arXiv subjects
Publications and source records attributed to Arno P. Kampf.
The tilt pattern of the CuO_6 octahedra in the LTT phase of the cuprate superconductors leads to planar anisotropies for the exchange coupling and hopping integrals. Here, we show that these anisotropies provide a possible structural mechanism for the orientation of stripes. A t_x-t_y-J_x-J_y model thus serves as an effective Hamiltonian to describe stripe formation and orientation in LTT-phase cuprates.
We have investigated the evolution of the electronic properties of the t-t'-U Hubbard model with hole doping and temperature. Due to the shape of the Fermi surface, scattering from short wavelength spin fluctuations leads to strongly anisotropic quasi-particle scattering rates at low temperatures near half-filling. As a consequence, significant variations with momenta near the Fermi surface emerge for the spectral functions and the corresponding ARPES signals. This behavior is quite in contrast to the intermediate doping regime and we discuss the possible relevance of our results for the interpretation of photoemission spectra in cuprate superconductors at different hole doping levels.
We present the results for a model calculation of resonant two-magnon Raman scattering in a spin density wave (SDW) antiferromagnet. The resonant enhancement of the two-magnon intensity is obtained from a microscopic analysis of the photon-magnon coupling vertex. By combining magnon-magnon interactions with `triple resonance` phenomena in the vertex function the resulting intensity line shape is found to closely resemble the measured two-magnon Raman signal in antiferromagnetic cuprates. Both, resonant and non-resonant Raman scattering are discussed for the SDW antiferromagnet and a comparison is made to the conventional Loudon-Fleury theory of two-magnon light scattering.
Two--magnon Raman scattering is a useful tool to verify recent suggestions concerning the value of the interplanar exchange constant in antiferromagnetic two--layer systems, such as $YBa_2Cu_3O_{6+x}$. We present a theory for Raman scattering in a two--layer antiferromagnet. We study the spectra for the electronic and magnetic excitations across the charge transfer gap within the one--band Hubbard model and derive the matrix elements for the Raman scattering cross section in a diagrammatic formalism. We analyze the effect of the interlayer exchange coupling $J_2$ for the Raman spectra in $A_{1g}$ and $B_{1g}$ scattering geometries both in the non--resonant regime (when the Loudon--Fleury model is valid), and at resonance. We show that within the Loudon--Fleury approximation, a nonzero $J_2$ gives rise to a finite signal in $A_{1g}$ scattering geometry. Both, in this approximation and at resonance, the intensity in the $A_{1g}$ channel has a peak at {\it small} transferred frequency equal to twice the gap in the spin--wave spectrum. We compare our results with experiments in $YBa_2Cu_3O_{6.1}$ and $Sr_2CuO_2Cl_2$ compounds and argue that the large value of $J_2$ suggested in a number of recent studies is incompatible with Raman experiments in $A_{1g}$ geometry.