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La1.6-xNd0.4SrxCuO4超导体的磁性和输运性质与条纹相的关系

Interplay among Stripe Phase, Magnetic and Charge Transport Properties in La1.6-xNd0.4SrxCuO4 Superconductor

【作者】 丁俊峰

【导师】 李晓光;

【作者基本信息】 中国科学技术大学 , 凝聚态物理, 2010, 博士

【摘要】 电荷有序是在掺杂的Mott绝缘体中的一种普遍存在的现象。在高温超导体中,很多实验都观察到了电荷条纹相的存在。电荷条纹相是指,在CuO2面内,载流子自发聚集形成一维链状结构,与反铁磁背景发生分离并形成周期性调制结构。实验表明电荷条纹以及反铁磁涨落与高温超导电性有着密切联系。本论文主要研究了具有静态电荷和自旋条纹序的La1.6-xNd0.4SrxCuO4单晶的磁各向异性,讨论了条纹对磁通钉扎及磁通运动的影响,以及电荷输运与自旋有序之间的关系。另外,我们还通过材料的电流-电压(I-V)特性,研究了体系中的本征Josephson结特性。论文内容共分为五章,每章的主要内容分别概括如下:第一章主要是综述了条纹相的物理图像以及条纹相在不同高温超导体中的实验证据,讨论了条纹相与超导电性的关系,介绍了在条纹相超导体中的赝能隙行为,最后阐述了磁场对自旋有序的影响。第二章首先介绍了光学浮熔区法生长La系超导单晶。通过研究La1.6-xNd0.4SrxCuO4单晶在H||ab面和H||c轴时的各向异性直流和交流磁化率,发现体系中存在着二维超导电性。这种二维超导电性发生在体超导转变温度Tc以上,具有明显的面外各向异性,可能是由静态条纹相引起的CuO2面层间退耦合导致的。这说明层间退耦合是静态条纹相压制Tc的主要因素之一第三章研究了La1.45Nd0.4Sr0.15CuO4单晶交流磁化率的面内各向异性。当磁场在CuO2面内转动时,交流磁化率和直流磁弛豫率对磁场的依赖都表现出四重对称性。这表明由条纹引起的面内超导序的不均匀性可以对磁通产生钉扎作用。在磁场平行于条纹方向时,条纹对磁通的钉扎力达到最大值,相应的临界电流最大,交流磁化率的抗磁性最强,而磁场平行于Cu-Cu方向时,抗磁性最弱。第四章研究了不同掺杂浓度的La1.6-xNd0.4SrxCuO4单晶的c轴磁阻行为。在温度低于自旋有序温度时,材料中立刻出现了反常磁阻行为。在磁场大于转变场后,电阻随着磁场升高逐渐降低,表明在条纹相体系中存在由磁场诱导的spin-flop转变。直流磁化率随温度的变化,特别是在χab上的宽峰可以用晶体场中的Nd3+离子磁性来描述,表明在条纹有序的LNSCO样品中磁化率主要由Nd3+离子决定,不能够反映出Cu2+自旋的贡献。第五章研究了LNSCO单晶c轴方向的I-V特性。发现样品的I-V特性表现出了本征Josephson结行为,其I-V曲线上出现了一个电压台阶以及回滞行为。随着掺杂浓度增加,结的类型从SIS结逐渐过渡到SNS结,表明了条纹相对Josephson耦合的影响。Ic对外加磁场的依赖在一个较大的磁场周期表现出了调制行为。磁场在CuO2面转动时,而由于静态条纹相对磁通的钉扎效应,Ic的对磁场角度的依赖表现出四重对称性。

【Abstract】 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 (Ⅰ-Ⅴ) 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.Ⅰ-Ⅴcurves 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.

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