受最近镧基铜氧化物非弹性中子散射（INS）实验的启发，基于费米学理论，我们研究了镧基（例如，La $_{2-x}$ Sr $_x$ CuO $_4$）和Y基（例如，YBa $_2$ Cu $_3$ O $_y$）铜酸盐。在超导态下，YBa $_2$ Cu $_3$ O $_y$的自旋激发由频率为40 meV的尖锐共振峰控制。在共振频率以下和以上，不相称（IC）峰发展并且峰的强度急剧降低。在正常状态下，不发生共振激发，IC峰合并成相称的峰。La $_{2-x}$ Sr $_x$ CuO $_4$的自旋激发与Y基的有很大的不同，即由于超导能隙的减小和可能的自旋条纹有序的存在，不存在共振峰。光谱仅在预期的共振频率（约18毫电子伏）增强，而它仍然是不相称的。另一方面，在频率55 meV的另一个频率尺度也被揭示，即频谱是相称的，在这个频率的局部最大值。我们详细阐述了所有的结果的基础上的费米面拓扑结构和d波超导性，并建议自旋条纹顺序也是重要的，在决定的La基铜氧化物的自旋激发。给出了Y基和La基铜氧化物的自旋激发相干图。
英文摘要：
Motivated by recent inelastic neutron scattering (INS) experiments on La-based cuprates and based on the fermiology theories, we study the spin susceptibility for La-based (e.g., La$_{2-x}$Sr$_x$CuO$_4$) and Y-based (e.g., YBa$_2$Cu$_3$O$_y$) cuprates, respectively. The spin excitation in YBa$_2$Cu$_3$O$_y$ is dominated by a sharp resonance peak at the frequency 40 meV in the superconducting state. Below and above the resonance frequency, the incommensurate (IC) peaks develop and the intensity of the peaks decreases dramatically. In the normal state, the resonant excitation does not occur and the IC peaks are merged into commensurate ones. The spin excitation of La$_{2-x}$Sr$_x$CuO$_4$ is significantly different from that of Y-based ones, namely, the resonance peak does not exist due to the decreasing of the superconducting gap and the presence of the possible spin-stripe order. The spectra are only enhanced at the expected resonance frequency (about 18 meV) while it is still incommensurate. On the other hand, another frequency scale at the frequency 55 meV is also revealed, namely the spectra are commensurate and local maximum at this frequency. We elaborate all the results based on the Fermi surface topology and the d-wave superconductivity, and suggest that the spin-stripe order be also important in determining the spin excitation of La-based cuprates. A coherent picture for the spin excitations is presented for Y-based and La-based cuprates.
We present a phenomenological Green's function to characterize the superconducting and pseudogap phases of the cuprates based on a microscopic theory of doped Mott insulators. In this framework, the "Fermi arc" and "kink" phenomena observed by angle-resolved photoemission spectroscopy (ARPES) experiments in the pseudogap phase can be systematically explained as a function of doping, which is further connected to the two-gap feature in the superconducting phase with dichotomy between the nodal and antinodal physics. We demonstrate that a phase-string-induced fractionalization plays the key role…
The recent development of x-ray scattering techniques revealed the charge-excitation spectrum in high-$T_c$ cuprate superconductors. While the presence of a dispersive signal in the high-energy charge-excitation spectrum is well accepted in the electron-doped cuprates, its interpretation and universality are controversial. Since charge fluctuations are observed ubiquitously in cuprate superconductors, the understanding of its origin is a pivotal issue. Here, we employ the layered $t$-$J$ model with the long-range Coulomb interaction and show that an acoustic-like plasmon mode with a gap at in-…
The nature of the spin excitations in superconducting cuprates is a key question toward a unified understanding of the cuprate physics from long-range antiferromagnetism to superconductivity. The intense spin excitations up to the over-doped regime revealed by resonant inelastic X-ray scattering bring new insights as well as questions like how to understand their persistence or their relation to the collective excitations in ordered magnets (magnons). Here, we study the evolution of the spin excitations upon hole-doping the superconducting cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ by disentangling…
Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum ${\bf q}_{\parallel}=(0,0)$ for various doping rates in electron-doped cuprates ${\rm La_{2-x}Ce_xCuO_4}$. We find that those new experimental data are well captured by acoustic-like plasmon excitations in a microscopic study of the layered $t$-$J$ model with the long-range Coulomb interaction. The acoustic-like plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping $t_z$.
A Unified Theory for the Cuprates, Iron-Based and Similar Superconducting Systems: Application for Spin and Charge Excitations in the Hole-Doped Cuprates
Collective nature of spin excitations in superconducting cuprates probed by resonant inelastic x-ray scattering
利用共振非弹性X射线散射（RIXS）技术，研究了高温超导体YBa_2Cu_3 O_（6 +x）在宽掺杂范围（0.1 < x < 1）内的色散自旋激发。当入射光子能量从共振条件失谐时，仔细监测激发分布，发现自旋激发能对所有x都与失谐无关。这些发现表明，掺杂铜氧化物中自旋翻转RIXS分布的最大部分来自磁集体模式，而不是最近理论上提出的非相干粒子-空穴激发[Benjamin等人，Phys. Rev. Lett. 112，247002（2014）]。在铜酸盐的电子系统的理论描述的影响进行了讨论。
Dispersive spin excitations in highly overdoped cuprates revealed by resonant inelastic x-ray scattering
利用共振非弹性X射线散射（RIXS）在铜$L$吸收边，我们已经观察到强烈的，分散的自旋激发在高度过掺杂的Tl $_2$ Ba $_2$ CuO $_{6+δ}$（超导$T_c =6$ K），一种化合物，其正常状态的电荷传输和热力学性质已被广泛研究，并显示出典型的费米液体行为。补充RIXS实验上稍过掺杂的Tl $_2$ Ba $_2$ CuO $_{6+δ}$（$T_c =89$ K）和Y $_{1-x}$ Ca $_{x}$ Ba $_2$ Cu $_3$ O $_{6+δ}$化合物跨越广泛的掺杂水平表明，这些激发表现出的能量和能量积分光谱权重密切类似的反铁磁振子在未掺杂的铜酸盐。令人惊讶的共存费米液体的电荷激发和高能量的自旋激发，让人想起反铁磁绝缘体在高度过掺杂的化合物提出了挑战，目前的理论模型的铜酸盐。
Recently it was reported experimentally that the monolayer graphene can be doped to beyond the Van Hove singularity. We study theoretically the possible superconductivity and the corresponding spin excitations of the monolayer graphene in this doping region. A static spin-density-wave state is favorable due to the nested Fermi surface as the Fermi level is doped to the Van Hove singularity point. Superconductivity may be realized upon further doping. The spin excitations in the superconducting state are studied theoretically based on the random phase approximation. The overall features are qua…
We propose that cuprate superconductors are in the vicinity of a spontaneous d-wave type Fermi surface symmetry breaking, often called a d-wave Pomeranchuk instability. This idea is explored by means of a comprehensive study of magnetic excitations within the slave-boson mean-field theory of the t-J model. We can naturally understand the pronounced xy anisotropy of magnetic excitations in untwinned YBa_{2}Cu_{3}O_{y} and the sizable change of incommensurability of magnetic excitations at the transition temperature to the low-temperature tetragonal lattice structure in La_{2-x}Ba_{x}CuO_{4}. In…
Unveiling the nature of collective charge excitations in a cuprate superconductor
The Chinese University of Hong Kong（香港中文大学）；Universität Zürich（苏黎世大学）；Diamond Light Source（钻石光源）；Korea Advanced Institute of Science and Technology（韩国科学技术院）；KTH Royal Institute of Technology（瑞典皇家理工学院）；The University of Tokyo（东京大学）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；Institut Néel, CNRS and Université Grenoble Alpes（内尔研究所，法国国家科学研究中心与格勒诺布尔阿尔卑斯大学）
The discovery of charge-density wave (CDW)-related effects in the resonant inelastic x-ray scattering (RIXS) spectra of cuprates holds the tantalizing promise of clarifying the interactions that stabilize the electronic order. Here, we report a comprehensive RIXS study of La2-xSrxCuO4 (LSCO) finding that CDW effects persist up to a remarkably high doping level of x = 0.21 before disappearing at x = 0.25. The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no d…
