293 research outputs found
Electronic charges and electric potential at LaAlO3/SrTiO3 interfaces studied by core-level photoemission spectroscopy
We studied LaAlO3/SrTiO3 interfaces for varying LaAlO3 thickness by
core-level photoemission spectroscopy. In Ti 2p spectra for conducting "n-type"
interfaces, Ti3+ signals appeared, which were absent for insulating "p-type"
interfaces. The Ti3+ signals increased with LaAlO3 thickness, but started well
below the critical thickness of 4 unit cells for metallic transport. Core-level
shifts with LaAlO3 thickness were much smaller than predicted by the polar
catastrophe model. We attribute these observations to surface
defects/adsorbates providing charges to the interface even below the critical
thickness
Temperature Dependent Polarity Reversal in Au/Nb:SrTiO3 Schottky Junctions
We have observed temperature-dependent reversal of the rectifying polarity in
Au/Nb:SrTiO3 Schottky junctions. By simulating current-voltage characteristics
we have found that the permittivity of SrTiO3 near the interface exhibits
temperature dependence opposite to that observed in the bulk, significantly
reducing the barrier width. At low temperature, tunneling current dominates the
junction transport due both to such barrier narrowing and to suppressed thermal
excitations. The present results demonstrate that novel junction properties can
be induced by the interface permittivity
Nanometer scale electronic reconstruction at the interface between LaVO3 and LaVO4
Electrons at interfaces, driven to minimize their free energy, are
distributed differently than in bulk. This can be dramatic at interfaces
involving heterovalent compounds. Here we profile an abrupt interface between V
3d2 LaVO3 and V 3d0 LaVO4 using electron energy loss spectroscopy. Although no
bulk phase of LaVOx with a V 3d1 configuration exists, we find a nanometer-wide
region of V 3d1 at the LaVO3/LaVO4 interface, rather than a mixture of V 3d0
and V 3d2. The two-dimensional sheet of 3d1 electrons is a prototypical
electronic reconstruction at an interface between competing ground states.Comment: 14 pages, 5 figure
Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations
Optical reflectivity experiments have been conducted on single crystals of
the Kondo insulator YbB_12 in order to obtain its optical conductivity,
\sigma(\omega). Upon cooling below 70 K, a strong supression of \sigma(\omega)
is seen in the far-infrared region, indicating the opening of an energy gap of
~ 25 meV. This gap development is coincident with a rapid decrease in the
magnetic susceptibility, which shows that the gap opening has significant
influence on magnetic properties. A narrow, asymmetric peak is observed at ~40
meV in \sigma(\omega), which is attributed to optical transitions between the
Yb 4f-derived states across the gap. In addition, a broad peak is observed at
~0.25 eV. This peak is attributed to transitions between Yb 4f-derived states
and p-d band, and is reminiscent of similar peaks previously observed for
rare-earth hexaborides.Comment: 4 pages, 4 figure
Open Semiclassical Strings and Long Defect Operators in AdS/dCFT Correspondence
We consider defect composite operators in a defect superconformal field
theory obtained by inserting an AdS_4 x S^2-brane in the AdS_5 x S^5
background. The one-loop dilatation operator for the scalar sector is
represented by an integrable open spin chain. We give a description to
construct coherent states for the open spin chain. Then, by evaluating the
expectation value of the Hamiltonian with the coherent states in a long
operator limit, a Landau-Lifshitz type of sigma model action is obtained. This
action is also derived from the string action and hence we find a complete
agreement in both SYM and string sides. We see that an SO(3)_H pulsating string
solution is included in the action and its energy completely agrees with the
result calculated in a different method. In addition, we argue that our
procedure would be applicable to other AdS-brane cases.Comment: 22 pages, 1 figure, LaTeX, minor corrections and references added.
v3) some new results added. shortened and accepted version in PR
Disorder Effects in the Bipolaron System TiO Studied by Photoemission Spectroscopy
We have performed a photoemission study of TiO around its two
transition temperatures so as to cover the metallic, high-temperature
insulating (bipolaron-liquid), and low-temperature insulating
(bipolaron-crystal) phases. While the spectra of the low-temperature insulating
phase show a finite gap at the Fermi level, the spectra of the high-temperature
insulating phase are gapless, which is interpreted as a soft Coulomb gap due to
dynamical disorder. We suggest that the spectra of the high-temperature
disordered phase of FeO, which exhibits a charge order-disorder
transition (Verwey transition), can be interpreted in terms of a Coulomb gap.Comment: 4 pages, 3 epsf figures embedde
Theory for Magnetic Anisotropy of Field-Induced Insulator-to-Metal Transition in Cubic Kondo Insulator YbB_{12}
Magnetization and energy gap of Kondo insulator YbB_{12} are calculated
theoretically based on the previously proposed tight-binding model composed of
Yb 5d and 4f orbitals. It is found that magnetization
curves are almost isotropic, naturally expected from the cubic symmetry, but
that the gap-closing field has an anisotropy: the gap closes faster for the
field in (100) direction than in (110) and (111) directions, in accord with the
experiments. This is qualitatively understood by considering the maximal
eigenvalues of the total angular momentum operators projected on each direction
of the magnetic field. But the numerical calculation based on the band model
yields better agreement with the experiment.Comment: 4 pages, 4 figures, to appear in J. Phys. Soc. Jp
Characterization of the Schottky Barrier in SrRuO3/Nb:SrTiO3 Junctions
Internal photoemission spectroscopy was used to determine the Schottky
barrier height in rectifying SrRuO3/Nb-doped SrTiO3 junctions for 0.01 wt % and
0.5 wt % Nb concentrations. Good agreement was obtained with the barrier height
deduced from capacitance-voltage measurements, provided that a model of the
nonlinear permittivity of SrTiO3 was incorporated in extrapolating the built-in
potential, particularly for high Nb concentrations. Given the generic
polarizability of perovskites under internal/external electric fields, internal
photoemission provides a valuable independent probe of the interface electronic
structure.Comment: 15 pages, 3 figure
Two-Dimensional Confinement of 3d1 Electrons in LaTiO3/LaAlO3 Multilayers
We report spectroscopic ellipsometry measurements of the anisotropy of the
interband transitions parallel and perpendicular to the planes of
(LaTiO3)n(LaAlO3)5 multilayers with n = 1-3. These provide direct information
about the electronic structure of the two-dimensional (2D) 3d^1 state of the Ti
ions. In combination with LDA+U calculations, we suggest that 2D confinement in
the TiO2 slabs lifts the degeneracy of the t_{2g} states leaving only the
planar d_xy orbitals occupied. We outline that these multilayers can serve as a
model system for the study of the t_{2g} 2D Hubbard model.Comment: 7 pages, 4 figures. Accepted for publication in Phys. Rev. Let
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