Low-Temperature Phase of the Cd2_2Re2_2O7_7 Superconductor: Ab initio Phonon Calculations and Raman Scattering

Using an {\it ab initio} approach, we report a phonon soft mode in the tetragonal structure described by the space group $I4_{1}22$ of the $1$ K $5d$ superconductor Cd$_2$Re$_2$O$_7$. It induces an orthorhombic distortion to a crystal structure described by the space group $F222$ which hosts the superconducting state. This new phase has a lower total energy than the other known crystal structures of Cd$_2$Re$_2$O$_7$. Comprehensive temperature dependent Raman scattering experiments on isotope enriched samples, $^{116}$Cd$_2$Re$_2{^{18}}$O$_7$, not only confirm the already known structural phase transitions but also allow us to identify a new characteristic temperature regime around $\sim 80$ K, below which the Raman spectra undergo remarkable changes with the development of several sharp modes and mode splitting. Together with the results of the \textit{ab initio} phonon calculations we take these observations as strong evidence for another phase transition to a novel low-temperature crystal structure of Cd$_2$Re$_2$O$_7$.Comment: 7 pages, 8 figure

SchemaSQL -- A Language for Interoperability in Relational Multi-database Systems

We provide a principled extension of SQL, called SchemaSQL , that offers the capability of uniform manipulation of data and meta-data in relational multi-database systems. We develop a precise syntax and semantics of SchemaSQL in a manner that extends traditional SQL syntax and semantics, and demonstrate the following. (1) SchemaSQL retains the flavour of SQL while supporting querying of both data and meta-data. (2) It can be used to represent data in a database in a structure substantially different from original database, in which data and meta-data may be interchanged. (3) It also permits the creation of views whose schema is dynamically dependent on the contents of the input instance. (4) While aggregation in SQL is restricted to values occurring in one column at a time, SchemaSQL permits "horizontal" aggregation and even aggregation over more general "blocks" of information. (5) SchemaSQL provides a great facility for interoperability and data/meta-data manage..

Information Integration and the Semantic Web

Introduction Information integration and interoperability among information sources are related problems that have received significant attention since early days of computer information processing. Initially, for a few decades, the focus was on integration/interoperability for a relatively small number of sources. This is the setting encountered in traditional business and service applications, for example when two companies merge or several services interoperate (which requires the integration of their information systems). Much of the work in this context of federated or multi-databases focused on integrating schemas by defining a global schema in an expressive data model and defining mappings from local schemas to the global one [19]. More recently, in the context of integration of data sources on the internet, the so-called global-as-view (GAV) and local-as-view (LAV) paradigms have emerged out of projects such as TSIMMIS [20] and Information Manifold (IM) [12]. Recently, the a