T. Susaki

Polar discontinuity doping of the LaVO_{3}/SrTiO_{3} interface.

We have investigated the transport properties of LaVO_{3}/SrTiO_{3} Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO_{2}/LaO/TiO_{2} polar discontinuity is conducting, exhibiting a LaVO3 thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO_{2}/SrO/TiO_{2} interface, formed by inserting a single layer of bulk metallic SrVO3 or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.

Magnetocapacitance and exponential magnetoresistance in manganite–titanate heterojunctions

We present a rectifying manganite–titanate heterojunction exhibiting a magnetic field tunable depletion layer. This creates a large positive magnetocapacitance, a direct measure of the field-induced reduction of the effective depletion width across the junction. Furthermore, the reduction of the junction barrier shifts the forward bias characteristics, giving exponentially enhanced differential magnetoresistance, occurring despite the absence of a spin filter. These results provide a unique probe of a Mott insulator∕band insulator interface, and further suggest electronic devices incorporating the magnetic field sensitivity of these strongly correlated electron materials.

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 and 0.5wt% 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.

Atomically flat (110) SrTiO3 and heteroepitaxy

We have prepared an atomically flat and insulating (110) SrTiO3 surface by annealing at high temperature under varying oxygen partial pressure. At low pressure, the polar surface is stabilized by oxygen vacancies, resulting in an atomically flat surface characterized by (110) unit-cell steps. The vacancies can be filled while maintaining this surface structure, providing an atomically ideal (110) substrate. We demonstrate two-dimensional homoepitaxial and heteroepitaxial growth, establishing the potential of this growth orientation for controlling interface states arising from polarity discontinuities in perovskite heterostructures.

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.

Low-Energy Electronic Structure of the Kondo Insulator YbB12

We have studied the low-energy electronic structure of a Kondo insulator YbB{sub 12} by high-resolution photoemission spectroscopy. A {open_quote}{open_quote}Kondo peak{close_quote}{close_quote} is observed {approximately}25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak. {copyright} {ital 1996 The American Physical Society.}

Modulation doping of a Mott quantum well by a proximate polar discontinuity

We present evidence for hole injection into LaAlO3/LaVO3/LaAlO3 quantum wells near a polar surface of LaAlO3 (001). As the surface is brought in proximity to the LaVO3 layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient is observed below a characteristic coupling length of 10-15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the AlO2-terminated LaAlO3 surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole-doping in oxide thin film heterostructures.

Low-Energy Electronic Structure of the Kondo Insulator YbB{sub {bold 12}}

We have studied the low-energy electronic structure of a Kondo insulator YbB{sub 12} by high-resolution photoemission spectroscopy. A {open_quote}{open_quote}Kondo peak{close_quote}{close_quote} is observed {approximately}25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak. {copyright} {ital 1996 The American Physical Society.}

Growth and epitaxial structure of LaVOx films

We have studied the phase diagram of LaVOx films grown on (001) SrTiO3 substrates by pulsed laser deposition. With increasing oxygen partial pressure, the growth phase diagram varies between epitaxial perovskite LaV3+O3 single crystal films and polycrystalline monoclinic LaV5+O4. An interesting feature is the lack of an accessible phase corresponding to V4+, resulting in an extended region of phase coexistence of LaVO3 and LaVO4. Atomically flat LaVO3 could be grown in both layer-by-layer and step-flow growth modes, making this a promising candidate for incorporating strongly correlated electrons in atomic-scale perovskite heterostructures.

Vanishing Hall Coefficient in the Extreme Quantum Limit in Photocarrier-Doped SrTiO3

We explore the extreme quantum limit of photogenerated electrons in quantum paraelectric SrTiO3. This regime is distinct from conventional semiconductors, due to the large electron effective mass and large dielectric constant. At low temperature, the magnetoresistance and Hall resistivity saturate at a high magnetic field, deviating from conventional behavior. As a result, the Hall coefficient vanishes on the scale of the ratio of the Landau level splitting to the thermal energy, indicating the essential role of lowest Landau level occupancy, as limited by thermal broadening.