Symmetry of Magnetically Ordered Quasicrystals

The notion of magnetic symmetry is reexamined in light of the recent observation of long range magnetic order in icosahedral quasicrystals [Charrier et al., Phys. Rev. Lett. 78, 4637 (1997)]. The relation between the symmetry of a magnetically-ordered (periodic or quasiperiodic) crystal, given in terms of a ``spin space group,'' and its neutron diffraction diagram is established. In doing so, an outline of a symmetry classification scheme for magnetically ordered quasiperiodic crystals is provided. Predictions are given for the expected diffraction patterns of magnetically ordered icosahedral crystals, provided their symmetry is well described by icosahedral spin space groups.Comment: 5 pages. Accepted for publication in Phys. Rev. Letter

Application of Face-Gear Drives in Helicopter Transmissions

The use of face gears in helicopter transmissions was explored. A light-weight, split torque transmission design utilizing face gears was described. Face-gear design and geometry were investigated. Topics included tooth generation, limiting inner and outer radii, tooth contact analysis, contact ratio, gear eccentricity, and structural stiffness. Design charts were developed to determine minimum and maximum face-gear inner and outer radii. Analytical study of transmission error showed face-gear drives were relatively insensitive to gear misalignment, but tooth contact was affected by misalignment. A method of localizing bearing contact to compensate for misalignment was explored. The proper choice of shaft support stiffness enabled good load sharing in the split torque transmission design. Face-gear experimental studies were also included and the feasibility of face gears in high-speed, high-load applications such as helicopter transmissions was demonstrated

Face-gear drives: Design, analysis, and testing for helicopter transmission applications

The use of face-gears in helicopter transmissions was explored. A light-weight, split-torque transmission design utilizing face-gears is described. Face-gear design and geometry were investigated. Topics included tooth generation, limiting inner and outer radii, tooth contact analysis, contact ratio, gear eccentricity, grinding, and structural stiffness. Design charts were developed to determine minimum and maximum face-gear inner and outer radii. An analytical study showed that the face-gear drive is relatively insensitive to gear misalignment with respect to transmission errors, but the tooth contact is affected by misalignment. A method of localizing the bearing contact to permit operation with misalignment was explored. Two new methods for grinding of the face-gear tooth surfaces were also investigated. The proper choice of shaft stiffness enabled good load sharing in the split-torque transmission design. Face-gear experimental studies were also conducted. These tests demonstrated the feasibility of face-gears in high-speed, high-load applications such as helicopter transmissions

Domain geometry engineering and domain average engineering in ferroics

Abstract Multidomain samples of ferroics (ferroelectrics, ferroelastics, and related materials) with fixed geometrical distribution of domains can offer new macroscopic properties required for particular applications. Two extreme cases of such applications are defined. In domain-geometry-engineered samples of ferroic crystals, the spatial distribution of domains and thus the spatial distribution of tensorial properties is tuned to correspond to the k-vectors of applied electric, optical or acoustic fields. For a given wavelength, the size, geometry, and distribution of domains give rise to a qualitatively new kind of response specified by the symmetry of the multidomain system. In domainaverage-engineered samples of ferroic crystals, the specimen is subdivided into a very large number of domains, representing µ domain states where µ is smaller than the theoretically allowed maximum number, and forming a regular or irregular pattern. Its response to external fields is roughly described by tensorial properties averaged over all of the domain states involved. The effective symmetry of the domain-average-engineered system is given by a point group H and we show how it can be determined. As an example, all groups H are specified for domain-average-engineered samples which can arise in a material undergoing the phase transition with symmetry change from m3m to 3m. Ferroic materials (and here we concentrate on non-magnetic materials, i.e. on ferroelectrics, ferroelastics, and higher-order ferroics) play an essential role in a number of technical applications. In some of them, dynamic domain processes are essential (e.g. thin-film memories, electron emitters) while in others the static distribution domains in the sample play the crucial role. In this contribution we concentrate on the latter case and wish to specify a clear distinction between two kinds of such static multidomain system. We have in mind materials undergoing a structural phase transition from the parent phase of point group G into the ferroic phase of symmetry F ⊂ G (such a material is referred t

Keeping creative writing on track: Co-designing a framework to support behavior change

The application of persuasive technology in web-based and mobile phone-based systems is well established, particularly in the health domain. However, a greater understanding of the effectiveness of the techniques deployed is needed to facilitate the successful transfer of research findings into practical applications. The context explored here is that of creative writing and the potential use of persuasive technology to foster and support a productive writing routine. Employing a user-centered design approach, we conducted surveys and a co-creation workshop with writers. Goal setting and regular writing, combined with self-monitoring, were key indicators of an effective writing practice. Group and mentor support were also highlighted. Based on our findings, we developed the architecture for a mobile personal writing coach. We evaluated the architecture against existing frameworks, finding good congruence. This supports our long-term goal of creating a universal framework, applicable to a wider range of behavior change interventions, domains and users. The design considerations reported in this paper go some way towards that goal

Complex magnetic ordering in the oxide selenide Sr2Fe3Se2O3

Sr2Fe3Se2O3 is a localised-moment iron oxide selenide in which two unusual coordinations for Fe2+ ions form two sublattices in a 2:1 ratio. In the paramagnetic region at room temperature the compound adopts the crystal structure first reported for Sr2Co3S2O3, crystallising in space group Pbam with a = 7.8121 Å, b = 10.2375 Å, c = 3.9939 Å and Z = 2. The sublattice occupied by two thirds of the iron ions (Fe2 site) is formed by a network of distorted mer-[FeSe3O3] octahedra linked via shared Se2 edges and O vertices forming layers, which connect to other layers by shared Se vertices. As shown by magnetometry, neutron powder diffraction and Mössbauer spectroscopy measurements, these moments undergo long range magnetic ordering below TN1 = 118 K, initially adopting a magnetic structure with a propagation vector (½–δ, 0, ½) (0 ≤ ≤ 0.1) which is incommensurate with the nuclear structure and described in the Pbam1’(a01/2)000s magnetic superspace group, until at 92 K (TINC) there is a first order lock-in transition to a structure in which these Fe2 moments form a magnetic structure with a propagation vector (½ , 0, ½) which may be modelled using a 2a × b × 2c expansion of the nuclear cell in space group 36.178 Bab21m (BNS notation). Below TN2 = 52 K the remaining third of the Fe2+ moments (Fe1 site) which are in a compressed trans-[FeSe4O2] octahedral environment undergo long range ordering, as is evident from the magnetometry, the Mössbauer spectra and the appearance of new magnetic Bragg peaks in the neutron diffractograms. The ordering of the second set of moments on the Fe1 sites results in a slight re-orientation of the majority moments on the Fe2 sites. The magnetic structure at 1.5 K is described by a 2a × 2b × 2c expansion of the nuclear cell in space group 9.40 Iab (BNS notation)

Effect of biologically active additives based on the homogenate of drone larvae on microcirculation and metabolism in Nordic skiers

Objective: to study the effect of biologically active additives (BAA) based on the homogenate of drone larvae on microcirculation and metabolism in Nordic skiers. Materials and methods: 58 male skiers at the age of 18-20 years ranging from 1 category to CMS were divided into two groups: the main group (30 athletes) and the comparison group (28 athletes). The skiers of the main group used the homogenate of drone larvae daily at a dose of 10 mg/kg of body weight during 21 days. Athletes from the comparison group received potato starch (placebo). The study of the microcirculation system was performed with a laser analyzer of the capillary blood flow «LAKK-M» before the BAA administration, at the 1st, 10th and 21st days of the study and 30 days after the end of the BAA administration. Results: A significant increase in perfusion was observed in the experimental group (121%,

New Symmetries in Crystals and Handed Structures

For over a century, the structure of materials has been described by a combination of rotations, rotation-inversions and translational symmetries. By recognizing the reversal of static structural rotations between clockwise and counterclockwise directions as a distinct symmetry operation, here we show that there are many more structural symmetries than are currently recognized in right- or left-handed handed helices, spirals, and in antidistorted structures composed equally of rotations of both handedness. For example, though a helix or spiral cannot possess conventional mirror or inversion symmetries, they can possess them in combination with the rotation reversal symmetry. Similarly, we show that many antidistorted perovskites possess twice the number of symmetry elements as conventionally identified. These new symmetries predict new forms for "roto" properties that relate to static rotations, such as rotoelectricity, piezorotation, and rotomagnetism. They also enable symmetry-based search for new phenomena, such as multiferroicity involving a coupling of spins, electric polarization and static rotations. This work is relevant to structure-property relationships in all material structures with static rotations such as minerals, polymers, proteins, and engineered structures.Comment: 15 Pages, 4 figures, 3 Tables; Fig. 2b has error

ART-XC: A Medium-energy X-ray Telescope System for the Spectrum-R-Gamma Mission

The ART-XC instrument is an X-ray grazing-incidence telescope system in an ABRIXAS-type optical configuration optimized for the survey observational mode of the Spectrum-RG astrophysical mission which is scheduled to be launched in 2011. ART-XC has two units, each equipped with four identical X-ray multi-shell mirror modules. The optical axes of the individual mirror modules are not parallel but are separated by several degrees to permit the four modules to share a single CCD focal plane detector, 1/4 of the area each. The 450-micron-thick pnCCD (similar to the adjacent eROSITA telescope detector) will allow detection of X-ray photons up to 15 keV. The field of view of the individual mirror module is about 18 x 18 arcminutes(exp 2) and the sensitivity of the ART-XC system for 4 years of survey will be better than 10(exp -12) erg s(exp -1) cm(exp -2) over the 4-12 keV energy band. This will allow the ART-XC instrument to discover several thousand new AGNs

Symmetric, gated, ballistic rings as tunable electron interferometers

Abstract In the present work we investigate the coherent electron transport in a symmetrically designed ballistic ring uniformly covered by a top metal gate. We ÿnd that as the Fermi energy is varied, the phase of Aharonov-Bohm (AB) oscillations near zero magnetic ÿeld switches between 0 and . It seems unlikely that this behaviour can be explained by some accidental asymmetry in the structures. We give a qualitative explanation of our results using a model where the ring is considered to be weakly coupled to the leads and the conductance is calculated on the basis of an exact energy spectrum of an ideal ring. This model predicts that a variation of the phase of AB oscillations with gate voltage may be observed in a symmetrical ring. ? 2000 Elsevier Science B.V. All rights reserved. Ever since the start of the mesoscopic physics, the properties of quantum ring interferometers have been intensively studie