Magnetic susceptibility of a CuO2 plane in the La2CuO4 system: I. RPA treatment of the Dzyaloshinskii-Moriya Interactions

Motivated by recent experiments on undoped La2CuO4, which found pronounced temperature-dependent anisotropies in the low-field magnetic susceptibility, we have investigated a two-dimensional square lattice of S=1/2 spins that interact via Heisenberg exchange plus the symmetric and anti-symmetric Dzyaloshinskii-Moriya anisotropies. We describe the transition to a state with long-ranged order, and find the spin-wave excitations, with a mean-field theory, linear spin-wave analysis, and using Tyablikov's RPA decoupling scheme. We find the different components of the susceptibility within all of these approximations, both below and above the N'eel temperature, and obtain evidence of strong quantum fluctuations and spin-wave interactions in a broad temperature region near the transition.Comment: 20 pages, 2 column format, 22 figure

Sr impurity effects on the magnetic correlations of LaSrCuO

We examine the low-temperature magnetic properties of moderately doped LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC transition as they are affected by Sr impurity disorder. New measurements of the low-temperature susceptibility in the SG phase show an increase of an anomalously small Curie constant with doping. This behaviour is explained in terms of our theoretical work that finds small clusters of AFM correlated regions separated by disordered domain walls. The domain walls lead to a percolating sequence of paths connecting the impurities. We predict that for this spin morphology the Curie constant should scale as $1/(2 \xi(x,T=0)^2)$, a result that is quantitatively in agreement with experiment. Also, we find that the magnetic correlations in the ground states in the SG phase are commensurate, and that this behaviour should persist at higher temperatures where the holes should move along the domain walls. However, our results show that incommensurate correlations develop continuously around 5 % doping, consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to be published in Physical Review B

A modular design strategy to integrate mechanotransduction concepts in scaffold-based bone tissue engineering

Repair or regeneration of load-bearing bones has long been an incentive for the tissue engineering community to develop a plethora of synthetic bone scaffolds. Despite the key role of physical forces and the mechanical environment in bone regeneration, the mechanotransduction concept has rarely been incorporated in structural design of bone tissue scaffolds, particularly those made of bioactive materials such as hydrogels and bioceramics. Herein, we introduce a modular design strategy to fabricate a load bearing device that can support a wide range of hydrogel- and ceramic-based scaffolds against complex in-vivo loading conditions to induce desirable mechanical strains for bone regeneration within the scaffolds. The device is comprised of a fenestrated polymeric shell and ceramic structural pillars arranged in a sophisticated configuration to provide ample internal space for the scaffold, also enabling it to purposely regulate the levels of strains and stresses within the scaffolds. Utilizing this top-down design approach, we demonstrate that the failure load of alginate hydrogels increases 3200-fold in compression, 300-fold in shear and 75-fold in impact, achieving the values that enable them to withstand physiological loads in weight-bearing sites, while allowing generation of osteoinductive strains (i.e., 0.2-0.4%) in the hydrogel. This modular design approach opens a broad range of opportunities to utilize various bioactive but mechanically weak scaffolds for the treatment of load-bearing defects and exploiting mechanobiology strategies to improve bone regeneration

Enhanced Bound State Formation in Two Dimensions via Stripe-Like Hopping Anisotropies

We have investigated two-electron bound state formation in a square two-dimensional t-J-U model with hopping anisotropies for zero electron density; these anisotropies are introduced to mimic the hopping energies similar to those expected in stripe-like arrangements of holes and spins found in various transition metal oxides. In this report we provide analytical solutions to this problem, and thus demonstrate that bound-state formation occurs at a critical exchange coupling, J_c, that decreases to zero in the limit of extreme hopping anisotropy t_y/t_x -> 0. This result should be contrasted with J_c/t = 2 for either a one-dimensional chain, or a two-dimensional plane with isotropic hopping. Most importantly, this behaviour is found to be qualitatively similar to that of two electrons on the two-leg ladder problem in the limit of t_interchain/t_intrachain -> 0. Using the latter result as guidance, we have evaluated the pair correlation function, thus determining that the bound state corresponds to one electron moving along one chain, with the second electron moving along the opposite chain, similar to two electrons confined to move along parallel, neighbouring, metallic stripes. We emphasize that the above results are not restricted to the zero density limit - we have completed an exact diagonalization study of two holes in a 12 X 2 two-leg ladder described by the t-J model and have found that the above-mentioned lowering of the binding energy with hopping anisotropy persists near half filling.Comment: 6 pages, 3 eps figure

Unifying the Phase Diagrams of the Magnetic and Transport Properties of La_(2-x)Sr_xCuO_4, 0 < x < 0.05

An extensive experimental and theoretical effort has led to a largely complete mapping of the magnetic phase diagram of La_(2-x)Sr_xCuO_4, and a microscopic model of the spin textures produced in the x < 0.05 regime has been shown to be in agreement with this phase diagram. Here we use this same model to derive a theory of the impurity-dominated, low temperature transport. Then, we present an analysis of previously published data for two samples: x = 0.002 data from Chen et. al., and x = 0.04 data from Keimer et. al. We show that the transport mechanisms in the two systems are the same, even though they are on opposite sides of the observed insulator-to-metal transition. Our model of impurity effects on the impurity band conduction, variable-range hopping conduction, and coulomb gap conduction, is similar to that used to describe doped semiconductors. However, for La_(2-x)Sr_xCuO_4 we find that in addition to impurity-generated disorder effects, strong correlations are important and must be treated on a equal level with disorder. On the basis of this work we propose a phase diagram that is consistent with available magnetic and transport experiments, and which connects the undoped parent compound with the lowest x value for which La_(2-x)Sr_xCuO_4 is found to be superconducting, x about 0.06.Comment: 7 pages revtex with one .ps figur

The Sheela-na-gig: An Inspirational Figure for Contemporary Irish Art

A Sheela-na-gig is an enigmatic, medieval stone carving of a female figure with exposed genitalia. It is exceptional both as a public image of a woman with an exaggerated vulva and because it is often located on a religious building. This thesis explores the connection between the Sheela-na-gig and Irish contemporary art considering theological, feminist and historical themes. The artists Barrie Cooke, Louise Walsh and Eilis O’Connell are reviewed with a focus on their works that were inspired by the Sheela-na-gig. The Killinaboy and Ballyvourney churches are focal points in this thesis because of their connection to the artists and because they have Sheelas that became linked to saints. The theological writings on virtues and vices are present in the early Irish texts of penitentials. These penitentials evolved into devotional rituals based on the Sheela, but were also used as a warning against immoral behaviour in monastic praxis. The thesis takes an interdisciplinary approach to inspiration and the forces that bring together the Sheela, the artists and certain social and theological issues in Ireland. The analysis is supported by interviews and oral histories that were collected as part of the research. In this way, the underlying themes of feminism, sexuality, form, repression, history and nature are revealed as key to understanding how the Sheela has served as an inspirational figure for selected contemporary Irish art

Elliptical supra-cellular topographies regulate stem cells migratory pattern and osteogenic differentiation

In living systems, the extracellular environment is structured in a hierarchal order assembling into tissues in a myriad of shapes and complex geometries. Residing within the extracellular matrix, cells are presented and influenced by geometrical cues at several scales. While there is an emerging body of evidence that substrate with symmetric supra-cellular scale geometries (e.g. cylinders and spheres) influence the cell behavior, the effect of physiologically relevant, non-symmetric geometries with varying mean curvatures remain unexplored. In this study, we systematically explore the migratory and differentiation behavior of adipose derived stem cells (ADSCs) on arrays of elliptical cylinders (up to 80 × cell size) with varying mean curvature made from hydroxyapatite. Here, we report a new substrate-driven cell response, which we term “ridge-effect” that leads to osteogenic differentiation and nuclear deformation of cells adhered on regions of highest mean curvature at the ridge of the elliptical cylinders. This phenomenon is observed in both expansion and osteogenic medium. Live imaging combined with functional analysis shows that cells travel along-side the zero mean curvature direction on elliptical cylinders and significantly promote expression of collagen I and osteocalcin compared to a flat surface, in the absence of osteogenic supplements. Altogether, this work identifies supra-cellular scale topographies, and suggest the “ridge-effect” as a physical cue for guiding cellular mechanoresponse and promoting osteogenic differentiation. This knowledge could be utilized as an important biomaterial design parameter for the development of biomedical interfaces and bone scaffolds in tissue engineering and regenerative medicine