10 research outputs found
Anisotropic Superexchange for nearest and next nearest coppers in chain, ladder and lamellar cuprates
We present a detailed calculation of the magnetic couplings between
nearest-neighbor and next-nearest-neighbor coppers in the edge-sharing
geometry, ubiquitous in many cuprates. In this geometry, the interaction
between nearest neighbor coppers is mediated via two oxygens, and the Cu-O-Cu
angle is close to 90 degrees. The derivation is based on a perturbation
expansion of a general Hubbard Hamiltonian, and produces numerical estimates
for the various magnetic energies. In particular we find the dependence of the
anisotropy energies on the angular deviation away from the 90 degrees geometry
of the Cu-O-Cu bonds. Our results are required for the correct analysis of the
magnetic structure of various chain, ladder and lamellar cuprates.Comment: 13 pages, Latex, 7 figure
Magnetic polarons in weakly doped high-Tc superconductors
We consider a spin Hamiltonian describing - exchange interactions
between localized spins of a finite antiferromagnet as well as -
interactions between a conducting hole () and localized spins. The spin
Hamiltonian is solved numerically with use of Lanczos method of
diagonalization. We conclude that - exchange interaction leads to
localization of magnetic polarons. Quantum fluctuations of the antiferromagnet
strengthen this effect and make the formation of polarons localized in one site
possible even for weak - coupling. Total energy calculations, including
the kinetic energy, do not change essentially the phase diagram of magnetic
polarons formation. For parameters reasonable for high- superconductors
either a polaron localized on one lattice cell or a small ferron can form. For
reasonable values of the dielectric function and - coupling, the
contributions of magnetic and phonon terms in the formation of a polaron in
weakly doped high- materials are comparable.Comment: revised, revtex-4, 12 pages 8 eps figure
Spin polaron damping in the spin-fermion model for cuprate superconductors
A self-consistent, spin rotational invariant Green's function procedure has
been developed to calculate the spectral function of carrier excitations in the
spin-fermion model for the CuO2 plane. We start from the mean field description
of a spin polaron in the Mori-Zwanzig projection method. In order to determine
the spin polaron lifetime in the self-consistent Born approximation, the
self-energy is expressed by an irreducible Green's function. Both, spin polaron
and bare hole spectral functions are calculated. The numerical results show a
well pronounced quasiparticle peak near the bottom of the dispersion at
(pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather
large damping away from the minimum and an asymmetry of the spectral function
with respect to the antiferromagnetic Brillouin zone. These findings are in
qualitative agreement with photoemission data for undoped cuprates. The direct
oxygen-oxygen hopping is responsible for a more isotropic minimum at
(pi/2,pi/2).Comment: 18 pages, 13 figure
Evolution of a Metal to Insulator Transition in CaNaCuOCl, as seen by ARPES
We present angle resolved photoemission (ARPES) data on Na-doped
CaCuOCl. We demonstrate that the chemical potential shifts upon
doping the system across the insulator to metal transition. The resulting low
energy spectra reveal a gap structure which appears to deviate from the
canonical form. To reconcile the measured
gap structure with d-wave superconductivity one can understand the data in
terms of two gaps, a very small one contributing to the nodal region and a very
large one dominating the anti-nodal region. The latter is a result of the
electronic structure observed in the undoped antiferromagnetic insulator.
Furthermore, the low energy electronic structure of the metallic sample
contains a two component structure in the nodal direction, and a change in
velocity of the dispersion in the nodal direction at roughly 50 meV. We discuss
these results in connection with photoemission data on other cuprate systems.Comment: 10 pages, 12 figures, accepted by PRB; a high quality pdf is
available at
http://helios.physics.utoronto.ca/~fronning/RonningNaCCOCResub.pdf (2.2MB
Growth and characterisation of bulk Sr2CuO2Cl2 single crystals
Large bulk single crystals of the Sr2CuO2Cl2 compound with dimensions of 15 x 6 x 4 mm(3) have been grown directly from the melt by the floating-zone method using a light-image furnace. The optimal growth conditions are found in a mixed atmosphere of 0.2 bar oxygen and 1.2 bar argon. Results of the characterisation have revealed thr high quality of the as-grown crystals. Furthermore, a possible congruent route of formation from the melt as well as the chemical stability of this compound is discussed
Single hole dynamics in the CuO2 plane at half filling
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
Single hole dynamics in the CuO2 plane at half filling
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