems.Comment: 31 pages, 11 figures, to appear in Annals of Physic

Chiral gauge theories with domain wall fermions

We have investigated a proposal to construct chiral gauge theories on the lattice using domain wall fermions. The model contains two opposite chirality zeromodes, which live on two domain walls. We couple only one of them to a gauge field, but find that mirror fermions which also couple to the gauge field always seem to exist.Comment: 3 pages. ref. added, some rewording at the end. contribution to Lattice'93. latex file, style file espcrc2.sty needed (appended), compressed tar file with two figures appended at the end (look for FIGURES

The continued spectral and temporal evolution of RX J0720.4-3125

RX J0720.4-3125 is the most peculiar object among a group of seven isolated X-ray pulsars (the so-called "Magnificent Seven"), since it shows long-term variations of its spectral and temporal properties on time scales of years. This behaviour was explained by different authors either by free precession (with a seven or fourteen years period) or possibly a glitch that occurred around $\mathrm{MJD=52866\pm73 days}$. We analysed our most recent XMM-Newton and Chandra observations in order to further monitor the behaviour of this neutron star. With the new data sets, the timing behaviour of RX J0720.4-3125 suggests a single (sudden) event (e.g. a glitch) rather than a cyclic pattern as expected by free precession. The spectral parameters changed significantly around the proposed glitch time, but more gradual variations occurred already before the (putative) event. Since $\mathrm{MJD\approx53000 days}$ the spectra indicate a very slow cooling by $\sim$2 eV over 7 years.Comment: seven pages, three figures, three tables; accepted by MNRA