Local Antiferromagnetic Correlations and dx2−y2d_{x^2-y^2} Pairing

The high $T_c$ cuprate superconductors doped near half-filling have short range antiferromagnetic correlations. Here we describe an intuitive local picture of why, if pairing occurs in the presence of short-range antiferromagnetic correlations, the orbital state will have $d_{x^2-y^2}$ symmetry.Comment: 8 pages and one figur

The electronic Hamiltonian for cuprates

A realistic many-body Hamiltonian for the cuprate superconductors should include both copper d and oxygen p states, hopping matrix elements between them, and Coulomb energies, both on-site and inter-site. We have developed a novel computational scheme for deriving the relevant parameters ab initio from a constrained occupation local density functional. The scheme includes numerical calculation of appropriate Wannier functions for the copper and oxygen states. Explicit parameter values are given for La2CuO4. These parameters are generally consistent with other estimates and with the observed superexchange energy. Secondly, we address whether this complicated multi-band Hamiltonian can be reduced to a simpler one with fewer basis states per unit cell. We propose a mapping onto a new two-band effective Hamiltonian with one copper d and one oxygen p derived state per unit cell. This mapping takes into account the large oxygen-oxygen hopping given by the ab initio calculations

<i>d</i>-wave superconductivity from electron-phonon interactions

I examine electron-phonon mediated superconductivity in the intermediate coupling and phonon frequency regime of the quasi-two-dimensional Holstein model. I use an extended Migdal-Eliashberg theory that includes vertex corrections and spatial fluctuations. I find a d-wave superconducting state that is unique close to half filling. The order parameter undergoes a transition to s-wave superconductivity on increasing filling. I explain how the inclusion of both vertex corrections and spatial fluctuations is essential for the prediction of a d-wave order parameter. I then discuss the effects of a large Coulomb pseudopotential on the superconductivity (such as is found in contemporary superconducting materials like the cuprates), which results in the destruction of the s-wave states, while leaving the d-wave states unmodified

The Effects of d_{x^2-y^2}-d_{xy} Mixing on Vortex Structures and Magnetization

The structure of an isolated single vortex and the vortex lattice, and the magnetization in a $d$-wave superconductor are investigated within a phenomenological Ginzburg-Landau (GL) model including the mixture of the $d_{x^2-y^2}$-wave and $d_{xy}$-wave symmetry. The isolated single vortex structure in a week magnetic field is studied both numerically and asymptotically. Near the upper critical field $H_{c2}$, the vortex lattice structure and the magnetization are calculated analytically.Comment: 14 pages, REVTeX, 2 EPS figures, Journal of Physics: Condensed Matter (in press

Critical current of a Josephson junction containing a conical magnet

We calculate the critical current of a superconductor/ferromagnetic/superconductor (S/FM/S) Josephson junction in which the FM layer has a conical magnetic structure composed of an in-plane rotating antiferromagnetic phase and an out-of-plane ferromagnetic component. In view of the realistic electronic properties and magnetic structures that can be formed when conical magnets such as Ho are grown with a polycrystalline structure in thin-film form by methods such as direct current sputtering and evaporation, we have modeled this situation in the dirty limit with a large magnetic coherence length ($\xi_f$). This means that the electron mean free path is much smaller than the normalized spiral length $\lambda/2\pi$ which in turn is much smaller than $\xi_f$ (with $\lambda$ as the length a complete spiral makes along the growth direction of the FM). In this physically reasonable limit we have employed the linearized Usadel equations: we find that the triplet correlations are short ranged and manifested in the critical current as a rapid oscillation on the scale of $\lambda/2\pi$. These rapid oscillations in the critical current are superimposed on a slower oscillation which is related to the singlet correlations. Both oscillations decay on the scale of $\xi_f$. We derive an analytical solution and also describe a computational method for obtaining the critical current as a function of the conical magnetic layer thickness.Comment: Extended version of the published paper. Additional information about the computational method is included in the appendi

Hierarchical task analysis: Developments, applications and extensions

Hierarchical Task Analysis (HTA) is a core ergonomics approach with pedigree of over thirty years continuous use. At its heart, HTA is based upon a theory of performance and has only three governing principles. Originally developed as a means of determining training requirements, there was no way the initial pioneers of HTA could have foreseen the extent of its success. HTA has endured as a way of representing a system sub-goal hierarchy for extended analysis. It has been used for a range of applications, including interface design and evaluation, allocation of function, job aid design, error prediction, and workload assessment. Ergonomists are still developing new ways of using HTA which has assured the continued use of the approach for the foreseeable future

Patterns of Aggression in Gulls: Asymmetries and Tactics in Different Social Categories

During contests in Western Gulls, Larus occidentalis, we examined patterns of aggression in relation to sex, age class, and territorial status. Data were collected on agonistic contests during parts of three reproductive seasons on Southeast Farallon Island. This colony has dense nesting territories and appears to be near saturation, making breeding space a limiting and valuable resource. Established, territory-holding male gulls were able to defeat nearly all opponents. The exceptions were a few unestablished adult males early in the reproductive period that appeared to be equal, or superior to, established males in fighting ability and persistence. Established females were able to defend their territories against territory-holding neighbors, immature gulls, and adult female intruders, but often were forced to retreat by unestablished male intruders. Both immature and unestablished adult intruders more readily challenged an established female than an established male. The value of breeding territories, the primary contested resource, appeared to increase throughout the reproductive period as established birds became increasingly inclined to escalate encounters after eggs were laid, and even more inclined to escalate after chicks appeared. In contrast, unestablished adults became less common and, when present, less inclined to escalate as the season progressed

Hybridization and Male Parental Investment in Birds

Hybridization within genera occurs more frequently in avian families and subfamilies where there is considerable male parental investment, less frequently in families with moderate levels of male parental investment, and rarely in lineages where males contribute only genetic material to their offspring. In addition, genera that show considerable male parental investment are typically less speciose than genera where there is reduced male parental investment. Species showing high levels of male parental investment, however, typically have more subspecies, indicating that local adaptation evolves in these groups, but reproductive isolation does not evolve. Some hybrid matings appear to involve females of one species showing an apparent preference for mating with larger or dominant males, even if these males are heterospecific (e.g., black and mallard ducks). Similar patterns occur in fishes, amphibians, and mammals, the other three vertebrate lineages that show extensive parental care. Hybridization in birds may be an evolutionary mechanism that allows increased genetic diversity and adaptability under changing environmental conditions, particularly environments disturbed by humans. It is also possible that new forms (incipient species) may arise through hybridization that are better adapted to disturbed environments than either parental species

System-Integrated Finite Element Analysis of a Full-Scale Helicopter Crash Test with Deployable Energy Absorbers

A full-scale crash test of an MD-500 helicopter was conducted in December 2009 at NASA Langley's Landing and Impact Research facility (LandIR). The MD-500 helicopter was fitted with a composite honeycomb Deployable Energy Absorber (DEA) and tested under vertical and horizontal impact velocities of 26-ft/sec and 40-ft/sec, respectively. The objectives of the test were to evaluate the performance of the DEA concept under realistic crash conditions and to generate test data for validation of a system integrated finite element model. In preparation for the full-scale crash test, a series of sub-scale and MD-500 mass simulator tests was conducted to evaluate the impact performances of various components, including a new crush tube and the DEA blocks. Parameters defined within the system integrated finite element model were determined from these tests. The objective of this paper is to summarize the finite element models developed and analyses performed, beginning with pre-test predictions and continuing through post-test validation