625 research outputs found
Exact diagonalization study of optical conductivity in two-dimensional Hubbard model
The optical conductivity \sigma(\omega) in the two-dimensional Hubbard model
is examined by applying the exact diagonalization technique to small square
clusters with periodic boundary conditions up to \sqrt{20} X \sqrt{20} sites.
Spectral-weight distributions at half filling and their doping dependence in
the 20-site cluster are found to be similar to those in a \sqrt{18} X \sqrt{18}
cluster, but different from 4 X 4 results. The results for the 20-site cluster
enable us to perform a systematic study of the doping dependence of the
spectral-weight transfer from the region of the Mott-gap excitation to
lower-energy regions. We discuss the dependence of the Drude weight and the
effective carrier number on the electron density at a large on-site Coulomb
interaction.Comment: 5 pages, 5 figure
Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2
Thermoelectric properties of the layered cobalt oxide system LixCoO2 were
investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase
bulk samples of LixCoO2 were successfully obtained through electrochemical
deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >=
0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35)
exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K
(S293K) significantly depends on the Li content (x). The S293K value is as
large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K
to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50.
This behavior is in sharp contrast to the results of x <= 0.40 for which the
S293K value is small and independent of x (+10 uV/K), indicating that a
discontinuous change in the thermoelectric characteristics takes place at x =
0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity
are found to coexist in a narrow range of Li composition at about x = 0.75. The
coexistence, which leads to an enhanced thermoelectric power factor, may be
attributed to unusual electronic structure of the two-dimensional CoO2 block.Comment: 29 pages, 1 table, 8 figure
Exciton doublet in the Mott-Hubbard LiCuVO insulator identified by spectral ellipsometry
Spectroscopic ellipsometry was used to study the dielectric function of
LiCuVO, a compound comprised of chains of edge-sharing CuO
plaquettes, in the spectral range (0.75 - 6.5) eV at temperatures (7-300) K.
For photon polarization along the chains, the data reveal a weak but
well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral
weight is strongly enhanced upon cooling near the magnetic ordering
temperature. We identify these features as an exciton doublet in the
Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction
between electrons on nearest and next-nearest neighbor sites along the chains.
Our results and methodology can be used to address the role of the long-range
Coulomb repulsion for compounds with doped copper-oxide chains and planes.Comment: 4 pages with 4 figures and EPAPS supplementary online material (3
pages with 4 figures), accepted in Phys. Rev. Let
Doping dependence of chemical potential and entropy in hole- and electron-doped high-Tc cuprates
We examine the thermodynamic properties of the hole- and electron-doped
cuprates by using the t-t'-t''-J model. We find that the chemical potential
shows different doping dependence between the hole and electron dopings. Recent
experimental data of the chemical potential shift are reproduced except for
lightly underdoped region in the hole doping where stripe and/or charge
inhomogeneity are expected to be important. The entropy is also calculated as a
function of the carrier concentration. It is found that the entropy of the
electron-doped system is smaller than that of the hole-doped systems. This is
related to strong antiferromagnetic short-range correlation that survives in
the electron-doped system.Comment: REVTeX4, 3 pages, 3 figures, to appear in Phys. Rev.
Mott gap excitations in twin-free YBa2Cu3O7-d (Tc = 93 K) studied by RIXS
Mott gap excitations in the high-Tc superconductor of the optimal doped
YBa2Cu3O7-d (Tc = 93 K) have been studied by the resonant inelastic x-ray
scattering method. Anisotropic spectra in the ab-plane are observed in a
twin-free crystal. The excitation from the one-dimensional CuO chain is
enhanced at 2 eV near the zone boundary of the b* direction, while the
excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of
the momentum transfer. Theoretical calculation based on the one-dimensional and
two-dimensional Hubbard model reproduces the observed spectra by taking the
different parameters of the on-site Coulomb energy. The fact of the Mott gap of
the CuO chain site is much smaller than that of CuO2 plane site is observed for
the first time
Electronic States in the Antiferromagnetic Phase of Electron-Doped High-Tc Cuprates
We investigate the electronic states in the antiferromagnetic (AF) phase of
electron-doped cuprates by using numerically exact diagonalization technique
for a t-t'-t''-J model. When AF correlation develops with decreasing
temperature, a gaplike behavior emerges in the optical conductivity.
Simultaneously, the coherent motion of carriers due to the same sublattice
hoppings is enhanced. We propose that the phase is characterized as an AF state
with small Fermi surface around the momentum k=(\pi,0) and (0,\pi). This is a
remarkable contrast to the behavior of hole-doped cuprates.Comment: RevTeX, 5 pages, 4 figures, to appear in Phys. Rev. B Brief Report
Momentum-resolved charge excitations in high-Tc cuprates studied by resonant inelastic x-ray scattering
We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of
high-Tc cuprates. Momentum-resolved charge excitations in the CuO2 plane are
examined from parent Mott insulators to carrier-doped superconductors. The Mott
gap excitation in undoped insulators is found to commonly show a larger
dispersion along the [pi,pi] direction than the [pi,0] direction. On the other
hand, the resonance condition displays material dependence. Upon hole doping,
the dispersion of the Mott gap excitation becomes weaker and an intraband
excitation appears as a continuum intensity below the gap at the same time. In
the case of electron doping, the Mott gap excitation is prominent at the zone
center and a dispersive intraband excitation is observed at finite momentum
transfer
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