Influence of the Pseudogap on the Inelastic Neutron Scattering Spectra of the Underdoped Lanthanum Cuprate

The influence of pseudogap on the inelastic neutron scattering spectra of the underdoped lanthanum cuprate is studied on the basis of the model which incorporates both the superconducting state and pseudogap state.It is found that the striking effects of the influence of the pseudogap on the incommensurability of the spin excitation spectrum are that in the superconducting state the pseudogap makes the intensity of the incommensurate peak increase,in the normal state the pseudogap not only makes the intensity of the incommensurate peak increase,but also sharpens the incommensurate peak and increases incommensurability.     

Study of Pseudogap in Underdoped Cuprate

A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the dependence of the energy gap which mimics the pseudogap near (π, 0) point at least. It implies that the pseudogap structure near (π, 0) is not sensitive to the long-range order and will survive leading to the pseudogap phenomenon in the underdoped metallic phase. On the other hand, in the long-range ordering antiferromagnetic region, the mean-field SDW theory holds and the pseudogap structure predicated by the theory should be observable experimentallyv. Then one prediction is that the pseudogap would smoothly extrapolate between itinerant antiferromagnetic phase and underdoped metallic phase.     

Study of Pseudogap in Underdoped Cuprate

A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the basis of the t-t^ˊ-U Hubbard model. It is surprised to find that a simple t^ˊ term will do the trick to introduce the momentum dependence of the energy gap which mimics the pseudogap near (π,0) point at least. It implies that the pseudogap structure near (π,0) is not sensitive to the long-range order and will survive leading to the pseudogap phenomenon in the underdoped metallic phase. On the other hand, in the long-range ordering antiferromagnetic region, the mean-field SDW theory holds and the pseudogap structure predicated by the theory should be observable experimentally. Then one prediction is that the pseudogap would smoothly extrapolate between itinerant antiferromagnetic phase and underdoped metallic phase.     

高温超导体中的赝隙与超导位相涨落

高温超导体赝隙态与超导态之间的关系一直是研究的焦点.交流电导和能斯特（Nernst）效应测量相继探测到超导转变温度Tc0以上温区一定范围内存在磁通涡旋激发,利用力矩技术的磁化率测量则探测到超导涨落引起的弱抗磁性.这些发现都支持了高温超导体赝隙相中存在有限的超导序参量振幅和强烈的位相涨落的图像,说明Tc0处的相变是由库珀对之间长程位相关联的消失所驱动的.文章首先简短地介绍高温超导体的电子态相图和赝隙态,以及能斯特效应的原理和测量方法,然后对能斯特效应的测量结果作一评述性介绍,还讨论了相关的理论模型.     

铜氧化物高温超导体正常态反常光电导性质的研究

在全电荷自旋重组的模型下,计算了铜氧化物高温超导体正常态的光电导性质,发现在欠掺杂和最佳掺杂区,铜氧化物高温超导体正常态的光电导谱由低能端的non-Drude峰和较高能量处的中红外带两部分组成,低能峰和中红外带之间存在电荷转移能隙.随着掺杂量的增加,中红外带不断移向低能峰,最后在极过掺杂时完全融入低能峰,光电导谱恢复为Drude形式.全电荷自旋重组模型下的自能体现了部分关联效应,包含了赝能隙的信息,被认为造成了光电导的反常特性.     

高温超导体中的赝隙与超导位相涨落

高温超导体赝隙态与超导态之间的关系一直是研究的焦点.交流电导和能斯特（Nernst）效应测量相继探测到超导转变温度Tc0以上温区一定范围内存在磁通涡旋激发,利用力矩技术的磁化率测量则探测到超导涨落引起的弱抗磁性.这些发现都支持了高温超导体赝隙相中存在有限的超导序参量振幅和强烈的位相涨落的图像,说明Tc0处的相变是由库珀对之间长程位相关联的消失所驱动的.文章首先简短地介绍高温超导体的电子态相图和赝隙态,以及能斯特效应的原理和测量方法,然后对能斯特效应的测量结果作一评述性介绍,还讨论了相关的理论模型.     

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     

浅析电子型掺杂铜氧化物超导体的退火过程

铜氧化物高温超导体的发现, 打破了基于电声子相互作用BCS理论所预言的超导转变温度极限, 掀开了高温超导材料探索和高温超导机理研究的序幕. 根据掺杂类型的不同, 铜氧化物超导材料可以分为空穴型掺杂和电子型掺杂两类. 受限于样品, 对电子型掺杂铜氧化物的研究工作远少于空穴型掺杂体系. 本文简要回顾有关电子型掺杂铜氧化物超导体近期研究成果, 通过对比电子型掺杂和空穴型掺杂铜氧化物的相图来阐明电子型掺杂铜氧化物的研究对探索高温超导机理的必要性, 并特别针对电子型掺杂样品制备中的关键因素“退火过程”展开讨论. 结合课题组最新实验结果和相关实验报道我们发现电子型掺杂铜氧化物超导体在制备过程中除受到温度和氧分压的影响外, 退火效果还受到界面应力的强烈调制. 在综合考虑样品生长过程中温度、气氛及应力等多种因素的基础上, 探讨了“保护退火”方法导致电子型体系化学掺杂相图变化的起因.     

Interlayer magnetotransport study in electron-doped Sm2−x Ce x CuO4−δ

Vortex and pseudogap states in electron-doped Sm_2−x Ce _x CuO_4−δ (x ∼ 0.14) are investigated by the interlayer transport in magnetic fields up to 45 T. To extract intrinsic properties, we fabricated small 30 nm-high mesa structures, sufficiently thin to be free of the recently reported partial decomposition problems. On cooling, the c-axis resistivity ρ_c of the mesa structures reveals a semiconductive upturn above T_c, followed by a sharp superconducting transition at 20 K. When the magnetic fieldH is applied along the c-axis, ρ_c(T) shows a parallel shift without significant broadening, as also observed in the hole-doped underdoped cuprates. Above the transition we observe negative magnetoresistance (MR), which can be attributed to the field suppression of the pseudogap, whose magnitude is as small as 38 K. Our results in thex ∼ 0.14 samples closely correspond to the interlayer transport behavior in the ‘overdoped’ regime of hole-doped Bi₂Sr₂ CaCu₂ O_8+y.     

Pseudogap phenomena in ultracold atomic Fermi gases

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tc superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum sinmlator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tc superconductivity. One obstacle to the ultimate understand- ing of high Tc superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics comnnmity, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fuctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tc superconductivity.     

利用比热手段证明氧化物超导体正常态有电子库珀对存在

文章简单介绍了氧化物超导体机理问题研究中最核心的问题之一,即超导与赝能隙关系的最新进展状态.介绍了利用高精度比热测量所发现的在欠掺杂样品的正常态有电子配对存在的证据,指出超导凝聚过程是非BCS型.比热数据显示,在重过掺区域,超导转变温度附近的比热跳变非常之陡,超导凝聚所造成的熵变满足守恒规律.然后到欠掺杂区,这个比热跳变变得很矮,而且在正常态仍然测量到一个与磁场相关的比热部分,表现在比热系数有一个很长的尾巴拖到高温区.计算到Tc 附近的超导熵不满足守恒律,但是积分到高温后,即考虑到尾巴部分后,熵就接近守恒.这说明,在欠掺杂超导体的正常态已经有电子配对,但是没有建立起宏观超导相干态.氧化物超导体中的超导凝聚过程不是BCS型的．     

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.     

Anti-Correlation between Energy-Gap and Phonon Energy for Cuprate Bi2212 Superconductor

Using the electron--phonon mechanism, we explain the spatial anti-correlation between the energy-gap and the energy of phonon mode for cuprate superconductor found in tunnelling spectrum by STM measurements of Bi2212, which is the direct effect of an important relationship (or constraint) I=const, where I is superconducting parameters. By relaxing above constraint, we study the correlation of energy gap and phonon energywhen I has a distribution. We calculate a map of transition temperature in space constructing by phonon energy and the parameter of electron--phonon interaction, which is helpful for understanding of the relation.     

SSeS超导结的约瑟夫逊效应

以推广的 Jacobson方法 ,从 Ginzburg- L andau方程出发 ,推导出了 SSe S超导结的直流约瑟夫逊效应 ,从而进一步求得了 SSe S超导结的临界电流与温度之间的关系 ,以及对结区厚度的依赖性     

Quantum Dynamical Approach to Various Scattering Mechanisms and Their Influences on Thermal Conductivity of Sr- and Zn-Doped La2CuO4 High-Temperature Superconductor Cuprate

JETP Letters - A theoretical investigation of the thermal conductivity of lightly Sr- and Zn-doped La2CuO4 high temperature superconductor cuprates has been analyzed auspiciously. We used a quantum...     

Charge order driven by Fermi-arc instability and its connection with pseudogap in cuprate superconductors

The recently discovered charge order is a generic feature of cuprate superconductors, however, its microscopic origin remains debated. Within the framework of the fermion-spin theory, the nature of charge order in the pseudogap phase and its evolution with doping are studied by taking into account the electron self-energy (then the pseudogap) effect. It is shown that the antinodal region of the electron Fermi surface is suppressed by the electron self-energy, and then the low-energy electron excitations occupy the disconnected Fermi arcs located around the nodal region. In particular, the charge order state is driven by the Fermi-arc instability, with a characteristic wave vector corresponding to the hot spots of the Fermi arcs rather than the antinodal nesting vector. Moreover, although the Fermi arc increases its length as a function of doping, the charge order wave vector reduces almost linearity with the increase of doping. The theory also indicates that the Fermi arc, charge order and pseudogap in cuprate superconductors are intimately related to each other, and all of them emanates from the electron self-energy due to the interaction between electrons by the exchange of spin excitations.     

Isotropic Bipolaron–Fermion Exchange Theory and Unconventional Pairing in Cuprate Superconductors

The appearance of unconventional pairing in superconducting cuprates is examined from a microscopic model, taking into account important properties of hole‐doped copper oxides. An exchange interaction between fermions and dominantly inter‐site bipolarons is considered to be the mechanism which leads to the pairing. Its momentum dependency is connected to the well‐established fermion–phonon anomalies in cuprate superconductors. Since charge carriers in these materials are strongly correlated, a screened Coulomb repulsion is added to this exchange term. Any ad hoc assumptions like anisotropy are avoided, but a microscopic explanation of unconventional pairing for coupling strengths that are in accordance with experimental facts is provided. One important outcome is a mathematically rigorous elucidation of the role of Coulomb repulsion in unconventional pairing, which is shown to be concomitant with a strong depletion of superconducting pairs. The theory, applied to the special case of LaSr 214, predicts at optimal doping i) a coherence length of $21\AA$, which is the same as that obtained from the Ginzburg–Landau critical magnetic field measured for this material, and ii) d‐wave pair formation in the pseudogap regime, that is, at temperatures much higher than the superconducting transition temperature.     

手征对称性动力学破缺和铜氧化物超导体相变的三维量子电动力学研究

利用三维量子电动力学理论中的Dyson-Schwinger方程方法, 研究了零温情况下平面铜氧化合物超导体的反铁磁相和d波超导相之间的相变. 通过在朗道规范下近似解析求解和数值求解完全耦合的Dyson-Schwinger方程、并将所得结果与1/N展开方法的结果相比较, 发现在半填充准费密子味道数约小于等于4的情况下, 通过手征对称性自发破缺, d波超导相可以演化到反铁磁相, 并且反铁磁相有可能与d波超导相共存. 通过进一步比较不同相的压强, 还说明反铁磁与d波超导共存相为稳定相, 从而反铁磁相确实可以与d波超导相共存.     

Critical Temperature Characteristics of Layered High-Temperature Superconductor Under Pressure

We consider a Ginzburg-Landau modified model of layered high-temperature superconductor under pressure. We have theoretically studied the relation between the pressure and the temperature of layered high-temperature superconductor. If the pressure is not a constant, we have a relation of quadratic equation between the pressure and the temperature of layered high-temperature superconductor. In a special case, we find the critical temperature decreases with further increasing pressure. In another special case, the critical temperature increases with furtherincreasing pressure.