Common phase diagram for low-dimensional superconductors

A phenomenological phase diagram which has been derived for high-temperature superconductors from NMR Knight-shift measurements of the pseudogap is compared to the phase diagram that is obtained for organic superconductors and spin-ladder superconductors, both low-dimensional systems. This is contrasted to the phase diagram of some Heavy Fermion superconductors, i.e. superconductors not constrained to a low dimensionality.     

高温超导体磁通动力学和混合态相图(II)

文章简要介绍了高温超导体磁通动力学和混合态物理在过去十余年的发展.高温超导体由于其自身的一些特点,使得它与常规超导体相比较拥有极其丰富的相图,磁通动力学也表现出了非常丰富的研究内容,很多新的概念被提出,新的现象被观察到.比如说涡旋玻璃态,集体钉扎和蠕动,磁通格子的一级和二级熔化相变,布拉格玻璃,峰值效应,二维涡旋饼态,Josephson 磁通运动等等,均是在高温超导体发现之后提出来的新的概念或新发现的现象.有些研究结果目前尚无定论,如关于涡旋玻璃态存在与否的争论至今仍然在进行,但是这些研究内容无疑会大大促进超导物理的发展.高温超导体磁通动力学纷繁复杂的研究内容可以归结为三个相互关联的数字：Ginzburg数(Tc/H2cεζ3)2/2,量子电阻Qu=(e2/)(ρn/εζ),和临界电流的比值jc/j0,这里ζ是相干长度,Hc是热力学临界磁场,ε是有效质量的各向异性度,ρn是正常态电阻率,jc是零温临界电流,j0是拆对临界电流.对于高温超导体前两个数值（Ginzburg数和量子电阻）很大,而临界电流比值较小,因此导致有强的热涨落和量子涨落,以及很强的磁通运动行为（对应小的实测临界电流）.磁通动力学的研究从更深层次影响超导体的临界电流问题和强电应用的发展,最后简要地介绍了这方面的情况.     

高温超导体磁通动力学和混合态相图(Ⅰ)

文章简要介绍了高温超导体磁通动力学和混合态物理在过去十余年的发展。高温超导体由于其自身的一些特点，使得它与常规超导体相比较拥有极其丰富的相图，磁通动力学也表现出了非常丰富的研究内容，很多新的概念被提出，新的现象被观察到。比如说涡旋玻璃态，集体钉扎和蠕动，磁通格子的一级和二级熔化相变，布拉格玻璃，峰值效应，二维涡旋饼态，Josephson磁通运动等等，均是在高温超导体发现之后提出来的新的概念或新发现的现象。有些研究结果目前尚无定论，如关于涡旋玻璃态存在与否的争论至今仍然在进行，但是这些研究内容无疑会大大促进超导物理的发展。高温超导体磁通动力学纷繁复杂的研究内容可以归结为三个相互关联的数字：Ginzburg数（T/He^2εξ^3）^2／2，量子电阻Qu=（e^2／h）（ρn/／εξ），和临界电流的比值jc/jo,这里ξ是相干长度，Hc是热力学临界磁场，ε是有效质量的各向异性度,ρn是正常态电阻率，jc是零温临界电流，jo是拆对临界电流。对于高温超导体前两个数值（Ginzburg数和量子电阻）很大，而临界电流比值较小，因此导致有强的热涨落和量子涨落，以及很强的磁通运动行为（对应小的实测临界电流）。磁通动力学的研究从更深层次影响超导体的临界电流问题和强电应用的发展，最后简要地介绍了这方面的情况。     

高温超导体磁通动力学和混合态相图(Ⅱ)

（上接2006年第1期第16页） 5涡旋态相图的发展 随着理论和实验工作的不断进展，高温超导体混合态相图变得丰富多彩．首先在涡旋玻璃态理论出现后，相图就较常规超导体有了很大的变化．在常规超导体中，相图上就只有两根标志相边界的线。     

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.     

铁基超导体系基于电子关联强度的统一相图

铁基超导和铜基超导具有诸多相似性,这为建立统一的高温超导机理图像提供了可能性.然而,对铁基超导体系中无论是进行电荷掺杂、还是等价掺杂来改变化学压力,都能产生定性上类似、而细节上纷繁复杂的相图,这对建立统一的图像造成了困难.研究化学掺杂效应如何在微观上影响电子结构和超导电性,区分主导超导电性演化的主要因素和次要因素,对建立统一图像和揭示高温超导机理至关重要.本文综述了对铁基超导体系中化学掺杂效应的一系列角分辨光电子能谱研究,涵盖了基于FeAs和FeSe面的多种代表性铁基超导体系,包括异价掺杂、等价掺杂、在元胞不同位置的化学掺杂,及其对电子体系在费米面结构、杂质散射、电子关联强度等方面的影响.实验结果表明：电子关联性或能带宽度是多个铁基超导相图背后的普适参数,不同的晶格和杂质散射效应导致了并不重要的复杂细节,而费米面拓扑结构与超导电性的关联并不强.这些结果对弱耦合机理图像提出了挑战,并促使人们通过局域反铁磁交换作用配对图像在带宽演化层面上统一地理解铁基超导.     

Numerical study of QCD phase diagram at high temperature and density by a histogram method

We study the QCD phase structure at high temperature and density adopting a histogram method. Because the quark determinant is complex at finite density, the Monte-Carlo method cannot be applied directly. We use a reweighting method and try to solve the problems which arise in the reweighting method, i.e. the sign problem and the overlap problem. We discuss the chemical potential dependence of the probability distribution function in the heavy quark mass region and examine the applicability of the approach in the light quark region.     

Anomalous transport properties and phase diagram of the FeAs-based SmFeAsO1-xFx superconductors

We report the detailed phase diagram and anomalous transport properties of Fe-based high-T_{c} superconductors SmFeAsO1-xFx. It is found that superconductivity emerges at x approximately 0.07, and optimal doping takes place in the x approximately 0.20 sample with the highest T_{c} approximately 54 K. T_{c} increases monotonically with doping; the anomaly in resistivity from structural phase or spin-density-wave order is rapidly suppressed, suggesting a quantum critical point around x approximately 0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the x<0.14 regime but at low temperatures just above T_{c} in the x>0.14 regime; a drop in carrier density evidenced by a pronounced rise in the Hall coefficient is observed below the temperature of the anomaly peak in resistivity. A scaling behavior is observed between the Hall angle and temperature: cottheta_{H} proportional, variantT;{1.5} for all samples with different x in SmFeAsO1-xFx system.     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

铜氧化物高温超导体中的电子有序态

铜氧化物高温超导现象自30年前被发现以来,对现代凝聚态物理的发展产生了极其重要的影响,然而其微观机制至今依然是一个谜。近年来,多种实验手段的研究结果发现,在铜氧化物高温超导体中电子除了形成库珀对,还可能形成多种其他新奇的有序态,例如自旋有序态、电荷有序态以及库珀对密度波等。这些有序态的起源及其与赝能隙态和超导态的关联对于理解高温超导机理可能具有重要的意义。文章将主要从实验的角度对铜氧化物超导体中的电子有序态做一个概述。     

Spin triplet pairing in high temperature superconductors

Various superconductivity mechanisms show that the electron pair is in spin singlet (S=0) belowTc. In contrast with these, our EPR measurements show that the electron pair is in spin triplet (S=1) rather than the spin singlet. Since an intense zero field absorption was observed in the high temperature superconductors of YBa₂Cu₃O_7–y , and YBa₂Cu₃O_7–y F₁, which violated various spin-pairing singlet superconductivity mechanisms.