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.

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.

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.

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.

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.

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.

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.

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.

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.

Total synthesis of the L-hexoses.

Enantiomerically pure polyhydroxylated natural products are synthesized by using a reiterative two-carbon extension cycle consisting of four steps. The generality and efficiency of this methodology are demonstrated in the total synthesis of all eight L-hexoses.

Enhancement of rabbit protein S anticoagulant cofactor activity in vivo by modulation of the protein S C4B binding protein interaction.

The carboxy-terminal region of protein S has been recently been observed to be involved in the interaction between protein S and C4b-binding protein (Walker, F. J. 1989. J. Biol. Chem. 264:17645-17658). A synthetic peptide, GVQLDLDEAI, corresponding to that region of protein S has been used to investigate the protein S/C4b-binding protein interaction in vitro and in vivo. Rabbit activated protein C possesses species-specific anticoagulant activity for which rabbit protein S functions as a cofactor. In plasma, rabbit protein S is found in complex with C4b-binding protein. GVQLDLDEAI can inhibit this interaction, resulting in enhancement of the anticoagulant activity of rabbit activated protein C. The effect of the peptide can be blocked by the concurrent addition of human or rabbit C4b-binding protein. When infused into rabbits, GVQLDLDEAI was cleared from the circulation with a half-life of 80 min. This is significantly less rapid than the clearance of similarly sized control peptides (half-life of 15 min), but much more than that of bovine protein S, a much larger protein (half-life of 15 h). Plasma samples removed from the rabbits after infusion with GVQLDLDEAI were found to have increased concentrations of free protein S and to show enhanced anticoagulation by rabbit activated protein C ex vivo in a dose-dependent manner. The concentration for half-maximal effect (5 microM) was very similar to that observed in vitro. These results suggest that the formation of a complex between protein S and C4b-binding protein is important in the regulation of protein S activity in vivo, and that modulation of this interaction allows one to influence the anticoagulant activity of the protein C pathway.

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.

Interactions between heparin and factor Xa. Inhibition of prothrombin activation.

The effects of heparin on prothrombin activation have been examined. Heparin was found to inhibit the rate of prothrombin activation by Factor Xa, calcium and phospholipid. In the absence of phospholipid, heparin had no effect on the rate of prothrombin activation. In contrast, heparin was found to increase the rate of activation of prethrombin-1 and prethrombin-2. Initial velocity studies indicated that heparin blocks lipid stimulation of prothrombin activation. In accord with this, binding studies demonstrated that heparin could displace Factor Xa, and in separate experiments, prothrombin, from phospholipid vesicles.

Inactivation of human factor VIII by activated protein C: evidence that the factor VIII light chain contains the activated protein C binding site.

Factor VIII is represented as a series of heterodimers composed of an 83(81) kDa light chain noncovalently bound to a variable size (93 to 210 kDa) heavy chain. Activated protein C inactivates factor VIII causing several cleavages of the factor VIII heavy chain(s). When factor VIII subunits were dissociated and component heavy and light chains isolated, the heavy chains were no longer a substrate for proteolysis by activated protein C. However, when factor VIII heavy chains were recombined with light chain, the reconstituted factor VIII activity was inactivated by activated protein C. The rate of factor VIII inactivation catalyzed by activated protein C was reduced by the presence of free light chain. The extent of this inhibition was dependent upon the concentration of light chain. Control experiments indicated that this protective effect of free light chain was not the result of inhibition of the activated protein C - lipid interaction. Fluorescence analysis demonstrated binding between the factor VIII light chain, chemically modified with eosin maleimide, and activated protein C, modified at its active site by dansyl-Glu-Gly-Arg chloromethyl ketone. Similar to proteolysis of factor VIII by activated protein C, this binding was dependent upon a lipid surface. Based upon the degree of fluorescence quenching, a spatial distance of 26 A was calculated separating the two fluorophores. These results demonstrate direct binding of activated protein C to the factor VIII light chain and suggest that this binding is an obligate step for activated protein C-catalyzed inactivation of factor VIII.

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.

The thrombotic diathesis associated with the presence of phospholipid antibodies may be due to low levels of free protein S.

PURPOSE: To determine if abnormalities in the protein C/protein S anticoagulant system exist in patients with phospholipid antibodies who had the primary clinical complaint of fetal wastage. PATIENTS AND METHODS: Eleven patients with fetal wastage and phospholipid antibodies were selected for study. Some patients also gave a history of previous thrombotic events related to oral contraceptives and/or pregnancy, but patients were not selected because of a history of clinical thrombosis. The levels of protein C (chromogenic assay), protein S (both free and bound) (Laurell rocket), and C4b-binding protein (Laurell rocket) were measured, and assays for the presence of antibodies against protein S or protein C were performed. RESULTS: Seven of the 11 patients were found to have low levels of free protein S. Total protein S and protein C levels were within the normal range in all patients. Antibodies to protein C and protein S were not found in any patient. These findings suggest that free protein S levels may be abnormally low in some patients with phospholipid antibodies. CONCLUSION: Free protein S levels are abnormally low in some patients with phospholipid antibodies, and this abnormality may be a factor contributing to the thrombotic diathesis associated with phospholipid antibodies.

Species specificity of the fibrinolytic effects of activated protein C.

Activated protein C (APC) has been shown to stimulate fibrinolysis in both in vitro and in vivo experimental systems. In order to test the importance of protein S in the fibrinolytic activity of APC we have compared the activity of APC, prepared from rabbit, bovine and human plasma, in the stimulation of whole blood clot lysis, the inactivation of plasminogen activator inhibitors and anticoagulant activity. When whole blood clot lysis was performed using tissue plasminogen activator in either human or rabbit blood, APC was found to enhance clot lysis in a species specific manner that paralleled the pattern observed for its anticoagulant activity. Bovine APC, was the poorest stimulator of fibrinolysis in human plasma. However, if bovine protein S was also added to human plasma, bovine APC was as effective in promoting fibrinolysis as human APC. In contrast, no species specific effects on the inactivation of plasminogen activator inhibitor activity was observed. Though substantial effects of APC on plasminogen activator inhibitor levels were made by rabbit, human and bovine activated protein C in human plasma, there was no effect of activated protein C on the rate of clot lysis of human plasma. These results suggest that protein S is important for the expression of the fibrinolytic activity of activated protein C and that the effect of protein S may be useful for the differentiation of fibrinolytic effects of activated protein C that may be related to the inactivation of plasminogen activator inhibitors and those that are not.

Purification and preliminary characterization of rabbit vitamin K-dependent coagulation proteins.

We describe herein a procedure for the purification of protein C, protein S, prothrombin, factor VII, and factor X to apparent homogeneity from rabbit plasma. The initial steps, which are common to the purification of vitamin K-dependent proteins from other mammalian species, include adsorption onto and elution from barium followed by anion exchange chromatography. Proteins were further purified using a variety of techniques, including affinity chromatography, gel filtration, and anion exchange chromatography in a Fast Protein Liquid Chromatography system. Significant structural homologies exist between rabbit, human, and bovine vitamin K-dependent proteins. As is true for protein C and factor X in human and bovine plasma, rabbit protein C and factor X are two-chain proteins which can be converted to active proteases by specific venom activators. Rabbit factor VII is also a two-chain protein and can restore coagulant activity to human or bovine plasma deficient in factor VII. In contrast, rabbit protein S and prothrombin are single chain proteins. In view of the well-described species specificity of many of the vitamin K-dependent proteins, purified rabbit coagulant and anticoagulant proteins should be useful in the development of animal models of coagulation and/or thrombosis.

Protein C deficiency in liver disease.

Protein C is a vitamin K-dependent zymogen of a serine protease that is found in blood plasma. The active form, activated protein C, can inhibit blood coagulation and stimulate fibrinolysis. Protein C is synthesized in the liver as a single chain protein. Its synthesis requires several post-translational modifications including carboxylation of glutamic acid residues, hydroxylation of aspartic acid residues, and glycosylation. Plasma protein C levels are sensitive to liver function. Protein C levels fall more rapidly than other vitamin K-dependent proteins when synthesis is altered by the administration of oral anticoagulants. In addition, low protein C levels are highly indicative of abnormal liver function. In one case, homozygous protein C deficiency has been corrected by liver transplantation. In liver transplantation for end-stage liver failure, plasma protein C levels may be a good indicator of the success of the transplantation.