Surface state scattering at a buried interface

The free-electron-like surface state of Mg(0001) is strongly modified in thin films grown on W(110). The long bulk penetration length of its wave function makes it sensitive to the reflective properties of the buried interface, and hence to the complex electronic structure of the substrate. In particular we find a many-fold splitting of the Mg surface band by entering a wide projected band gap of W(110). There is a strong thickness-dependent two-band splitting, which is a clear signature of the formation of a surface-interface resonant state. An additional split-off from these two surface bands is explained by the substrate induced spin-orbit interaction.     

Modified magnetism at a buried Co/Pd interface resolved with X-ray standing waves

Soft x-ray standing waves produced by a multilayer interference substrate add depth sensitivity to magnetic circular dichroism to resolve changes in Co magnetism across a 1 nm distance from the Co center to the Co-on-Pd interface of a Pd/Co/Pd trilayer with an in-plane magnetization. Large enhancements of the number of Co d holes, and of in-plane orbital and spin magnetic moments, are strongly localized at a thin, chemically modified interface layer. These results provide new insight into magnetic anisotropy at interfaces, and suggest a broad applicability of such standing wave measurements to interface magnetism studies.     

Generation and propagation of a picosecond acoustic pulse at a buried interface: time-resolved x-ray diffraction measurements

We report on the propagation of coherent acoustic wave packets in (001) surface oriented Al0.3Ga0.7As/GaAs heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along [001] and [001] directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.     

Electrode reaction at platinum / ceria surface modified YSZ interface

The electrode reaction was examined on ceria coated YSZ by a platinum point electrode in H₂-H₂O atmosphere at 973 K- 1173 K. The thickness of the ceria coating layer was altered from 0 to 2.5 μm, fabricated by a laser ablation and by a vacuum vapor deposition method on YSZ single crystals. The electrode / electrolyte interface conductivity increased with 1/4 powers ofp(H₂) andp(H₂O) on both ceria coated and non-coated YSZ. The interface conductivity was significantly improved on a thicker ceria coating surface than 1 μm. The effective electrode reaction radius also increased in a thick ceria coating. The¹⁸O/¹⁶O exchange experiment at low oxygen partial pressure revealed that the oxygen surface exchange rate of ceria is not high compared with that of YSZ. It can be concluded that the bulk ionic conduction of ceria makes a more effective contribution to the electrode reaction than the surface catalytic activity in H₂-H₂O atmosphere. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

High resolution transmission electron microscopy to study very thin crystalline layers buried at an amorphous-crystalline interface

Structure characterisation of interfaces is a field of widespread application of high resolution transmission electron microscopy for its very high spatial resolution. Specimen thickness and electron optical condition have a deep influence on the high resolution electron transmission microscopy image contrast. Hence, in many cases, the real structure of the sample can be understood from experimental images only by comparison with the relevant simulation. Moreover, the understanding of the contrast variation of a few A at an interface is a task in which even the use of simulation could not produce an unequivocal solution of the experimental result. In this paper high resolution transmission electron microscopy image simulations show that two monolayers of crystalline material buried at an amorphous-crystalline interface can be successfully revealed and interpreted. The simulated images reproduce the experimental results as obtained from the Al/Si-As/n-GaAs (001) heterostructure.     

Apparent slip at a polymer-polymer interface

We consider a planar interface between strongly-segregated homopolymers subjected to steady shear in the plane of the interface. We develop a constitutive equation for stress relaxation in an inhomogeneous system for chains obeying Rouse dynamics. Using this equation, the interfacial viscosity for a symmetric blend is found to be in agreement with a scaling prediction due to de Gennes, where is the bead friction coefficient, b is the segment length, is the segment volume and is the Flory-Huggins interaction parameter driving the phase separation. We generalize our results to asymmetric blends and describe a phenomenological extension to entangled melts. Received: 18 August 1997 / Received in final form: 1 December 1997 / Accepted: 2 December 1997     

Magnetic polaritons at a superconductor-ferromagnet interface

The propagation of surface electromagnetic waves at the interface between a high-temperature superconductor and a ferromagnet is considered. Dispersion relations are derived for transverse electric and transverse magnetic surface eigenwaves in the case of various orientations of the crystallographic axes of the anisotropic superconductor with respect to the direction of the surface wave vector. A nonreciprocal character of the propagation of transverse electric polaritons is revealed, as well as a significant dependence of dispersion characteristics and the penetration depth on the external magnetic field.     

Bandstructure matching at a heterojunction interface

An analytic theory for the matching of the bandstructure of different crystals at the interface of a heterojunction is presented. The Generalized Wannier functions serve as a basis. For a simple uniform band, the Hamiltonian matrix elements then reduce to the k-space Fourier coefficients of the bandstructure along the superlattice direction. The technique therefore accounts for non-effective mass effects, and the lower and upper valleys, and enables us to invoke both the quasi-k-space periodicity together with the spatial variations of the bandstructure. The superlattice Hamiltonian is a system of difference equations taking the form of a band matrix. A new definition of current not based on the effective-mass is introduced for this higher-order Schroedinger equation. The enforcement of the continuity of the elemental currents leads to analytic connection rules for the overlap Hamiltonian matrix elements. A maximum transparency for all energies is achieved only for geometrically related bandstructures. Non-effective-mass effects are demonstrated in resonant-tunneling structures. The technique presented offers new theoretical insights together with efficient numerical tools for the study of non-effective-mass superlattices.     

Boundary conditions at a fluid-solid interface

We study the boundary conditions at a fluid-solid interface using molecular dynamics simulations covering a broad range of fluid-solid interactions and fluid densities and both simple and chain-molecule fluids. The slip length is shown to be independent of the type of flow, but rather is related to the fluid organization near the solid, as governed by the fluid-solid molecular interactions.     

Surface acoustic waves at a free-metallized interface

Light scattering was used to study surface acoustic waves at a free-metalized interface on piezoelectric lithium niobate. An interaction region of several acoustic wavelengths was observed in which a reflected surface wave and a continuous spectrum of bulk waves are generated.