Beads as Chronological Indicators in West African Archaeology: A Reexamination

Drawing primarily on data obtained from recent excavations at Elmina, Ghana, this report examines the potential use of beads as temporal markers in West African archaeology. It is argued that although beads from West-African contexts are difficult to date, they provide more information than has previously been suggested. The Elmina beads are of particular interest as they can be closely dated by associated European trade materials. Preliminary results from the analysis of the 30,000 European and locally-made glass beads are discussed and findings from other West-African sites are evaluated

Evidence for impurity-induced frustration in La2CuO4

Zero-field muon spin rotation and magnetization measurements were performed in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by M=Mg2+ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization (M(x)) is nearly the same for both compounds, the N\'eel temperature (T_N(x)) decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn2+ impurity onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.Comment: 4 pages, 4 figure

Field evolution of the magnetic structures in Er2_2Ti2_2O7_7 through the critical point

We have measured neutron diffraction patterns in a single crystal sample of the pyrochlore compound Er$_2$Ti$_2$O$_7$ in the antiferromagnetic phase (T=0.3\,K), as a function of the magnetic field, up to 6\,T, applied along the [110] direction. We determine all the characteristics of the magnetic structure throughout the quantum critical point at $H_c$=2\,T. As a main result, all Er moments align along the field at $H_c$ and their values reach a minimum. Using a four-sublattice self-consistent calculation, we show that the evolution of the magnetic structure and the value of the critical field are rather well reproduced using the same anisotropic exchange tensor as that accounting for the local paramagnetic susceptibility. In contrast, an isotropic exchange tensor does not match the moment variations through the critical point. The model also accounts semi-quantitatively for other experimental data previously measured, such as the field dependence of the heat capacity, energy of the dispersionless inelastic modes and transition temperature.Comment: 7 pages; 8 figure

Ghana\u27s Vanishing Past: Development, Antiquities, and the Destruction of the Archaeological Record

Ghana\u27s past is being destroyed at a rapid rate. Although the Ghana Museums and Monuments Board has in some instances successfully intervened to stop the illicit trading of antiquities, the destruction of archaeological sites as a consequence of development over the past two decades has been staggering and the pace is accelerating. The potential of the legislation that established the Ghana Museums and Monuments Board and empowered it to manage and preserve the country\u27s archaeological past has not been realized. The lack of political action, limited relevant public education, insufficient funding, and the poverty of the majority of the Ghanaian populace have allowed for the widespread destruction of both sites and historic buildings. Conspicuously, both the absence of integrated development planning by the Ghanaian government and the inability of development partners (both foreign and domestic) to recognize the potential value of cultural resources have contributed significantly to the continued loss of the archaeological record. While the antiquities trade is a continuing threat to Ghana\u27s cultural resources, it is, in fact, tourism and economic development that pose the major menace to the country\u27s archaeological past. This article reviews the history of cultural resource management in Ghana, including both traditional attitudes toward preservation and current legislation. Case studies are used to illustrate the problems faced

Evidence for impurity-induced frustration in La2CuO4

Zero-field muon spin rotation and magnetization measurements were performed in La2Cu{1-x}MxO4, for 0<x< 0.12, where Cu2+ is replaced either by M=Zn2+ or by M=Mg2+ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization (M(x)) is nearly the same for both compounds, the N\'eel temperature (T_N(x)) decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn2+ impurity onto the Cu2+ antiferromagnetic lattice. In fact, from T_N(x) and M(x) the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.Comment: 4 pages, 4 figure