Superconductivity in water intercalated Na CoO2 thin films

• Applicants: Professor Dr. Lambert Alff; Professor Dr. Bernhard Keimer; Professor Dr. Peter Lemmens
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2006 to 2010
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 20845042

Sodium cobaltate is a material with a large variety of interesting properties that originate in strong electron correlation effects. While its large Seebeck coefficient and low resistivity makes it a candidate for thermoelectric device applications, the layered sodium cobalt oxyhydrate NaxCo02 ¿ yH2O (x < 0.35, y < 1.3) has recently been shown to be superconducting with TC w 4.5K. The phase diagram of this superconductor with a triangular cobalt-oxygen lattice resembles that of the high-temperature superconductors (HTSC) with square copper-oxygen layers. The study of this new material has thus far been restricted to single crystals or poly crystalline ceramic materials. Only very recently, the first thin films of sodium cobaltate have been successfully prepared. The work proposed here will take advantage of the unique opportunities offered by the synthesis of high-quality thin films. The phase purity and surface quality of epitaxial thin films grown by pulsed laser deposition (PLD) are far superior to those of ceramic samples. High-quality films can be fabricated over a wider range of Na concentrations than single crystals. This will enable a comprehensive characterization of the optical properties by infrared and Raman scattering experiments over the entire phase diagram. Moreover, water-intercalated superconducting thin films have recently been prepared by the applicants, opening up the tantalizing possibility of carrying out phase-sensitive Josephson interference experiments to determine the pairing symmetry, as well as a search for pseudogap phenomena well known from HTSC. Altogether, these studies will provide deepened insight into a new superconducting system where coupled spin and charge dynamics are expected to play an essential role for superconductivity.

• DFG Programme: Research Grants
• Major Instrumentation: HelioxVL refrigenerator
• Instrumentation Group: 8550 Spezielle Kryostaten (für tiefste Temperaturen)
• Participating Person: Professor Dr. Hanns-Ulrich Habermeier