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Novel equilibrium phase diagram and dynamics of interlayer Josephson vortices in high-T_c superconductors - computer simulation approach

高温超导体层间约瑟夫森涡旋的新颖平衡相图和动力学 - 计算机模拟方法

基本信息

批准号：
15540355
负责人：
HU Xiao
金额：
$ 1.92万
依托单位：
National Institute for Materials Science
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2003
资助国家：
日本
起止时间：
2003 至 2004
项目状态：
已结题

项目摘要

(1) Possible phases and phase transitions of interlayer Josephson vortex-lines are explored by computer simulation and the density functional theory. In high magnetic fields where vortices reside in every layer (m=1), computer simulations reveal a novel KT phase in which the intralayer vortex correlations are quasi-long-ranged while the interlayer ones are short-ranged. This novel phase transforms into low-temperature 3D lattice phase via a 2^<nd> order phase transition and into high-temperature liquid phase via a KT transition. As the magnetic field decreases, a vortex smectic phase, which shows a short-ranged intralayer vortex correlations while the block layers with denser vortices appear in every other layers (period m=2) in a long-ranged way, is stabilized by layer pinning in materials such as YBCO, accompanied by a sequential second- and first-order transitions upon cooling. In low magnetic fields, the effect of layer pinning is suppressed drastically because of high transition t … More emperature and large discrepancy between the wave-length of layer pinning and the vortex lattice constant, which naturally recovers the anisotropy scaling of melting line. These results are reported in many international conferences as invited talks, and attracting considerable attentions. Experiments designed for testing these theoretical results are scheduled in Europe, USA and Japan.(2) By means of computer simulation we find that when the angle between the magnetic field and the ab plane is not so large the distorted triangular lattice of Josephson vortices dominates the system such that pancake vortices look like to sit on chains as observed in experiment. As the tilt angle increases, the flux lines are aligned essentially in the same way as Abrikosov vortex lattice. There is a first-order phase transition between the two structures.(3) Combining group theoretical analysis of the hexagonal system with energetics based on fermiology, we found within a single band picture, that among the likely pairing states is a novel chiral spin-triplet state, which takes advantage of the nesting among different segments of the Fermi surface. This is similar to the pairing mechanism proposed for the tetragonal material SrRuO_4, though the hexagonal case would benefit further from the nesting effect. More recent developments incorporate orbital effects. There again, the physical picture of pairing induced by scattering between Fermi segments, resulting in a triplet state continues to be an important possibility pursued both experimentally and theoretically. Less

(1)利用计算机模拟和密度泛函理论探讨了层间约瑟夫森涡线可能存在的相位和相变。在高磁场中，每一层都有涡旋存在（m=1），计算机模拟揭示了一种新的KT相，其中层内涡旋相关是准远程的，而层间涡旋相关是短程的。这种新型相通过2^<和>阶相变转变为低温三维晶格相，并通过KT转变为高温液相。当磁场减弱时，在YBCO等材料中，通过层钉固定（layer pin）来稳定具有较密集漩涡的块层，并伴随着冷却时的连续二阶和一阶转变，一个显示出短距离层内漩涡关联的涡近晶相，在每隔一层（周期m=2）中以长期方式出现更密集漩涡的块层。在低磁场条件下，由于层钉的过渡温度高，层钉的波长与涡晶格常数的差异大，层钉的作用被大大抑制，自然恢复了熔点的各向异性尺度。这些成果在许多国际会议上作为特邀报告会进行了报道，引起了相当大的关注。为验证这些理论结果而设计的实验计划在欧洲、美国和日本进行。(2)通过计算机模拟发现，当磁场与ab面夹角不太大时，约瑟夫森涡的扭曲三角形晶格占主导地位，使得实验中观察到的煎饼涡看起来像坐在链上。随着倾斜角的增加，通量线基本上以与阿布里科索夫涡晶格相同的方式排列。两种结构之间存在一阶相变。(3)将六方体系的群理论分析与基于费米学的能量学相结合，发现在单个带图中，可能的配对态中存在一种新的手性自旋三重态，该态利用了费米表面不同段之间的嵌套。这与四边形材料SrRuO_4的配对机制类似，尽管六边形的情况将进一步受益于嵌套效应。最近的发展包括轨道效应。在那里，由费米段之间的散射引起的配对的物理图景，导致三重态仍然是实验和理论上追求的重要可能性。少