高温超导体组合薄膜和相图表征高通量方法

铜氧化物超导体和铁基高温超导体是已知的两类高温超导体,研究高温超导机理是如今超导领域最具有挑战性的前沿课题.构建高温超导的高维精确相图、寻找决定超导转变温度的关键物理量可以为高温超导机理做好实验铺垫.对于铜氧化物高温超导体,多种自由度的相互关联与耦合使其相图呈现出复杂性与多样性.现有的研究方法在构建高维“全息”相图及获取定量化物理规律等方面面临着难以克服的困难,而材料的高通量制备与表征技术可以在相图空间实现参量的线扫描甚至面扫描,有望快速建立可靠的高温超导高维相图和高温超导关键参量数据库,并从中提取重要的统计物理规律.本文从阳离子掺杂、母体氧掺杂、双电层晶体管(静电场/电化学)、磁场等几个调控维度,回顾了主要基于输运手段获得的铜氧化物电子态相图,介绍了基于脉冲激光沉积技术和分子束外延技术的组合薄膜生长方法以及与之匹配的跨尺度选区输运测量技术,展示了高通量技术在高温超导研究中的初步应用.高通量实验技术与超导研究结合,逐步形成了新兴的高通量超导研究范式,将在构建高维精确相图、突破高温超导机理、推进超导材料实用化等方面发挥不可替代的作用.     

中国科学技术大学高温超导物理研究新进展

在中国科学技术大学(以下简称中国科大)建校50周年之际,文章作者对近年来中国科大在高温超导物理方面的最新研究进展情况作一介绍,包括新型高温超导材料探索研究和高温超导机理实验研究.在新型高温超导材料探索研究方面,文章作者首次发现了除高温超导铜基化合物以外第一个超导温度突破麦克米兰极限(39 K)的非铜基超导体--铁基砷化物SmO1-xFxFeAs,该类材料的最高超导转变温度可达到55K;中国科大还成功地制备出大量高质量的超导化合物单晶,包括Nd2-xCexCuO4,NaxCoO2,CuxTiSe2等.在高温超导机理实验研究方面,中国科大系统地研究了SmO1-xFxFeAs体系的电输运性质给出了该体系的电子相图;发现了在电子型高温超导体中存在反常的热滞现象和电荷-自旋强烈耦合作用;在NaxCoO2体系中也开展了系列的工作,并且首次明确了电荷有序态中小自旋的磁结构问题;此外,还系统地研究了CuxTiSe2体系中电荷密度波与超导的相互关系.     

高温超导材料、物理、应用和实验方法研究进展

文章介绍了中国科学院物理研究所超导国家重点实验室在新超导材料探索合成、高质量样品的制备、超导机理和物理性质研究、以及仪器建设方面的研究进展.在超导材料方面,生长出具有国际一流水平的氧化物高温超导单晶;在铁基高温超导材料方面,获得最高的超导转变温度;在超导机理的研究方面,建立了一些行之有效的手段,如低温比热、隧道谱、角分辨光电子谱、红外光电导谱等等,利用这些重要手段获得了一些重要结果,如从比热角度提出欠掺杂氧化物超导体应该具有一个费米弧基态,利用高精度角分辨光电子谱测量第一次观察到115meV和150meV两个新的高能量精细结构.另外,还在介观尺度超导薄膜上面观察到一些有趣的现象,如磁通系统和人工微结构之间的匹配效应.     

电子型超导体Sm1.85Ce0.15CuO4单晶的赝能隙行为研究

测量了在O2中退火不同时间的Sm1.85Ce0.15CuO4单晶样品的热电势S与电阻率ρ的温度依赖关系.所有的样品电阻率高温下呈现线性温度依赖行为.未退火的样品在148K发生超导转变,而退火后的样品在低温下发生金属半导体相变,其超导电性消失,表明退火引起了载流子浓度下降,体系进入欠掺杂态.随着温度降低,所有的样品ST和ρT曲线在200K附近(T)都发生斜率的改变,可以用赝能隙现象解释.热电势S在低温下出现一个正的曳引峰,意味着载流子符号发生改变,由电子型转变为空穴型 关键词： 电子型超导体 热电势 赝能隙     

电子型超导材料的超流密度响应

超导电性是凝聚态物理学中研究的热点.超导材料基本构成分为电子型与空穴型.对于空穴型超导材料,其电子配对对称性普遍接受的是具有dx2-y2波对称性;而对于电子型超导材料,电子配对对称性是dx2-y2波对称性还是s波对称性,目前还存在着很大的争议.在线性近似下,dx2-y2波超导体超流密度是温度的线性函数,而s波超导体的超流密度是温度的指数函数.因此通过对超流密度的研究可以明确电子型超导材料的电子配对对称性.本文首先从微观t-t′-t″-J模型出发,通过隶波色子平均场近似,计及反铁磁关联序和超导配对项,得到两带超导哈密顿量,进而推导出两带超流密度.最后验证了电子掺杂型超导体电子配对对称性和空穴掺杂型一样,同样具有dx2-y2波对称性,和早期唯象的两带模型的研究相一致.明确超流密度随掺杂的变化,是两带准粒子随掺杂变化相互作用的结果.     

转角铜氧化物中的约瑟夫森效应

当前常压下超导转变温度最高的材料仍然来自铜氧化物家族.然而,铜氧化物超导的微观机理仍未被完全建立起来,成为了凝聚态物理领域最具挑战性的问题之一.测定配对波函数的相位部分是全面理解高温超导机理不可或缺的一环.该实验往往需要将不同晶向的铜氧化物拼接成高质量的约瑟夫森结,十分考验样品的合成制备技术.近年来,利用二维材料中发展起来的范德瓦耳斯堆垛技术,研究者们构建了具有原子级平整界面的转角铜氧化物双晶结,研究了不同掺杂浓度、不同转角下的约瑟夫森隧穿,探索了其中出现s波、d波、以及由于界面耦合演生出的d+id波配对的可能性.本文将回顾转角铜氧化物约瑟夫森结的研究进展,介绍近年来发展起来的转角结制备技术,讨论当前实验测量的结果及其意义,提出尚待解决的关键性问题.     

铁基高温超导体电子结构的角分辨光电子能谱研究

铜氧化物超导体和铁基超导体是人类相继发现的两类高温超导家族,它们的高温超导机理是凝聚态物理领域中长期争论但悬而未决的重大问题.对铁基超导体广泛而深入的研究,以及与铜氧化物高温超导体的对比,对于发展新的量子固体理论、解决高温超导机理、探索新的超导体以及超导实际应用都具有重要意义.固体材料的宏观物性由其微观电子结构所决定,揭示高温超导材料的微观电子结构是理解高温超导电性的前提和基础.由于角分辨光电子能谱技术具有独特的同时对能量、动量甚至自旋的分辨能力,已成为探测材料微观电子结构的最直接、最有力的实验手段,在高温超导体的研究中发挥了重要作用.本文综述了在不同体系铁基超导体中费米面拓扑结构、超导能隙大小和对称性、轨道三维性和选择性、电子耦合模式等的揭示和发现,为甄别和提出铁基超导新理论、解决高温超导机理问题提供重要依据.     

在La2CuO4+y中超导与长程非公度磁有序的共存

铜氧化物超导体的相图具有如下共同特征:未掺杂或很低掺杂浓度对应反铁磁区;最佳掺杂及欠掺杂的低温相是超导区;当掺杂浓度介于上述二者之间时,材料既不超导也不具有反铁磁长程序.一般认为,超导与长程磁有序不能共存.     

铜氧化物超导体两能隙问题的电子拉曼散射理论研究

电子拉曼实验表明在空穴型掺杂的铜氧化物超导体中存在两能隙行为,即在欠掺杂区,随着掺杂浓度的降低,一个能隙逐渐增大而且在超导转变温度以上仍然存在,而另一个能隙逐渐减小且在DDW态依然存在.解释两能隙行为非常重要因为它与赝能隙的机理密切相关.本文计算了超导序和d-density-wave(DDW)序竞争机理下相图上不同区域的电子拉曼谱,发现欠掺杂区能隙表现出两能隙行为,与实验一致.特别地,本文发现B_1g峰对应能量由超导和DDW序共同决定,且随着掺杂浓度的降低而增大,在D 关键词： 两能隙 电子拉曼散射 竞争序     

铁基超导体的量子临界行为

铁基超导体呈现丰富的电子相图, 各种有序态相互交叠. 本文主要介绍利用核磁共振手段在空穴型和电子型掺杂的BaFe₂As₂以及LaFeAsO_1-xF_x这三种具有代表性的铁基超导体中探测到的反铁磁序与超导序的微观共存、量子临界点和量子临界行为. 实验发现, 无论在空穴型还是电子型掺杂的铁基超导体中, 反铁磁相变温度都随着掺杂被抑制, 并最终在某个掺杂量降到零温而形成量子临界点. 在反铁磁转变温度之上存在结构相变, 其转变温度也随着掺杂而降低. 核磁共振谱证实结构相变也形成一个量子临界点. 本文介绍核磁共振及输运测量揭示的这两种量子临界点附近存在的量子临界行为, 共存态下奇异的超导性质等.     

Symmetrical aspects of the cooperative pseudo Jahn-Teller effect in high-Tc superconductors

A microscopical model for the structural phase transitions in high-T_c La₂CuO₄-type superconductors is presented. This model is based on the cooperative pseudo Jahn-Teller effect. The most attention is paid to the symmetrical aspects of the approach. The developed theory predicts the possibility of the re-entrant structural phase transition from the orthorhombic phase to the tetragonal one in the doped superconductors. This prediction is in agreement with the experimental observation. The important consequences for the mechanism of superconductivity that followed from the structural phase transitions are under consideration too.     

Nuclear magnetic resonance study of the electron-doped high-temperature superconducting cuprates

Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr_2−xCe_xCuO₄ and Sr_0.9La_0.1CuO₂ that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a d_x²−y² symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin–lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y_1−xCa_xBa₂Cu₃O_7−δ for a hole concentration range of 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.     

Hole distribution and madelung energy in the high-Tc oxide superconductors

Evidence is presented that in high-T_c superconductors the electrostatic interaction as expressed by the Madelung energy is the key factor in determining the charge distribution and in particular the distribution of the holes between the CuO₂-planes and other structural subunits. Specifically, this explains the phase diagram (T_c as a function of oxygen content and oxidizing power) of cation-substituted YBa₂Cu₃O_6+y and the oxygen ordering in YBa₂Cu₃O_6.5. Moreover, we argue that the dependence of T_c on strontium content in La_2−xSr_xCuO₄ is similarly due to the fact that holes go into the (La,Sr)O-sheets as well as into the CuO₂-planes.     

Superconductivity, Ca content and oxygen deficiency of Ga(Sr,Ca)2(Yb,Ca)Cu2O7

Superconducting transition temperature (T_c), Ca content and oxygen deficiency are studied on GaSr_1.8Ca_0.2Yb_1−xCa_xCu₂O₇ (x≤0.35). Superconducting samples with T_c=52 K are prepared after the annealing at 20 MPa of oxygen. The T_c is reduced through a slight oxygen loss accompanied by annealing in air above 650°C. The oxygen loss suggests the presence of short Cu–O chains in the GaO₄ slab. The formal valence of planar Cu required for the appearance of superconductivity depends on oxygen and Ca contents. The critical formal Cu valences are 2.105 and 2.125 for the samples annealed in air at 600°C and at 835°C, respectively. The values are higher than those of usual high-T_c superconductors. This can be explained by a high concentration of localized holes in the CuO₅ slab.     

Microwave absorption and paracoherence excess conductivity studies in high-Tc superconductors

Single phase polycrystalline samples of La_1.8Sr_0.2CuO₄, YBa₂Cu₃O_7−δ and Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y high-T_c superconductors have been studied using XRD, SEM, EDAX, (T), χ_AC and EPR techniques. Detailed investigations on low-field-dependent microwave absorption have been carried out in order to investigate the effects of field exposure, field cooling, zero field cooling and amplitude of field variation on the samples. The results have been discussed on the basis of granular model via Josephson junctions of high-T_c superconductors. Over and above, paracoherence excess conductivity analysis has been carried out in all the three samples. The critical exponent (γ) estimated for all the three samples in close vicinity to their respective transition temperatures has an excellent agreement with the theoretically predicted value for 3-dimensional XY-ferromagnet (γ = 1.33 for D = 3, N = 2).     

Electronic structure and Fermi surface topology of the infinite-layered superconductor Sr1−xCaxCuO2

Superconductivity reported at 110 K, and recently at 150–170 K, in the infinite-layered (Sr_xCa_1−x)CuO₂ system is investigated by means of the full potential linear muffin-tin orbital (FLMTO) method. As in other high-T_c cuprates, the electronic structure of the parent compounds, CaCuO₂ and SrCuO₂, and of the separately calculated composition Sr_0.7Ca_0.3CuO₂ using the experimental lattice parameters, displays strong 2D features including a low density of states at E_F (lower than in the other cuprates) and a simple 2D Fermi surface (rounded square) with strong nesting due to a single hybridized Cu d-O p band. As in La₂CuO₄, a major van Hove saddle-point singularity exists near E_F. The drastic changes of the Fermi surface when Ca/Sr vacancies shift the Fermi energy to the van Hove singularity may have a strong influence on the superconducting properties of the compounds and indicate the need for Sr/Ca vacancies in inducing the high T_c.     

Temperature and doping dependent flat-band superconductivity on the Lieb-lattice

We consider the superconducting properties of Lieb lattice, which produces a flat-band energy spectrum in the normal state under the strong electron–electron correlation. Firstly, we show the hole-doping dependent superconducting order amplitude with various electron–electron interaction strengths in the zero-temperature limit. Secondly, we obtain the superfluid weight and Berezinskii–Kosterlitz–Thouless(BKT) transition temperature with a lightly doping level. The large ratio between the gap-opening temperature and BKT transition temperature shows similar behavior to the pseudogap state in high-T_c superconductors. The BKT transition temperature versus doping level exhibits a dome-like shape in resemblance to the superconducting dome observed in the high-T_c superconductors. However, unlike the exponential dependence of T_c on the electron–electron interaction strength in the conventional high-T_c superconductors, the BKT transition temperature for a flat band system depends linearly on the electron–electron interaction strength. We also show the doping-dependent superconductivity on a lattice with the staggered hoping parameter in the end. Our predictions are amenable to verification in the ultracold atoms experiment and promote the understanding of the anomalous behavior of the superfluid weight in the high-T_c superconductors.     

Structurally tuned superconductivity in heavy-fermion CeMIn5 (M=Co, Ir, Rh)

We discuss systematic trends in the high-temperature physical properties of the heavy Fermion superconductors (HFS) CeCoIn₅ (T_c=2.3 K, γ=300 mJ/molK²), CeIrIn₅ (T_c=0.4 K, γ=750 mJ/molK²), and CeRhIn₅ (P_c=16 kbar, T_c=2.1 K, γ=400 mJ/molK²) in terms of crystalline-electrical-field effects(CEF). We suggest the possibility that the interplay between the symmetry of the CEF ground-state (or low-T CEF scheme of levels) and the f–s hybridization could generate spin fluctuations relevant to the superconducting pairing mechanism in these materials. This hypothesis may provide insight into the fact that some crystal structures appear to favor superconductivity. Further, CeMIn₅ (M=Co, Ir, Rh) appear to be structural relatives of the cubic heavy Fermion superconductor CeIn₃, but with much higher T_c's. We argue that structural layering inherent in the tetragonal CeMIn₅ crystal structure determines the magnetic and electronic anisotropy responsible for the enhanced T_c's. We also describe similarities and differences between these compounds and the high-T_c cuprates.     

A possible mechanism of superconductivity in high-Tc cuprates

This paper derives a generic T_c formula by using the long-range phase coherence condition in quantum phase fluctuation of the order parameter. Taking the two-local-spin-mediated interaction (TLSMI) proposed by Liu and Chen [Phys. Rev. B 58 (1998) 8812] as a Cooper pair potential, and the T_c formula, this paper explains five basic experimental facts in high-T_c cuprates. The aim of this paper is to show that TLSMI is a possible pairing mechanism of superconductivity in high-T_c cuprates.     

Nuclear magnetic resonance studies of vortices in high temperature superconductors

The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin–lattice relaxation has been reported in cuprates, including YBa₂Cu₃O_7-x, Bi₂SrCa₂Cu₂O_8+δ, and Tl₂Ba₂CuO_6+δ. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi₂SrCa₂Cu₂O_8+δ. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.