Abstract. Two different polymorphs of the metal nitride halides MNX (M ¼ Ti, Zr, Hf; X ¼ Cl, Br, I) are known to crystallize in layered structures. The two crystal structures differ in the way 2 1 {X[M 2 N 2 ]X} slabs are stacked along the c-axes. Metal atoms and/or organic molecules can be intercalated into the van-der-Waals gap between these layers. After such an electron-doping via intercalation the prototypic band insulators change into superconductors with moderate high critical temperatures T c up to 25.5 K. This review gathers information on synthesis routes, structural characteristics and properties of the prototypic nitride halides and the derivatives after electron-doping with a focus on superconductivity.

The actual limitations for the trapped field in YBa2Cu3O7−δ (YBCO) monoliths are discussed. The influence of the sample geometry and of the critical current density on the trapped field is investigated by numerical calculations. The field dependence of the critical current density strongly influences the trapped field. A nonlinear relationship between the sample size, the critical current density and the resulting trapped field is derived. The maximum achievable trapped field in YBCO at 77 K is found to be around 2.5 T. This limit is obtained for reasonable geometries and high but realistic critical current densities. Such high fields have not been reached experimentally so far, due to non-optimized flux pinning and material inhomogeneities. These inhomogeneities can be directly assessed by the magnetoscan technique, and their influence is discussed. Significant differences between the a- and the c-growth sectors were found. Limitations due to cracks and non-superconducting inclusions (e.g. 211 particles) are estimated and found to be candidates for variations of Jc on a millimetre length scale, as observed in experiments.

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Ferromagnetic semiconductors (FS) with high Curie temperatures are of great interest for applications in spinbased, multifunctional devices. In particular, dilute magnetic semiconductors (DMS) have been created by doping semiconducting host materials with magnetic transition-metal ions, yielding magnetic ordering transitions near room temperature. [1,2] The ferromagnetism of DMS is currently thought to result from a local anti-ferromagnetic coupling between carriers (e.g., holes in (Ga,Mn)As) and the 3d (Mn) magnetic moments, leading to long-range ferromagnetic ordering. Nevertheless, concerns have been raised that clustering of randomly placed magnetic dopants leads to an inhomogeneous ferromagnetic state. Furthermore, DMS remain controversial from both experimental and theoretical points of view.

Melt-processed samples of YBa2Cu3O7−δ (YBCO) doped with Li additions substituting at Cu(2) plane sites were used to study the effects of doping on the superconducting temperature, critical current density, irreversibility field, upper critical field, coherence length, and magnetic relaxation as a function of temperature and magnetic field. The intrinsic superconducting properties were found only slightly influenced by Li additions at some optimal concentration (0.6at.%). At this Li concentration the volume pinning force of doped material is about five times that of undoped YBCO in intermediate magnetic fields at 77K. No significant change of the maximum volume pinning force with oxygen defects after long-time oxygenation experiment (∼450h) has been observed. The normalized pinning force of the sample optimally doped with Li obeys the same scaling law as a function of magnetic field and temperature before and after additional heat treatment, indicative of a single type of pinning centers. The time dependence of the magnetization in the standard and in the Li-doped sample has been analyzed in the framework of collective pinning theory. The magnetic relaxation measurements combined with the B-T phase diagram data provide evidence of an elastic-plastic transition. The pinning parameters estimated show a considerable enhancement of the critical current density in the optimally doped sample, but only an insignificant change in the effective activation energy, pointing to a weak pinning of pointlike pinning centers, created by Li additions.