Picoscale structural insight into superconductivity of monolayer FeSe/SrTiO3.

Remarkable enhancement of the superconducting transition temperature (T c) has been observed for monolayer (ML) FeSe films grown on SrTiO3 substrates. The atomic-scale structure of the FeSe/SrTiO3 interface is an important determinant of both the magnetic and interfacial electron-phonon interactions and is a key ingredient to understanding its high-T c superconductivity. We resolve the atomic-scale structure of the FeSe/SrTiO3 interface through a complementary analysis of scanning transmission electron microscopy and in situ surface x-ray diffraction. We find that the interface is more strongly bonded for a particular registration, which leads to a coherently strained ML. We also determine structural parameters, such as the distance between ML FeSe and the oxide, Se─Fe─Se bond angles, layer-resolved distances between Fe─Se, and registry of the FeSe lattice relative to the oxide. This picoscale structure determination provides an explicit structural framework and constraint for theoretical approaches addressing the high-T c mechanism in FeSe/SrTiO3.

Orbital Engineering in Nickelate Heterostructures Driven by Anisotropic Oxygen Hybridization rather than Orbital Energy Levels.

Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO_{3}-LaNiO_{3}-3×(LaAlO_{3}), a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.

Engineered Unique Elastic Modes at a BaTiO_{3}/(2×1)-Ge(001) Interface.

The strong interaction at an interface between a substrate and thin film leads to epitaxy and provides a means of inducing structural changes in the epitaxial film. These induced material phases often exhibit technologically relevant electronic, magnetic, and functional properties. The 2×1 surface of a Ge(001) substrate applies a unique type of epitaxial constraint on thin films of the perovskite oxide BaTiO_{3} where a change in bonding and symmetry at the interface leads to a non-bulk-like crystal structure of the BaTiO_{3}. While the complex crystal structure is predicted using first-principles theory, it is further shown that the details of the structure are a consequence of hidden phases found in the bulk elastic response of the BaTiO_{3} induced by the symmetry of forces exerted by the germanium substrate.

The modification of ferroelectric LiNbO3(0001) surfaces using chromium oxide thin films.

The impact of ferroelectric polarization on the chemical and electronic properties of atomically thin layers of non-polar chromium oxide deposited on positively and negatively poled LiNbO3(0001) was studied. Chromium(III) oxide readily forms on LiNbO3; however, annealing at high temperatures was required to maintain well-ordered films as the thickness increased. Prolonged heating at these temperatures caused Cr diffusion into the LiNbO3 substrate. Comparing Cr 2p X-ray photoelectron spectroscopy (XPS) peak positions as a function of temperature and substrate polarization revealed no evidence of shifts from the peak positions expected for Cr2O3. The lack of any band offset between Cr2O3 on the oppositely poled surface suggests that charge compensation of the ferroelectric substrate occurs at least predominantly at the surface of the film, as opposed to the film-substrate interface. No evidence of shifts due to oxidation or reduction of the Cr was observed indicating that charge compensation did not involve a change in the ionic state of the Cr. Exposing the films to reactive oxygen species emitted from an oxygen plasma, however, caused a distinct high binding energy shoulder on the Cr 2p3/2 XPS peaks that could be associated with oxygen adsorption on surface Cr and concomitant oxidation to Cr(5+). This feature was used to gauge the concentration of O adatoms on the surfaces as a function of temperature for oppositely poled substrates; these measurements did not reveal any significant polarization dependence for oxygen desorption. Further, temperature programmed desorption measurements for a Cr2O3 film on α-Al2O3 showed a similar trend in O2 desorption. Therefore, it is concluded that the reactivity of Cr2O3 toward O is at least largely independent of substrate polarization despite data suggestive of charge compensation at the film surfaces.

Intrinsic interfacial phenomena in manganite heterostructures.

We review recent advances in our understanding of interfacial phenomena that emerge when dissimilar materials are brought together at atomically sharp and coherent interfaces. In particular, we focus on phenomena that are intrinsic to the interface and review recent work carried out on perovskite manganites interfaces, a class of complex oxides whose rich electronic properties have proven to be a useful playground for the discovery and prediction of novel phenomena.

Orbital engineering in symmetry-breaking polar heterostructures.

We experimentally demonstrate a novel approach to substantially modify orbital occupations and symmetries in electronically correlated oxides. In contrast to methods using strain or confinement, this orbital tuning is achieved by exploiting charge transfer and inversion symmetry breaking using atomically layered heterostructures. We illustrate the technique in the LaTiO_{3}-LaNiO_{3}-LaAlO_{3} system; a combination of x-ray absorption spectroscopy and ab initio theory reveals electron transfer and concomitant polar fields, resulting in a ∼50% change in the occupation of Ni d orbitals. This change is sufficiently large to remove the orbital degeneracy of bulk LaNiO_{3} and creates an electronic configuration approaching a single-band Fermi surface. Furthermore, we theoretically show that such three-component heterostructuring is robust and tunable by choice of insulator in the heterostructure, providing a general method for engineering orbital configurations and designing novel electronic systems.

Dynamic evanescent phonon coupling across the La(1-x)Sr(x)MnO3/SrTiO3 interface.

The transport and magnetic properties of correlated La0.53Sr0.47MnO3 ultrathin films, grown epitaxially on SrTiO3, show a sharp cusp at the structural transition temperature of the substrate. Using a combination of experiment and first principles theory we show that the cusp is a result of evanescent cross-interface coupling between the charge carriers in the film and a soft phonon mode in the SrTiO3, mediated through linked oxygen octahedral motions. The amplitude of the mode diverges at the transition temperature, and phonons are launched into the first few atomic layers of the film, affecting its electronic state.

Origin of the magnetoelectric coupling effect in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} Multiferroic heterostructures.

The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in the absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, opening a new pathway to the electronic control of spin in complex oxide materials.

X-ray microdiffraction analysis of radiation-induced defects in single grains of polycrystalline Fe.

Single-crystal diffuse X-ray scattering was used to characterize radiation-induced defects in individual grains of a polycrystalline proton-irradiated Fe foil. The grains were probed with an intense 1 microm X-ray beam to demonstrate that both polycrystalline and micrometer-scale samples can be studied with single-crystal-like signal-to-noise. Scattering was measured with an X-ray-sensitive area detector, which measures intensity over a surface in reciprocal space. By scanning the X-ray energy, the intensity was measured over reciprocal-space volumes. Since the sample is not rotated, the real-space scattering volume does not change. Methods to minimize experimental artifacts arising from the use of an area detector are described.

Interface-induced polarization and inhibition of ferroelectricity in epitaxial SrTiO3/Si.

We use SrTiO3/Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed (pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.

Atomic structure of the epitaxial BaO/Si(001) interface.

We present the structure of the interface responsible for epitaxy of crystalline oxides on silicon. Using synchrotron x-ray diffraction, we observe a 2 x 1 unit cell reconstruction at the interface of BaO grown on Si(001) terminated with 1/2 ML of Sr. Since this symmetry is not present in bulk BaO or Si, only the interface contributes to diffracted intensity. First principles calculations accurately predict the observed diffraction and identify the structure of the BaO/Si interface, including the elemental composition and a sub-A rumpling due to epitaxial strain of the 7 adjacent BaO and Si layers.

Role of strontium in oxide epitaxy on silicon (001).

Epitaxial oxide-Si heterostructures, which integrate the functionality of crystalline oxides with Si technology, are made possible by a submonolayer of Sr deposited on Si (001). We find by electron diffraction studies using single termination Si wafers that this Sr submonolayer replaces the top layer of Si when deposited at 650 degrees C. Supported by first-principles calculations, we propose a model for the reaction dynamics of Sr on the Si surface and its effect on oxide epitaxy. This model predicts, and we experimentally confirm, an unexplored 25 degrees C pathway to crystalline oxide epitaxy on Si.

Ultrahigh vacuum-compatible fabrication and electrical characterization systems for environmentally sensitive metal oxide semiconductor capacitors.

We describe an integrated, ultrahigh vacuum system for metal oxide semiconductor (MOS) device fabrication and characterization. This system is advantageous for electrical property measurements on electronic devices with environmentally sensitive materials and is especially important as device dimensions approach the nanoscale. Without exposure to atmosphere, MOS capacitors were fabricated by evaporatively depositing gate metal on molecular-beam-epitaxy (MBE) grown dielectrics through a shadow mask in an UHV electrode-patterning chamber. Finished devices were transferred in UHV to an in situ UHV electrical characterization probe station. We obtained excellent agreement between air-ambient ex situ and in situ probe station measurements with less than 0.3% systemic error for frequencies from 20 Hz to 1 MHz. We have successfully measured MOS capacitors with sensitivity to a density of interface states of 1x10(10) states cm(-2) eV(-1). These measurements show 0.5% systematic error for measurement frequencies from 20 Hz to 1 kHz and less than 0.1% from 1 kHz to 1 MHz. The integrated system presented here is one where complex, MBE-grown MOS heterostructures can be synthesized and tested rapidly to explore new field-effect-device physics and functionality.

The interface phase and the Schottky barrier for a crystalline dielectric on silicon.

The barrier height for electron exchange at a dielectric-semiconductor interface has long been interpreted in terms of Schottky's theory with modifications from gap states induced in the semiconductor by the bulk termination. Rather, we show with the structure specifics of heteroepitaxy that the electrostatic boundary conditions can be set in a distinct interface phase that acts as a "Coulomb buffer." This Coulomb buffer is tunable and will functionalize the barrier-height concept itself.

Physical structure and inversion charge at a semiconductor interface with a crystalline oxide.

We show that the physical and electrical structure and hence the inversion charge for crystalline oxides on semiconductors can be understood and systematically manipulated at the atomic level. Heterojunction band offset and alignment are adjusted by atomic-level structural and chemical changes, resulting in the demonstration of an electrical interface between a polar oxide and a semiconductor free of interface charge. In a broader sense, we take the metal oxide semiconductor device to a new and prominent position in the solid-state electronics timeline. It can now be extensively developed using an entirely new physical system: the crystalline oxides-on-semiconductors interface.

Influenza, pneumococcal, and tetanus toxioid vaccination of adults--United States, 1993-7.

PROBLEM/CONDITION: An increasing proportion of adults have received recommended vaccinations against influenza, pneumococcal infection, and tetanus. However, in 1995, fewer than 60% of adults were vaccinated as recommended. REPORTING PERIOD COVERED: 1993-1997. DESCRIPTION OF SYSTEM: Data were obtained from the state-based Behavioral Risk Factor Surveillance System (BRFSS) for 1993, 1995, and 1997 and from the National Health Interview Survey (NHIS) for 1995 to describe national, regional, and state-specific patterns of use of influenza and pneumococcal vaccines and tetanus toxoid among noninstitutionalized adults aged > or = 18 years. RESULTS: Among adults aged > or = 65 years in 1995, 58% reported receiving an influenza vaccination during the previous 12 months, and 34% reported ever receiving a pneumococcal vaccination. In this age group, non-Hispanic whites were more likely to report receipt of influenza (61%) and pneumococcal vaccines (36%) than non-Hispanic blacks (40% and 22%, respectively) and Hispanics (50% and 23%, respectively). Among the 50 states and the District of Columbia, the median vaccination level among older adults (i.e., persons aged > or = 65 years) increased from 51% in 1993 to 66% in 1997 for influenza vaccine, and from 28% in 1993 to 46% in 1997 for pneumococcal vaccine. Adults with chronic medical conditions had low vaccination levels. Those aged 50-64 years were more likely than those aged 18-49 years to report influenza (38% versus 20%) and pneumococcal vaccination (20% versus 12%). In 1995, the proportion of adults who reported receiving a tetanus vaccination during the previous 10 years decreased with age, from 65% among those aged 18-49 years to 54% among those aged 50-64 years and to 40% among those aged > or = 65 years. In each age group, women were less likely than men to report receiving tetanus toxoid; and among adults aged > or = 65 years, Hispanics and Asians/Pacific Islanders were least likely among all racial/ethnic groups to report receiving tetanus toxoid. INTERPRETATION: By 1995, the Healthy People 2000 objective to increase to at least 60% the proportion of persons aged > or = 65 years who had received annual influenza vaccination had been achieved among non-Hispanic whites (objective 20.11). However, substantial improvement is needed among non-Hispanic blacks, Hispanics, and adults aged < 65 years with high-risk medical conditions. PUBLIC HEALTH ACTIONS: Continued surveillance of vaccine coverage among adults will direct attention to undervaccinated populations that may be disproportionately affected by vaccine-preventable diseases. Vaccination coverage data can be used to guide efforts to increase awareness among health-care providers and the public about the benefits of vaccination, establish systems to ensure that every contact with the health-care system is used to update vaccinations, and further support financial mechanisms to increase vaccine delivery.

Activated protein C-catalyzed proteolysis of factor VIIIa alters its interactions within factor Xase.

Factor VIIIa, the cofactor for the factor IXa-dependent conversion of factor X to factor Xa, is proteolytically inactivated by activated protein C (APC). APC cleaves at two sites in factor VIIIa, Arg336, near the C terminus of the A1 subunit; and Arg562, bisecting the A2 subunit (Fay, P., Smudzin, T., and Walker, F. (1991) J. Biol. Chem. 266, 20139-20145). Factor VIIIa increased the fluorescence anisotropy of fluorescein-Phe-Phe-Arg factor IXa (Fl-FFR-FIXa; Kd = 42.4 nM), whereas cleavage of factor VIIIa by APC eliminated this property. Isolation of the APC-cleaved A1/A3-C1-C2 dimer (A1336/A3-C1-C2), and the fragments derived from cleaved A2 subunit (A2N/A2C), permitted dissection of the roles of individual cleavages in cofactor inactivation. Intact A1/A3-C1-C2 dimer increased Fl-FFR-FIXa anisotropy and bound factor X in a solid phase assay, while these activities were absent in the A1336/A3-C1-C2. However, the residues removed by this cleavage, Met337 Arg372, did not directly participate in these functions since neither a synthetic peptide to this sequence nor an anti-peptide polyclonal antibody blocked these activities using intact dimer. CD spectral analysis of the intact and truncated dimers indicated reduced alpha and/or beta content in the latter. The A1/A3-C1-C2 dimer plus A2 subunit reconstitutes cofactor activity and produced a factor VIIIa-like effect on the anisotropy of Fl-FFR-FIXa. However, when A2 was replaced by the A2N/A2C fragments, the resulting fluorescence signal was equivalent to that observed with the dimer alone. These results indicate that APC inactivates the cofactor at two levels within the intrinsic factor Xase complex. Cleavage of either subunit modulates the factor IXa active site, suggesting an essential synergy of interactive sites in factor VIIIa. Furthermore, cleavage of the A1 site alters the conformation of a factor X binding site within that subunit, thereby reducing the affinity of cofactor for substrate.

Factor IXa protects factor VIIIa from activated protein C. Factor IXa inhibits activated protein C-catalyzed cleavage of factor VIIIa at Arg562.

Factor VIIIa is inactivated by both factor IXa and activated protein C. The latter protease rapidly attacked a site at Arg562 (A2 subunit), whereas both proteases slowly cleaved factor VIIIa at Arg336 (A1 subunit). Cofactor inactivation catalyzed by activated protein C was 8-fold faster than that catalyzed by factor IXa. Simultaneous reaction of factor VIIIa with the two enzymes resulted in a rate of inactivation intermediate to that observed for the individual proteases. Under these conditions, the activated protein C-catalyzed cleavage at Arg562 was inhibited such that cofactor inactivation resulted primarily from cleavage at Arg336. Substitution of factor IXa modified in its active site with 6-(dimethylamino)-2-naphthalenesulfonyl-glutamylglycylarginyl++ + chloromethyl ketone (DEGR-IXa) for the native enzyme yielded a similar rate of activated protein C-catalyzed cleavage at the A1 site, whereas cleavage at the A2 site was virtually eliminated. However, the inclusion of protein S resulted in a marked increase in cleavage at the A2 site that correlated with an increased rate of cofactor inactivation. Active site-modified activated protein C inhibited the factor IXa-dependent enhancement of factor VIIIa reconstitution from isolated subunits. In addition, the factor VIIIa-dependent fluorescence enhancement of DEGR-activated protein C was inhibited by EGR-IXa. These results indicate that factor IXa can reduce the rate of activated protein C-catalyzed cofactor inactivation by selectively blocking cleavage at the A2 domainal site, an effect reversed by protein S. One mechanism consistent with the reciprocal inhibitory effects of the proteases is that activated protein C and factor IXa occupy overlapping sites on the cofactor. Thus, factor IXa may protect factor VIIIa by preventing activated protein C binding.