434 research outputs found
Mars oxygen production system design
The design and construction phase is summarized of the Mars oxygen demonstration project. The basic hardware required to produce oxygen from simulated Mars atmosphere was assembled and tested. Some design problems still remain with the sample collection and storage system. In addition, design and development of computer compatible data acquisition and control instrumentation is ongoing
Average stresses and force fluctuations in non-cohesive granular materials
A lattice model is presented for investigating the fluctuations in static
granular materials under gravitationally induced stress. The model is similar
in spirit to the scalar q-model of Coppersmith et al., but ensures balance of
all components of forces and torques at each site. The geometric randomness in
real granular materials is modeled by choosing random variables at each site,
consistent with the assumption of cohesionless grains. Configurations of the
model can be generated rapidly, allowing the statistical study of relatively
large systems. For a 2D system with rough walls, the model generates
configurations consistent with continuum theories for the average stresses
(unlike the q-model) without requiring the assumption of a constitutive
relation. For a 2D system with periodic boundary conditions, the model
generates single-grain force distributions similar to those obtained from the
q-model with a singular distribution of q's.Comment: 18 pages, 10 figures. Uses aps,epsfig,graphicx,floats,revte
Quantum Collective Creep: a Quasiclassical Langevin Equation Approach
The dynamics of an elastic medium driven through a random medium by a small
applied force is investigated in the low-temperature limit where quantum
fluctuations dominate. The motion proceeds via tunneling of segments of the
manifold through barriers whose size grows with decreasing driving force .
In the limit of small drive, at zero-temperature the average velocity has the
form . For strongly
dissipative dynamics, there is a wide range of forces where the dissipation
dominates and the velocity--force characteristics takes the form
, with the
action for a typical tunneling event, the force dependence being determined by
the roughness exponent of the -dimensional manifold. This result
agrees with the one obtained via simple scaling considerations. Surprisingly,
for asymptotically low forces or for the case when the massive dynamics is
dominant, the resulting quantum creep law is {\it not} of the usual form with a
rate proportional to ; rather we find corresponding to and , with the naive scaling exponent for massive
dynamics. Our analysis is based on the quasi-classical Langevin approximation
with a noise obeying the quantum fluctuation--dissipation theorem. The many
space and time scales involved in the dynamics are treated via a functional
renormalization group analysis related to that used previously to treat the
classical dynamics of such systems. Various potential difficulties with these
approaches to the multi-scale dynamics -- both classical and quantum -- are
raised and questions about the validity of the results are discussed.Comment: RevTeX, 30 pages, 8 figures inserte
Quantizing Charged Magnetic Domain Walls: Strings on a Lattice
The discovery by Tranquada et al. of an ordered phase of charged domain walls
in the high-Tc cuprates leads us to consider the possible existence of a
quantum domain-wall liquid. We propose minimal models for the quantization, by
meandering fluctuations, of isolated charged domain walls. These correspond to
lattice string models. The simplest model of this kind, a directed lattice
string, can be mapped onto a quantum spin chain or on a classical
two-dimensional solid-on-solid surface model. The model exhibits a rich phase
diagram, containing several rough phases with low-lying excitations as well as
ordered phases which are gapped.Comment: 4 two-column pages, including the 3 Postscript figure
Reentrant Melting of Soliton Lattice Phase in Bilayer Quantum Hall System
At large parallel magnetic field , the ground state of bilayer
quantum Hall system forms uniform soliton lattice phase. The soliton lattice
will melt due to the proliferation of unbound dislocations at certain finite
temperature leading to the Kosterlitz-Thouless (KT) melting. We calculate the
KT phase boundary by numerically solving the newly developed set of Bethe
ansatz equations, which fully take into account the thermal fluctuations of
soliton walls. We predict that within certain ranges of , the
soliton lattice will melt at . Interestingly enough, as temperature
decreases, it melts at certain temperature lower than exhibiting
the reentrant behaviour of the soliton liquid phase.Comment: 11 pages, 2 figure
Phase transitions in the antiferromagnetic XY model with a kagome lattice
The ground state of the antiferromagnetic XY model with a kagome lattice is
characterized by a well developed accidental degeneracy. As a consequence the
phase transition in this system consists in unbinding of pairs of fractional
vortices. Addition of the next-to-nearest neighbors (NNN) interaction leads to
stabilization of the long-range order in chirality (staggered chirality). We
show that the phase transition, related with destruction of this long-range
order, can happen as a separate phase transition below the temperature of the
fractional vortex pairs unbinding only if the NNN coupling is extremely weak,
and find how the temperature of this transition depends on coupling constants.
We also demonstarte that the antiferromagnetic ordering of chiralities and,
accordingly, the presence of the second phase transition are induced by the
free energy of spin wave fluctuations even in absence of the NNN coupling.Comment: 10 pages (Revtex) + 8 figures (in 2 postscript files
On defining the Hamiltonian beyond quantum theory
Energy is a crucial concept within classical and quantum physics. An
essential tool to quantify energy is the Hamiltonian. Here, we consider how to
define a Hamiltonian in general probabilistic theories, a framework in which
quantum theory is a special case. We list desiderata which the definition
should meet. For 3-dimensional systems, we provide a fully-defined recipe which
satisfies these desiderata. We discuss the higher dimensional case where some
freedom of choice is left remaining. We apply the definition to example toy
theories, and discuss how the quantum notion of time evolution as a phase
between energy eigenstates generalises to other theories.Comment: Authors' accepted manuscript for inclusion in the Foundations of
Physics topical collection on Foundational Aspects of Quantum Informatio
Intensivist supervision of resident-placed central venous catheters decreases the incidence of catheter-related blood stream infections
Catheter-related blood stream infections (CRBSI) cause significant morbidity and mortality. A retrospective study of a performance improvement project in our teaching hospital's surgical intensive care unit (SICU) showed that intensivist supervision was important in reinforcing maximal sterile barriers (MSB) use during the placement of a central venous catheter (CVC) in the prevention of CRBSI. A historical control period, 1 January 2001–31 December 2003, was established for comparison. From 1 January 2003–31 December 2007, MSB use for central venous line placement was mandated for all operators. However, in 2003 there was no intensivist supervision of CVC placements in the SICU. The use of MSB alone did not cause a significant change in the CRBSI rate in the first year of the project, but close supervision by an intensivist in years 2004–2007, in conjunction with MSB use, demonstrated a significant drop in the CRBSI rate when compared to the years before intensivist supervision (2001–2003), p < .0001. A time series analysis comparing monthly rates of CRBSI (2001–2007) also revealed a significant downward trend, p = .028. Additionally, in the first year of the mandated MSB use (2003), 85 independently observed resident-placed CVCs demonstrated that breaks in sterile technique (34/85), as compared those placements that had no breaks in technique (51/85), had more CRBSI, 6/34 (17.6%) vs. 1/51 (1.9%), p < .01. Interventions to reduce CRBSI in our SICU needed emphasis on adequate supervision of trainees in CVC placement, in addition to use of MSB, to effect lower CRBSI rates
Spanish Teachers\u27 Sense of Humor and Student Performance on the National Spanish Exams
Research suggests that second/foreign language teachers\u27 sense of humor is directly related to many outcomes for teachers and their students. This research investigates the relationship between the perceived sense of humor of in-service Spanish teachers\u27 (n = 102) and their students\u27 (n = 5,419) score on the National Spanish Exams using the affective filter hypothesis as a conceptual framework. Statistical analyses indicate that Spanish teacher sense of humor is related to student achievement on the exams. This research has implications for language teachers and teacher educators
GCOM-W AMSR2 Soil Moisture Product Validation Using Core Validation Sites
The Advanced Microwave Scanning Radiometer 2 (AMSR2) is part of the Global Change Observation Mission-Water (GCOM-W). AMSR2 has filled the gap in passive microwave observations left by the loss of the Advanced Microwave Scanning RadiometerEarth Observing System (AMSR-E) after almost 10 years of observations. Both missions provide brightness temperature observations that are used to retrieve soil moisture estimates at the near surface. A merged AMSR-E and AMSR2 data product will help build a consistent long-term dataset; however, before this can be done, it is necessary to conduct a thorough validation and assessment of the AMSR2 soil moisture products. This study focuses on the validation of the AMSR2 soil moisture products by comparison with in situ reference data from a set of core validation sites around the world. A total of three soil moisture products that rely on different algorithms were evaluated; the Japan Aerospace Exploration Agency (JAXA) soil moisture algorithm, the Land Parameter Retrieval Model (LPRM), and the Single Channel Algorithm (SCA). JAXA, SCA and LPRM soil moisture estimates capture the overall climatological features. The spatial features of the three products have similar overall spatial structure. The JAXA soil moisture product shows a lower dynamic range in the retrieved soil moisture with a satisfactory performance matrix when compared to in situ observations (ubRMSE0.059 m3m3, Bias-0.083 m3m3, R0.465). The SCA performs well over low and moderately vegetated areas (ubRMSE0.053 m3m3, Bias-0.039 m3m3, R0.549). The LPRM product has a large dynamic range compared to in situ observations with a wet bias (ubRMSE0.094 m3m3, Bias0.091 m3m3, R0.577). Some of the error is due to the difference in observation depth between the in situ sensors (5 cm) and satellite estimates (1 cm). Results indicate that overall the JAXA and SCA have the best performance based upon the metrics considered
- …