Remnant Fermi Surfaces in Photoemission

Recent experiments have introduced a new concept for analyzing the photoemission spectra of correlated electrons -- the remnant Fermi surface (rFs), which can be measured even in systems which lack a conventional Fermi surface. Here, we analyze the rFs in a number of interacting electron models, and find that the results fall into two classes. For systems with pairing instabilities, the rFs is an accurate replica of the true Fermi surface. In the presence of nesting instabilities, the rFs is a map of the resulting superlattice Brillouin zone. The results suggest that the gap in Ca_2CuO_2Cl_2 is of nesting origin.Comment: 4 pages LaTex, 3 ps figure

A competing order scenario of two-gap behavior in hole doped cuprates

Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature $T_c$, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order.Comment: 9 pages, 7 fig

Theory of non-Fermi liquid and pairing in electron-doped cuprates

We apply the spin-fermion model to study the normal state and pairing instability in electron-doped cuprates near the antiferromagnetic QCP. Peculiar frequency dependencies of the normal state properties are shown to emerge from the self-consistent equations on the fermionic and bosonic self-energies, and are in agreement with experimentally observed ones. We argue that the pairing instability is in the $d_{x^{2}-y^{2}}$ channel, as in hole-doped cuprates, but theoretical $T_{c}$ is much lower than in the hole-doped case. For the same hopping integrals and the interaction strength as in hole-doped materials, we obtain $T_{c}\sim10$K at the end point of the antiferromagnetic phase. We argue that a strong reduction of $T_{c}$ in electron-doped cuprates compared to hole-doped ones is due to critical role of the Fermi surface curvature for electron-doped materials. The $d_{x^{2}-y^{2}}$-pairing gap $\Delta(\mathbf{k},\omega)$ is strongly non-monotonic along the Fermi surface. The position of the gap maxima, however, does not coincide with the hot spots, as the non-monotonic $d_{x^{2}-y^{2}}$ gap persists even at doping when the hot spots merge on the Brillouin zone diagonals.Comment: 16 page

Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates

The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping from a pseudogap state in the underdoped cuprates to a superconducting state at optimal and overdoping, has been interpreted as evidence that the pseudogap must be due to precursor pairing. We suggest an alternative explanation, that the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the pseudogap could actually be due to any of a number of nesting instabilities, including charge or spin density waves or more exotic phases. We present a detailed analysis of this competition for one particular model: the pinned Balseiro-Falicov model of competing charge density wave and (s-wave) superconductivity. We show that most of the anomalous features of both tunneling and photoemission follow naturally from the model, including the smooth crossover, the general shape of the pseudogap phase diagram, the shrinking Fermi surface of the pseudogap phase, and the asymmetry of the tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1 and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be described in detail by this model, but we suggest a simple generalization to account for inhomogeneity, which does provide an adequate description. We show that it should be possible, with a combination of photoemission and tunneling, to demonstrate the extent of pinning of the Fermi level to the Van Hove singularity. A preliminary analysis of the data suggests pinning in the underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure

Renormalization of f-levels away from the Fermi energy in electron excitation spectroscopies: Density functional results of Nd2−x_{2-x}Cex_xCuO4_4

Relaxation energies for photoemission, when an occupied electronic state is excited, and for inverse photoemission, when an empty state is filled, are calculated within the density functional theory with application to Nd$_{2-x}$Ce$_x$CuO$_4$. The associated relaxation energies are obtained by computing differences in total energies between the ground state and an excited state in which one hole or electron is added into the system. The relaxation energies of f-electrons are found to be of the order of several eV's, indicating that f-bands will appear substantially away from the Fermi energy ($E_F$) in their spectroscopic images, even if these bands lie near $E_F$. Our analysis explains why it would be difficult to observe f electrons at the $E_F$ even in the absence of strong electronic correlations.Comment: 6 pages, 1 figure, 1 tabl

Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model

Slave boson calculations have been carried out in the three-band tJ model for the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode phonons. Phonon-induced Van Hove nesting leads to a phase separation between a hole-doped domain and a (magnetic) domain near half filling, with long-range Coulomb forces limiting the separation to a nanoscopic scale. Strong correlation effects pin the Fermi level close to, but not precisely at the Van Hove singularity (VHS), which can enhance the tendency to phase separation. The resulting dispersions have been calculated, both in the uniform phases and in the phase separated regime. In the latter case, distinctly different dispersions are found for large, random domains and for regular (static) striped arrays, and a hypothetical form is presented for dynamic striped arrays. The doping dependence of the latter is found to provide an excellent description of photoemission and thermodynamic experiments on pseudogap formation in underdoped cuprates. In particular, the multiplicity of observed gaps is explained as a combination of flux phase plus charge density wave (CDW) gaps along with a superconducting gap. The largest gap is associated with VHS nesting. The apparent smooth evolution of this gap with doping masks a crossover from CDW-like effects near optimal doping to magnetic effects (flux phase) near half filling. A crossover from large Fermi surface to hole pockets with increased underdoping is found. In the weakly overdoped regime, the CDW undergoes a quantum phase transition ($T_{CDW}\to 0$), which could be obscured by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes, esp. in Sect. 3, Figs 1-4,6 replace

Phase Separation Models for Cuprate Stripe Arrays

An electronic phase separation model provides a natural explanation for a large variety of experimental results in the cuprates, including evidence for both stripes and larger domains, and a termination of the phase separation in the slightly overdoped regime, when the average hole density equals that on the charged stripes. Several models are presented for charged stripes, showing how density waves, superconductivity, and strong correlations compete with quantum size effects (QSEs) in narrow stripes. The energy bands associated with the charged stripes develop in the middle of the Mott gap, and the splitting of these bands can be understood by considering the QSE on a single ladder.Comment: significant revisions: includes island phase, 16 eps figures, revte

Systematic Low-Energy Effective Field Theory for Electron-Doped Antiferromagnets

In contrast to hole-doped systems which have hole pockets centered at $(\pm \frac{\pi}{2a},\pm \frac{\pi}{2a})$, in lightly electron-doped antiferromagnets the charged quasiparticles reside in momentum space pockets centered at $(\frac{\pi}{a},0)$ or $(0,\frac{\pi}{a})$. This has important consequences for the corresponding low-energy effective field theory of magnons and electrons which is constructed in this paper. In particular, in contrast to the hole-doped case, the magnon-mediated forces between two electrons depend on the total momentum $\vec P$ of the pair. For $\vec P = 0$ the one-magnon exchange potential between two electrons at distance $r$ is proportional to $1/r^4$, while in the hole case it has a $1/r^2$ dependence. The effective theory predicts that spiral phases are absent in electron-doped antiferromagnets.Comment: 25 pages, 7 figure

Anisotropic softening of collective charge modes in the vicinity of critical doping in a doped Mott insulator

Momentum resolved inelastic resonant x-ray scattering is used to map the evolution of charge excitations over a large range of energies, momenta and doping levels in the electron doped Mott insulator class Nd$_{2-x}$Ce$_x$CuO$_4$. As the doping induced AFM-SC (antiferromagnetic-superconducting) transition is approached, we observe an anisotropic softening of collective charge modes over a large energy scale along the Gamma to (\pi,\pi)-direction, whereas the modes exhibit broadening ($\sim$ 1 eV) with relatively little softening along Gamma to (\pi,0) with respect to the parent Mott insulator (x=0). Our study indicates a systematic collapse of the gap consistent with the scenario that the system dopes uniformly with electrons even though the softening of the modes involves an unusually large energy scale.Comment: 5 pages + 5 Figure

Dispersion of Ordered Stripe Phases in the Cuprates

A phase separation model is presented for the stripe phase of the cuprates, which allows the doping dependence of the photoemission spectra to be calculated. The idealized limit of a well-ordered array of magnetic and charged stripes is analyzed, including effects of long-range Coulomb repulsion. Remarkably, down to the limit of two-cell wide stripes, the dispersion can be interpreted as essentially a superposition of the two end-phase dispersions, with superposed minigaps associated with the lattice periodicity. The largest minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk) Van Hove singularity. The calculated spectra are dominated by two features -- this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a strong correlation between these two features and the experimental photoemission results of a two-peak dispersion in La$_{2-x}$Sr$_x$CuO$_4$, and the peak-dip-hump spectra in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The differences are suggestive of the role of increasing stripe fluctuations. The 1/8 anomaly is associated with a quantum critical point, here expressed as a percolation-like crossover. A model is proposed for the limiting minority magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure