The in-plane infrared and visible (3 meV-3 eV) reflectivity of Bi2Sr2CaCu2O 8+δ (Bi-2212) thin films is measured between 300 K and 10 K for different doping levels with unprecedented accuracy. The optical conductivity is derived through an accurate fitting procedure. We study the transfer of spectral weight from finite energy into the superfluid as the system becomes superconducting. In the over-doped regime, the superfluid develops at the expense of states lying below 60 meV, a conventional energy of the order of a few times the superconducting gap. In the underdoped regime, spectral weight is removed from up to 2 eV, far beyond any conventional scale. The intraband spectral weight change between the normal and superconducting state, if analyzed in terms of a change of kinetic energy is ∼ 1 meV. Compared to the condensation energy, this figure addresses the issue of a kinetic energy driven mechanism.
The influence of 57 Fe-doping and strain effects on the structural and magnetotransport properties of undoped and lightly doped 57 Fe ͑1 and 3% at Mn site͒ La 2/3 Ca 1/3 MnO 3 thin films and bulk powder samples have been studied. Thin films were grown on ͑100͒-SrTiO 3 ͑STO͒ and ͑100͒-LaAlO 3 ͑LAO͒ single crystal substrates, via high O 2 pressure ͑500 mTorr͒ using dc magnetron sputtering. Conversion electron Mössbauer ͑CEM͒ spectra measured at room temperature in the paramagnetic regime of the Fe-doped samples do not show significant differences in the isomeric shift for the case of the La 2/3 Ca 1/3 MnO 3 films doped with 1 and 3% iron. The isomeric shift values correspond to the presence of Fe in the 3+ state with octahedral coordination, thus indicating that Fe is incorporated into the structure by substituting Mn. The absence of further states in the spectra indicates that Fe is not involved in forming other additional impurity phases. The x-ray -2 scan showed that all thin films on LAO and STO have single phase and c-axis strong orientation along the growth direction and the Fe doping gives rise to a relaxation of the epitaxial strain. Finally, we have observed that the saturation magnetization, Curie temperature, metal-insulator transition, and magnetoresistance vary nonmonotonically with increased Fe concentration. This behavior can be understood in terms of competing influences from the strain relaxation, which enhances the tendency to order ferromagnetically, and the reduced double exchange, which is detrimental to the ferromagnetic order.
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