10 research outputs found
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
Anisotropic optical response of the mixed-valent Mott-Hubbard insulator NaCu2O2
We report the results of a comprehensive spectroscopic ellipsometry study of
NaCu2O2, a compound composed of chains of edge-sharing Cu2+O4 plaquettes and
planes of Cu1+ ions in a O-Cu1+-O dumbbell configuration, in the spectral range
0.75-6.5 eV at temperatures 7 -300 K. The spectra of the dielectric function
for light polarized parallel to the Cu1+ planes reveal a strong in-plane
anisotropy of the interband excitations. Strong and sharp absorption bands
peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along
(perpendicular) to the Cu2+O2 chains. They are superimposed on flat and
featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on
density-functional calculations, the anomalous absorption peaks can be assigned
to transitions between bands formed by Cu1+ 3dxz(dyz) and Cu2+ 3dxy orbitals,
strongly hybridized with O pstates. The major contribution to the background
response comes from transitions between Cu1+ 3dz2 and 4px(py) bands. This
assignment accounts for the measured in-plane anisotropy. The dielectric
response along the Cu2+O2 chains develops a weak two-peak structure centered at
2.1 and 2.65 eV upon cooling below 100 K, along with the appearance of spin
correlations along the Cu2+O2 chains. These features bear a striking
resemblance to those observed in the single-valent Cu2+O2 chain compound
LiCuVO4, which were identified as an exciton doublet associated with
transitions to the upper Hubbard band that emerges as a consequence of the
long-range Coulomb interaction between electrons on neighboring Cu2+ sites
along the chains. An analysis of the spectral weights of these features yields
the parameters characterizing the on-site and long-range Coulomb interactions.Comment: 12 pages, 12 figure
Signatures of Electronic Correlations in Optical Properties of LaFeAsOF
Spectroscopic ellipsometry is used to determine the dielectric function of
the superconducting LaFeAsOF ( = 27 K) and undoped LaFeAsO
polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 350 K. The free charge carrier response in both samples is heavily
damped with the effective carrier density as low as 0.0400.005 electrons
per unit cell. The spectral weight transfer in the undoped LaFeAsO associated
with opening of the pseudogap at about 0.65 eV is restricted at energies below
2 eV. The spectra of superconducting LaFeAsOF reveal a
significant transfer of the spectral weight to a broad optical band above 4 eV
with increasing temperature. Our data may imply that the electronic states near
the Fermi surface are strongly renormalized due to electron-phonon and/or
electron-electron interactions.Comment: 4 pages, 4 figures, units in Fig.2 adde
Eliashberg approach to superconductivity-induced infrared anomalies in Ba0.68K0.32Fe2As2
We report the full complex dielectric function of high-purity
single crystals
with determined by wide-band spectroscopic
ellipsometry at temperatures . We discuss the
microscopic origin of superconductivity-induced infrared optical anomalies in
the framework of a multiband Eliashberg theory with two distinct
superconducting gap energies $2\Delta_{\mathrm{A}}\approx6\
k_{\mathrm{B}}T_{\mathrm{c}}2\Delta_{\mathrm{B}}\approx2.2\
k_{\mathrm{B}}T_{\mathrm{c}}14\
k_{\mathrm{B}}T_{\mathrm{c}}$ can be ascribed to spin-fluctuation--assisted
processes in the clean limit of the strong-coupling regime.Comment: 4 pages, 4 figures; suppl. material: 3 pages, 2 figures, 1
interactive simulation (Fig. S3
Doping-Dependent Raman Resonance in the Model High-Temperature Superconductor HgBa2CuO4+d
We study the model high-temperature superconductor HgBa2CuO4+d with
electronic Raman scattering and optical ellipsometry over a wide doping range.
The resonant Raman condition which enhances the scattering cross section of
"two-magnon" excitations is found to change strongly with doping, and it
corresponds to a rearrangement of inter-band optical transitions in the 1-3 eV
range seen by ellipsometry. This unexpected change of the resonance condition
allows us to reconcile the apparent discrepancy between Raman and x-ray
detection of magnetic fluctuations in superconducting cuprates. Intriguingly,
the strongest variation occurs across the doping level where the antinodal
superconducting gap reaches its maximum.Comment: 4 pages, 4 figures, contact authors for Supplemental Materia
Dimensionality Control of Electronic Phase Transitions in Nickel-Oxide Superlattices
The competition between collective quantum phases in materials with strongly
correlated electrons depends sensitively on the dimensionality of the electron
system, which is difficult to control by standard solid-state chemistry. We
have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap
insulator LaAlO3 with atomically precise layer sequences. Using optical
ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as
thin as two unit cells are shown to undergo a sequence of collective
metalinsulator and antiferromagnetic transitions as a function of decreasing
temperature, whereas samples with thicker LaNiO3 layers remain metallic and
paramagnetic at all temperatures. Metal-oxide superlattices thus allow control
of the dimensionality and collective phase behavior of correlated-electron
systems