Primarily motivated by the similarities between the underdoped superconducting cuprates and the granular systems in regards of electric conductivity, phase fluctuations of the order parameter, and nuclear spin-lattice relaxation, a study has been carried out in a NbN(111) textured film at controlled granularity by means of superconducting quantum interference device magnetization and N 93 b NMR measurements. The Meissner diamagnetism in zero-field-cooling and field-cooling conditions and for different orientation of the magnetic field and the isothermal magnetization curves around the superconducting transition temperature Tc, are studied. N 93 b spectra and relaxation measurements have been performed for two values of the external magnetic field in parallel and perpendicular geometry, in the temperature range 4-300 K. In the superconducting phase the experimental findings for the textured film are similar to the one in bulk NbN. The nuclear spin-lattice relaxation process is the same as in bulk NbN in the temperature range 50-300 K, confirming a dominant contribution to the density of states at the Fermi energy arising from the Nb 4d band. At variance, on cooling from about 40 K down to Tc (H), the N 93 b relaxation rate in the film dramatically departs from the expected behavior for the Fermi gas and mimics the opening of a spin gap. The interpretation of the spin-gap opening in terms of depletion in the density of states at the Fermi energy can justify the anomalous temperature behavior of the N 93 b relaxation rate on approaching Tc (H) from above. The experimental findings suggest the occurrence of superconducting fluctuations (density-of-states term) in one-dimensional regime, coupled to a reduction in the time of flight of the electrons, both effects being related to the granularity. The data also suggest that the spin-gap phase in underdoped cuprates could be connected more to granularity, rather than to exotic mechanisms of magnetic origin.
|Titolo:||Superconducting properties of a textured NbN film from 93Nb NMR relaxation and magnetization measurements|
|Settore Scientifico Disciplinare:||Settore FIS/01 - Fisica Sperimentale|
Settore FIS/03 - Fisica della Materia
|Data di pubblicazione:||2009|
|Digital Object Identifier (DOI):||10.1103/PhysRevB.80.104505|
|Appare nelle tipologie:||01 - Articolo su periodico|