3 research outputs found
Mott-Hubbard exciton in the optical conductivity of YTiO3 and SmTiO3
In the Mott-Hubbard insulators YTiO3 and SmTiO3 we study optical excitations
from the lower to the upper Hubbard band, d^1d^1 -> d^0d^2. The multi-peak
structure observed in the optical conductivity reflects the multiplet structure
of the upper Hubbard band in a multi-orbital system. Absorption bands at 2.55
and 4.15 eV in the ferromagnet YTiO3 correspond to final states with a triplet
d^2 configuration, whereas a peak at 3.7 eV in the antiferromagnet SmTiO3 is
attributed to a singlet d^2 final state. A strongly temperature-dependent peak
at 1.95 eV in YTiO3 and 1.8 eV in SmTiO3 is interpreted in terms of a Hubbard
exciton, i.e., a charge-neutral (quasi-)bound state of a hole in the lower
Hubbard band and a double occupancy in the upper one. The binding to such a
Hubbard exciton may arise both due to Coulomb attraction between
nearest-neighbor sites and due to a lowering of the kinetic energy in a system
with magnetic and/or orbital correlations. Furthermore, we observe anomalies of
the spectral weight in the vicinity of the magnetic ordering transitions, both
in YTiO3 and SmTiO3. In the G-type antiferromagnet SmTiO3, the sign of the
change of the spectral weight at T_N depends on the polarization. This
demonstrates that the temperature dependence of the spectral weight is not
dominated by the spin-spin correlations, but rather reflects small changes of
the orbital occupation.Comment: Strongly extended version; new data of SmTiO3 included; detailed
discussion of temperature dependence include
Collective orbital excitations in orbitally ordered YVO3 and HoVO3
We study orbital excitations in the optical absorption spectra of YVO3 and
HoVO3. We focus on an orbital absorption band observed at 0.4 eV for
polarization E parallel c. This feature is only observed in the intermediate,
monoclinic phase. By comparison with the local crystal-field excitations in
VOCl and with recent theoretical predictions for the crystal-field levels we
show that this absorption band cannot be interpreted in terms of a local
crystal-field excitation. We discuss a microscopic model which attributes this
absorption band to the exchange of two orbitals on adjacent sites, i.e., to the
direct excitation of two orbitons. This model is strongly supported by the
observed dependence on polarization and temperature. Moreover, the calculated
spectral weight is in good agreement with the experimental result.Comment: 12 pages, 9 figure