The Architecture of Racial Segregation: The Challenges of Preserving the Problematical Past

The article examines racial segregation as a spatial system and proposes a conceptual framework for assessing its significance. It analyzes how the ideology of white supremacy influenced design form in the United States and how Jim Crow architecture appeared on the landscape. For African Americans, the settings for everyday life were not simply the confines of this imposed architecture; the article analyzes responses such as the construction of alternative spaces. The discussion concludes by considering the architecture of segregation from the perspective of historic preservation

L4Fe2As2Te1-xO4-yFy (L = Pr, Sm, Gd): a layered oxypnictide superconductor with Tc up to 45 K

The synthesis, structural and physical properties of iron lanthanide oxypnictide superconductors, L4Fe2As2Te1-xO4 (L = Pr, Sm, Gd), with transition temperature at ~ 25 K are reported. Single crystals have been grown at high pressure using cubic anvil technique. The crystal structure consists of layers of L2O2 tetrahedra separated by alternating layers of chains of Te and of Fe2As2 tetrahedra: -L2O2-Te-L2O2-Fe2As2-L2O2-Te-L2O2- (space group: I4/mmm, a ~ 4.0, c ~ 29.6 {\AA}). Substitution of oxygen by fluorine increases the critical temperature, e.g. in Gd4Fe2As2Te1-xOyF4-y up to 45 K. Magnetic torque measurements reveal an anisotropy of the penetration depths of ~31.Comment: 8 figures, 4 table

Synthesis and Bulk Properties of Oxychloride Superconductor Ca2-xNaxCuO2Cl2

Polycrystalline samples and submillimeter size single crystals of Na-doped Ca2CuO2Cl2 have been synthesized under high pressure. A series of experiments showed that the Na content depends not only on the pressure during the synthesis but also on the synthesis temperature and time. From a comparison of the Na-CCOC data with those of structurally related La214 cuprate superconductors we concluded that chlorine at the apical site is less effective that oxygen in supplying charge carriers to the CuO2 plans. As a result, the coupling between the CuO2 planes is weakened, the transition temperature Tc is reduced and the anisotropic nature is enhanced.Comment: 7 pages, 7 figures, 1 table, presenthed at the Eucas 2007 conference. Accepted for "Journal of Physics: Conference Series (JPCS)" 2008 and European News Forum, Issue 3 (2008

Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor

The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with increasing field.Comment: 7 pages, 5 figure

Anisotropic superconducting properties of single-crystalline FeSe0.5Te0.5

Iron-chalcogenide single crystals with the nominal composition FeSe$_{0.5}$Te$_{0.5}$ and a transition temperature of $T_{c}\simeq14.6$ K were synthesized by the Bridgman method. The structural and anisotropic superconducting properties of those crystals were investigated by means of single crystal X-ray and neutron powder diffraction, SQUID and torque magnetometry, and muon-spin rotation. Room temperature neutron powder diffraction reveals that 95% of the crystal volume is of the same tetragonal structure as PbO. The structure refinement yields a stoichiometry of Fe_1.045Se_0.406Te_0.594. Additionally, a minor hexagonal Fe_7Se_8 impurity phase was identified. The magnetic penetration depth \lambda at zero temperature was found to be 491(8) nm in the ab-plane and 1320(14) nm along the c-axis. The zero-temperature value of the superfluid density \rho_s(0) \lambda^-2(0) obeys the empirical Uemura relation observed for various unconventional superconductors, including cuprates and iron-pnictides. The temperature dependences of both \lambda_ab and \lambda_c are well described by a two-gap s+s-wave model with the zero-temperature gap values of \Delta_S(0)=0.51(3) meV and \Delta_L(0)=2.61(9) meV for the small and the large gap, respectively. The magnetic penetration depth anisotropy parameter \gamma_\lambda(T)=\lambda_c(T)/\lambda_{ab}(T) increases with decreasing temperature, in agreement with \gamma_\lambda(T) observed in the iron-pnictide superconductors

Study of the mixed state of La_{1.83}Sr_{0.17}CuO_{4} by means of muon-spin rotation and magnetization experiments in a low magnetic field

Muon-spin rotation (muSR) experiments are often used to study the magnetic field distribution in type-II superconductors in the vortex state. Based on the determination of the magnetic penetration depth it is frequently speculated---also controversially---about the order-parameter symmetry of the studied superconductors. This article reports on a combined muSR and magnetization study of the mixed state in the cuprate high-temperature superconductor La_{1.83}Sr_{0.17}CuO_{4} in a low magnetic field of 20 mT applied along the c axis of a single crystal. The macroscopic magnetization measurements reveal substantial differences for various cooling procedures. Yet, indicated changes in the vortex dynamics between different temperature regions as well as the results of the microscopic muSR experiments are virtually independent of the employed cooling cycles. Additionally, it is found that the mean magnetic flux density, locally probed by the muons, strongly increases at low temperatures. This can possibly be explained by a non-random sampling of the spatial field distribution of the vortex lattice in this cuprate superconductor caused by intensified vortex pinning.Comment: 10 pages, 8 figures, accepted for publication in Physical Review

Evidence for Two Distinct Anisotropies in the Oxypnictide Superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2

Single crystals of the oxypnictide superconductors SmFeAsO0.8F0.2 and NdFeAsO0.8F0.2 with T c in the range of 44 to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the magnetic penetration depth anisotropy γ λ differently than the upper critical field anisotropy γ H , provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result γ λ differs significantly in magnitude and in temperature dependence from γ H , analogous to MgB2 but with a reversed sign of slope. This scenario strongly suggests a new multi-band mechanism in the novel class of oxypnictide high-temperature superconductor

Intrinsic magnetic properties of NdFeAsO0.9_{0.9}F0.1_{0.1} superconductor from local and global measurements

Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO$_{0.9}$F$_{0.1}$ (NFAOF). Individual crystallites up to $\sim200\times100\times30$ $\mu m^{3}$ in size could be mapped at various temperatures. The in-grain, persistent current density is about $j\sim10^{5}$ A/cm$^{2}$ and the magnetic relaxation rate in a remanent state peaks at about $T_{m}\sim38$ K. By comparison with with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO$_{0.9}$F$_{0.1}$ is similar to a layered high-$T_{c}$, compound such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ and exhibits a $3D\to2D$ crossover in the vortex structure. The 2D Ginzburg parameter is about $Gi^{2D}$ implying electromagnetic anisotropy as large as $\epsilon \sim1/30$. Below $T_{m}$, the static and dynamic behaviors are consistent with collective pinning and creep