963 research outputs found
Theory of Phonon-Assisted Multimagnon Optical Absorption and Bimagnon States in Quantum Antiferromagnets
We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in a perovskite like antiferromagnet and we compute the
spectra for two magnon absorption using interacting spin-wave theory. The full
set of equations for the interacting two magnon problem is presented in the
random phase approximation for arbitrary total momentum of the magnon pair. The
spin wave theory results fit very well the primary peak of recent measured
bands in the parent insulating compounds of cuprate superconductors. The line
shape is explained as being due to the absorption of one phonon plus a new
quasiparticle excitation of the Heisenberg Hamiltonian that consists off a long
lived virtual bound state of two magnons (bimagnon). The bimagnon states have
well defined energy and momentum in a substantial portion of the Brillouin
zone. The higher energy bands are explained as one phonon plus higher
multimagnon absorption processes. Other possible experiments for observing
bimagnons are proposed. In addition we predict the line shape for the spin one
system LaNiO.Comment: Modified version of the paper to be published in PR
Charged excitons in doped extended Hubbard model systems
We show that the charge transfer excitons in a Hubbard model system including
nearest neighbor Coulomb interactions effectively attain some charge in doped
systems and become visible in photoelectron and inverse photoelectron
spectroscopies. This shows that the description of a doped system by an
extended Hubbard model differs substantially from that of a simple Hubbard
model. Longer range Coulomb interactions cause satellites in the one electron
removal and addition spectra and the appearance of spectral weight if the gap
of doped systems at energies corresponding to the excitons of the undoped
systems. The spectral weight of the satellites is proportional to the doping
times the coordination number and therefore is strongly dependent on the
dimension.Comment: 10 pages revtex, 5 figures ps figures adde
Bond disproportionation and dynamical charge fluctuations in the perovskite rare earth nickelates
We present a theory describing the local electronic properties of the
perovskite rare earth nickelates--materials which have negative charge transfer
energies, strong O -- Ni covalence, and breathing mode lattice
distortions at the origin of highly studied metal-insulator and
antiferromagnetic ordering transitions. Utilizing a full orbital, full
correlation double cluster approach, we find strong charge fluctuations in
agreement with a bond disproportionation interpretation. The unique double
cluster formulation permits the inclusion of necessary orbital degeneracies and
Coulomb interactions to calculate resonant x-ray spectral responses, with which
we find excellent agreement with well-established experimental results. This
previously absent, crucial link between theory and experiment provides
validation of the recently proposed bond disproportionation theory, and
provides an analysis methodology for spectroscopic studies of engineered phases
of nickelates and other high valence transition metal compounds
Non-Zhang-Rice singlet character of the first ionization state of T-CuO
We argue that tetragonal CuO (T-CuO) has the potential to finally settle
long-standing modelling issues for cuprate physics. We compare the one-hole
quasiparticle (qp) dispersion of T-CuO to that of cuprates, in the framework of
the strongly-correlated () limit of the three-band
Emery model. Unlike in CuO, magnetic frustration in T-CuO breaks the
rotational symmetry and leads to strong deviations from the Zhang-Rice singlet
picture in parts of the reciprocal space. Our results are consistent with
angle-resolved photoemission spectroscopy data but in sharp contradiction to
those of a one-band model previously suggested for them. These differences
identify T-CuO as an ideal material to test a variety of scenarios proposed for
explaining cuprate phenomenology.Comment: 4 pages, 2 figure
The role of the lattice structure in determining the magnon-mediated interactions between charge carriers doped into a magnetically ordered background
We use two recently proposed methods to calculate exactly the spectrum of two
spin- charge carriers moving in a ferromagnetic background, at zero
temperature, for three types of models. By comparing the low-energy states in
both the one-carrier and the two-carrier sectors, we analyze whether complex
models with multiple sublattices can be accurately described by simpler
Hamiltonians, such as one-band models. We find that while this is possible in
the one-particle sector, the magnon-mediated interactions which are key to
properly describe the two-carrier states of the complex model are not
reproduced by the simpler models. We argue that this is true not just for
ferromagnetic, but also for antiferromagnetic backgrounds. Our results question
the ability of simple one-band models to accurately describe the low-energy
physics of cuprate layers.Comment: 15 pages, 10 figure
t-J model of coupled CuO ladders in SrCaCuO
Starting from the proper charge transfer model for CuO coupled
ladders in SrCaCuO we derive the low energy
Hamiltonian for this system. It occurs that the widely used ladder t-J model is
not sufficient and has to be supplemented by the Coulomb repulsion term between
holes in the neighboring ladders. Furthermore, we show how a simple mean-field
solution of the derived t-J model may explain the onset of the charge density
wave with the odd period in SrCaCuO.Comment: 8 pages, 4 figures, 2 table
Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons
Ferromagnetism and superconductivity are in most cases adverse. However,
recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3.
We analyze the magnetic state within density functional theory and provide
evidence that magnetism is not an intrinsic property of the two-dimensional
electron liquid at the interface. We demonstrate that the robust ferromagnetic
state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer.
This allows for the notion that areas with increased density of oxygen
vacancies produce ferromagnetic puddles and account for the previous
observation of a superparamagnetic behavior in the superconducting state.Comment: 5 pages, 4 figures, to appear in Physical Review B (Rapid
Communications
Electronic structure and transport properties of atomic NiO spinvalves
Ab-initio quantum transport calculations show that short NiO chains suspended
in Ni nanocontacts present a very strong spin-polarization of the conductance.
The generalized gradient approximation we use here predicts a similiar
polarization of the conductance as the one previously computed with non-local
exchange, confirming the robustness of the result. Their use as nanoscopic
spinvalves is proposed.Comment: 2 pages, 1 figure; accepted in JMMM (Proceedings of ICM'06, Kyoto
Fermi pockets and correlation effects in underdoped YBa2Cu3O6.5
The detection of quantum oscillations in the electrical resistivity of
YBa2Cu3O6.5 provides direct evidence for the existence of Fermi surface pockets
in an underdoped cuprate. We present a theoretical study of the electronic
structure of YBa2Cu3O7-d (YBCO) aiming at establishing the nature of these
Fermi pockets, i.e. CuO2 plane versus CuO chain or BaO. We argue that electron
correlation effects, such as orbital-dependent band distortions and highly
anisotropic self-energy corrections, must be taken into account in order to
properly interpret the quantum oscillation experiments.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/YBCO_OrthoII_LDA.pd
Non conventional screening of the Coulomb interaction in low dimensional and finite size system
We study the screening of the Coulomb interaction in non polar systems by
polarizable atoms. We show that in low dimensions and small finite size systems
this screening deviates strongly from that conventionally assumed. In fact in
one dimension the short range interaction is strongly screened and the long
range interaction is anti-screened thereby strongly reducing the gradient of
the Coulomb interaction and therefore the correlation effects. We argue that
this effect explains the success of mean field single particle theories for
large molecules.Comment: 4 pages, 5 figure
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