David Lederman

LARGE EXCHANGE BIAS AND ITS CONNECTION TO INTERFACE STRUCTURE IN FEF2-FE BILAYERS

Large exchange bias effects (ΔE∼1.1 erg/cm2) were observed in antiferromagnetic (FeF2)–ferromagnetic (Fe) bilayers grown on MgO. The FeF2 grows along the spin‐compensated (110) direction. The FeF2–Fe interface roughness was characterized using specular and diffuse x‐ray diffraction and atomic force microscopy. The magnitude of the exchange bias field HE increases as the interface roughness decreases. These results imply that magnetic domain creation in the antiferromagnet plays an important role.

Perpendicular coupling at Fe–FeF2 interfaces

We have studied the exchange anisotropy of ferromagnetic Fe films grown on antiferromagnetic FeF2 single crystals. The behavior of the hysteresis loops of the Fe above and below the Neel temperature TN of FeF2 indicates a 90° rotation of the ferromagnetic easy axis due to the antiferromagnetic ordering. By examining the Fe hysteresis loops together with the FeF2 susceptibility behavior we infer that below TN the ferromagnetic and antiferromagnetic spins are coupled perpendicular to each other. This behavior can be explained by recent micromagnetic calculations on exchange bias systems, or by magnetoelastic effects.

EVOLUTION OF STRAIN-DEPENDENT TRANSPORT PROPERTIES IN ULTRATHIN LA0.67SR0.33MNO3 FILMS

We report on a systematic investigation of strain-induced lattice distortion effects on the crystal structure and transport properties of as-grown and postannealed ultrathin La0.67Sr0.33MnO3 epitaxial films grown on LaAlO3 (001) substrates by pulsed laser deposition. The resistivity of the as-grown films is critically dependent on the thickness, i.e., 100 A thick films show insulating behavior, 300 A thick films show metal-insulator transitions, and 500 A thick films show metallic behavior. However, all the annealed films show identical metallic behavior. Conventional θ–2θ x-ray scans with momentum transfer (q) along the [001] direction, and θ–2θ scans of the (103) and (113) peaks, with q having a component perpendicular to the growth direction, were used to measure the out-of-plane and in-plane lattice parameters of the sample. Φ scans of the (103) and (113) peaks revealed a fourfold symmetry which is consistent with a tetragonal unit cell. These data conclusively show that significantly elongated tetrag...

A GENERAL APPROACH TO THE EPITAXIAL GROWTH OF RARE-EARTH-TRANSITION-METAL FILMS

The growth of epitaxial rare‐earth‐transition‐metal thin films is reported by magnetron sputtering on single‐crystal MgO substrates. The use of epitaxial W buffer layers demonstrates a general approach to control the phase and orientation of the films. Structure and magnetism results for SmFe12(001) on W(100) and magnetically hard Sm2Co7 (110) and (001) on W(100) and (110), respectively, are highlighted to illustrate the utility of the approach.

Molecular beam epitaxy and characterization of thin Bi2Se3 films on Al2O3 (110)

The structural and electronic properties of thin Bi2Se3 films grown on Al2O3 (110) by molecular beam epitaxy are investigated. The epitaxial films grow in the Frank-van der Merwe mode and are c-axis oriented. They exhibit the highest crystallinity, the lowest carrier concentration, and optimal stoichiometry at a substrate temperature of 200 °C determined by the balance between surface kinetics and desorption of Se. The crystallinity of the films improves with increasing Se/Bi flux ratio. Our results enable studies of thin topological insulator films on inert, non-conducting substrates that allow optical access to both film surfaces.

Ultrafast carrier dynamics in thin-films of the topological insulator Bi2Se3

Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi2Se3 reveal a strong dependence of the carrier relaxation time on the film thickness. For thicker films, the relaxation dynamics are similar to those of bulk Bi2Se3, where the contribution of the bulk insulating phase dominates over that of the surface metallic phase. The carrier relaxation time shortens with decreasing film thickness, reaching values comparable to those of noble metals. This effect may result from the hybridization of Dirac cone states at the opposite surfaces for the thinnest films.

Electrocatalytic Drug Metabolism by CYP2C9 Bonded to A Self-Assembled Monolayer-Modified Electrode

Cytochrome P450 (P450) enzymes typically require the presence of at least cytochrome P450 reductase (CPR) and NADPH to carry out the metabolism of xenobiotics. To address whether the need for redox transfer proteins and the NADPH cofactor protein could be obviated, CYP2C9 was bonded to a gold electrode through an 11-mercaptoundecanoic acid and octanethiol self-assembled monolayer (SAM) through which a current could be applied. Cyclic voltammetry demonstrated direct electrochemistry of the CYP2C9 enzyme bonded to the electrode and fast electron transfer between the heme iron and the gold electrode. To determine whether this system could metabolize warfarin analogous to microsomal or expressed enzyme systems containing CYP2C9, warfarin was incubated with the CYP2C9-SAM-gold electrode and a controlled potential was applied. The expected 7-hydroxywarfarin metabolite was observed, analogous to expressed CYP2C9 systems, wherein this is the predominant metabolite. Current-concentration data generated with increasing concentrations of warfarin were used to determine the Michaelis-Menten constant (Km) for the hydroxylation of warfarin (3 μM), which is in good agreement with previous literature regarding Km values for this reaction. In summary, the CYP2C9-SAM-gold electrode system was able to carry out the metabolism of warfarin only after application of an electrical potential, but in the absence of either CPR or NADPH. Furthermore, this system may provide a unique platform for both studying P450 enzyme electrochemistry and as a bioreactor to produce metabolites without the need for expensive redox transfer proteins and cofactors.

Molecular beam epitaxy and characterization of thin Bi{sub 2}Se{sub 3} films on Al{sub 2}O{sub 3} (110)

The structural and electronic properties of thin Bi2Se3 films grown on Al2O3 (110) by molecular beam epitaxy are investigated. The epitaxial films grow in the Frank-van der Merwe mode and are c-axis oriented. They exhibit the highest crystallinity, the lowest carrier concentration, and optimal stoichiometry at a substrate temperature of 200 °C determined by the balance between surface kinetics and desorption of Se. The crystallinity of the films improves with increasing Se/Bi flux ratio. Our results enable studies of thin topological insulator films on inert, non-conducting substrates that allow optical access to both film surfaces.

Molecular beam epitaxy of YMnO3 on c-plane GaN

Epitaxial YMnO3 films were grown on (0001) GaN-on-sapphire templates using molecular beam epitaxy. The YMnO3 maintained the (0001) orientation with an in-plane YMnO3∕GaN epitaxial relationship of (0001)‖(0001); [11¯00]‖[112¯0]. The YMnO3 was ferroelectric at room temperature with a remanent polarization of ∼3.2μC∕cm2 and a saturation polarization of ∼12μC∕cm2. This heterostructure is a promising candidate for multifunctional structures that integrate ferroelectrics with GaN-based high-power and short-wavelength light-emitting devices.

Photoinduced superconductivity and structural changes in high temperature superconducting films

The illumination of PryGd1−yBa2Cu3Ox semiconducting and superconducting thin films increases their critical temperatures and decreases their normal state resistivities if and only if the films are oxygen deficient. Moreover, these changes are enhanced near the Pr‐induced metal‐insulator transition. Light also causes a contraction of the c‐axis in YBa2Cu3Ox which is correlated with the observed photoinduced resistivity changes. These changes are similar to those observed when oxygen‐deficient YBa2Cu3Ox is enriched with oxygen or annealed at room temperature after quenching from high temperatures.