A critical assessment of the superconducting pairing symmetry in NaxCoO2·yH2O

Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

“…The manner in which electrons form these pairs can be strongly influenced by the shape of the Fermi surface. Questions regarding the origin of the pairing interaction and the nature of the superconductivity in the cobaltates has stimulated significant theoretical speculation, most of which has focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, [5][6][7][8] but the outcome of direct attempts to look for them has instead cast serious doubt over their existence. [10][11][12][13][14] Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodium cobalt oxides using the technique of x-ray Compton scattering.…”

“…In particular, an analysis of permitted order-parameter symmetries has indicated that an unconventional f wave ͓x͑x 2 −3y 2 ͒ẑ͔ would be the most probable if the pockets exist. 7 With 70% of the density-ofstates at the Fermi level, these pockets are central to several models of spin-fluctuation-mediated superconductivity, developing either via imperfect nesting between the Fermi surface pockets, 5 or as a consequence of these disconnected pieces of Fermi surface. 6,8 A series of angle-resolved photoemission spectroscopy ͑ARPES͒ results, the consensus of which report observation of the a 1g sheet at ⌫ but crucially not the e g Ј pockets, has fuelled the controversy surrounding the elliptical pockets.…”

Superconductivity in epitaxial thin films of NaxCoO2∙yD2O

“…Such experiments could confirm that, following the discovery of p-wave superconductivity in Sr 2 RuO 4 [13], the compound Na x CoO 2 · yH 2 O is another metallic solid-state analog to liquid 3 He with an even higher transition temperature. In a very recent paper, it was even suggested that Na x CoO 2 · yH 2 O is an f -wave superconductor [14] which would make this material the most exotic one discovered so far. Following the successful example of high-temperature superconductors, high-quality superconducting thin film samples of waterintercalated sodium cobaltate thus promise to enhance our knowledge about this complex material.…”

Probing the Fermi surface by positron annihilation and Compton scattering

“…The discovery 114 of superconductivity at 5 K in hydrated Na x CoO 2 was quickly followed by theoretical speculation about the role of Fermi surface nesting, 115 and a series of attempts to measure the Fermi surface (see, for example, Refs. [116][117][118].…”