78 research outputs found
Phonons and Magnetic Excitations in Mott-Insulator LaTiO
The polarized Raman spectra of stoichiometric LaTiO (T K) were
measured between 6 and 300 K. In contrast to earlier report on half-metallic
LaTiO, neither strong background scattering, nor Fano shape of the
Raman lines was observed. The high frequency phonon line at 655 cm
exhibits anomalous softening below T: a signature for structural
rearrangement. The assignment of the Raman lines was done by comparison to the
calculations of lattice dynamics and the nature of structural changes upon
magnetic ordering are discussed. The broad Raman band, which appears in the
antiferromagnetic phase, is assigned to two-magnon scattering. The estimated
superexchange constant meV is in excellent agreement with the
result of neutron scattering studies.Comment: 4 pages, 5 figure
Determination of the orbital moment and crystal field splitting in LaTiO
Utilizing a sum-rule in a spin-resolved photoelectron spectroscopic
experiment with circularly polarized light, we show that the orbital moment in
LaTiO is strongly reduced both below and above the N\'{e}el temperature.
Using Ti x-ray absorption spectroscopy as a local probe, we found
that the crystal field splitting in the subshell is about 0.12-0.30
eV. This large splitting does not facilitate the formation of an orbital
liquid
Orbital order in the low-dimensional quantum spin system TiOCl probed by ESR
We present electron spin resonance data of Ti (3) ions in single
crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g
tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is
predominantly occupied and owing to the occurrence of orbital order a linear
spin chain forms along the crystallographic b axis. This result corroborates
recent theoretical LDA+U calculations of the band structure. The temperature
dependence of the parameters of the resonance signal suggests a strong coupling
between spin and lattice degrees of freedom and gives evidence for a transition
to a nonmagnetic ground state at 67 K.Comment: revised version, accepted for publication in Phys. Rev. B, Rapid Com
Crystal and magnetic structure of LaTiO3 : evidence for non-degenerate -orbitals
The crystal and magnetic structure of LaTiO3 ~ has been studied by x-ray and
neutron diffraction techniques using nearly stoichiometric samples. We find a
strong structural anomaly near the antiferromagnetic ordering, T=146 K. In
addition, the octahedra in LaTiO3 exhibit an intrinsic distortion which implies
a splitting of the t2g-levels. Our results indicate that LaTiO3 should be
considered as a Jahn-Teller system where the structural distortion and the
resulting level splitting are enhanced by the magnetic ordering.Comment: 4 pages 5 figure
Hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet
Superconductivity in layered copper-oxide compounds emerges when charge
carriers are added to antiferromagnetically-ordered CuO2 layers. The carriers
destroy the antiferromagnetic order, but strong spin fluctuations persist
throughout the superconducting phase and are intimately linked to
super-conductivity. Neutron scattering measurements of spin fluctuations in
hole-doped copper oxides have revealed an unusual `hour-glass' feature in the
momentum-resolved magnetic spectrum, present in a wide range of superconducting
and non-superconducting materials. There is no widely-accepted explanation for
this feature. One possibility is that it derives from a pattern of alternating
spin and charge stripes, an idea supported by measurements on stripe-ordered
La1.875Ba0.125CuO4. However, many copper oxides without stripe order also
exhibit an hour-glass spectrum$. Here we report the observation of an
hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the
family of superconducting copper oxides. Our system has stripe correlations and
is an insulator, which means its magnetic dynamics can conclusively be ascribed
to stripes. The results provide compelling evidence that the hour-glass
spectrum in the copper-oxide superconductors arises from fluctuating stripes.Comment: 13 pages, 4 figures, to appear in Natur
Optical study of orbital excitations in transition-metal oxides
The orbital excitations of a series of transition-metal compounds are studied
by means of optical spectroscopy. Our aim was to identify signatures of
collective orbital excitations by comparison with experimental and theoretical
results for predominantly local crystal-field excitations. To this end, we have
studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10,
ranging from early to late transition-metal ions, from t_2g to e_g systems, and
including systems in which the exchange coupling is predominantly
three-dimensional, one-dimensional or zero-dimensional. With the exception of
LaMnO3, we find orbital excitations in all compounds. We discuss the
competition between orbital fluctuations (for dominant exchange coupling) and
crystal-field splitting (for dominant coupling to the lattice). Comparison of
our experimental results with configuration-interaction cluster calculations in
general yield good agreement, demonstrating that the coupling to the lattice is
important for a quantitative description of the orbital excitations in these
compounds. However, detailed theoretical predictions for the contribution of
collective orbital modes to the optical conductivity (e.g., the line shape or
the polarization dependence) are required to decide on a possible contribution
of orbital fluctuations at low energies, in particular in case of the orbital
excitations at about 0.25 eV in RTiO3. Further calculations are called for
which take into account the exchange interactions between the orbitals and the
coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved
calculation of orbital excitation energies in TiOCl, figure 16 improved,
references updated, 33 pages, 20 figure
G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO_3-type distortion in RTiO_3 with R=La, Pr, Nd and Sm
The origin of the antiferromagnetic order and puzzling properties of LaTiO_3
as well as the magnetic phase diagram of the perovskite titanates are studied
theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually
lifted by the La cations in the GdFeO_3-type structure, which generates a
crystal field with nearly trigonal symmetry. This allows the description of the
low-energy structure of LaTiO_3 by a single-band Hubbard model as a good
starting point. The lowest-orbital occupation in this crystal field stabilizes
the AFM(G) state, and well explains the spin-wave spectrum of LaTiO_3 obtained
by the neutron scattering experiment. The orbital-spin structures for RTiO_3
with R=Pr, Nd and Sm are also accounted for by the same mechanism. We point out
that through generating the R crystal field, the GdFeO_3-type distortion has a
universal relevance in determining the orbital-spin structure of the perovskite
compounds in competition with the Jahn-Teller mechanism, which has been
overlooked in the literature. Since the GdFeO_3-type distortion is a universal
phenomenon as is seen in a large number of perovskite compounds, this mechanism
may also play important roles in other compounds of this type.Comment: 20 pages, 15 figure
Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies
We provide a complete and detailed characterization of the
temperature-dependent response to ac electrical fields of LaTiO3, a
Mott-Hubbard insulator close to the metal-insulator transition. We present
combined dc, broadband dielectric, mm-wave, and infrared spectra of ac
conductivity and dielectric constant, covering an overall frequency range of 17
decades. The dc and dielectric measurements reveal information on the
semiconducting charge-transport properties of LaTiO3, indicating the importance
of Anderson localization, and on the dielectric response due to ionic
polarization. In the infrared region, the temperature dependence of the phonon
modes gives strong hints for a structural phase transition at the magnetic
ordering temperature. In addition, a gap-like electronic excitation following
the phonon region is analyzed in detail. We compare the results to the
soft-edge behavior of the optical spectra characteristic for Mott-Hubbard
insulators. Overall a consistent picture of the charge-transport mechanisms in
LaTiO3 emerges.Comment: 11 pages, 8 figures, 1 tabl
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