层状铜氧化物超导体的有限温Landau理论

采用有限温的Landau理论来描述层状铜氧化物超导体中的竞争序和层间Josephson隧道效应 .通过讨论单层和双层铜氧面结构的超导体中序参量的相图，并与实验相比较来确定理论中 唯象参数的合理范围，同时也解释了在最佳掺杂区附近赝能隙的转变温度远高于与之对应的 能量尺度这个颇具疑惑的实验事实.再将理论应用到多层的超导体系，计算了超导转变温度 T_c随体系层数N的变化，在合理的参数空间里，最佳掺杂区附近的计算结果与现 有的实验相一致，而在极度欠掺杂区和极度过掺杂区的单调性上升可作为理论预言 关键词： 竞争序 层间Josephson隧道效应 电荷不平衡 赝能隙     

空穴掺杂超导体的超导电性

在二维空穴掺杂t-t′-J-U模型和重整化平均场理论的框架下,用Gutzwiller方法研究了电子次近邻跃迁对空穴掺杂超导体的超导电性的影响。在掺杂浓度小于0.1的欠掺杂区,随着电子次近邻跃迁的增大,超导序参数在欠掺杂区域受到抑制,而在过掺杂区得到加强。     

扶手椅型单壁碳纳米管中的B/N对共掺杂

利用第一性原理电子结构计算方法,研究了扶手椅型单壁碳纳米管（SWCNT）的B/N对掺杂效应.研究发现,对于扶手椅型SWCNT两个不同的掺杂位点,B/N对更容易发生在与管轴线成30°角的P₁位点上.B/N对的掺杂使得金属性SWCNT能隙打开,且能隙随着B/N对轴向掺杂浓度的升高而逐渐增大.同时,还发现两B/N对掺杂后SWCNT的电子结构敏感地依赖于B/N对在圆周上的相对位置,能隙随着B/N对相对距离的增大而增大.这归结于B/N对的掺入影响了原有的电荷分布,这种影响是局域的 关键词： 单壁碳纳米管 B/N对掺杂 电子结构 第一性原理     

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.     

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.     

轨道序对半掺杂锰氧化物光学性质的影响

在两轨道双交换模型基础上，讨论了电子关联作用对半掺杂锰氧化物轨道序的影响，推导出能计算各个相的光电导公式.结果显示，光吸收与轨道序之间存在关联现象.对于铁磁相，当格点库仑相互作用(U)从无到有逐渐增加时，铁磁相会从无轨道序过渡到有轨道序，相变前 后非相干光吸收有一个从无能隙到有能隙的明显变化.对于层状反铁磁的A相，U的增加会使 轨道序更明显，非相干光吸收部分的能隙随之也增大. 关键词： 轨道序 光电导 锰氧化物 库仑相互作用     

电子型掺杂铜氧化物超导体准粒子激发谱研究

文中工作采用t-t′-t″-J模型研究了电子型掺杂铜氧化物超导体在超导序和反铁磁序共存情况下的准粒子激发谱,分别计算了在欠掺杂区、最佳掺杂区和过掺杂区的谱权重,计算结果与角分辨光电子谱实验结果符合较好.     

物理新闻与动态

角分辨光发射谱实验证实MgB2 中的多能隙　　对MgB2 超导机理的理解关键在于其超导能隙的性质(详见《物理》 ,2 0 0 3,32 :32 5 ) .比热、光发射谱、隧穿谱和拉曼谱等实验已经提示 :在MgB2 中存在两种类型的超导能隙 ,这一点既不同于传统超导体 ,也不同于铜氧化物高温超导体 .不幸的是 ,上述实验不可能区分超导电子的动量 ,因此只能提供在整个动量空间的平均信息 .另一方面 ,能带计算给出了能隙对晶体波矢k的详细依赖关系 .从这个意义上讲 ,实验测定能隙在布里渊区内的分布 ,对验证理论和理解MgB2 中的超导机制具有重要意义 .按照Choi(…     

Bi_2Sr_2CuOy掺La－和掺Pb－样品的磁性质

研究了掺Ｌａ－和掺Ｐｂ－的Ｂｉ２２０１样品在不同磁场下（Ｈ＝０．０１，０．０３和５Ｔ）正常态的直流磁化率与温度的关系．样品包括了从欠掺杂到过掺杂区域．实验结构表明，当增加空穴浓度时，从欠掺杂区域到开始发生超导转变区域，居里项的贡献逐渐减弱；而在超导区和金属区，居里项随着空穴浓度的增加而增加．除金属区外，居里项随着外加磁场的增加而增加．这表明在费米面附近存在一个窄能带．在低温下对于非理想超导体，其磁矩冻结且有序化，而在高场下（５Ｔ）则被抑制．在金属区，高温下是Ｃｕｒｉｅ－Ｗｅｉｓｓ行为，但居里常数不随外场而变化．我们对这种反常磁性行为提出了一种解释．     

在La_2CuO_(4 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 .然而 ,在过去的 10年中 ,核磁共振、μ子自旋共振 (μ -ＳＲ)和中子散射实验已经证明 :动态反铁磁关联是高温超导体的固有属性 .最近 ,研究兴趣转移到了观察高温超导体中的静态非公度磁有序 .对于Ｌａ1 6 -ｘＮｄ0 4 ＳｒｘＣｕＯ4 ,Ｔｒａｎｑｕａｄａ等的研究表明 ,存在电荷以及自旋的静态有序 ,并且调制相对于晶格…     

Mechanism of Pseudogap Detected by Electronic Raman Scattering： Phase Fluctuation or Hidden Order？

We study the electronic Raman scattering in the cuprates to distinguish the two possible scenarios of the pseudogap normal state. In one scenario, the pseudogap is assumed to be caused by phase fluctuations of the preformed Cooper pairs. We find that pair-breaking peaks appear in both the B1g and B2g Raman channels, and they axe smeared and tend to shift to the same energy with the increasing strength of phase fluctuations. Thus both channels reflect the same pairing energy scale, irrespectively of the doping level. In another scenario, the pseudogap is assumed to be caused by a hidden order that competes with the superconducting order. As an example, we assume that the hidden order is the d-density-wave （DDW） order. We find analytically and numerically that in the DDW normal state there is no Raman peak in the B2g channel in a tight-binding model up to the second nearest-neighbor hopping, while the Raman peak in the Big channel reflects the energy gap caused by the DDW order. This behavior is in agreement with experiments in the pseudogap normal state. To gain further insights, we also calculate the Raman spectra in the DDW＋SC state. We study the doping and temperature dependence of the peak energy in both channels and find a two-gap behavior, which is in agreement with recent Raman experiments. Therefore, our results shed light on the hidden order scenario for the pseudogap.     

Anomalous momentum dependence of the superconducting coherence peak and its relation to the pseudogap of La1.85Sr0.15CuO4

We performed high-resolution angle-resolved photoemission spectroscopy on La1.85Sr0.15CuO4 to study the nature of the single-particle excitation gap. We found that there is a well-defined superconducting coherence peak in the off-nodal region while it is strongly suppressed around the antinode. The momentum dependence of the single-particle excitation gap shows a striking deviation from the dx-y2--wave symmetry with anomalous enhancement around the antinode in both the superconducting and the pseudogap state. The observed close correlation between the superconducting coherence peak and the pseudogap suggests a substantial contribution of the pseudogap to the anomalous behavior of the gap in the superconducting state.     

Interplay between superconductivity and pseudogap state in bilayer cuprate superconductors

The interplay between the superconducting gap and normal-state pseudogap in the bilayer cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. It is shown that the charge carrier interaction directly from the interlayer coherent hopping in the kinetic energy by exchanging spin excitations does not provide the contribution to the normal-state pseudogap in the particle–hole channel and superconducting gap in the particle–particle channel, while only the charge carrier interaction directly from the intralayer hopping in the kinetic energy by exchanging spin excitations induces the normal-state pseudogap in the particle–hole channel and superconducting gap in the particle–particle channel, and then the two-gap behavior is a universal feature for the single layer and bilayer cuprate superconductors.     

Andreev and single-particle tunneling spectra of underdoped cuprate superconductors

We study tunneling spectroscopy between a normal metal and an underdoped cuprate superconductor modeled by a phenomenological theory in which the pseudogap is a precursor to the undoped Mott insulator. In the low barrier tunneling limit, the spectra are enhanced by Andreev reflection only within a voltage region of the small superconducting energy gap. In the high barrier tunneling limit, the spectra show a large energy pseudogap associated with single particle tunneling. Our theory semiquantitatively describes the two gap behavior observed in tunneling experiments.     

Non-Fermi-liquid behavior in the fluctuating gap model: From the pole to a zero of the Green’s function

We analyze the non-Fermi-liquid (NFL) behavior of the fluctuating gap model (FGM) of pseudogap behavior in both one and two dimensions. A detailed discussion of quasiparticle renormalization (Z-factor) is given, demonstrating a kind of marginal Fermi-liquid or Luttinger-liquid behavior and topological stability of the bare Fermi surface (the Luttinger theorem). In the two-dimensional case, we discuss the effective picture of the Fermi surface destruction both in the hot spot model of dielectric (AFM, CDW) pseudogap fluctuations and for the qualitatively different case of superconducting d-wave fluctuations, reflecting the NFL spectral density behavior and similar to that observed in ARPES experiments on copper oxides. The text was submitted by the authors in English.     

Phenomenological description of the two energy scales in underdoped cuprate superconductors

Raman and angle-resolved photoemission spectroscopy experiments have demonstrated that in superconducting underdoped cuprates nodal and antinodal regions are characterized by two energy scales instead of the one expected in BCS theory. The nodal scale decreases with underdoping while the antinodal one increases. Contrary to the behavior expected for an increasing energy scale, the antinodal Raman intensity decreases with decreasing doping. Using the Yang-Rice-Zhang model, we show that these features are a consequence of the nonconventional nature of the superconducting state in which superconductivity and pseudogap correlations are both present and compete for the phase space.     

High-resolution photoemission study of MgB2

We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s like with the gap value ( Delta) of 4.5+/-0.3 meV at 15 K. The temperature dependence (15-40 K) of the gap value follows well the BCS form, suggesting that 2Delta/k(B)T(c) at T = 0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.     

Evidence for two energy scales in the superconducting state of optimally doped (Bi,Pb)2(Sr,La)2CuO6+delta

We use angle-resolved photoemission spectroscopy to investigate the energy gap(s) in (Bi,Pb)2(Sr,La)2CuO6+delta. We find that the spectral gap has two components in the superconducting state: a superconducting gap and pseudogap. Differences in their momentum and temperature dependence suggest that they represent two separate energy scales. Spectra near the node reveal a sharp peak with a small gap below T(c) that closes at T(c). Near the antinode, spectra are broad with a large energy gap of approximately 40 meV above and below T(c). The latter spectral shape and gap magnitude are almost constant across T(c), indicating that the pseudogap state coexists with the superconducting state below T(c), and it dominates spectra around the antinode. We speculate that the pseudogap state competes with the superconductivity by diminishing spectral weight in antinodal regions, where the superconducting gap is largest.     

Single-particle spectral function of the Hubbard chain: frustration induced

The one-electron spectral function of a frustrated Hubbard chain is computed by making use of the cluster perturbation theory. The spectral weight we found turns out to be strongly dependent on the frustrating next-nearest-neighbor hopping t'. A frustration induced pseudogap arises when the system evolves from a gapful Mott insulator to a gapless conductor for an intermediate value of the frustration parameter |t'|. Furthermore, the opening of a pseudogap in the density of states already in the metallic side leads to a continuous opening of the true gap in the insulator. For the hole-doped case, the pseudogap is pinned at the Fermi energy, while the Mott gap is shifted in energy with increasing Hubbard interaction U. The separation of the pseudogap and Mott gap in the hole-doped system demonstrates the validity of the existence of a pseudogap.     

Manifestations of the pseudogap and superconducting gap of cuprates in photoemission

The degree to which the interpretation of the existence of a pseudogap and a superconducting gap in cuprates on the basis t-t′-U the Hubbard-model corresponds to the data obtained from the photoemission spectra is discussed. The pseudogap in the model is interpreted as the work function of electrons from the insulating parts of the Brillouin zone boundary. On this basis one can explain the angle dependence of the gap measured in the photoemission spectra and its evolution on changes in doping and temperature. In particular, an explanation is found for the decline in the ratio of the angle derivative of the gap near the site, v _Δ = (1/2)dΔ(?)/d?, to the maximum value of the gap, Δ_max, with decreasing doping. That behavior and the different temperature dependence of the gap Δ(?) for different angles are due to the presence of two contributions to Δ with different anisotropies—from the pseudogap and from the superconducting gap. The calculation of the spectral functions confirms the sharp Fermi boundary observed in the direction and the smeared edge of the distribution along the path Γ(0, 0)-M(π, 0)-Y(π, π).