Possible isotope effect on the resonance peak formation in high-Tc_c cuprates

Starting from the three-band $p-d$ Hubbard Hamiltonian we derive an effective $t-J$ model including electron-phonon interaction of quasiparticles with optical phonons. Within the effective Hamiltonian we analyze the influence of electronic correlations and electron-phonon interaction on the dynamical spin susceptibility in layered cuprates. We find a huge isotope effect on the resonance peak in the magnetic spin susceptibility, ${Im}\chi({\bf q},\omega)$, seen by inelastic neutron scattering. It results from both the electron-phonon coupling and the electronic correlation effects taken into account beyond random phase approximation(RPA) scheme. We find at optimal doping the isotope coeffiecient $\alpha_{res} \approx 0.35$ which can be further tested experimentally.Comment: revised version, new figure is added. Phys. Rev. B 69, 0945XX (2004); in pres

Fractal characteristics of seismic process in rock mass surrounding the excavation at mining. Mathematical modelling and analysis

It is shown in the paper that the system of equations of solid mechanics, which has a mixed type, demonstrate the most common features of evolution of nonlinear dynamic systems. Previous investigations of seismic process were carried out on the base of simplified (sand-pile, land-slide) models which gave a graph of recurrence of seismic events and information about the state of self-organized criticality (SOC). However, these simplified models do not contain the information about the stress-strain state of the loaded geomedia and its proximity to the critical state. In the proposed paper the model of rock mass with excavation is constructed and general step of roof caving is modelled. On the base of these modelling the formation of critical state in loaded geomedia is studied. The fluctuations of stress-strain state at different points of geomedia are studied as the reflection of fracture process occurring in the main elements of rock mass: roof and floor, when the coal face is advanced. It is shown that the PDF dependencies, amplitude-frequency characteristics reflect the state of the rock mass and might be considered as the fractal characteristics of fracture process within. The evolution of these dependencies shows the dramatic change when the critical state is formed in the rock mass surrounding the underground opening

Dynamical charge and spin susceptibilities in a frame of t-J-G model

We have calculated the dynamical charge and spin susceptibilities using the new analytical expression obtained beyond a conventional random phase approximation scheme. Both susceptibilities are strongly peaked along a contour around wave vector Q = (π, π). We have analyzed the dispersions of the collective excitations near Q = (π, π) corresponding to a spin density wave and charge density wave modes, respectively. In addition we have calculated the momentum dependence of the imaginary part of the charge and spin susceptibilities along the instability contour and show that both susceptibilities display a maximum around the points (π, ±q 0), (±q0,π) in Brillouine zone with decreasing temperature that indicates that the stripe-like instability may become preferable. © 2002 Plenum Publishing Corporation

Possible isotope effect on the resonance peak formation in high-T c cuprates

Within effective t-J Hamiltonian we analyze the influence of electronic correlations and electron-phonon interaction on the dynamical spin susceptibility in layered cuprates. We find an isotope effect on the resonance peak in the magnetic spin susceptibility Im χ(q,ω), seen by inelastic neutron scattering. It results from both the electron-phonon coupling and the electronic correlation effects taken into account beyond random-phase approximation scheme. We find at optimal doping the isotope coefficient αres≈0.4 which can be further tested experimentally

Unified picture of the distribution of electric field gradients at Cu, O, and Tm sites in ReBa2Cu3O7-δ

An abundant set of published experimental NMR/NQR data on electric field gradients in ReBa2Cu3O7-δ high temperature conductors, where Re=Y and Tm, is used as a test for the singlet-correlated band theory. Because of the unusual spectral weight ("capacity") of this band, it has been possible to match the number of holes per lattice site to photoemission data on the energy spectrum and the location of the Fermi level. In the framework of a unified picture of the distribution of holes (charges) it has been possible for the first time to explain satisfactorily the observed electric field gradients at the Cu(1), Cu(2), O(1), O(2), O(3), and O(4) sites, as well as NMR and inelastic neutron scntteriti" datn on the rrvstnl electric field nt the Tm inns. © 1997 American Institute of Physics

Itinerant in-plane magnetic fluctuations and many-body correlations in Nax_xCoO2_2

Based on the {\it ab-initio} band structure for Na$_x$CoO$_2$ we derive the single-electron energies and the effective tight-binding description for the $t_{2g}$ bands using projection procedure. Due to the presence of the next-nearest-neighbor hoppings a local minimum in the electronic dispersion close to the $\Gamma$ point of the first Brillouin zone forms. Correspondingly, in addition to a large Fermi surface an electron pocket close to the $\Gamma$ point emerges at high doping concentrations. The latter yields the new scattering channel resulting in a peak structure of the itinerant magnetic susceptibility at small momenta. This indicates dominant itinerant in-plane ferromagnetic fluctuations above certain critical concentration $x_m$, in agreement with neutron scattering data. Below $x_m$ the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We further analyze the many-body effects on the electronic and magnetic excitations using various approximations applicable for different $U/t$ ratio.Comment: 10 page

Electronic theory for itinerant in-plane magnetic fluctuations in Nax_xCoO2_2

Starting from {\it ab-initio} band structure for Na$_x$CoO$_2$, we derive the single-electron energies and the effective tight-binding description for the $t_{2g}$ bands using a projection procedure. We find that due to the presence of the next-nearest-neighbor hoppings a local minimum in the electronic dispersion close to the $\Gamma$ point of the first Brillouin zone forms. Therefore, in addition to a large Fermi surface an electron pocket close to the $\Gamma$ point emerges at high doping concentrations. The latter yields the new scattering channel resulting in a peak structure of the itinerant magnetic susceptibility at small momenta. This indicates itinerant in-plane ferromagnetic state above certain critical concentration $x_m$, in agreement with neutron scattering data. Below $x_m$ the magnetic susceptibility shows a tendency towards the antiferromagnetic fluctuations. We estimate the value of $0.58 < x_m < 0.7$ within the rigid band model and within the Hubbard model with infinite on-site Coulomb repulsion consistent with the experimental phase diagram.Comment: 4 pages, 4 figures; LDA calculations were done with Na in the symmetric 2d position contrary to the 6h position in a previous version of this pape