Transport Related Phenomena In Y_1-xHo_xNi₂B₂C Superconductors And PMN-PT Relaxor Ferroelectrics

Transport phenomena are the most important phenomena in solids when referring to the motion of carriers under the drive of external force (i.e. electrons subjected to an electric field). Studying the transport properties of superconductors is tremendously important to understand the superconducting properties, and this kind of transport could be understood as electrons related phenomena. Compared with the electrons, ions could also transport in solids and arise the electric breakdown phenomena in oxides, and studying this kind of ions transport induced electric breakdown phenomena is of great industrial importance. Based on these motives, this thesis studied the transport related phenomena in two extremely different territories of solids—superconductors with Y1-xHoxNi2B2C as the model system and relaxor ferroelectrics with Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-0.3PT) as the model system. This thesis is of great academic interest as well as industrial importance for two main reasons:ⅰ) In the academic side, this thesis performed the first Nernst effect measurement in YNi2B2C and strongly evidenced its multiband nature. Furthermore, through doping magnetic Ho atoms into YNi2B2C matrix, a new reentrant behavior was discovered and discussed when Ho concentration is 0.75, which is thought to originate from the nature of magnetic disorder.ⅱ) In the engineering side, this thesis filled the gap of dc resistance degradation study in relaxor ferroelectrics. The derived activation energy and voltage acceleration factor which control the degradation process are extremely important to guide the design of reliable ferroelectric materials in industrial applications.In partⅠ, the transport and magnetic properties of Y1-xHoxNi2B2C superconductors were discussed with two emphases, namely, the recently suggested multiband superconductivity nature and the influence of magnetic ions Ho3+ doping. Experimentally, a high sensitive transport technique, Nernst effect, was employed to investigate the multiband nature. It was found that a huge anomalous Nernst coefficient appears in the normal state, which is suggested to originate from the invalidation of Sondheimer cancellation. This invalidation could be a direct evidence of the multiband nature. Furthermore, the substitution effect by Ho3+ doping was studied based on the suppression of superconducting transition temperature Tc and the curvature evolution of upper critical field Hc2(T). It was found that Tc(x) is dominated magnetic pair-breaking induced by Ho3+ doping and decreases almost linearly as the Ho3+ concentration increases, which basically obeys the Abrikosov—Gor'kov magnetic pair-breaking theory. Hc2(T) as a function of Ho3+ doping level x was also determined from the resistivity measurements under applied magnetic fields. It was found that Hc2(T) decreases as the Ho3+ doping level increases, implying a strong magnetic pair-breaking effect. Meanwhile, the Hc2(T) profile changed dramatically when Ho3+ doping level x is above 0.5, indicating the upper critical field curvature is more controlled by the doped magnetic ions rather than the multiband nature. Finally, the fine interaction between magnetism and superconductivity was discussed in a specific composition with x=0.75. It was found that compared with the case of HoNi2B2C, the reentrant behavior in Ho0.75Y0.25Ni2B2C demonstrates the shifted reentrant peaks in resistivity and the disappearance of deep minimum in Hc2(T). Based on the study of magnetic structure, we suggest that the magnetic disorder induced by the existence of Y3+ could be responsible for the difference in reentrant behavior between HoNi2B2C single crystal and Ho0.75Y0.25Ni2B2C single crystal.In partⅡ, the transport phenomena of point defects in a specific relaxor ferroelectric material, single crystalline Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-0.3PT) solid solution, were discussed. It was found that oxygen vacancies are the dominating point defects in this PMN-0.3PT system, and under dc biasing condition, oxygen vacancies are transported towards the cathode region, which induces the so called dc electric breakdown or dc resistance degradation phenomena. Highly Accelerated Lifetime Tests (HALTs), Thermally Stimulated Depolarization Current (TSDC) and Impedance Spectroscopy (IS) techniques were employed to generate a physical model, based on which the degradation mechanisms and related energetics were discussed.