Professor Paolo G. Radaelli OSI

Abstract:

This work aims at understanding the large reduction of superconducting critical temperature Tc observed in YSr2Cu3O6+χ as compared to its YBa2Cu3O6+χ counterpart (ΔTc=-30 K). We report on a combined study of structural and electronic properties of RESr2Cu3O6+χ (RE=Y, Yb) polycrystalline samples. Neutron diffraction data and Cu NQR spectra show that, contrary to REBa2Cu3O6+χ RESr2Cu3O6+χ is locally tetragonal and no CuO chains are formed. This arises from the random occupancy of oxygen along the a- or b- direction in the basal planes. Ab-initio calculations of the electronic structure using the full-potential linearized-augmented-plane-wave method (FLAPW) in the local density approximation (LDA) suggest that the CuO chains are not formed because of the large elastic strain associated with the orthorhombic distortion produced by the chain formation. In addition, by using a 2α√ x 2α√ supercell simulating the absence of chains, we find that oxygen disorder greatly alters the band structure near the Fermi level. Our analysis indicates that this alteration leads to a reduction of hole transfer from the CuO chains to the CuO2 planes, which accounts for the reduction of Tc experimentally observed.

Abstract:

MnAs is a commercially available material, intensively studied, both theoretically and experimentally, since the beginning of the last century. Interest in this compound could come up again as a consequence of new ideas and conjectures formulated during the last decade in connection with the study of the colossal magnetoresistance (CMR) response in Mn perovskites. Among these ideas is the invocation of a phase separation scenario for CMR manganese oxides and related materials that might be of particular relevance in systems, like MnAs, where first-order phase transitions occur.

Abstract:

MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal hexagonal phase to a paramagnetic, lower-spin insulator orthorhombic phase at T(C)=313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal phase above T(C). The phase transformation is reversible and constitutes the first demonstration of a bond-breaking transition induced by a magnetic field. The field-induced phase transition is accompanied by an enhanced magnetoresistance of about 17% at 310 K. The phenomenon appears to be similar to that of the colossal magnetoresistance response observed in the Mn [corrected] perovskite family.