346 research outputs found
Photoemission of a doped Mott insulator: spectral weight transfer and qualitative Mott-Hubbard description
The spectral weight evolution of the low-dimensional Mott insulator TiOCl
upon alkali-metal dosing has been studied by photoelectron spectroscopy. We
observe a spectral weight transfer between the lower Hubbard band and an
additional peak upon electron-doping, in line with quantitative expectations in
the atomic limit for changing the number of singly and doubly occupied sites.
This observation is an unconditional hallmark of correlated bands and has not
been reported before. In contrast, the absence of a metallic quasiparticle peak
can be traced back to a simple one-particle effect.Comment: 4 pages, 4 figures, related theoretical work can be found in
arXiv:0905.1276; shortene
Two pressure-induced structural phase transitions in TiOCl
We studied the crystal structure of TiOCl up to pressures of =25~GPa at
room temperature by x-ray powder diffraction measurements. Two pressure-induced
structural phase transitions are observed: At 15~GPa emerges
an 22 superstructure with -axis unique monoclinic
symmetry (space group P2/). At 22~GPa all lattice
parameters of the monoclinic phase show a pronounced anomaly. A fraction of the
sample persists in the ambient orthorhombic phase (space group ) over the
whole pressure range.Comment: 5 pages, 5 figures; accepted for publication in Phys. Rev.
Are the renormalized band widths in TTF-TCNQ of structural or electronic origin? - An angular dependent NEXAFS study
We have performed angle-dependent near-edge x-ray absorption fine structure
measurements in the Auger electron yield mode on the correlated
quasi-one-dimensional organic conductor TTF-TCNQ in order to determine the
orientation of the molecules in the topmost surface layer. We find that the
tilt angles of the molecules with respect to the one-dimensional axis are
essentially the same as in the bulk. Thus we can rule out surface relaxation as
the origin of the renormalized band widths which were inferred from the
analysis of photoemission data within the one-dimensional Hubbard model.
Thereby recent theoretical results are corroborated which invoke long-range
Coulomb repulsion as alternative explanation to understand the spectral
dispersions of TTF-TCNQ quantitatively within an extended Hubbard model.Comment: 6 pages, 5 figure
Pressure-induced metallization and structural phase transition of the Mott-Hubbard insulator TiOBr
We investigated the pressure-dependent optical response of the
low-dimensional Mott-Hubbard insulator TiOBr by transmittance and reflectance
measurements in the infrared and visible frequency range. A suppression of the
transmittance above a critical pressure and a concomitant increase of the
reflectance are observed, suggesting a pressure-induced metallization of TiOBr.
The metallic phase of TiOBr at high pressure is confirmed by the presence of
additional excitations extending down to the far-infrared range. The
pressure-induced metallization coincides with a structural phase transition,
according to the results of x-ray powder diffraction experiments under
pressure.Comment: 4 pages, 3 figure
Momentum-resolved single-particle spectral function for TiOCl from a combination of density functional and variational cluster calculations
We present results for the momentum-resolved single-particle spectral
function of the low-dimensional system TiOCl in the insulating state, obtained
by a combination of ab initio Density Functional Theory (DFT) and Variational
Cluster (VCA) calculations. This approach allows to combine a realistic band
structure and a thorough treatment of the strong correlations. We show that it
is important to include a realistic two-dimensional band structure of TiOCl
into the effective strongly-correlated models in order to explain the spectral
weight behavior seen in angle-resolved photoemission (ARPES) experiments. In
particular, we observe that the effect of the interchain couplings is a
considerable redistribution of the spectral weight around the Gamma point from
higher to lower binding energies as compared to a purely one-dimensional model
treatment. Hence, our results support a description of TiOCl as a
two-dimensional compound with strong anisotropy and also set a benchmark on the
spectral features of correlated coupled-chain systems.Comment: 9 pages, 7 figures, version to appear in Phys. Rev.
Two-Spinon and Orbital Excitations of the Spin-Peierls System TiOCl
We combine high-resolution resonant inelastic x-ray scattering with cluster
calculations utilizing a recently derived effective magnetic scattering
operator to analyze the polarization, excitation energy, and momentum dependent
excitation spectrum of the low-dimensional quantum magnet TiOCl in the range
expected for orbital and magnetic excitations (0 - 2.5 eV). Ti 3d orbital
excitations yield complete information on the temperature-dependent
crystal-field splitting. In the spin-Peierls phase we observe a dispersive
two-spinon excitation and estimate the inter- and intra-dimer magnetic exchange
coupling from a comparison to cluster calculations
Mott-Hubbard gap closure and structural phase transition in the oxyhalides TiOBr and TiOCl under pressure
Pressure-dependent transmittance and reflectance spectra of TiOBr and TiOCl
single crystals at room temperature suggest the closure of the Mott-Hubbard
gap, i.e., the gap is filled with additional electronic states extending down
to the far-infrared range. According to pressure-dependent x-ray powder
diffraction data the gap closure coincides with a structural phase transition.
The transition in TiOBr occurs at slightly lower pressure (=14 GPa) compared
to TiOCl (=16 GPa) under hydrostatic conditions, which is discussed in terms
of the chemical pressure effect. The results of pressure-dependent
transmittance measurements on TiOBr at low temperatures reveal similar effects
at 23 K, where the compound is in the spin-Peierls phase at ambient pressure.Comment: 11 pages, 12 figures; to appear in Phys. Rev.
Electronic structure of the two-dimensional Heisenberg antiferromagnet VOCl: a multi-orbital Mott insulator
We have studied the electronic structure of the two-dimensional Heisenberg
antiferromagnet VOCl using photoemission spectroscopy and density functional
theory including local Coulomb repulsion. From calculated exchange integrals
and the observed energy dispersions we argue that the degree of
one-dimensionality regarding both the magnetic and electronic properties is
noticeably reduced compared to the isostructural compounds TiOCl and TiOBr.
Also, our analysis provides conclusive justification to classify VOCl as a
multi-orbital Mott insulator. In contrast to the titanium based compounds
density functional theory here gives a better description of the electronic
structure. However, a quantitative account of the low-energy features and
detailed line shapes calls for further investigations including dynamical and
spatial correlations.Comment: 7 pages, 6 figure
Heat conductivity of the spin-Peierls compounds TiOCl and TiOBr
We report experimental results on the heat conductivity \kappa of the S=1/2
spin chain compounds TiOBr and TiOCl for temperatures 5K<T<300K and magnetic
fields up to 14. Surprisingly, we find no evidence of a significant magnetic
contribution to \kappa, which is in stark contrast to recent results on S=1/2
spin chain cuprates. Despite this unexpected result, the thus predominantly
phononic heat conductivity of these spin-Peierls compounds exhibits a very
unusual behavior. In particular, we observe strong anomalies at the phase
transitions Tc1 and Tc2. Moreover, we find an overall but anisotropic
suppression of \kappa in the intermediate phase which extends even to
temperatures higher than Tc2. An external magnetic field causes a slight
downshift of the transition at Tc1 and enhances the suppression of \kappa up to
Tc2. We interprete our findings in terms of strong spin-phonon coupling and
phonon scattering arising from spin-driven lattice distortions.Comment: 6 pages, 3 figure
Cluster Dynamical Mean-field calculations for TiOCl
Based on a combination of cluster dynamical mean field theory (DMFT) and
density functional calculations, we calculated the angle-integrated spectral
density in the layered quantum magnet TiOCl. The agreement with recent
photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is
found to be good. Th e improvement achieved with this calculation with respect
to previous single-site DMFT calculations is an indication of the correlated
nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe
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