Superstructures, Miscibility Gaps and Superconductivity in Two-Band Electronic Systems

Non-Technical Summary

Superconductivity, where a material loses its resistivity, is an interesting and technologically important phenomenon. For instance, medical diagnostic systems, MRIs, use superconductors to generate the high magnetic fields needed, and power transmission lines may use superconductors to transmit electrical power without losses. However, superconductors are sensitive to magnetic fields, with superconductivity suppressed if the magnetic field exceeds a material specific threshold.

This threshold is usually related to the temperature where a material becomes superconducting, and represents an upper limit. Research into materials that substantially exceed this limit indicates that the superconductivity in this class of materials may be due to different effects than in the classic systems. This project, supported by the Solid State and Materials Chemistry Program in NSF’s Division of Materials Research, will allow researchers at Florida State University to explore possible origins of this effect, and is expected to provide insights into ways of improving the performance of superconductors.

A material consisting of niobium, palladium and sulfur or selenium is the focus of this research, where its crystal structure and electrical resistivity depends on the palladium content, and the magnetic field threshold exceeds the expected value more than four-fold, making this compound well suited to study this effect. In this particular compound, the electrons carrying the current experience additional interactions that affect the superconductivity, and thus, the magnetic field threshold.

This research further develops the work force for tomorrow’s technology needs, and advances the understanding of exotic superconductivity. It combines the discovery and growth of materials with novel structural features with an in-depth characterization of their properties, an interdisciplinary activity that requires a variety of skills that are applicable in many fields.

Training students in the art and science of crystal growth and characterization of materials at undergraduate, graduate and postgraduate levels is instrumental to the next generation of scientists and engineers that will be active in this field. Technical Summary

Unconventional multi-band superconductivity has been observed in ternary niobium-palladium and tantalum-palladium chalcogenides, phases with variable palladium stoichiometry. The interactions between charge, spin and lattice are at the core of these effects, giving rise to unconventional physical behavior. This project, supported by the NSF’s Division of Materials Research, focuses on the interplay of intercalation, miscibility gaps, structural order, superstructure formation, and electronic behavior in Nb2PdxX5 (X=chalcogen) and related systems, where the palladium atoms can be considered the intercalating atoms.

Superconductivity in these phases is associated with a record high ratio of the upper critical field Hc2 to the superconducting transition temperature Tc. In these systems, the palladium atoms order in long-range incommensurate superstructures for different palladium content, and induce miscibility gaps, where certain palladium concentrations are not found.

The superconductivity is linked to the Pd stoichiometry and the development of these superstructures, where the derived superconducting coherence length is of the same order as the superstructure periodicity, suggesting an intimate coupling of the two effects. Single crystals of Nb2PdxX5 will be grown and characterized using X-ray diffraction to investigate the details of the superstructures, and their correlation with the superconducting transition temperature and the upper critical field Hc2.

NSF supported National Facilities are crucial to this research, where X-ray diffraction at synchrotron sources and high magnetic field measurements will be carried out.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.