6,408 research outputs found
Doping evolution of spin and charge excitations in the Hubbard model
To shed light on how electronic correlations vary across the phase diagram of
the cuprate superconductors, we examine the doping evolution of spin and charge
excitations in the single-band Hubbard model using determinant quantum Monte
Carlo (DQMC). In the single-particle response, we observe that the effects of
correlations weaken rapidly with doping, such that one may expect the random
phase approximation (RPA) to provide an adequate description of the
two-particle response. In contrast, when compared to RPA, we find that
significant residual correlations in the two-particle excitations persist up to
hole and electron doping (the range of dopings achieved in the
cuprates). These fundamental differences between the doping evolution of
single- and multi-particle renormalizations show that conclusions drawn from
single-particle processes cannot necessarily be applied to multi-particle
excitations. Eventually, the system smoothly transitions via a
momentum-dependent crossover into a weakly correlated metallic state where the
spin and charge excitation spectra exhibit similar behavior and where RPA
provides an adequate description.Comment: 5 pages, 4 figures, plus supplementary materia
Finite temperature spin-dynamics and phase transitions in spin-orbital models
We study finite temperature properties of a generic spin-orbital model
relevant to transition metal compounds, having coupled quantum Heisenberg-spin
and Ising-orbital degrees of freedom. The model system undergoes a phase
transition, consistent with that of a 2D Ising model, to an orbitally ordered
state at a temperature set by short-range magnetic order. At low temperatures
the orbital degrees of freedom freeze-out and the model maps on to a quantum
Heisenberg model. The onset of orbital excitations causes a rapid scrambling of
the spin spectral weight away from coherent spin-waves, which leads to a sharp
increase in uniform magnetic susceptibility just below the phase transition,
reminiscent of the observed behavior in the Fe-pnictide materials.Comment: 4 page
Doping Evolution of Oxygen K-edge X-ray Absorption Spectra in Cuprate Superconductors
We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate
the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate
superconductors. Using large-scale exact diagonalization of the three-orbital
Hubbard model, we observe the effect of strong correlations manifesting in a
dynamical spectral weight transfer from the upper Hubbard band to the ZRS band.
The quantitative agreement between theory and experiment highlights an
additional spectral weight reshuffling due to core-hole interaction. Our
results confirm the important correlated nature of the cuprates and elucidate
the changing orbital character of the low-energy quasi-particles, but also
demonstrate the continued relevance of the ZRS even in the overdoped region.Comment: Original: 5 pages, 4 figures. Replaced: 6 pages and 4 figures, with
updated title and conten
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