Although high-temperature superconductor cuprates have been discovered for more than 25 years, superconductors for high-field application are still based on low-temperature superconductors, such as Nb(3)Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Here we demonstrate that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over low-temperature superconductors at 4.2 K. With a CeO(2) buffer, critical current densities >10(6)  A cm(-2) were observed in iron-chalcogenide FeSe(0.5)Te(0.5) films grown on single-crystalline and coated conductor substrates. These films are capable of carrying critical current densities exceeding 10(5) A cm(-2) under 30 tesla magnetic fields, which are much higher than those of low-temperature superconductors. High critical current densities, low magnetic field anisotropies and relatively strong grain coupling make iron-chalcogenide-coated conductors particularly attractive for high-field applications at liquid helium temperatures.

Understanding the behaviour of the dielectric constant in ferroelectric thin films remains a challenging problem. These ferroelectric materials have high static dielectric constants, and so are important for their applications in high-storage-density capacitor structures such as dynamic random access memory (DRAM). But the dielectric constant tends to be significantly reduced in thin films, thereby limiting the potential benefit of ferroelectrics for memory devices. Extensive studies have shown that this phenomenon could be caused by a 'dead layer' of very low dielectric constant between the ferroeletric film and the electrode. And, although very few direct measurements are in fact available, it has been recognized that the lattice dynamical properties in the thin films should also play a key role in the reduction of the dielectric constant. Here we report far-infrared ellipsometry and low-frequency dielectric measurements in SrTiO3 thin films, which demonstrate that the Lyddane-Sachs-Teller relation between the optical-phonon eigenfrequencies and the dielectric constant is fully maintained, as is the case in the bulk material. This indicates that the dramatic reduction of the dielectric constant is a consequence of a profound change of the lattice dynamical properties, in particular of the reduced softening of its lowest optical-phonon mode. Our results therefore provide a better understanding of the fundamental limitations of the dielectric constant values in ferroelectric thin films.

We have studied lattice dynamic properties of SrTiO 3 thin films from 5 to 300 K using metaloxide bilayer Raman scattering. First-order zone-center optical phonons, symmetry forbidden in single crystals, have been observed in the thin films, indicating strain-induced lowering of symmetry. The asymmetric line shape of the TO 2 phonon is interpreted as evidence for micropolar regions in the thin films, likely due to oxygen vacancies. The optical phonon lines and the asymmetry persist up to room temperature. [S0031-9007(99)09277-7]

The high upper critical field characteristic of the recently discovered iron-based superconducting chalcogenides opens the possibility of developing a new type of non-oxide high-field superconducting wires. In this work, we utilize a buffered metal template on which we grow a textured FeSe 0.5 Te 0.5 layer, an approach developed originally for high temperature superconducting coated conductors. These tapes carry high critical current densities (>1×10 4 A/cm 2 ) at about 4.2 K under magnetic field as high as 25 T, which are nearly isotropic to the field direction. This demonstrates a very promising future for iron chalcogenides for high field applications at liquid helium temperatures. Flux pinning force analysis indicates a point defect pinning mechanism, creating prospects for a straightforward approach to conductor optimization.

Evolution of ferromagnetic clustering in Pr0.5Ca0.5MnO3 and its effect on the critical temperature of YBa2Cu3O7 thin film

We have epitaxially grown Ba0.6Sr0.4TiO3 (BST-0.4) thin films on MgO(001) substrates. By inserting a very thin Ba1−xSrxTiO3 (x=0.1–0.7) interlayer between the MgO substrate and the main layer of BST-0.4, we are able to manipulate the degree of the stress in BST-0.4 films. We have controlled the stress states, i.e., the lattice distortion ratio (D=in-plane lattice constant/out-of-plane lattice constant) of the BST-0.4 films by varying the chemical composition of the interlayers. We have found that small variations of D value can result in significantly large changes of dielectric properties. A BST-0.4 film under small tensile stress, which has a D value of 1.0023, shows the largest dielectric permittivity and tunability.

We have measured the dielectric loss in SrTiO3 thin films grown on SrRuO3 electrode layers with thickness ranging from 25 nm to 2.5 μm. The loss depends strongly on the thickness but differently above and below T≈80 K: as the thickness increases, the loss decreases at high temperatures but becomes higher at low temperatures. Our result suggests that, in the high temperature regime, the interfacial dead layer effect dominates while, in the low temperature regime, the losses related to the structural phase transition and quantum fluctuations are important.

Iron chalcogenides are of great interest for both basic physics and high-field applications. Although their superconducting transition temperatures are typically lower than those of iron pnictides, iron chalcogenides exhibit lower anisotropies with very high upper critical field slopes near the superconducting transition temperatures. They also have the simplest structures among the iron-based superconductors. This review covers recent progress in the field of superconducting thin films of iron chalcogenides, with primary focus on FeSe 1−x Te x (0 x 1). High quality superconducting thin films with x = 0 and 0.5 have been fabricated by several groups. Of particular interest is that some of them exhibit significantly higher superconducting transition temperatures than those of bulk polycrystalline samples and single crystals over the entire doping regime. Upon the incorporation of small amounts of oxygen, superconductivity is seen to emerge in a thin film of FeTe (parent compound of the iron chalcogenides), but not in the bulk. Advances in superconducting-tape fabrication of iron chalcogenides are also described since the very high upper critical fields and critical current densities of these films suggest that they are prospective candidates for high-field applications. In addition, we present a brief comparison between iron chalcogenide films and iron pnictide films.