848 research outputs found
Asymmetry of the electronic states in hole- and electron-doped cuprates: Exact diagonalization study of the t-t'-t''-J model
We systematically examine the asymmetry of the electronic states in the hole-
and electron-doped cuprates by using the t-t'-t''-J model. Numerically exact
diagonalization method is employed for a 20-site square lattice. We impose
twisted boundary conditions (BC) instead of standard periodic BC. For static
and dynamical correlation functions, averaging procedure over the twisted BC is
used to reduce the finite-size effect. We find that antiferromagnetic spin
correlation remains strong in electron doping in contrast to the case of hole
doping, being similar to the case of the periodic BC. This leads to a
remarkable electron-hole asymmetry in the dynamical spin structure factor and
two-magnon Raman scattering. By changing the twist, the single-particle
spectral function is obtained for all momenta in the Brillouin zone. Examining
the spectral function in detail, we find a gap opening at around the k=(pi,0)
region for 10% doping of holes (the carrier concentration x=0.1), leading to a
Fermi arc that is consistent with experiments. In electron doping, however, a
gap opens at around k=(pi/2,pi/2) and persists up to x=0.2, being correlated
with the strength of the antiferromagnetic correlation. We find that the
magnitude of the gaps is sensitive to t' and t''. A pseudogap is also seen in
the optical conductivity for electron doping, and its magnitude is found to be
the same as that in the spectral function. We compare calculated quantities
with corresponding experimental data, and discuss similarities and differences
between them as well as their implications.Comment: 14 pages, 17 figures, Replaced figures, to be published in Phys. Rev.
Temperature and Dimensionality Dependences of Optical Absorption Spectra in Mott Insulators
We investigate the temperature dependence of optical absorption spectra of
one-dimensional (1D) and two-dimensional (2D) Mott insulators by using an
effective model in the strong-coupling limit of a half-filed Hubbard model. In
the numerically exact diagonalization calculations on finite-size clusters, we
find that in 1D the energy position of the absorption edge is almost
independent of temperature, while in 2D the edge position shifts to lower
energy with increasing temperature. The different temperature dependence
between 1D and 2D is attributed to the difference of the coupling of the charge
and spin degrees of freedom. The implications of the results on experiments are
discussed in terms of the dimensionality dependence.Comment: 5 pages, 4 figure
Modeling Antiferromagnetic Phase in Iron Pnictides: Weakly Ordered State
We examine electronic states of antiferromagnetic phase in iron pnictides by
mean-field calculations of the optical conductivity. We find that a five-band
model exhibiting a small magnetic moment, inconsistent with the
first-principles calculations, reproduces well the excitation spectra
characterized by a multi-peak structure emerging below the N\'{e}el temperature
at low energy, together with an almost temperature-independent structure at
high energy. Investigating the interlayer magnetoresistance for this model, we
also predict its characteristic field dependence reflecting the Fermi surface
Numerical approach to low-doping regime of the t-J model
We develop an efficient numerical method for the description of a single-hole
motion in the antiferromagnetic background. The method is free of finite-size
effects and allows calculation of physical properties at an arbitrary
wavevector. Methodical increase of the functional space leads to results that
are valid in the thermodynamic limit. We found good agreement with cumulant
expansion, exact- diagonalization approaches on finite lattices as well as
self-consistent Born approximations. The method allows a straightforward
addition of other inelastic degrees of freedom, such as lattice effects. Our
results confirm the existence of a finite quasiparticle weight near the band
minimum for a single hole and the existence of string-like peaks in the
single-hole spectral function.Comment: 6 pages, 6 figures, accepted for publication in PR
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