126 research outputs found
Stability of Transparent Spherically Symmetric Thin Shells and Wormholes
The stability of transparent spherically symmetric thin shells (and
wormholes) to linearized spherically symmetric perturbations about static
equilibrium is examined. This work generalizes and systematizes previous
studies and explores the consequences of including the cosmological constant.
The approach shows how the existence (or not) of a domain wall dominates the
landscape of possible equilibrium configurations.Comment: 12 pages, 7 figures, revtex. Final form to appear in Phys. Rev.
Theory of Magnetic Field Induced Spin Density Wave in High Temperature Superconductors
The induction of spin density wave (SDW) and charge density wave (CDW)
orderings in the mixed state of high superconductors (HTS) is
investigated by using the self-consistent Bogoliubov-de Gennes equations based
upon an effective model Hamiltonian with competing SDW and d-wave
superconductivity interactions. For optimized doping sample, the modulation of
the induced SDW and its associated CDW is determined by the vortex lattice and
their patterns obey the four-fold symmetry. By deceasing doping level, both SDW
and CDW show quasi-one dimensional like behavior, and the CDW has a period just
half that of the SDW along one direction. From the calculation of the local
density of states (LDOS), we found that the majority of the quasi-particles
inside the vortex core are localized. All these results are consistent with
several recent experiments on HTS
Expansion of Vortex Cores by Strong Electronic Correlation in LaSrCuO at Low Magnetic Induction
The vortex core radius \rv, defined as the peak position of the supercurrent
around the vortex, has been determined by muon spin rotation measurements in
the mixed state of \lscox for , 0.15, and 0.19. At lower doping (x=0.13
and 0.15), \rv(T) increases with decreasing temperature T, which is opposite to
the behavior predicted by the conventional theory. Moreover, \rv(T\to0) is
significantly larger than the Ginsburg-Landau coherence length determined by
the upper critical field, and shows a clear tendency to decrease with
increasing the doping x. These features can be qualitatively reproduced in a
microscopic model involving antiferromagnetic electronic correlations.Comment: 6 pages, 4 figures, to be published in Phys. Rev.
Faculty experiences and motivations in design thinking teaching and learning
Introduction: Design thinking (DT) is a creative, iterative approach to generating solutions that are desirable, feasible, and viable. Given its role in fostering creativity and innovation, a growing number of higher education instructors are teaching DT. Exploring how and what instructors know about DT and why they might teach it could provide critical insight into the ways in which DT is operationalized in higher education teaching and learning. Materials and methods: A convergent parallel mixed methods design was used for data collected from online surveys administered to faculty teaching DT. The survey included items about DT practices, outcomes from DT, demographic characteristics, and course characteristics. Five open-text survey items queried participants about their definition of DT, why they teach DT, and what additional outcomes they observed. Descriptive statistics were used to analyze quantitative items and thematic analysis was used to analyze qualitative items. Results: Participants (n = 49) represented various academic ranks, disciplines, types of institutions, and geographic locations. Analyses indicated clear congruence between quantitative and qualitative data. Definitions of DT aligned with well-known models of DT. Motivations for teaching DT included the promotion of personal development, DT proficiency, impact, and interpersonal skill development. Other positive student outcomes observed included increases in enthusiasm, self-awareness, empowerment, optimism, and a sense of belonging. Negative student outcomes included time constraints, teamwork conflicts, and student frustration. Conclusion: Faculty believe that DT leads to highly valuable social innovation skill sets for students. This cross-institutional, multi-disciplinary study provides critical insight into faculty experiences and motivations for teaching DT, offering various strategies for instructors and institutions interested in fostering the uptake of DT within higher education
Neutron scattering study of the effects of dopant disorder on the superconductivity and magnetic order in stage-4 La_2CuO_{4+y}
We report neutron scattering measurements of the structure and magnetism of
stage-4 La_2CuO_{4+y} with T_c ~42 K. Our diffraction results on a single
crystal sample demonstrate that the excess oxygen dopants form a
three-dimensional ordered superlattice within the interstitial regions of the
crystal. The oxygen superlattice becomes disordered above T ~ 330 K, and a fast
rate of cooling can freeze-in the disordered-oxygen state. Hence, by
controlling the cooling rate, the degree of dopant disorder in our
La_2CuO_{4+y} crystal can be varied. We find that a higher degree of quenched
disorder reduces T_c by ~ 5 K relative to the ordered-oxygen state. At the same
time, the quenched disorder enhances the spin density wave order in a manner
analogous to the effects of an applied magnetic field.Comment: 4 figures included in text; submitted to PR
Effect of a magnetic field on the spin- and charge-density wave order in La1.45Nd0.4Sr0.15CuO4
The spin-density wave (SDW) and charge-density wave (CDW) order in
superconducting La1.45Nd0.4Sr0.15CuO4 were studied under an applied magnetic
field using neutron and X-ray diffraction techniques. In zero field,
incommensurate (IC) SDW order appears below ~ 40 K, which is characterized by
neutron diffraction peaks at (1/2 +/- 0.134, 1/2 +/- 0.134, 0). The intensity
of these IC peaks increases rapidly below T_Nd ~ 8 K due to an ordering of the
Nd^3+ spins. The application of a 1 T magnetic field parallel to the c-axis
markedly diminishes the intensity below T_Nd, while only a slight decrease in
intensity is observed at higher temperatures for fields up to 7 T. Our
interpretation is that the c-axis field suppresses the parasitic Nd^3+ spin
order at the incommensurate wave vector without disturbing the stripe order of
Cu^2+ spins. Consistent with this picture, the CDW order, which appears below
60 K, shows no change for magnetic fields up to 4 T. These results stand in
contrast to the significant field-induced enhancement of the SDW order observed
in superconducting La2-xSrxCuO4 with x ~ 0.12 and stage-4 La2CuO4+y. The
differences can be understood in terms of the relative volume fraction
exhibiting stripe order in zero field, and the collective results are
consistent with the idea that suppression of superconductivity by vortices
nucleates local patches of stripe order.Comment: 7 pages, 5 figure
Checkerboard local density of states in striped domains pinned by vortices
Within a Green's function formalism we calculate the electronic structure
around static extended magnetic and non-magnetic perturbations in a d-wave
superconductor. In partucular, we discuss recent elastic neutron scattering and
scanning tunneling experiments on High-T_c cuprates exposed to an applied
magnetic field. A physical picture consisting of antiferromagnetic vortex cores
operating as pinning centers for surrounding stripes is qualitatively
consistent with the neutron data provided the stripes have the usual antiphase
modulation. The low energy electronic structure in such a region reveals a
checkerboard interference pattern consistent with recent scanning tunneling
experiments.Comment: 5 pages, 4 figure
Integrated chronological control on an archaeologically significant Pleistocene river terrace sequence: the Thames-Medway, eastern Essex, England
Late Middle Pleistocene Thames-Medway deposits in eastern Essex comprise both large expanses of Palaeolithic artefact-bearing river sands/gravels and deep channels infilled with thick sequences of fossiliferous fine-grained estuarine sediments that yield valuable palaeoenvironmental information. Until recently, chronological control on these deposits was limited to terrace stratigraphy and limited amino-acid racemisation (AAR) determinations. Recent developments in both this and optically stimulated luminescence (OSL) dating make them potentially powerful tools for improving the chronological control on such sequences. This paper reports new AAR analyses and initial OSL dating from the deposits in this region. These results will help with ongoing investigation of patterns of early human settlement.
Using AAR, the attribution by previous workers of the interglacial channel deposits to both MIS 11 (Tillingham Clay) and MIS 9 (Rochford and Shoeburyness Clays) is reinforced. Where there are direct stratigraphic relationships between AAR and OSL as with the Cudmore Grove and Rochford Clays and associated gravels, they agree well. Where OSL dating is the only technique available, it seems to replicate well, but must be treated with caution since there are relatively few aliquots. It is suggested on the basis of this initial OSL dating that the gravel deposits date from MIS 8 (Rochford and Cudmore Grove Gravels) and potentially also MIS 6 (Dammer Wick and Barling Gravels). However, the archaeological evidence from the Barling Gravel and the suggested correlations between this sequence and upstream Thames terraces conflict with this latter age estimate and suggest that it may need more investigation
Vortex structure in d-density wave scenario of pseudogap
We investigate the vortex structure assuming the d-density wave scenario of
the pseudogap. We discuss the profiles of the order parameters in the vicinity
of the vortex, effective vortex charge and the local density of states. We find
a pronounced modification of these quantities when compared to a purely
superconducting case. Results have been obtained for a clean system as well as
in the presence of a nonmagnetic impurity. We show that the competition between
superconductivity and the density wave may explain some experimental data
recently obtained for high-temperature superconductors. In particular, we show
that the d-density wave scenario explains the asymmetry of the gap observed in
the vicinity of the vortex core.Comment: 8 pages, 10 figure
Diffuse inverse Compton and synchrotron emission from dark matter annihilations in galactic satellites
Annihilating dark matter particles produce roughly as much power in electrons
and positrons as in gamma ray photons. The charged particles lose essentially
all of their energy to inverse Compton and synchrotron processes in the
galactic environment. We discuss the diffuse signature of dark matter
annihilations in satellites of the Milky Way (which may be optically dark with
few or no stars), providing a tail of emission trailing the satellite in its
orbit. Inverse Compton processes provide X-rays and gamma rays, and synchrotron
emission at radio wavelengths might be seen. We discuss the possibility of
detecting these signals with current and future observations, in particular
EGRET and GLAST for the gamma rays.Comment: 13 pages, 5 figure
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