X-ray scattering studies of charge stripes in transition-metal oxides

Abstract

This thesis describes the development of single crystal x-ray scattering applied to the study of charge stripes in some exotic transition-metal oxides. Charge stripes in the cuprates and manganites are strongly associated with high T(_c) superconductivity and colossal magnetoresistance, and can be characterised by the satellite reflections located around related Bragg peaks below the charge ordering transition temperature ( T(_co). The intensities of these extremely weak satellite reflections are between l0(^-4) and 10(^-6) less than those of Bragg reflections, thus very high brilliant rotating-anode x-ray and synchrotron radiation sources are required to study them. The structure factors and correlation lengths of the charge stripes can be directly obtained by measurements of the integrated intensities and peak widths of the charge ordering satellite reflections as a function of temperature. The charge stripes in single crystals of Bi(_0.24)Ca(_o.76)MnO(_3), Nd(_0.5)Sr(_0.5)MnO(_3) and La(_5/3)Sr(_1/3)NiO(_4) have been comprehensively characterised using both laboratory and synchrotron radiation x-ray scattering. The measurements on Bi(_0.24)Ca(_0.76)MnO(_3) directly demonstrate a strong relationship between the charge stripe ordering and the first-order structural phase transition, which is common in many perovskite manganites. Direct evidence for the existence of perpendicular charge-ordered domains in manganites was obtained for the first time by x-ray scattering in Nd(_0.5)Sr(_0.5)MnO(_3), and this result demonstrates that charge stripes have complex structures in the mesoscopic length scale. Scattering was also observed at the positions in reciprocal space associated with spin ordering into stripes. These showed evidence of different critical behaviour than the corresponding charge stripe satellites. The quasi two-dimensional feature of the charge stripes in La(_5/3)Sr(_1/3)NiO(_4) was successfully demonstrated by the critical exponents of the charge stripe melting and by the measurements of their m-plane and out-of-plane correlation lengths. Experimental evidence for the existence of electronic liquid crystal phases in La(_5/3)Sr(_1/3)NiO(_4) was obtained, in agreement with recent theoretical predictions