Interplay Among Stripe Phase, Magnetic And Charge Transport Properties In La_1.6-xNd_0.4Sr_xCuO₄ Superconductor

The charge inhomogeneity is one of the intrinsic properties in the doped Mott antiferromagnets. For the high temperature superconductors, charge stripe has been observed by many experiments. In a stripe phase, the doped carriers accommodate in striped pattern and separate the antiferromagnetic domains in the CuO2 plane. It is believed that the charge and spin stripes are closely related to the high temperature superconductivity.In this dissertation, the anisotropic susceptibilities were studied in the La1.6-xNd0.4SrxCuO4 single crystals with static charge and spin stripes. The influence of stripe order on the vortex pinning and motion, and the relation between the charge transport and spin order were discussed. Furthermore, the intrinsic Josephson junction behavior in the stripe ordered superconductor was studied through the current-voltage (â… -â…¤) characteristics along the c-axis.In chapter one, the picture and experiment evidences of the stripe phase were introduced. The relation between the stripe phase and superconductivity was discussed. The pseudogap in the stripe ordered superconductor and the effect of magnetic field on the spin order were also introduced.In chapter two, the crystal growth process by optical floating-zone single crystal furnace was firstly introduced. Then, the dc and ac susceptibilities of the stripe-ordered La1.6-xNd0.4SrxCuO4 single crystals were studied under different magnetic field orientations. A particular two dimensional (2D) superconductivity, which is closely related to the decoupling between CuO2 planes caused by static stripe phase, was found. This 2D superconductivity rising at a temperature higher than the bulk Tc suggests that the decoupling is one of the most important facts for the suppression of Tc by static stripe phase.In chapter three, the angular dependencies of the ac susceptibility and dc magnetic relaxation of a stripe-ordered La1.45Nd0.4Sr0.15CuO4 single crystal were studied below the superconducting transition temperature Tc. Due to the anisotropic vortex pinning of the static stripe phase in the sample, both the ac susceptibility and the dc magnetic relaxation rate show the fourfold symmetry for the magnetic field rotated in the CuO2 plane. For the field along the stripes, the vortex pinning force gets its maximum, which leads to the largest diamagnetization and the lowest normalized relaxation rate.In chapter four, the field and temperature dependencies of the spin structures in the normal states of La1.6-xNd0.4SrxCuO4 single crystals were studied by measuring the magnetoresistance and susceptibility. A negative magnetoresistance appears just below the spin-ordering temperature for the magnetic fields parallel to the CuO2 plane, which can be attributed to the spin-flop transition of the special spin structure in the normal state of the system. The anisotropic variations of susceptibilities with temperature for all the three specimens can be described in the framework of the crystal-field theory. The well fitted broad peaks of the in-plane susceptibilities Xab for the specimens suggest that the susceptibilities are dominated by Nd3+, and thus the spin reorientation of Cu2+ in the CuO2 plane can not be observed from the study of the susceptibility.In chapter five, the intrinsic Josephson junction properties were observed in the bulk La1.6-xNd0.4SrxCuO4 single crystals.â… -â…¤curves along c-axis exhibit voltage jumps at a critical current and a clear hysteresis without multiple branches below the superconducting transition temperatures. The change of the junction type from SIS to SNS with increasing doping level shows the influence of the stripe on the interlayer Josephson coupling. The field dependent critical current Ic exhibits periodical perturbation with a large scale of magnetic field. Due to the vortex pinning effect of static stripe, the angular dependence of Ic shows fourfold symmetry for the field rotated in the CUO2 plane.