Effects of the Next Nearest Neighbor Hopping on Superconductivity and Antiferromagnetism of Gossamer Superconductivity

The two dimensions hole-doped t-t '-J-U model was studied based on the Gutzwiller approach and the renormalized mean-field theory.The phase diagrams of gossamer superconductors and the effects of the next-nearestneighbor hopping(t ') on superconductivity and antiferromagnetism based on the t-t '-J-U model were investigated.The results show that the qualitative feature of the phase diagrams in the t-t '-J-U model is the same as in the case of the t-J-U model.The antiferromagnetic order coexists with the d-wave superconductivity(dSC) in the underdoped region below the doping δ≈ 0.1 and is enhanced by the t '.The dSC order is slightly suppressed by t ' in the underdoped region and greatly enhanced in the overdoped region.The dSC order is pushed to a larger doping region and the coexistence region of the AF and dSC extends to higher doping.     

Enhancement of the superconducting transition temperature of La2-xSrxCuO4 bilayers: role of pairing and phase stiffness

The superconducting transition temperature T(c) of bilayers comprising underdoped La(2-x)Sr(x)CuO(4) films capped by a thin heavily overdoped metallic La(1.65)Sr(0.35)CuO(4) layer, is found to increase with respect to T(c) of the bare underdoped films. The highest T(c) is achieved for x=0.12, close to the "anomalous" 1/8 doping level, and exceeds that of the optimally doped bare film. Our data suggest that the enhanced superconductivity is confined to the interface between the layers. We attribute the effect to a combination of the high pairing scale in the underdoped layer with an enhanced phase stiffness induced by the overdoped film.     

Enhancement of pairing correlation by t' in the two-dimensional extended t-J model

We investigate the effects of the next-nearest-neighbor (t') and the third-nearest-neighbor (t") hopping terms on superconductivity correlation in the 2D hole-doped extended t-J model based on the variational Monte Carlo, mean-field calculation and exact diagonalization method. Despite the diversity of the methods employed, the results all point to a consistent conclusion: While the d-wave superconductivity correlation is slightly suppressed by t' and t" in underdoped regions, it is greatly enhanced in the optimal and overdoped regions. The optimal Tc is a result of the balance of these two opposite trends.     

Phase fluctuations and the pseudogap in YBa(2)Cu(3)O (x)

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa(2)Cu(3)O (x) single crystals and Monte Carlo simulations of the anisotropic 3D- XY model. We directly show that T(c) of underdoped YBa(2)Cu(3)O (x) is strongly suppressed from its mean-field value (T(MF)(c)) by phase fluctuations of the superconducting order parameter. For overdoped YBa(2)Cu(3)O (x) fluctuation effects are greatly reduced and T(c) approximately T(MF)(c). We find that T(MF)(c) exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.     

Temperature dependence of the gap size near the brillouin-zone nodes of HgBa2CuO4+delta superconductors

Although more than 20 years have passed, the identification of the superconducting order parameter in cuprates is still under debate. Here, we show that the gap size near the nodes is a good candidate for the order parameter: it scales with the critical temperature Tc over a wide doping range and displays a significant temperature dependence below Tc in both the underdoped and the overdoped regimes. In contrast, the gap size at the antinodes does not scale with Tc in the underdoped regime and appears to be controlled by the pseudogap which persists below Tc.     

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。     

Marginal fermi liquid theory in the Hubbard model

We find marginal-Fermi-liquid- (MFL) like behavior in the Hubbard model on a square lattice for a range of hole doping and on-site interaction parameter U. Thereby we use a self-consistent projection operator method. It enables us to compute the momentum and frequency dependence of the single-particle excitations with high resolution. The Fermi surface is found to be holelike in the underdoped regime and electronlike in the overdoped regime. Our calculations concern normal state properties of the system. When a comparison is possible, we find consistency with finite temperature quantum Monte Carlo results. We also find a discontinuous change with doping concentration from a MFL to a Fermi-liquid behavior resulting from a collapse of the lower Hubbard band. This renders Luttinger's theorem inapplicable in the underdoped regime.     

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     

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.     

Tunneling into the normal state of Pr2-xCexCuO4

The temperature dependence of the tunneling conductance was measured for various doping levels of Pr(2-x)CexCuO4 using planar junctions. A normal state gap is seen at all doping levels studied, x=0.11 to x=0.19. We find it to vanish above a certain temperature T*. T* is greater than T(c) for the underdoped region and it follows T(c) on the overdoped side. This behavior suggests finite pairing amplitude above T(c) on the underdoped side.     

Same superconducting criticality for underdoped and overdoped La2-xSrxCuO4 single crystals

By measuring the superconducting diamagnetic moments for an underdoped and an overdoped La2-xSrxCuO4 single crystal with equal quality and roughly equal transition temperatures, it is found that the underdoped sample has only one transition which corresponds to H(c2), but the overdoped sample has two transitions with the higher one at H(c2). Further investigation reveals the same upper-critical field H(c2) for both samples although the overall charge densities are very different, indicating the possibility of a very direct and detailed equivalence of the superconducting condensation process in the two doping limits. The second transition for the overdoped sample can be understood as the bulk coupling between the superconducting clusters produced by macroscopic phase separation.     

Oxygen isotope effect in disordered underdoped and overdoped La2−xSrxCu1−yZnyO4 superconductors

The effect of oxygen isotopic substitution on the superconducting transition temperature has been studied for heavily underdoped and overdoped La_2?xSr_xCu_1?yZn_yO₄ compounds with different Zn contents in the CuO₂ plane. The effect of Zn on the isotope coefficient, α, was significantly more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with disorder content can be described quite well within a model based solely on Cooper pair-breaking in the case of the underdoped compounds. This model fails to describe the behavior of α(y) for the overdoped samples, even though Zn still suppresses T_c very effectively at this hole (Sr) content, indicating that the Zn induced pair-breaking is still very much at play. We discuss the implications of these findings in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as hole content in the CuO₂ plane, p (≡x), is varied.     

Excitation gap in the normal and superconducting state of underdoped Bi2Sr2Ca1-xDyxCu2O8+\delta thin film and single crystals

We summarize photoemission data from underdoped Bi₂Sr₂Ca_1-xDy_xCu₂O_8+\delta that revealed anomalous properties: i) We observed an excitation gap in the normal state of underdoped samples, and this normal state gap closes in overdoped samples; ii) The normal state gap has similar magnitude and momentum dependence as the superconducting gap, which is consistent with a d_x ² _-y ² order parameter; iii) The normal state gap persists to a temperature range much higher than T_c; iv) The superconducting gap in the underdoped regime does not scale with T_c. These results are consistent with theoretical models that suggest the underdoped regime can be characterized by two temperatures, a mean‐field temperature below which there is pairing, and a lower superconducting transition temperature at which the pairs become phase coherent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic fluctuations in coupled inequivalent Hubbard layers as a model for

We investigate, within the fluctuation-exchange approximation, a correlated-electron model for represented by two inequivalent Hubbard layers coupled by an interlayer hopping . An energy offset is introduced in order to produce a different charge carrier concentration in the two layers. We compare several single-particle and magnetic excitations, namely, the single particle scattering rate, the spectral function and the spin lattice as well as spin-spin relaxation times in the two layers as a function of . We show that the induced interlayer magnetic coupling produces a tendency to “equalization” of the magnetic properties in the two layers whereby antiferromagnetic fluctuations are suppressed in the less doped layer and enhanced in the heavily doped one.The strong antiferromagnetic bilayer coupling causes the charge carriers in the plane with larger doping concentration to behave similar to those of the underdoped layer, they are coupled to. This effect grows for decreasing temperature. For high temperatures or if both layers are optimally or overdoped, i.e. when the antiferromagnetic correlation length becomes of the order or smaller than one lattice site the charge carrier and magnetic dynamics of the two layers is disconnected and the equalization effect disappears. These results are in good agreement with NMR experiments on by Stern et al. [Phys. Rev B 51, 15478 (1995)]. We also compare the results with calculations on bilayer systems with equivalent layers as models for the constituent compounds and . Received: 28 August 1998     

A comparison of μSR and thermopower in Hg 1:2:0:1 high‐Tc cuprates

We present the results of μSR measurements of the transverse field relaxation rate below T_c for seven samples of HgBa₂ CuO_4+\delta, spanning the underdoped, optimally doped, and nominally overdoped regions of the superconducting phase. On comparison with the “anomalous” thermopower data for these compounds, we find that nominally “overdoped” Hg 1:2:0:1 resembles underdoped compounds in certain respects. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.     

Magnitude of spin and charge density wave amplitudes in underdoped cuprates

Self-consistent calculation of spin (charge) density wave (S(C)DW) order parameters have been performed for bilayered cuprates on the basis of a singlet-correlated band model. Evolution of the Fermi surface in the strongly underdoped regime is described by a two-band approach. The smooth development of the pseudogap formation temperature from underdoped to overdoped states is explained and the Fourier amplitudes 〈s_q〉 (spin) and 〈e_q〉 (charge) modulations are calculated. We have found a maximum of the incommensurability for doping 0.09 ÷ 0.11 holes per copper site.     

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.     

Effects of Cooper pair tunneling on the electronic structure of trilayer cuprate superconductors

Recent angle-resolved photoemission spectroscopy (ARPES) experiment on the optimally doped trilayer cuprate superconductors Bi2223 has revealed a layer variation of both doping density and d-wave gap. In particular, the two outer layers are overdoped with a gap which is larger than the gap for optimally doped single layer cuprates while the inner layer is underdoped with an even larger gap. Here we propose a minimal model composed of three layer t-J Hamiltonian, single particle interlayer tunneling as well as Cooper pair tunneling terms. By using renormalized mean field method, we study the superconducting order parameters and their dependence of tunneling parameters. Some relevant physical quantities have been calculated. Both tunneling effects influence the electronic structure of the trilayer system and their cooperative action may qualitatively explain the ARPES results.     

Indications of an electronic phase transition in two-dimensional superconducting YBa2Cu3O(7-x) thin films induced by electrostatic doping

We successfully tuned an underdoped ultrathin YBa2Cu3O(7-x) film into the overdoped regime by means of electrostatic doping using an ionic liquid as a dielectric material. This process proved to be reversible. Transport measurements showed a series of anomalous features compared to chemically doped bulk samples and a different two-step doping mechanism for electrostatic doping was revealed. The normal resistance increased with carrier concentration on the overdoped side and the high temperature (180 K) Hall number peaked at a doping level of p～0.15. These anomalous behaviors suggest that there is an electronic phase transition in the Fermi surface around the optimal doping level.