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The optical conductivity σ(ω) of undoped and very low doped YBa2Cu3O6.0δ is studied in detail in the midinfrared range. We collect experimental evidence for a magnetic origin of the observed absorption. The main resonance peak is well described by a bimagnon-plus-phonon absorption spectrum calculated within spin-wave theory. But spin-wave theory fails to describe further resonances at higher frequencies with significant spectral weight, which grows with both temperature and doping. This underlines the importance of quantum fluctuations for the description of short-wavelength magnetic excitations in a two-dimensional spin-1/2 system.

To explain the experimental spectra for angle-resolved photoemission we consider a modified t-J model. The modified model includes next-nearest(t') and next-next-nearest (t '') hopping as well as corrections due to ''virtual'' charge fluctuations to the spectral weights. A Dyson equation that relates the single-hole Green's functions for a given pseudospin and given spin is derived and is compared to experimental results. [S0163-1829(97)04438-X].

We present an exact diagonalization study of the single-particle spectral function in the so-called t-t'-t ''-J model in two dimensions. As a key result, we find that hole doping leads to a major reconstruction of the quasiparticle band structure near (pi,0): whereas for the undoped system the quasiparticle states near (pi,0) are deep below the top of the band near (pi/2,pi/2), hole doping shifts these states up to E(F), resulting in extended flat band regions close to E(F) and around (pi,0).

We present a k-dependent study of the single hole states in Sr2CuO2Cl2. We demonstrate that the controversial ''1 eV peaks'' in the high T-c's are quasiparticles derived from the same O 2p states as the Zhang-Rice (ZR) singlets, but of different symmetry and intensity in those regions of the Brillouin zone where the hybridization with the correlated Cu 3d states vanishes by symmetry. We use this new source of information to estimate the quasiparticle weight of the ZR singlets, discuss the quasiparticle line shape, and suggest a strong k dependence of the self-energy.

International audience; X-ray magnetic circular dichroism provides evidence of an out-of-plane spin moment in undoped and doped cuprates. In La2CuO4 this moment is related to the canting of the antiferromagnetically ordered Cu2+ spins caused by the Dzyaloshinskii-Moriya interaction within the CuO2 planes. This canting gives rise to the well-known weak ferromagnetism in high magnetic fields. We find a similar behavior in doped compounds, both in the normal and in the superconducting state, but with a different temperature dependence typical of paramagnetic systems. This result suggests that, together with short-range in-plane antiferromagnetic correlations, the Dzyaloshinskii-Moriya interaction survives up to optimal doping and in the superconducting state.

We compare the theoretical predictions of the previous article [L. Benfatto and M. B. Silva Neto, cond-mat/0602419], with Raman spectroscopy experiments in Sr(2)CuO(2)Cl(2) and untwinned La(2)CuO(4) single crystals. We construct the magnetic point group for the magnetically ordered phase of the two compounds, Sr(2)CuO(2)Cl(2) and La(2)CuO(4), and we classify all the Raman active one-magnon excitations according to the irreducible co-representations for the associated magnetic point group. We then measure the evolution of the one-magnon Raman energies and intensities for low and moderate magnetic fields along the three crystallographic directions. In the case of La(2)CuO(4), we demonstrate that from the jump of the Dzyaloshinskii-Moriya gap at the critical magnetic field H_c ~ 6.6 T for the weak-ferromagnetic transition one can determine the value of the interlayer coupling J_\perp/J ~ 3.2 x 10^-5. We furthermore determine the components of the anisotropic gyromagnetic tensor as g_s^a=2.0, g_s^b=2.08, and the upper bound g_s^c=2.65. For the case of Sr(2)CuO(2)Cl(2), we compare the Raman data obtained in an in-plane magnetic field with previous magnon-gap measurements done by ESR. Using the very low magnon gap estimated by ESR (~ 0.05 meV), the data for the one-magnon Raman energies agree reasonably well with the theoretical predictions for the case of a transverse field (only hardening of the gap)., Comment: 18 pages, 14 figures, final version. Part I can be found at cond-mat/0602419

We consider the single hole dynamics in a modified $t-J$ model at finite temperature. The modified model includes a next nearest ($t'$) and next-next nearest ($t''$) hopping. The model has been considered before in the zero temperature limit to explain angle resolved photo-emission measurements. We extend this consideration to the case of finite temperature where long-range anti-ferromagnetic order is destroyed, using the self-consistent Born approximation. The Dyson equation which relates the single hole Green's functions for a fixed pseudo-spin and for fixed spin is derived. The Green's function with fixed pseudo-spin is infrared stable but the Green's function with fixed spin is close to an infrared divergency. We demonstrate how to renormalize this Green's function in order to assure numerical convergence. At non-zero temperature the quasi-particle peaks are found to shift down in energy and to be broadened., 7 pages, RevTex, 5 Postscript figures

We present a k-dependent study of the single hole states in Sr2CuO2Cl2. We demonstrate that the controversial ''1 eV peaks'' in the high T-c's are quasiparticles derived from the same O 2p states as the Zhang-Rice (ZR) singlets, but of different symmetry and intensity in those regions of the Brillouin zone where the hybridization with the correlated Cu 3d states vanishes by symmetry. We use this new source of information to estimate the quasiparticle weight of the ZR singlets, discuss the quasiparticle line shape, and suggest a strong k dependence of the self-energy.

We present an exact diagonalization study of the single particle spectral function in the so-called t-t'-t''-J model in 2D. As a key result, we find that unlike the `pure' t-J model, hole doping leads to a major reconstruction of the quasiparticle band structure near (pi,0): whereas for the undoped system the quasiparticle states near (pi,0) are deep below the top of the band at (pi/2,pi/2), hole doping shifts these states up to E_F, resulting in extended flat band regions close to E_F and around (pi,0). This strong doping-induced deformation can be directly compared to angle resolved photoemission results on Sr_2 Cu Cl_2 O_2, underdoped Bi2212 and optimally doped Bi2212. We propose the interplay of long range hopping and decreasing spin correlations as the mechanism of this deformation., 4 pages, Revtex, with 4 embedded eps figures. Hardcopies of figures (or the entire manuscript) can be obtained by e-mail request to eder@vsf1.phys.rug.nl

We have exploited angle-resolved photoelectron spectroscopy to study changes in the quasiparticle excitations of CuO2 planes as a function of the carrier density, in optimally doped and underdoped Bi2Sr2CaCuO8+x and in Sr2CuO2Cl2. We have observed large changes in the E versus k dispersion of the quasiparticle bands, and in the topology of the Fermi surface away from optimal doping. We have also found that the carrier density has a nontrivial effect on the symmetry properties of these states.

Using angle-resolved photoemission and linearly polarized synchrotron radiation, we measured the electronic band structure of electronic states of CuO2 plane materials ranging from insulators (Sr2CuO2Cl2) to overdoped superconductors (Bi2Sr2CaCu2O8+x). We report three results: (i) The CuO2 containing insulator possesses a spin-density-wave (SDW) ground state; (ii) there are precursors df the SDW state for underdoped Bi2Sr2CaCu2O8+x; (iii) an extended saddle-point-type van Hove singularity is neither a necessary nor a sufficient condition for a high superconducting transition temperature, T-c.