铜氧化物高温超导中铜氧面电子结构的扫描隧道显微镜观测

铜氧化物高温超导体的超导机理是近30年来凝聚态物理研究领域的一个重大科学难题.赝能隙和d波配对对称性是对铜氧化物超导体研究所得出的重要实验观察.然而,过去的宏观测量描述的是超导层和电荷库层性质叠加的结果;而微观表面探测主要在实验上易获得的电荷库层进行,无法保证其测量结果能真实表征夹在电荷库层之间的超导结构单元(CuO_2).因此,获得铜氧化物高温超导体的超导层CuO_2且对其进行直接测量是非常必要的,对揭示其高温超导机理具有重要的科学意义.本文综述最近五年内对铜氧化物中CuO_2层相关的扫描隧道显微镜研究结果,对铜氧化物中赝能隙及其与超导电性的关系进行讨论.这些研究为理解高温超导机理提供了重要的思路.     

高温超导的物理进展

铜氧化物高温超导体的机理研究依然是凝聚态物理中的前沿课题,其物理性质显著不同于常规金属,超导的配对机制仍然悬而未决.为此回顾了最近几年该领域内重要的实验和理论,这些进展有助于人们理解高温超导体的物理特性.     

高温超导体中超导位相涨落:Nernst效应研究

高温超导体赝隙态具有许多反常的现象,与高温超导机理之间有密切联系,一直是研究的焦点.有理论提出在赝隙态存在预超导配对.能斯特（Nernst）效应测量探测到了超导转变温度TC0以上温区一定范围内存在磁通涡旋激发,支持了赝隙态中存在有限的超导序参量振幅和强烈的位相涨落的图象,说明TC0处的相变是由Cooper对之间长程位相关联的消失所驱动的.     

t''''次近邻跃迁对角分析光电子谱的影响

以SDW波模拟赝能隙,基于具有自旋密度波(SDW)和d-波超导对称性(DSC)的Hubbard唯象模型,同时考虑近邻、次近邻跃迁效应,在平均场近似下计算谱函数及态密度,研究赝能隙和次近邻跃迁(t')对ARPES的影响.发现考虑次近邻跃迁后得到的结果与铜氧化物高温超导体的性质符合的更好.     

t′次近邻跃迁对角分析光电子谱的影响

以SDW波模拟赝能隙,基于具有自旋密度波(SDW)和d-波超导对称性(DSC)的Hub bard唯象模型,同时考虑近邻、次近邻跃迁效应,在平均场近似下计算谱函数及态密度,研究赝能隙和次近邻跃迁(t′)对ARPES的影响。发现考虑次近邻跃迁后得到的结果与铜氧化物高温超导体的性质符合的更好。     

高T_c氧化物超导体的LO耦合机制

本文利用Eliashberg超导理论,分析了高T_c氧化物超导体的LO耦合机制,给出了此类超导体e-LO耦合参数λ、库仑赝势μ~n、临界温度T_c和0K能隙△_0与T_c比值的计算公式。     

铜氧化物高温超导体的比热容

 基于Nozieres Schmitt Rink的强耦合超导理论和推广的Luttinger理论,文中分别计算了低电子浓度和高电子浓度体系的电子比热容及线性比热系数,描绘出线性比热系数随温度的变化曲线。电子线性比热系数的计算结果表明,在Nozieres Schmitt Rink理论和推广的Luttinger理论框架下,铜氧化物高温超导体中产生赝隙。     

铁基超导体初期部分研究工作介绍

高温超导电性是一个有巨大应用前景和重要科学意义的研究课题。1986年铜氧化物高温超导体的发现，在世界范围内掀起了研究和探索高温超导电性的热潮，时至今日依然是凝聚态物理领域最受关注的问题之一。     

高温超导薄膜的原子尺度制备和表征

高温超导体因其丰富的物理性质和潜在的应用价值已成为30余年来凝聚态物理学备受关注的前沿方向之一.探索和发现新型高温超导体并以此建立非常规高温超导电性的物理机制是超导物理学家们长期追求的目标.本文从高温超导体的研究现状和瓶颈出发,介绍基于异质外延薄膜高温超导电性的原子尺度研究,阐述近年来分子束外延技术在高温超导薄膜制备和量子调控方面取得的研究进展.在外延薄膜超导特性表征方面,着重介绍基于原位扫描隧道显微镜对超导层的原子尺度研究.在新型高温超导体探索方面,主要介绍基于异质外延薄膜界面超导体系的构筑.本文侧重于展示实验设计思路、研究方法以及对高温超导电性微观机制的理解,力图以此启发相关领域的研究人员.     

氧化物材料中赝能隙对输运性质的影响

在ｔ－Ｊ模型和Ｆｅｒｍｉｏｎ－Ｓｐｉｏｎ理论的框架下,研究了强关联氧化物超导材料的输运性质,在最佳掺杂区,系统的电阻与温度是线性关系。在欠掺杂区域,由于在低温情况下存在赝能隙减弱了带电粒子的散射,导致了电阻由在高温时与温度的线性关系转变成为低温时偏离线性关系。     

超导材料的超快光谱研究

电子、晶格、自旋和轨道微观自由度对超导材料的宏观特性起到至关重要的作用.在超导体系中,特别是非常规超导材料,这些自由度衍生出具有不同能量尺度的玻色激发和有序态.前者如声子、磁振子、电荷密度波、自旋密度波、自旋涨落、向列涨落等;后者如超导态、赝能隙态、向列相、反铁磁/铁磁等.前者与后者的形成密切相关.尤其是,不同的玻色激发在频域内纠缠在一起彼此相互作用,同时又与电子(或准粒子)耦合,构建出复杂而又丰富的平衡态和非平衡态物理过程.超快光谱技术的独特性在于具有宽能量范围和高时间分辨率的特点,利用光(电磁波)与超导材料相互作用中的线性和非线性响应,可以共振或非共振地探测与调控这类材料中的准平衡或非平衡态动力学属性.因为桌面超快光谱系统功能全面且具有很大的灵活性,它不仅被应用于超导体系,而且被广泛应用于其他各种无机和有机材料.由于非平衡态理论,特别是与关联电子体系相关的,目前还处在快速发展的阶段,所以本综述主要介绍了常用的桌面超快光谱技术和目前被广泛使用的相关分析理论,聚焦于讨论超导材料中超快光谱实验数据涌现出来的一些普适性趋势及进展.所涉及的超导材料包含了常规超导体、铜氧化物超导体、铁基超导体和重费米子超导体.     

The origin of multiple superconducting gaps in MgB2

Magnesium diboride, MgB2, has the highest transition temperature (T(c) = 39 K) of the known metallic superconductors. Whether the anomalously high T(c) can be described within the conventional BCS (Bardeen-Cooper-Schrieffer) framework has been debated. The key to understanding superconductivity lies with the 'superconducting energy gap' associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments; this is in contrast to both conventional and high-T(c) superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the sigma and pi bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor.     

The origin of the anomalous superconducting properties of MgB(2)

Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.     

Study on Growth of GdBaSrCu_3O_(7-δ) Single Crystal

1 Results It is generally known that CuO2 planes of layered cuprate superconductors play a major role on the variation of critical temperature,Tc.In order to investigate their microscopic and electronic properties,preparation of such materials in single crystal form with highly structure orientation is very important.Crystal growth techniques for copper oxide materials have greatly improved since the discovery of high Tc superconductor materials[1].However,a strong reaction between CuO flux and crucible material has been a serious problem in crystal growth by using crucible,especially in a self-flux slow cooling method that needs long time.It is important to avoid the contamination from crucible material and also the ‘creeping out' problem of CuO flux during long time and high sintering temperature.In this study,we attempted to grew the GdBaSrCu3O7-δ single crystals via self-flux slow cooling technique.They were grown from CuO-rich nonstoichiometric solutions as similar as the YBCO case with approximately cation and powder ratio used as reported before[2-3].     

Structural and Chemical Diversity of Tl－Based Cuprate Superconductors

The Tl-based cuprate superconductor family is the largest family in crystal structure and chemical composition among all high Tc cuprate superconductors. The Tl family can be divided into two sub-families, the Tl single layer family and the Tl double layer family, based on their crystal structural characteristics. The Tl single layer family is an ideal material for investigating the evolution of crystalline formation, charge carrier density, chemical composition, transport properties, superconductivity and their relationships. The Tl family contains almost all possible crystal structures discovered in high-Tc cuprate superconductors. Tl cuprate superconductors are of great importance not only in studying high-temperature superconductivity but also in commercial applications.     

高温超导体的核磁共振最新研究进展

核磁共振作为一种重要的谱学研究手段,在高温超导体的机理研究中发挥了极其重要的作用.近年来,随着新型铁基高温超导材料家族的发现以及基于强磁场下核磁共振技术的发展,相关高温超导方面的核磁共振研究也有了许多新的进展,这些工作对高温超导电性的机理研究起到了积极的推动作用.本文将就核磁共振技术在铜氧化物高温超导体和铁基高温超导体这两大类高温超导材料中的若干最新研究进展进行一个有针对性的概述和梳理,希望对后续高温超导电性的机理研究以及材料探索能起到一些启示作用.     

电负性的密度泛函理论计算值与氧化物超导电性

研究了氧化物的超导电性与元素电负性之间的关系。笔者用电负性均衡值Xcq作为氧化物超导电性的新的经验判据,并用一个式子确定氧化物中元素电负性的均衡值Xcq。笔者用该式计算了230种氧化物超导体的电负性均衡值Xcq,结果表明,所有氧化物超导体的电负性均衡值Xcq集中在5．00到5．63这样窄的范围内,而电负性均衡值在此范围之外的氧化物都不具有超导性。该判据几乎适用于一切氧化物超导体。显然,笔者的工作对进一步探讨氧化物的超导机制是有益的。     

Inhomogeneity, dynamical symmetry, and complexity in high-temperature superconductors: Reconciling a universal phase diagram with rich local disorder

A model for high-temperature superconductors incorporating antiferromagnetism, d-wave superconductivity, and no double latticesite occupancy can give energy surfaces delicately balanced between antiferromagnetic and superconducting order for specific ranges of doping and temperature. The resulting properties can reconcile a universal cuprate phase diagram with rich inhomogeneity, relate that inhomogeneity to pseudogaps, give a fundamental rationale for giant proximity effects and other emergent behavior, and provide an objective framework to separate essential from peripheral in the superconducting mechanism. high-temperature superconductivity, pseudogap, critical dynamical symmetry, inhomogeneity, complexity, emergent behavior     

Vortex dynamics in superconducting MgB2 and prospects for applications

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are 'pinned' in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the 'persistent' supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.