High Tc Superconductivity

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-perfo...

Iron based superconductors have a high superconducting transition temperature (Tc), a large critical current density (Jc), a very high upper critical field (Hc2) and a low anisotropy (γ). Electronic properties of the iron based superconductor are critical for understanding its phase diagram, mechanism and transport properties. In this paper we describe the progress of research on material classification, electronic structure, phase diagram, superconducting properties and applications of iron based superconductors.

Recently, high-transition-temperature (high-T c ) superconductivity was discovered in the iron pnictide RFeAsO 1-x F x (R, rare-earth metal) family of materials. We use neutron scattering to study the structural and magnetic phase transitions in CeFeAsO 1-x F x as the system is ...

We report variation of lattice structure in diborides (AB2) with different A atoms to identify the special case of MgB2 showing high Tc superconductivity. High purity MgB2 (Tc~39 K) has been prepared by direct reaction of the elements and the lattice parameters are measured by x-ray diffraction determining the strain e of the B-B distance. The results show that the superconductivity in these intermetallics appear near a critical point in a two variables (strain and charge density) phase diagram at (ec, rc).

We describe the correlated electronic structure of a prototype Fe-pnictide superconductor, SmO1-xFxFeAs , using local-density approximation plus dynamical mean-field theory. Strong, multiorbital electronic correlations generate a low-energy pseudogap in the undistorted phase, giving a bad, incoherent metal in qualitative agreement with observations. Very good semiquantitative agreement with the experimental spectral functions is seen (for U=4.0eV and JH=0.7eV ) and interpreted within a correlated, multiorbital picture. Our results show that Fe pnictides should be understood as low-carrier density, incoherent metals, in resemblance to the underdoped cuprate superconductors.

We examine the possibility of probing superconductivity effects in metal nanoclusters via diamond magnetometry. Metal nanoclusters have been proposed as constitutive elements of high T$_c$ superconducting nanostructured materials. Magnetometry based on the detection of spin-selective fluorescence of nitrogen-vacancy (NV) centers in diamond is capable of nanoscale spatial resolution and could be used as a tool for investigating the properties of single or multiple clusters interacting among each other or with a surface. We have carried out sensitivity estimates and experiments to understand how these magnetometers could be used in such a situation. We studied the behavior of the sensor as a function of temperature and detected the flux exclusion effect in a superconductor by monitoring the [111]-orientation fine structure spectrum in high-NV density diamond. Our results show that phase transitions can be ascertained in a bulk superconductor with this technique while the principal zero field splitting parameter ($D$) demonstrates a temperature dependence that may require compensation schemes.

Magnesium diboride (MgB2) is attractive for RF cavity application for particle accelerators because it might not show an increase of RF surface losses at high magnetic surface fields, a phenomenon that has prevented high-Tc superconducting materials such as YBCO from being used for this application. We have measured the RF surface resistance (Rs) at 21 GHz of a MgB2 sample