用两带Ginzburg-Landau理论分析两带超导体Lu2Fe3Si5的表面临界磁场

用两带Ginzburg-Landau(GL)理论分析了Lu₂Fe₃Si₅的表面临界磁场,当超导体的表面与任一主晶面重合,且外磁场平行于超导体的表面时,比值H_c3/H_c2(H_C2是Lu₂Fe₃Si₅的上临界磁场)强烈依赖于温度. 当超导体的表面是bc平面,且 关键词： 两带超导体 GL理论 2Fe₃Si₅')" href="#">Lu₂Fe₃Si₅ 表面临界磁场     

超导材料NbS2上临界磁场的理论分析

根据两带Ginzburg-Landau 理论计算了层状超导材料NbS₂的上临界磁场, 以及上临界磁场各向异性参数随温度的变化情况, 并将NbS₂与MgB₂, NbSe₂上临界磁场的各向异性进行比较. 所得计算结果与已有实验数据符合得很好, 充分说明了NbS₂的超导电性具有两能隙特征. NbS₂上临界磁场各向异性参数在5.0 K附近逐渐变小, 这与MgB₂和NbSe₂有相似之处. 但NbS₂的上临界磁场各向异性参数大约为7.3, 明显大于MgB₂和NbSe₂. 计算结果还表明, NbS₂较大能隙所对应能带的有效质量比约为54, 另一能带的有效质量基本为各向同性.     

二带超导体上临界磁场的各向异性

采用二带Ginziburg-Landau(G-L)理论模型计算了各向异性超导体的上临界磁场,发现上临界磁场Hc2的解析式中包含序参量的各向异性和有效质量的各向异性;上临界磁场ab-面内分量Hc|2|ab、c-轴方向分量Hc|2|ab与实验数据符合;上临界磁场比率在实验许可范围之内。     

MgB2超导体中的临界电流和上临界场

20 0 1年 3月 12日的夜晚 ,在西雅图召开的美国物理学年会重演了 1987年 (高温超导发现 )年会火爆的场面 .10 0 0名以上的物理学家涌入报告大厅(还有更多的人站在外边 ) ,为了聆听日本科学家Ａｋｉｍｉｔｓｕ关于发现ＭｇＢ2 超导体的讲演 .接下来数十位报告人的发言 ,被严格限制在 2分钟以内 ,外加 1分钟的提问 .Ｔｃ～ 4 0Ｋ的ＭｇＢ2 超导体是Ａｋｉｍｉｔｓｕ领导的研究小组于 2 0 0 1年 1月发现的 .他们的原始论文已发表在 2 0 0 1年 3月 1日出版的Ｎａｔｕｒｅ周刊上 .这个小组原本是想以化学试剂商店可大量供应的ＭｇＢ2 为原料 ,制…     

手性ABM态p波超导体上临界磁场的角依赖关系

利用格林函数方法,通过Klemm-Clem变换,计算具有正交晶格结构椭球形费米面的手性ABM态p波超导体上临界磁场的角依赖关系.超导序参量选取具有手性ABM对称性的等自旋配对单分量形式.当椭球形费米面满足一定条件时,上临界磁场随角度呈现非单调变化,表明除配对电子有效质量的各向异性外,手性ABM态p波超导体也具有超导序参量的各向异性.计算结果可用于判断重费米子超导体Sr2RuO4的配对电子空间结构.     

超导体MgB2中临界电流密度与磁场和温度之间的关系

用SQUID磁强计测量了在不同温度一直到5T的MgB2磁滞回线,确定了下临界磁场和样品的退磁因子.临界电流密度对磁场和温度的关系可以用一个简单的函数拟合.钉扎力密度符合Kramer标度律,但是峰值位置随温度移动.     

掺杂对二硼化镁超导体上临界磁场的影响

采用二带Ginzburg-Landau(GL)理论模型,考虑到能带有效质量的各向异性,讨论了MgB2超导体的上临界磁场对角度、温度和相干长度的依赖关系.结果表明,替代杂质越多,上临界磁场越强,这与实验结果符合.     

层状铁基超导体的Ginzburg-Landau方程（英文）

基于铁基超导体的多带能隙特征,采用两带模型,假设带内电子配对为各项同性,带间电子配对序参量为coskxcosky形式.利用Gro′kov理论,推导了层状铁基超导体的二维Ginzburg-Landau方程,并给出了自由能密度和垂直于二维载流子运动平面的上临界场表达式.计算表明,上临界场在转变温度附近随温度变化的曲线呈对数形,该特征与最近的一些实验结果相符合,并且与三维体系的上临界场有显著不同,这一特点是由系统的准二维特征所决定.     

MgB2超导体参量的理论研究

在各向同性的双带GL理论的框架内,研究了块体MgB2的一些超导态参量:包括关联长度ξ(T),临界Cooper对速度υc(T),London穿透深度λ(T)以及临界电流密度jc(T)随温度的变化关系;并将穿透深度λ(T)和临界电流密度jc(T)的理论计算结果和实验数据作了比较,符合得较好.     

第二类超导体的下临界磁场的新计算

从Ginzburg-Landau理论出发,对理想第二类超导体的下临界磁场做计算。通过比较两相的吉布斯自由能,特别是考虑相变时的熵增,最终求得下临界场的新公式,得到纯超导体的下临界场对温度的依赖与经验公式有所不同。作者把这种不同归结为超导体的G-L参数对温度的依赖。     

Microscopic measurements in La-NQR of the ternary carbide superconductor LaNiC2

¹³⁹La-NQR measurements have been carried out in the ternary carbide superconductor LaNiC₂. The nuclear quadrupole frequency and the asymmetry parameter of ¹³⁹La in LaNiC₂ were estimated to be about 1.9 MHz and 0, respectively. In the normal state, the nuclear spin relaxation rate (1/T₁) in the ¹³⁹La NQR signal was proportional to temperature (T) in zero external field above the superconducting transition temperature (T_c) or in an external field larger than the superconducting critical field, which means the system is in the Fermi-liquid state. In the superconducting state, on the other hand, 1/T₁ decreases no more linearly with T, but decreases rapidly exponentially as exp (−Δ/k_BT) at low T with an appreciable enhancement just below T_c. The value of the superconducting energy gap, 2Δ, was estimated to be 3.34k_BT_c, compared with 3.52k_BT_c of the BCS-value. This result strongly suggests that the superconductivity in LaNiC₂ is of a conventional BCS type.     

The upper critical field in two-band layered superconductors

The upper critical field of clean MgB₂ is investigated using the two-band layered Ginzburg--Landau (GL) theory. The calculated results are fitted to the experimental data of clean MgB₂ crystal very well in a broad temperature range. Based on the GL theory for clean superconductors, a phenomenological theory for dirty superconductor is proposed. Selecting appropriate parameters, two-band layered GL theory is successfully applied to the crystal of Mg(B_1-xC_x)₂ and the neutron irradiation samples of MgB₂.     

Mean-field calculations on the magnetic properties of the two-gap superconductor Lu2Fe3Si5

Based on Ginzburg-Landau theory, we study the temperature dependence of upper critical field and London penetration depth for the superconducting crystal Lu₂Fe₃Si₅. All the results fit the experimental data well. We also indicate that the larger gap opens in the quasi-one-dimensional part of Fermi surfaces.     

Very high upper critical fields of F-doped Fe-based layered superconductors NdO0.88F0.12FeAs and CeO0.88F0.12FeAs

Using the solid state reaction method, we have synthesized the polycrystalline F-doped NdO_0.88F_0.12FeAs and CeO_0.88F_0.12FeAs with the superconducting transition temperatures at about 48 and 40 K, respectively. To obtain the upper critical field H _c2 of Nd(Ce)O_0.88F_0.12FeAs samples, we measured the electrical resistivity under magnetic field up to 14 T. Based on the Werthamer-Helfand-Hohenberg (WHH) relation together with the H _c2(T) curves in a relatively high field, we estimated that these superconductors have a rather high upper critical field of about 115 T for Nd-based and 107 T for Ce-based samples, indicating the similarities between these ReO_1−x F _x FeAs (Re = rare earth element) superconductors and high T _c cuprate superconductors. Recommended by Prof. Nie Yuxin, Executive Editor of Science in China Series G-Physics, Mechanics & Astronomy Supported by the National Basic Research Program of China (973 Program)(Grant No. 2006CB9213001) and the National Natural Science Foundation of China (Grant No. 10774181)     

Anomalous upper critical field in ternary iron-silicide superconductor Lu2Fe3Si5

We report the upper critical field H_c2 in a ternary iron-silicide superconductor Lu₂Fe₃Si₅ with T_c  6 K obtained by the resistivity measurements. We find that H_c2 increases linearly with decreasing temperature down to T_c/3, and H_c2(T = 0) exceeds the orbital depairing field described by the Werthamer–Helfand–Hohenberg theory. We also find that the anisotropy of H_c2 is nearly independent of temperature and the angular dependence of H_c2 is well-described by the anisotropic Ginzburg–Landau model. These results strongly indicate the presence of two distinct superconducting gaps in Lu₂Fe₃Si₅ although the behavior is slightly different from that of the typical two-gap superconductor MgB₂.     

Surface critical magnetic field H<Subscript>c3</Subscript>(T) of a bulk superconductor MgB<Subscript>2</Subscript> using two-band Ginzburg-Landau theory

Two-band Ginzburg-Landau (TB G–L) equations for a bulk MgB₂ were solved analytically to determine the temperature dependence of surface critical magnetic fieldH _c3 (T). It is shown thatH _c3 (T) has the same temperature dependence with H_c2 (T), similar to the case of a single-band superconductor,H _c3 (T) = 166H _c2 (T). We use an elimination procedure for the decoupling of G–L equations of two-band superconductivity, which eases the calculations. It is expected that the temperature dependence forH _c3 (T) gives positive curvature nearT _c .     

Anisotropy of the upper critical field in c-axis oriented MgB2 thin films

C-axis oriented MgB_2 thin films were synthesized on single-crystal MgO (111) substrates using a chemical vapour deposition technique. The as-formed films revealed a sharp superconducting transition temperature of 38K with the transition width 0.2K. The temperature dependence of the upper critical magnetic field H_{c2}(T) in the films was determined via resistivity for magnetic field H parallel and perpendicular to the c axis of the films. Using the Werthamer－Helfand－Hohenberg formula, we obtained the anisotropy ratio of the upper critical field γ=1.2.