The parent compound of the copper-oxide high temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high temperature superconductors and the superconductivity mechanism. Here we report high resolution angle-resolved photoemission measurement on heavily underdoped Bi2Sr2-xLaxCuO6+d system. The electronic structure of the lightly-doped samples exhibit a number of characteristics: existence of an energy gap along the nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of a coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetism and superconductivity.
Motivated by the observation in copper-oxide high-temperature superconductors, we investigate the appearance of kinks in the electronic dispersion due to coupling to phonons for a system with strong electronic repulsion. We study a Hubbard model supplemented by an electron-phonon coupling of Holstein type within Dynamical Mean Field Theory (DMFT) utilizing Numerical Renormalization Group as impurity solver. Paramagnetic DMFT solutions in the presence of large repulsion show a kink only for large values of the electron-phonon coupling $λ$ or large doping and, contrary to the conventional electr…
An outstanding problem in the field of high-transition-temperature (high Tc) superconductivity is the identification of the normal state out of which superconductivity emerges in the mysterious underdoped regime. The normal state uncomplicated by thermal fluctuations is effectively accessed by the use of applied magnetic fields sufficiently strong to suppress long-range superconductivity at low temperatures. Proposals in which the normal ground state is characterised by small Fermi surface pockets that exist in the absence of symmetry breaking have been superseded by models based on the existe…
We report the hole doping dependencies of the pseudogap phase energy scale, $2Δ_{\rm PG}$, the anti-nodal (nodal) superconducting energy scales $2Δ^{AN}_{\rm SC}$ ($2Δ^{N}_{\rm SC}$) and the charge density wave energy scale, $2Δ_{\rm CDW}$. They have been extracted from the electronic Raman responses of distinct copper oxide families. For all the cuprates studied, we reveal universal doping dependencies which suggest that $2Δ_{\rm PG}$, $2Δ^{AN}_{\rm SC}$ and $2Δ_{\rm CDW}$ are governed by common microscopic interactions and that these interactions become relevant well above the superconductin…
The three-band Emery model is applied to study the selected principal features of the $d$-$wave$ superconducting phase in the copper-based compounds. The electron-electron correlations are taken into account by the use of the diagrammatic expansion of the Guztwiller wave function (DE-GWF) method. The nodal Fermi velocity, Fermi momentum and effective mass are all determined in the paired state and show relatively good agreement with the available experimental data, as well as with the corresponding single-band calculations. Additionally, the influence of the next-nearest neighbor oxygen-oxygen…
The spin-fluctuation mechanism of superconductivity usually results in the presence of gapless or nodal quasiparticle states in the excitation spectrum. Nodal quasiparticle states are well established in copper-oxide, and heavy-fermion superconductors, but not in iron-based superconductors. Here, we study the pairing symmetry and mechanism of a new class of plutonium-based high-Tc superconductors and predict the presence of a nodal $s^{+-}$ wave pairing symmetry in this family. Starting from a density-functional theory (DFT) based electronic structure calculation we predict several three-dimen…
We study systematically the scattering processes and the conductance spectra in nodal-line semimetalsuperconductor junctions using the extended Blonder-Tinkham-Klapwijk theory. The coexistence of peculiar quadruple reflections are found, which are the specular normal reflection, the retro-normal reflection, the specular Andreev reflection and the retro-Andreev reflection. The incident angle dependence and the quasiparticle energy dependence of the double normal reflections and the double Andreev reflections are investigated under various values of parameters such as the interfacial barrier hei…
Doping-dependent evolution of low-energy excitations and quantum phase transitions within effective model for High-Tc copper oxides
本文发展了p型和n型高$T_c$铜氧化物自旋液体顺磁非超导相的平均场论。这一理论适用于有效的$t-t'-t''-J^*$模型与{ \it ab initio}计算的参数和三个网站相关的跳跃。自洽地计算了Hubbard-I近似下的静态自旋-自旋关联函数和运动学关联函数。费米面和带色散的演变得到了广泛的掺杂浓度$x$。对于p-型系统的三种不同类型的行为被发现，这些类型之间的过渡是伴随着费米表面拓扑结构的变化。因此，在$x=0.15$和$x=0.23$发生量子相变。由于费米面拓扑结构的不同，我们发现n型铜氧化物只有一个量子临界浓度，$x=0.2$。节点费米速度和有效质量的掺杂依赖性的计算结果与实验数据符合得很好。
Despite the intensive efforts for determining the mechanism that causes high-temperature superconductivity in copper oxide materials, no consensus on the pairing mechanism has been reached. Recent advances in high resolution angle-resolved photoemission spectroscopies have suggested that a sizeable electron-phonon coupling exists as the principal cause for kinks in the dispersion relations (energy versus wave vector) of the electronic states. Here, we report on a systematic study about the influence of the electron-phonon coupling parameter "$λ$" in the electronic quasiparticle dispersions alo…
By paying special attention to the fact that the doped holes induce deformation of CuO6 octahedrons (or CuO5 pyramids) in cuprate superconductors, we develop a non-rigid band theory treating doping-induced alterations of energy-band structures in copper oxide superconductors. Thanks to this theory, we obtain a complete picture of the doping-induced alteration in the electronic structure of La2CuO4, from the spin-disordered insulating phase to the metallic phase. We conclude that the Fermi surface structure of this cuprate in the underdoped region consists of Fermi pockets in the antinodal regi…
We discuss the physics of the high temperature superconductivity in hole doped copper oxide ceramics in the pseudogap region. Starting from an effective reduced Hamiltonian relevant to the dynamics of holes injected into the copper oxide layers proposed in a previous paper, we determine the superconductive condensate wavefunction. We show that the low-lying elementary condensate excitations are analogous to the rotons in superfluid $^4He$. We argue that the rotons-like excitations account for the specific heat anomaly at the critical temperature. We discuss and compare with experimental observ…
A nodeless superconducting (SC) gap was reported in a recent scanning tunneling spectroscopy experiment of a copper-oxide monolayer grown on the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) substrate [Y. Zhong {\it et al.}, Sci. Bull. {\bf 61}, 1239 (2016)], which is in stark contrast to the nodal d-wave pairing gap in the bulk cuprates. Motivated by this experiment, we first show with first-principles calculations that the tetragonal CuO (T-CuO) monolayer on the Bi2212 substrate is more stable than the commonly postulated CuO$_{2}$ structure. The T-CuO monolayer is composed of two CuO$_2$ layers sh…
The doping of charge carriers into the CuO2 planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping conc…
The issue of whether the quantum critical point (QCP) is hidden inside unconventional superconductors is a matter of hot debate. Although a prominent experiment on London penetration depth has demonstrated the existence of the QCP in the isovalent-doped iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$, with the observation of a sharp peak in the penetration depth in the vicinity of the disappearance of magnetic order at zero temperature, the nature of such an emerging QCP remains unclear. Here, we provide a unique picture to understand well the phenomena of the QCP based on the framewor…
The pursuit of a comprehensive understanding of the dynamical nature of intertwined orders in quantum matter has fueled the development of several new experimental techniques, including time- and angle-resolved photoemission spectroscopy (TR-ARPES). In this regard, the study of copper-oxide high-temperature superconductors, prototypical quantum materials, has furthered both the technical advancement of the experimental technique, as well as the understanding of their correlated dynamical properties. Here, we provide a brief historical overview of the TR-ARPES investigations of cuprates, and re…
A theory of the underlying metallic state of large-U$_d$ high-Tc cuprates is presented starting from the covalent Cu-2O 3-band model associated with a sufficiently small copper occupancy. U$_d=\infty$ is dealt by the slave fermion approach. Diagrammatic low order NCA theory in terms of the Cu-O hopping is supplemented with two independent slave particle chemical potentials which implement the U(1) local gauge invariance "at average", thus avoiding the mean-field approximations. The resulting hole spectra consist of itinerant and localized states. The itinerant states close to the Fermi level a…
Electronic Raman scattering measurements have been performed on hole doped copper oxide superconductors as a function of temperature and doping level. In the superconducting state coherent Bogoliubov quasiparticles develop preferentially over the nodal region in the underdoped regime. We can then define the fraction of coherent Fermi surface, $f_c$ around the nodes for which quasiparticles are well defined and superconductivity sets in. We find that $f_c$ is doping dependent and leads to the emergence of two energy scales. We then establish in a one single gap shem, that the critical temperatu…
The unusual dispersion of electron states in antinodal direction for $n_{h}\simeq\frac{1}{8}$ holes/copper is explained. In photoelectron excitations, transformations of the $\mathbf{k}$ space as a consequence of symmetry breaking proves to be an exceptional phenomenon distinctly reflected by the experiments. Paired photoelectron emissions are predicted that enable a new energetic upward shift termed as \emph{pair shake-up effect} which explains the experimentally observed sharp transition between non-dispersive and dispersive behaviour. The disappearance of antinodal states for $\mathbf{k}_{|…
Since the discovery of the cuprate high-temperature superconductivity in 1986, a universal phase diagram has been constructed experimentally and numerous theoretical models have been proposed. However, there remains no consensus on the underlying physics thus far. Here, we theoretically investigate the phase diagram of hole-doped cuprates based on an itinerant-localized dual fermion model, with the charge carriers doped on the oxygen sites and localized holes on the copper $d_{x^{2}-y^{2}}$ orbitals. We analytically demonstrate that the puzzling anomalous normal state or the strange metal coul…
The normal state in the hole underdoped copper oxide superconductors has proven to be a source of mystery for decades. The measurement of a small Fermi surface by quantum oscillations on suppression of superconductivity by high applied magnetic fields, together with complementary spectroscopic measurements in the hole underdoped copper oxide superconductors, point to a nodal electron pocket from charge order in YBa2Cu3O6+x. Here we report quantum oscillation measurements in the closely related stoichiometric material YBa2Cu4O8, which reveal similar Fermi surface properties to YBa2Cu3O6+x, desp…
High-temperature superconductivity in iron-arsenic materials (pnictides) near an antiferromagnetic phase raises the possibility of spin-fluctuation-mediated pairing. However, the interplay between antiferromagnetic fluctuations and superconductivity remains unclear in the underdoped regime, which is closer to the antiferromagnetic phase. Here we report that the superconducting gap of the underdoped pnictides scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the SC gap develops on underdoping. This pseudogap occurs on Fermi surface sheets connected b…
