Ground State Phase Diagram of Generic XY Pyrochlore Magnets with Quantum Fluctuations

Motivated by recent experimental and theoretical progress on the Er2Ti2O7 pyrochlore XY antiferromagnet, we study the problem of quantum order-by-disorder in pyrochlore XY systems. We consider the most general nearest-neighbor pseudo spin-1/2 Hamiltonian for such a system characterized by anisotropic spin-spin couplings J_e = [J_\pm, J_{\pm\pm}, J_{z\pm}, J_{zz}] and construct zero-temperature phase diagrams. Combining symmetry arguments and spin-wave calculations, we show that the ground state phase boundaries between the two candidate ground states of the \Gamma_5 irreducible representation, the \psi_2 and \psi_3 (basis) states, are rather accurately determined by a cubic equation in J_{\pm}J_{\pm\pm})/J_{z\pm}^2. Depending on the value of J_{zz}, there can be one or three phase boundaries that separate alternating regions of \psi_2 and \psi_3 states. In particular, we find for sufficiently small J_{zz}/J_{\pm} a narrow \psi_2 sliver sandwiched between two \psi_3 regions in the J_{\pm\pm}/J_\pm vs J_{z\pm}/J_\pm phase diagram. Our results further illustrate the very large potential sensitivity of the ground state of XY pyrochlore systems to minute changes in their spin Hamiltonian. Using the experimentally determined J_3 and g-tensor values for Er2Ti2O7, we show that the heretofore neglected long-range 1/r^3 magnetostatic dipole-dipole interactions do not change the conclusion that Er2Ti2O7 has a \psi_2 ground state induced via a quantum order-by-disorder mechanism. We propose that the CdDy2Se4 chalcogenide spinel, in which the Dy^{3+} ions form a pyrochlore lattice and may be XY-like, could prove interesting to investigate.Comment: 8 pages, 3 figures. Version is the same as the published one, within epsilo

Examining the validity and reliability of the Chinese version of the International Physical Activity Questionnaire, long form (IPAQ-LC)

Objective : To investigate the reliability and the validity of the long format, Chinese version of the International Physical Activity Questionnaire (IPAQ-LC). Design : Cross-sectional study, examining the reliability and validity of the IPAQ-LC compared with a physical activity log (PA-log) and objective accelerometry. Setting : Self-reported physical activity (PA) in Hong Kong adults. Subjects : A total of eighty-three Chinese adults (forty-seven males, thirty-six females) were asked to wear an ActiTrainer accelerometer (MTI-ActiGraph, Fort Walton Beach, FL, USA) for >10 h over 7 d, to complete a PA-log at the end of each day and to complete the IPAQ-LC on day 8. On a sub-sample of twenty-eight adults the IPAQ-LC was also administered on day 11 to assess its reliability. Results : The IPAQ-LC had good test–retest reliability for grouped activities, with intra-class correlation coefficients ranging from 0·74 to 0·97 for vigorous, moderate, walking and total PA, with between-test effect sizes that were small (<0·49). The Spearman correlation coefficients were statistically significant for vigorous PA (r = 0·28), moderate + walking PA (r = 0·27), as well as overall PA (r = 0·35), when compared with the accelerometry-based criterion measures, but none of the IPAQ activity categories correlated significantly with the PA-log. In absolute units, only the IPAQ light and overall PA did not differ significantly from the accelerometry measures, yet overall PA was able to faithfully discriminate between quartiles of PA (P = 0·019) when compared to accelerometry. Conclusions : The IPAQ-LC demonstrated adequate reliability and showed sufficient evidence of validity in assessing overall levels of habitual PA to be used on Hong Kong adults

Application of data warehouse and Decision Support System in construction management

Author name used in this publication: K. W. Chau2002-2003 > Academic research: refereed > Publication in refereed journalAccepted ManuscriptPublishe

Energy Efficiency Improvements in Dry Drilling with Optimised Diamond-Like Carbon Coating

We demonstrate enhancements of performance and energy efficiency of cutting tools by deposition of diamond-like carbon (DLC) coatings on machine parts. DLC was deposited on steel drill bits, using plasma enhanced chemical vapour deposition (PECVD) with the acetylene precursor diluted with argon, to produce a surface with low friction and low wear rate. Drill bit performance in dry drilling of aluminium was quantified by analysis of power consumption and swarf flow. Optimised deposition conditions produced drill bits with greatly enhanced performance over uncoated drill bits, showing a 25% reduction in swarf clogging, a 36% reduction in power consumption and a greater than five-fold increase in lifetime. Surface analysis with scanning electron microscopy shows that DLC coated drills exhibit much lower aluminium build up on the trailing shank of the drill, enhancing the anti-adhering properties of the drill and reducing heat generation during operation, resulting in the observed improvements in efficiency. Variation of drilling efficiency with argon dilution of precursor is related to changes in the microstructure of the DLC coating

Virialization of high redshift dark matter haloes

We present results of a study of the virial state of high redshift dark matter haloes in an N-body simulation. We find that the majority of collapsed, bound haloes are not virialized at any redshift slice in our study ($z=15-6$) and have excess kinetic energy. At these redshifts, merging is still rampant and the haloes cannot strictly be treated as isolated systems. To assess if this excess kinetic energy arises from the environment, we include the surface pressure term in the virial equation explicitly and relax the assumption that the density at the halo boundary is zero. Upon inclusion of the surface term, we find that the haloes are much closer to virialization, however, they still have some excess kinetic energy. We report trends of the virial ratio including the extra surface term with three key halo properties: spin, environment, and concentration. We find that haloes with closer neighbors depart more from virialization, and that haloes with larger spin parameters do as well. We conclude that except at the lowest masses (M < 10^6 \Msun), dark matter haloes at high redshift are not fully virialized. This finding has interesting implications for galaxy formation at these high redshifts, as the excess kinetic energy will impact the subsequent collapse of baryons and the formation of the first disks and/or baryonic structures.Comment: 5 pages, Accepted to MNRA

Brownian molecular motors driven by rotation-translation coupling

We investigated three models of Brownian motors which convert rotational diffusion into directed translational motion by switching on and off a potential. In the first model a spatially asymmetric potential generates directed translational motion by rectifying rotational diffusion. It behaves much like a conventional flashing ratchet. The second model utilizes both rotational diffusion and drift to generate translational motion without spatial asymmetry in the potential. This second model can be driven by a combination of a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven) depending on the chosen parameters. In the third model, elements of both the Brownian motor and powerstroke mechanisms are combined by switching between three distinct states. Relevance of the model to biological motor proteins is discussed.Comment: 11 pages, 8 figure

Cryptosporidium cuniculus - new records in human and kangaroo in Australia

BACKGROUND: To date, Cryptosporidium cuniculus has been found exclusively in rabbits and humans. The present study provides the first published molecular evidence for C. cuniculus in an Australian human patient as well as a kangaroo. FINDINGS: Using PCR-based sequencing of regions in the actin, 60 kDa glycoprotein (gp60) and small subunit of ribosomal RNA (SSU) genes, we identified a new and unique C. cuniculus genotype (akin to VbA25) from a human, and C. cuniculus genotype VbA26 from an Eastern grey kangaroo (Macropus giganteus) in Australia. CONCLUSIONS: The characterisation of these genotypes raises questions as to their potential to infect humans and/or other animals in Australia, given that C. cuniculus has been reported to cause cryptosporidiosis outbreaks in Europe

An intelligent decision support system in construction management by data warehousing technique

Author name used in this publication: K. W. Chau2002-2003 > Academic research: refereed > Publication in refereed journalAccepted ManuscriptPublishe