Passenger ride comfort technology for transport aircraft situations

A brief overview is given of NASA research in ride comfort and of the resultant technology. Three useful relations derived from the technology are presented together with five applications of these relations to illustrate their effectiveness in addressing various ride comfort situations of passenger transports

Exploring the high-pressure materials genome

A thorough in situ characterization of materials at extreme conditions is challenging, and computational tools such as crystal structural search methods in combination with ab initio calculations are widely used to guide experiments by predicting the composition, structure, and properties of high-pressure compounds. However, such techniques are usually computationally expensive and not suitable for large-scale combinatorial exploration. On the other hand, data-driven computational approaches using large materials databases are useful for the analysis of energetics and stability of hundreds of thousands of compounds, but their utility for materials discovery is largely limited to idealized conditions of zero temperature and pressure. Here, we present a novel framework combining the two computational approaches, using a simple linear approximation to the enthalpy of a compound in conjunction with ambient-conditions data currently available in high-throughput databases of calculated materials properties. We demonstrate its utility by explaining the occurrence of phases in nature that are not ground states at ambient conditions and estimating the pressures at which such ambient-metastable phases become thermodynamically accessible, as well as guiding the exploration of ambient-immiscible binary systems via sophisticated structural search methods to discover new stable high-pressure phases.Comment: 14 pages, 6 figure

Critical behavior of loops and biconnected clusters on fractals of dimension d < 2

We solve the O(n) model, defined in terms of self- and mutually avoiding loops coexisting with voids, on a 3-simplex fractal lattice, using an exact real space renormalization group technique. As the density of voids is decreased, the model shows a critical point, and for even lower densities of voids, there is a dense phase showing power-law correlations, with critical exponents that depend on n, but are independent of density. At n=-2 on the dilute branch, a trivalent vertex defect acts as a marginal perturbation. We define a model of biconnected clusters which allows for a finite density of such vertices. As n is varied, we get a line of critical points of this generalized model, emanating from the point of marginality in the original loop model. We also study another perturbation of adding local bending rigidity to the loop model, and find that it does not affect the universality class.Comment: 14 pages,10 figure

Monte Carlo Study of Short-Range Order and Displacement Effects in Disordered CuAu

The correlation between local chemical environment and atomic displacements in disordered CuAu alloy has been studied using Monte Carlo simulations based on the effective medium theory (EMT) of metallic cohesion. These simulations correctly reproduce the chemically-specific nearest-neighbor distances in the random alloy across the entire Cu\$_x\$Au\$_{1-x}\$ concentration range. In the random equiatomic CuAu alloy, the chemically specific pair distances depend strongly on the local atomic environment (i.e. fraction of like/unlike nearest neighbors). In CuAu alloy with short-range order, the relationship between local environment and displacements remains qualitatively similar. However the increase in short-range order causes the average Cu-Au distance to decrease below the average Cu-Cu distance, as it does in the ordered CuAuI phase. Many of these trends can be understood qualitatively from the different neutral sphere radii and compressibilities of the Cu and Au atoms.Comment: 9 pages, 5 figures, 2 table

Stretching dependence of the vibration modes of a single-molecule Pt-H2-Pt bridge

A conducting bridge of a single hydrogen molecule between Pt electrodes is formed in a break junction experiment. It has a conductance near the quantum unit, G_0 = 2e^2/h, carried by a single channel. Using point contact spectroscopy three vibration modes are observed and their variation upon stretching and isotope substitution is obtained. The interpretation of the experiment in terms of a Pt-H_2-Pt bridge is verified by Density Functional Theory calculations for the stability, vibrational modes, and conductance of the structure.Comment: 5 pages, 4 figure

Bulk, surface and corner free energy series for the chromatic polynomial on the square and triangular lattices

We present an efficient algorithm for computing the partition function of the q-colouring problem (chromatic polynomial) on regular two-dimensional lattice strips. Our construction involves writing the transfer matrix as a product of sparse matrices, each of dimension ~ 3^m, where m is the number of lattice spacings across the strip. As a specific application, we obtain the large-q series of the bulk, surface and corner free energies of the chromatic polynomial. This extends the existing series for the square lattice by 32 terms, to order q^{-79}. On the triangular lattice, we verify Baxter's analytical expression for the bulk free energy (to order q^{-40}), and we are able to conjecture exact product formulae for the surface and corner free energies.Comment: 17 pages. Version 2: added 4 further term to the serie

Neutron scattering study of spin ordering and stripe pinning in superconducting La1.93_{1.93}Sr0.07_{0.07}CuO4_4

The relationships among charge order, spin fluctuations, and superconductivity in underdoped cuprates remain controversial. We use neutron scattering techniques to study these phenomena in La$_{1.93}$Sr$_{0.07}$CuO$_4$, a superconductor with a transition temperature of $T_c = 20$~K. At $T\ll T_c$, we find incommensurate spin fluctuations with a quasielastic energy spectrum and no sign of a gap within the energy range from 0.2 to 15 meV. A weak elastic magnetic component grows below $\sim10$~K, consistent with results from local probes. Regarding the atomic lattice, we have discovered unexpectedly strong fluctuations of the CuO$_6$ octahedra about Cu-O bonds, which are associated with inequivalent O sites within the CuO$_2$ planes. Furthermore, we observed a weak elastic $(3\bar{3}0)$ superlattice peak that implies a reduced lattice symmetry. The presence of inequivalent O sites rationalizes various pieces of evidence for charge stripe order in underdoped \lsco. The coexistence of superconductivity with quasi-static spin-stripe order suggests the presence of intertwined orders; however, the rotation of the stripe orientation away from the Cu-O bonds might be connected with evidence for a finite gap at the nodal points of the superconducting gap function.Comment: 13 pages, 11 figures; accepted versio

Oxygen Isotope Measurements of a Rare Murchison Type A CAI and Its Rim

Ca-, Al-rich inclusions (CAIs) from CV chondrites commonly show oxygen isotope heterogeneity among different mineral phases within individual inclusions reflecting the complex history of CAIs in both the solar nebula and/or parent bodies. The degree of isotopic exchange is typically mineral-specific, yielding O-16-rich spinel, hibonite and pyroxene and O-16-depleted melilite and anorthite. Recent work demonstrated large and systematic variations in oxygen isotope composition within the margin and Wark-Lovering rim of an Allende Type A CAI. These variations suggest that some CV CAIs formed from several oxygen reservoirs and may reflect transport between distinct regions of the solar nebula or varying gas composition near the proto-Sun. Oxygen isotope compositions of CAIs from other, less-altered chondrites show less intra-CAI variability and 16O-rich compositions. The record of intra-CAI oxygen isotope variability in CM chondrites, which commonly show evidence for low-temperature aqueous alteration, is less clear, in part because the most common CAIs found in CM chondrites are mineralogically simple (hibonite +/- spinel or spinel +/- pyroxene) and are composed of minerals less susceptible to O-isotopic exchange. No measurements of the oxygen isotope compositions of rims on CAIs in CM chondrites have been reported. Here, we present oxygen isotope data from a rare, Type A CAI from the Murchison meteorite, MUM-1. The data were collected from melilite, hibonite, perovskite and spinel in a traverse into the interior of the CAI and from pyroxene, melilite, anorthite, and spinel in the Wark-Lovering rim. Our objectives were to (1) document any evidence for intra-CAI oxygen isotope variability; (2) determine the isotopic composition of the rim minerals and compare their composition(s) to the CAI interior; and (3) compare the MUM-1 data to oxygen isotope zoning profiles measured from CAIs in other chondrites

Mechanical properties and formation mechanisms of a wire of single gold atoms

A scanning tunneling microscope (STM) supplemented with a force sensor is used to study the mechanical properties of a novel metallic nanostructure: a freely suspended chain of single gold atoms. We find that the bond strength of the nanowire is about twice that of a bulk metallic bond. We perform ab initio calculations of the force at chain fracture and compare quantitatively with experimental measurements. The observed mechanical failure and nanoelastic processes involved during atomic wire fabrication are investigated using molecular dynamics (MD) simulations, and we find that the total effective stiffness of the nanostructure is strongly affected by the detailed local atomic arrangement at the chain bases.Comment: To be published in Phys. Rev. Lett. 4 pages with 3 figure