9 research outputs found
Dipole-active optical phonons in YTiO_3: ellipsometry study and lattice-dynamics calculations
The anisotropic complex dielectric response was accurately extracted from
spectroscopic ellipsometry measurements at phonon frequencies for the three
principal crystallographic directions of an orthorhombic (Pbnm) YTiO_3 single
crystal. We identify all twenty five infrared-active phonon modes allowed by
symmetry, 7B_1u, 9B_2u, and 9B_3u, polarized along the c-, b-, and a-axis,
respectively. From a classical dispersion analysis of the complex dielectric
functions \tilde\epsilon(\omega) and their inverses -1/\tilde\epsilon(\omega)
we define the resonant frequencies, widths, and oscillator strengths of the
transverse (TO) and longitudinal (LO) phonon modes. We calculate
eigenfrequencies and eigenvectors of B_1u, B_2u, and B_3u normal modes and
suggest assignments of the TO phonon modes observed in our ellipsometry spectra
by comparing their frequencies and oscillator strengths with those resulting
from the present lattice-dynamics study. Based on these assignments, we
estimate dynamical effective charges of the atoms in the YTiO_3 lattice. We
find that, in general, the dynamical effective charges in YTiO_3 lattice are
typical for a family of perovskite oxides. By contrast to a ferroelectric
BaTiO_3, the dynamical effective charge of oxygen related to a displacement
along the c-axis does not show the anomalously large value. At the same time,
the dynamical effective charges of Y and ab-plane oxygen exhibit anisotropy,
indicating strong hybridization along the a-axis.Comment: 8 pages, 7 figure
Crucial role of orbital structure in formation of frustrated magnetic structure in BiMnO
Hyperfine interactions in titanates: Study of orbital ordering and local magnetic properties
Theory of Magnetic Resonance as an Orbital State Probe
It is demonstrated that magnetic resonance in a magnetically ordered state is a sensitive indirect method for the investigation of the orbital ground state. This idea is illustrated for two perovskite titanates: LaTiO3 and YTiO3. In contrast to the spin-wave energy spectra, antiferromagnetic resonance spectra in an external magnetic field reveal clear dependence on the orbital state and it can distinguish the state with strong orbital fluctuations from the static orbital order. Our theoretical analysis is based on the model, which explicitly takes into consideration the strong correlation among lattice, orbital, and magnetic degrees of freedom. © 2009 The American Physical Society.We thank I. V. Solovyev for careful reading of the paper and extremely useful discussions. We also acknowledge fruitful communications with G. Khaliullin, M. Braden, M. V. Eremin, and D. V. Zakharov. This work was partially supported by the “Dynasty” foundation, by the CRDF REC-005, and by the Russian Federal Scientific Program “The development of scientific potential of the higher school.