High-resolution X-ray scattering studies of charge ordering in highly correlated electron systems
Many important properties of transition metal oxides such as, copper oxide high- temperature superconductivity and colossal magnetoresistance (CMR) in manganites are due to strong electron-electron interactions, and hence these systems are called highly correlated systems. These materials are characterised by the coexistence of different kinds of order, including charge, orbital, and magnetic moment. This thesis contains high-resolution X-ray scattering studies of charge ordering in such systems namely the high-T(_c) copper oxides isostructural system, La(_2-x)Sr(_x)NiO(_4)) with various Sr concentrations (x = 0.33 - 0.2), and the CMR manganite system, Nd(_1/2)Sr(_1/2)MnO(_3)). It also includes a review of charge ordering in a large variety of transition metal oxides, such as ferrates, vanadates, cobaltates, nickelates, manganites, and cuprates systems, which have been reported to date in the scientific literature. Using high-resolution synchrotron X-ray scattering, it has been demonstrated that the charge stripes exist in a series of single crystals of La(_2-x)Sr(_x)NiO(_4)) with Sr concentrations (x = 0.33 - 0.2) at low temperatures. Satellite reflections due to the charge ordering were found with the wavevector (2Ɛ, 0, 1) below the charge ordering transition temperature, Tco, where 2 Ɛ is the amount of separation from the corresponding Bragg peak. The charge stripes are shown to be two-dimensional in nature both by measurements of their correlation lengths and by measurement of the critical exponents of the charge stripe melting transition with an anomaly at x = 0.25. The results show by decreasing the hole concentration from the x = 0.33 to 0.2, the well-correlated charge stripes change to a glassy state at x = 0.25. The electronic transition into the charge stripe phase is second-order without any corresponding structural transition. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. In a single-crystal of Nd(_1/2)Sr(_1/2)MnO(_3) a series of phase transitions were observed using high-resolution synchrotron X-ray scattering. Above the charge ordering transition temperature, Tco, by measuring the peak profiles of Bragg reflections as a function of temperature, it was found that this crystal undergoes two transitions corresponding to the transition from a paramagnetic- to a ferromagnetic state at T ≈ 252 K, and the formation of a mixture of the antiferromagnetic and ferromagnetic phases below T ≈ 200 K. Below the charge ordering temperature, Tco =162 K, additional satellite reflections with the wavevector, q = (1/2, 0, 0), were observed due to Jahn-Teller distortion of the MnO(_6) octahedra caused by charge- and orbital ordering in sample. This transition was observed to be of first-order with a hysteresis width of 10 K. In addition, another very weak satellites with wavevector (1/2, 1, 1/2) were observed possibly due to spin ordering.