Study On Isotope Effect, Phase Separation And Related Effects In Co-based Correlated Electronic Systems

The transition metal oxides have been widely studied for their novel properties, such as superconductivity, colossal magnetoresistance, abnormal Hall effect, huge ther-moelectrical effect and multiferronics. In these system charge, spin, orbital and lattice are coupled to each other, and complicated phase diagrams are introduced by the competition among these interactions, giving rise to some interesting phenomena such as charge ordering, magnetic ordering, glassy behavior, phase separation, metal-insulator transition and spin-state transition. These are important not only for scientific purpose, but also for the possibility of application.In this dissertation, we have studied the coupling among charge, spin and lattice in (Pr_1-xSm_x)_0.7Ca_0.3CoO₃, and the relationship between superconductivity and stripe phase in La_1.6-xNd_0.4Sr_xCuO₄, via the substitution of ¹⁶O by ¹⁸O. We also found a glassy behavior in BaCo_0.9Ni_0.1S_1.947, which shows us a slow, nonexponential relaxation and "physical aging" behavior. Some of the results were published in Phys. Rev. B, and some were submitted.The contents are arranged as six parts:1. Brief overview of the research in Co-based and cuprate high temperature superconductorsIn the first chapter, the author reviews the hot topics in recent condensed matter physics studies, especially these systems which are related to the studies in this dissertation. The contents are as follows: The review of the theoretical study history in strong correlated electronic system; The introduction to the conventional superconductivity and the high temperature superconductivity in cuprates, including stucture, transport properties, magnetic properties and a hot topic in the high temperature superconductivity studies - the stripe phase; The review to the cobalt oxides, especially the spin-state transition in perovskite-related structures; The introduction to both the experimental and the theoretical studies in oxygen isotope substitution; The background of Ba(Co,Ni)S_2y system, including structure, transport and magnetic properties.2. Giant isotope effect and spin state transition induced by oxygen isotope exchange in (Pr_1-xSm_x)_0.7Ca_0.3CoO₃In this chapter, we systematically investigate the effect of the oxygen isotope in (Pr_1-xSm_x)_0.7Ca_0.3CoO₃ which shows a crossover with x from ferromagnetic metal to the insulator with spin-state transition. A striking feature is that effect of oxygen iso- tope on the ferromagnetic transition is negligible in the metallic phase, while replacing ¹⁶O with ¹⁸O leads to a giant up shift of the spin-state transition temperature T_s in the insulating phase, especially T_s shifts from 36 to 54 K with isotope componentαs=-4.7 for the sample with x=0.175. A metal-insulator transition is induced by oxygen isotope exchange in the sample x=0.172 being close to the insulating phase. The contrasting behaviors observed in the two phases can be well explained by occurrence of static Jahn-Teller distortions in the insulating phase, while the absence of them in the metallic phase.3. Oxygen isotope effect on the superconductivity and stripe phase in La_1.6-xNd_0.4Sr_xCu0₄In this chapter, the oxygen isotope effect on the superconductivity, stripe phase and structure transition is systematically investigated in La_1.6-xNd_0.4Sr_xCuO₄ with static stripe phase. Substitution of ¹⁶O by ¹⁸O leads to a decrease in superconducting transition temperature T_c, while enhances the temperature of the structural transition from low-temperature-orthorhombic (LTO) phase to low-temperature-tetragonal (LTT) phase. Compared to the Nd free sample, a larger isotope effect on T_c is observed in La_1.6-xNd_0.4Sr_xCuO₄. These results indicate that the distortion of CuO₂ plane suppresses the superconductivity, giving a direct evidence for the competing of stripe phase and superconductivity because the distortion of CuO₂ plane enhances the stripe phase.4. Synthesis and magnetic properties of Layered cobalt oxides Pr_2zCa_xCo0₄ (0.1≤x≤0.9)In this chapter, we synthesized layered oxides Pr_2-xCa_xCoO₄ (0.1≤x≤0.9) and investigated their structure and magnetic properties. With increasing Ca doping level, the antiferromagnetic correlation is suppressed and ferromagnetic correlation is enhanced for x>0.5. In the sample with x=0.9, glassy ferromagnetic cluster phase is observed. The reversed magnetic susceptibility departs from the Curie-Weiss law at a temperature higher than the glassy ferromagnetic cluster temperature, indicating the appearance of ferromagnetic fluctuation(Griffiths phase).5. Novel dynamical effects and glassy response in strongly correlated electronic systemIn this chapter, we find an unconventional nucleation of low temperature paramagnetic metal (PMM) phase with monoclinic structure from the matrix of high-temperature antiferromagnetic insulator (AFI) phase with tetragonal structure in strongly correlated electronic system BaCo_0.9Ni_0.1S_1.97. Such unconventional nucle- ation leads to a decease in resistivity by several orders with relaxation at a fixed temperature without external perturbation. The novel dynamical process could arise from the competition of strain fields, Coulomb interactions, magnetic correlations and disorders. Such competition may frustrate the nucleation, giving rise to a slow, nonexponential relaxation and"physical aging" behavior.6. Phase separation induced by oxygen deficiency in La_0.67Ca_0.33Mn0₃ thin filmsIn this chapter, epitaxial La_0.67Ca_0.33MnO₃ thin films deposited on (001)-LaAlO₃ under different oxygen partial pressures (Po₂) from 50 mTorr to 500 mTorr are investigated. An oxygen-deficiency-activated phase separation is observed by X-Ray diffraction (XRD), microstructure and temperature coefficient of resistance (TCR) studies. The transport properties are strongly related to the strain state of films. All films show phase separation behavior except for the film deposited at 50 mTorr at which most of the strain was relieved by serious crystal defects. The films deposited at 300 and 350 mTorr show insulating behavior in the whole temperature range, which is attributed to charge-ordered (CO) state and can be suppressed by applied field.