206 research outputs found
Spin chain in magnetic field: limitations of the large-N mean-field theory
Motivated by the recent success in describing the spin and orbital spectrum
of a spin-orbital chain using a large-N mean-field approximation [Phys. Rev. B
91, 165102 (2015)], we apply the same formalism to the case of a spin chain in
the external magnetic field. It occurs that in this case, which corresponds to
N=2 in the approximation, the large-N mean-field theory cannot qualitatively
reproduce the spin excitation spectra at high magnetic fields, which polarize
more than 50% of the spins in the magnetic ground state. This, rather
counterintuitively, shows that the physics of a spin chain can under some
circumstances be regarded as more complex than the physics of a spin-orbital
chain.Comment: 4 pages, 3 figures; appeared in the proceedings of the European
Conference "Physics of Magnetism" (2014
Using RIXS to uncover elementary charge and spin excitations in correlated materials
Despite significant progress in resonant inelastic x-ray scattering (RIXS)
experiments on cuprates at the Cu L-edge, a theoretical understanding of the
cross-section remains incomplete in terms of elementary excitations and the
connection to both charge and spin structure factors. Here we use
state-of-the-art, unbiased numerical calculations to study the low energy
excitations probed by RIXS in undoped and doped Hubbard model relevant to the
cuprates. The results highlight the importance of scattering geometry, in
particular both the incident and scattered x-ray photon polarization, and
demonstrate that on a qualitative level the RIXS spectral shape in the
cross-polarized channel approximates that of the spin dynamical structure
factor. However, in the parallel-polarized channel the complexity of the RIXS
process beyond a simple two-particle response complicates the analysis, and
demonstrates that approximations and expansions which attempt to relate RIXS to
less complex correlation functions can not reproduce the full diversity of RIXS
spectral features
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