Stripe formation in electron-doped cuprates

We investigate the formation of charge domain walls in an electron-doped extended Hubbard model for the superconducting cuprates. Within an unrestricted Hartree-Fock approach, extended by slave bosons to obtain a better treatment of strong correlations, we demonstrate the occurrence of stripes in the (1,1) and (1,-1) directions having one doped electron per stripe site. The different filling, direction, and width of these electron-doped stripes with respect to those obtained in the hole-doped systems have interesting observable consequences.     

Single-particle spectrum in the electron-doped cuprates

We study the evolution of the single-particle spectrum with electron doping in a scheme which adds multiple exchange of transverse spin excitations to the mean-field antiferromagnetic insulator. Away from half-filling small Fermi surface pockets appear first around the X points, and simultaneously new spectral weight grows in the insulating gap. With further doping the in-gap states develop the character of a renormalized quasiparticle band near the chemical potential. The essential features in momentum-energy space agree well with recent studies using angle-resolved photoemission spectroscopy on electron-doped cuprates. We interpret the origins and the nature of the in-gap states using a simple variational wave function.     

Models of bands and Fermi surfaces for electron-doped cuprates

The properties of Fermi surfaces and electron bands in electron-doped cuprates have been studied. The possible origins of a hole pocket in the nodal direction and a pseudogap at hot spots are discussed, including stripe phases and double bands in an antiferromagnetically correlated Fermi liquid. Within the framework of the mean field method, it is shown that both t-t′-t″-U Hubbard model solutions with a homogeneous antifer-romagnetic spin structure and those with a diagonal stripe structure can reproduce the fragmentar character of the Fermi surface. The appearance of hole pockets in various structures is related either to states in the lower Hubbard band or to states localized on domain walls. The behavior of a gap at the leading edge of the energy distribution of photoelectrons and its dependence on oxygen removal in the course of annealing are considered.     

On electron pairing in superconducting cuprates

The superconducting properties of materials of layered structure containing copper and other metal oxides are compared with the expectations ofa recently proposed electron pairing model 1. The role of the oxygen content of samples is emphasized. Evidence is found showing that superconduction is originated only in presence of coupled layers of metal oxides holding unpaired electrons. Received 8 November 2000     

Local electronic structures in electron-doped cuprates with coexisting orders

Motivated by the observation of a so-called non-monotonic gap in recent angle-resolved photoemission spectroscopy measurement, we study the local electronic structure near impurities in electron-doped cuprates by considering the influence of antiferromagnetic (AF) spin-density-wave (SDW) order. We find that the evolution of density of states (DOS) with AF SDW order clearly indicates the non-monotonic d-wave gap behavior. More interestingly, the local DOS for spin-up is much different from that for spin-down with increasing AF SDW order. As a result, the impurity induced resonance state near the Fermi energy exhibits a spin-polarized feature. These features can be detected by spin-polarized scanning tunneling microscopy experiments.     

Pseudogap and spin fluctuations in the normal state of the electron-doped cuprates

We present reliable many-body calculations for the t-t(')-t(')-U Hubbard model that explain in detail the results of recent angle-resolved photoemission experiments on electron-doped high-temperature superconductors. The origin of the pseudogap is traced to two-dimensional antiferromagnetic spin fluctuations whose calculated temperature-dependent correlation length also agrees with recent neutron scattering measurements. We make specific predictions for photoemission, for neutron scattering, and for the phase diagram.     

Superconductivity and topological Fermi surface transitions in electron-doped cuprates near optimal doping

We discuss evolution of the Fermi surface (FS) topology with doping in electron-doped cuprates within the framework of a one-band Hubbard Hamiltonian, where antiferromagnetism and superconductivity are assumed to coexist in a uniform phase. In the lightly doped insulator, the FS consists of electron pockets around the (π,0) points. The first change in the FS topology occurs in the optimally doped region when an additional hole pocket appears at the nodal point. The second change in topology takes place in the overdoped regime (∼18%) where antiferromagnetism disappears and a large (π,π)-centered metallic FS is formed. Evidence for these two topological transitions is found in recent Hall effect and penetration depth experiments on Pr_2-xCe_xCuO_4-δ (PCCO) and with a number of spectroscopic measurements on Nd_2-xCe_xCuO_4-δ (NCCO).     

Nuclear magnetic resonance study of the electron-doped high-temperature superconducting cuprates

Nuclear magnetic resonance (NMR) measurements have been made on two of the electron-doped high-temperature superconducting cuprates (HTSCs), Pr_2−xCe_xCuO₄ and Sr_0.9La_0.1CuO₂ that represent the two known electron-doped structures. The results are compared with the more-studied hole-doped HTSCs. We show that the electron and hole-doped HTSCs probe a similar antiferromagnetic spin fluctuation spectrum in the normal state, which provides support for theories of superconductivity where the pairing is mediated by antiferromagnetic spin fluctuations and the superconducting order parameter has a d_x²−y² symmetry. Contrary to results from underdoped and hole-doped HTSCs, there is no evidence for a normal-state pseudogap in the NMR data from measurements on the electron-doped HTSCs. Therefore, the electron-doped HTSCs can be better compared with overdoped and hole-doped HTSCs where the normal-state pseudogap is absent. The antiferromagnetic spin fluctuation spectrum as probed by the Cu spin–lattice relaxation rate, is independent of the doped electrons per Cu. A similar effect is observed in the overdoped and hole-doped HTSC, Y_1−xCa_xBa₂Cu₃O_7−δ for a hole concentration range of 0.063. The anomalous Cu NMR linewidth anisotropy observed in the electron-doped HTSCs suggests a small and static spin variation for temperatures up to room temperature.     

Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements

The in-plane magnetic penetration depth, lambda(T), was measured down to 0.4 K in single crystals of electron-doped superconductors, Pr(1.85)Ce(0.15)CuO(4-delta) (PCCO) and Nd(1.85)Ce(0.15)CuO(4-delta) (NCCO). In PCCO, the superfluid density varies as T2 from 0.025 up to roughly 0.3T/T(c) suggestive of a d-wave state with impurities. In NCCO, lambda(T) shows a pronounced upturn for T<4 K due to the paramagnetic contribution of Nd3+ ions. Fits to an s-wave order parameter over the standard BCS range (T/T(c) = 0.32) limit any gap to less than Delta(min)(0)/T(c) = 0.57 in NCCO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of Delta(min)(0)/T(c) = 0.2.     

Competition between antiferromagnetism and superconductivity in the electron-doped cuprates triggered by oxygen reduction

We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr(2-x)CexCuO4 and Pr(1-x)LaCexCuO4 electron-doped cuprates. In contrast with the common belief, neither the band filling nor the band parameters are significantly affected by the oxygen reduction process. Instead, we show that the main electronic role of the reduction process is to remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.     

Phase-sensitive evidence for d-wave pairing symmetry in electron-doped cuprate superconductors

We present phase-sensitive evidence that the electron-doped cuprates Nd(1.85)Ce(0.15)CuO(4-y) (NCCO) and Pr(1.85)Ce(0.15)CuO(4-y) (PCCO) have d-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in c-axis-oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be frustrated for a d(x(2)-y(2)) order parameter. Samples with two other configurations, designed to be unfrustrated for a d-wave superconductor, do not show the half-flux quantum effect.     

Behaviour of superconductivity energetic characteristics in electron-doped cuprates. A simple model

A simple model to describe the energetic phase diagram of electron-doped cuprate superconductor is developed. Interband pairing operates between the UHB and the defect states created by doping and supplied by both extincting HB-s. Two defect subbands correspond to the (π,0) and (π/2,π/2) momentum regions. Extended doping quenches the bare normal state gaps (pseudogaps). Maximal transition temperature corresponds to overlapping bands ensemble intersected by the chemical potential. Illustrative results for Tc, pseudo- and superconducting gaps are calculated on the whole doping scale. Major characteristic features on the phase diagram are reproduced. Anticipated manifestation of gaps doping dynamics is discussed.     

Non-fermi liquid behavior in nearly charge ordered layered metals

Non-Fermi liquid behavior is shown to occur in two-dimensional metals which are close to a charge ordering transition driven by the Coulomb repulsion. A linear temperature dependence of the scattering rate together with an increase of the electron effective mass occur above T*, a temperature scale much smaller than the Fermi temperature. It is shown that the anomalous temperature dependence of the optical conductivity of the quasi-two-dimensional organic metal alpha-(BEDT-TTF)2MHg(SCN)4, with M = NH4 and Rb, above T* = 50-100K, agrees qualitatively with predictions for the electronic properties of nearly charge ordered two-dimensional metals.     

Non-fermi liquid criticality and superuniversality in the quantum hall regime

The results of a microscopic theory, based on the topological concept of a θ vacuum, which show that the Coulomb potential, unlike any finite-ranged interaction potential, renders the long-standing problem of the plateau transitions in the quantum Hall regime like a non-Fermi liquid are reported. These results, which are important for quantum-phase transitions in general and composite fermion ideas in particular, provide a novel understanding of the critical exponent values that have recently been (re-)taken from a series of state-of-the-art quantum Hall samples. The text was submitted by the authors in English.     

Spin dynamics of the electron-doped high-T(c) superconducting cuprates

We show that a basic difference between the electron- and hole-doped cuprates is their proximity to two different quantum critical points in a 2D free fermion system on a square lattice and that the spin dynamics observed recently for the first time in the electron-doped Nd2-xCexCuO4, very different from that in the hole-doped cuprates, can be understood as a consequence of this effect.     

Strong pairing fluctuations and lattice anomalies in the pseudogap phase of the cuprates

Abstract

The pseudogap state in underdoped cuprates shows many very anomalous features. Among them are an extended temperature region of pairing fluctuations above the superconducting transition temperature and an unusual giant phonon anomaly in the same temperature and hole density range. A recent theoretical proposal that ascribes these anomalies to the presence of strong phase fluctuations related to a Leggett mode, is summarised.     

A Cud-d excitation model for the pairing in the high-T c cuprates

A new model for the pairing mechanism in the ceramic superconductors is presented. Like the magnetic models, we assume the limit of large correlation energies for the Cud electrons. We postulate that the pairing of the Op conduction holes occurs viad–d orbital excitations within thee _g manifold of thed hole of Cu⁺⁺, which is split because of tetragonal or lower symmetry at the Cu sites. This valence conserving charge degree of freedom has been ignored in the magnetic pairing models. Thed–d excitation model may provide a simple qualitative understanding of many experimental results.