Force induced triple point for interacting polymers

We show the existence of a force induced triple point in an interacting polymer problem that allows two zero-force thermal phase transitions. The phase diagrams for three different models of mutually attracting but self avoiding polymers are presented. One of these models has an intermediate phase and it shows a triple point but not the others. A general phase diagram with multicritical points in an extended parameter space is also discussed.Comment: 4 pages, 8 figures, revtex

Photoluminescence Spectroscopy of the Molecular Biexciton in Vertically Stacked Quantum Dot Pairs

We present photoluminescence studies of the molecular neutral biexciton-exciton spectra of individual vertically stacked InAs/GaAs quantum dot pairs. We tune either the hole or the electron levels of the two dots into tunneling resonances. The spectra are described well within a few-level, few-particle molecular model. Their properties can be modified broadly by an electric field and by structural design, which makes them highly attractive for controlling nonlinear optical properties.Comment: 4 pages, 5 figures, (v2, revision based on reviewers comments, published

Dynamics and Transport in Random Antiferromagnetic Spin Chains

We present the first results on the low-frequency dynamical and transport properties of random antiferromagnetic spin chains at low temperature ($T$). We obtain the momentum and frequency dependent dynamic structure factor in the Random Singlet (RS) phases of both spin-1/2 and spin-1 chains, as well as in the Random Dimer phase of spin-1/2 chains. We also show that the RS phases are unusual `spin-metals' with divergent low-frequency conductivity at T=0, and follow the spin conductivity through `metal-insulator' transitions tuned by the strength of dimerization or Ising anisotropy in the spin-1/2 case, and by the strength of disorder in the spin-1 case.Comment: 4 pages (two-column format). Presentation substantially revised to accomodate new result

Systems with Multiplicative Noise: Critical Behavior from KPZ Equation and Numerics

We show that certain critical exponents of systems with multiplicative noise can be obtained from exponents of the KPZ equation. Numerical simulations in 1d confirm this prediction, and yield other exponents of the multiplicative noise problem. The numerics also verify an earlier prediction of the divergence of the susceptibility over an entire range of control parameter values, and show that the exponent governing the divergence in this range varies continuously with control parameter.Comment: Four pages (In Revtex format) with 4 figures (in Postcript

Ground-State Phase Diagram of the Two-Dimensional Quantum Heisenberg Mattis Model

The two-dimensional $S=1/2$ asymmetric Heisenberg Mattis model is investigated with the exact diagonalization of finite clusters. The N\'eel order parameter and the spin glass order parameter can be smoothly extrapolated to the thermodynamic limit in the antiferromagnetic region, as in the pure Heisenberg antiferromagnet. The critical concentration of the N\'eel phase is consistent with that of the two-dimensional Ising Mattis model, and the spin glass order parameter increases monotonously as the ferro-bond concentration increases. These facts suggest that quantum fluctuation does not play an essential role in two-dimensional non-frustrated random spin systems. KEYWORDS: quantum spin system, ground state, randomness, Mattis model, N\'eel order, spin glass orderComment: 10 pages, LaTeX, 6 compressed/uuencoded postscript figures, J. Phys. Soc. Jpn. 65 (1996) No. 2 in pres

OH far-infrared emission from low- and intermediate-mass protostars surveyed with Herschel-PACS

OH is a key species in the water chemistry of star-forming regions, because its presence is tightly related to the formation and destruction of water. This paper presents OH observations from 23 low- and intermediate-mass young stellar objects obtained with the PACS integral field spectrometer on-board Herschel in the context of the Water In Star-forming Regions with Herschel (WISH) key program. Most low-mass sources have compact OH emission (< 5000 AU scale), whereas the OH lines in most intermediate-mass sources are extended over the whole PACS detector field-of-view (> 20000 AU). The strength of the OH emission is correlated with various source properties such as the bolometric luminosity and the envelope mass, but also with the OI and H2O emission. Rotational diagrams for sources with many OH lines show that the level populations of OH can be approximated by a Boltzmann distribution with an excitation temperature at around 70 K. Radiative transfer models of spherically symmetric envelopes cannot reproduce the OH emission fluxes nor their broad line widths, strongly suggesting an outflow origin. Slab excitation models indicate that the observed excitation temperature can either be reached if the OH molecules are exposed to a strong far-infrared continuum radiation field or if the gas temperature and density are sufficiently high. Using realistic source parameters and radiation fields, it is shown for the case of Ser SMM1 that radiative pumping plays an important role in transitions arising from upper level energies higher than 300 K. The compact emission in the low-mass sources and the required presence of a strong radiation field and/or a high density to excite the OH molecules points towards an origin in shocks in the inner envelope close to the protostar.Comment: Accepted for publication in Astronomy and Astrophysics. Abstract abridge

Empirical approaches for the investigation of toxicant-induced loss of tolerance.

It has been hypothesized that sensitivity to low-level chemical exposures develops in two steps: initiation by an acute or chronic chemical exposure, followed by triggering of symptoms by low levels of previously tolerated chemical inhalants, foods, or drugs. The Working Group on Toxicant-induced Loss of Tolerance has formulated a series of research questions to test this hypothesis: Do some individuals experience sensitivity to chemicals at levels of exposure unexplained by classical toxicological thresholds and dose-response relationships, and outside normally expected variation in the population? Do chemically sensitive subjects exhibit masking that may interfere with the reproducibility of their responses to chemical challenges? Does chemical sensitivity develop because of acute, intermittent, or continuous exposure to certain substances? If so, what substances are most likely to initiate this process? An experimental approach for testing directly the relationship between patients' reported symptoms and specific exposures was outlined in response to the first question, which was felt to be a key question. Double-blind, placebo-controlled challenges performed in an environmentally controlled hospital facility (environmental medical unit) coupled with rigorous documentation of both objective and subjective responses are necessary to answer this question and to help elucidate the nature and origins of chemical sensitivity

A Persistent Disk Wind in GRS 1915+105 with NICER

The bright, erratic black hole X-ray binary GRS 1915+105 has long been a target for studies of disk instabilities, radio/infrared jets, and accretion disk winds, with implications that often apply to sources that do not exhibit its exotic X-ray variability. With the launch of NICER, we have a new opportunity to study the disk wind in GRS 1915+105 and its variability on short and long timescales. Here we present our analysis of 39 NICER observations of GRS 1915+105 collected during five months of the mission data validation and verification phase, focusing on Fe XXV and Fe XXVI absorption. We report the detection of strong Fe XXVI in 32 (>80%) of these observations, with another four marginal detections; Fe XXV is less common, but both likely arise in the well-known disk wind. We explore how the properties of this wind depends on broad characteristics of the X-ray lightcurve: mean count rate, hardness ratio, and fractional RMS variability. The trends with count rate and RMS are consistent with an average wind column density that is fairly steady between observations but varies rapidly with the source on timescales of seconds. The line dependence on spectral hardness echoes known behavior of disk winds in outbursts of Galactic black holes; these results clearly indicate that NICER is a powerful tool for studying black hole winds.Comment: Accepted for publication in ApJL. Comments welcom

Data Recovery at Justiceburg Reservoir (Lake Alan Henry), Garza and Kent Counties, Texas: Phase III, Season 1

Phase III data recovery investigations at one historic and three prehistoric sites, augmented by additional survey and off-site geological investigations, were conducted at Lake Alan Henry (formerly Justiceburg Reservoir) on the Double Mountain Fork of the Brazos River in Garza and Kent counties, Texas, during the fall and winter of 1990-1991. Descriptive data from this first of three field seasons of data recovery are presented here in anticipation of a final synthetic report that will relate all the investigations to a series of research design hypotheses. Analysis of surface distributions at 41KT42, a late nineteenth- to early twentieth-century ranch line camp known locally as the Ed Scott Cabin, groups the artifacts into material and functional categories that demonstrate primary and secondary refuse accumulations adjacent to the cabin. The distribution of primary refuse shows kitchen artifacts are spatially distinct from other types that include architectural, firearm, ranching, clothing, and personal items. The primary discard pattern is obfuscated by redistribution attributed to secondary refuse discard. The uppermost of two superimposed hearths at 41GR484, the Grape Creek Bench Site, is radiocarbon dated to 260 B.P. and yielded charcoal identified as Carya sp. Excavations were halt~d at this Late Prehistoric II site because of extremely low artifact recovery. Geoarcheological investigations demonstrate that most of the site has been destroyed by cutbank erosion associated with the meanderings of Grape Creek. At 41KT33, the Late Prehistoric I South Sage Creek Site, stone-lined hearth features dated to 1005 B.P. are surrounded by clusters of artifacts. Petrographic analysis of brownware ceramics suggests affiliations with the local Palo Duro Complex and nonlocally with the Pecos River valley. The Gobbler Creek Bridge Site, 4IGR383, spans the late Archaic and Late Prehistoric I periods. Intact· and dispersed stone-lined hearths radiocarbon dated to 1865-1215 B.P. are surrounded by artifact clusters. Both of the sites appear to have been multifunctional campsites, and repeated occupations are likely. Dense concentrations of fire-cracked rocks at these sites may represent secondary refuse disposal. Lithic analysis indicates extensive use of nonlocal materials derived from Cretaceous formations that occur in the Callahan Divide and Edwards Plateau to the south. Freshwater mussels are the only faunal remains recovered from either of these sites. The meat is presumed to have been consumed as food, while the shells sometimes were made into ornaments. An additional 440 acres of land were surveyed for cultural resources; 360 of these acres were subsequently acquired by the City of Lubbock for use as wildlife mitigation lands. Nine prehistoric archeological sites and one isolated fmd were documented and evaluated for their eligibility for listing on the National Register of Historic Places. Five of these sites are considered to be ineligible for listing on the National Register, and five are potentially eligible for listing. Avoidance is recommended at these five sites, with testing recommended to determine their National Register eligibility if avoidance is not possible. Geological investigations provide information about depositional environments in and near the archeological sites. Two soils buried in the Double Mountain Fork floodplain are radiocarbon dated to 8~00 B.P. and 1700-1300 B.P.; fluvial sediments extend as much as 15 m below these soils in portions of the floodplain. An older terrace previously was identified 18 m above the modem river channel. Current work obtained a terminal date of 8690 B.P. from a buried soil that caps this older terrace. Another radiocarbon date of 3320 B.P. was obtained from fill in a gully that is eroded into the older terrace. These dates bracket an erosional episode when the older terrace was heavily dissected. The erosion coincides with the Altithermal, a period believed to have been characterized by a hot and dry climatic regime