Electron energy loss spectroscopy with parallel readout of energy and momentum

We introduce a high energy resolution electron source that matches the requirements for parallel readout of energy and momentum of modern hemispherical electron energy analyzers. The system is designed as an add-on device to typical photoemission chambers. Due to the multiplex gain, a complete phonon dispersion of a Cu(111) surface was measured in seven minutes with 4 meV energy resolution

Dielectric response effects in attosecond time-resolved streaked photoelectron spectra of metal surfaces

The release of conduction-band electrons from a metal surface by a sub-femtosecond extreme ultraviolet (XUV) pulse, and their propagation through the solid, provokes a dielectric response in the solid that acts back on the photoelectron wave packet. We calculated the (wake) potential associated with this photoelectron self-interaction in terms of bulk and surface plasmon excitations and show that it induces a considerable, XUV-frequency-dependent temporal shift in laser-streaked XUV photoemission spectra, suggesting the observation of the ultrafast solid-state dielectric response in contemporary streaked photoemission experiments.Comment: 4 pages and 4 figures, submitted to PR

Перспективы использования магнезиальных пород

A new operation mode of the lens system in electron energy loss spectrometers is proposed which brings the cardinal plane of the lenses closer to the sample. Experiments as well as electron optical calculations show an intensity gain by a factor of 3-4. The required voltages on the lens elements are calculated as function of the impact energy at the sample. Furthermore, the lens system is characterized with respect to the solid angle probed by the spectrometer and the transmission which enables the evaluation of spectral intensities in terms of the absolute scattering probability for elastic and inelastic scattering. The results apply to most existing electron energy loss spectrometers as they stem from the same design and use identical lens systems. (C) 2012 Elsevier B.V. All rights reserved

Effective mass in quasi two-dimensional systems

The effective mass of the quasiparticle excitations in quasi two-dimensional systems is calculated analytically. It is shown that the effective mass increases sharply when the density approaches the critical one of metal-insulator transition. This suggests a Mott type of transition rather than an Anderson like transition.Comment: 3 pages 3 figure

First-principles calculations of the dispersion of surface phonons of the unreconstructed and reconstructed Pt(110)

We present result of calculations of the surface phonon dispersion curves for Pt(110) using density functional theory in the local density approximation and norm conserving pseudopotentials in a mixed-basis approach. Linear response theory is invoked and both the unreconstructed, and the missing row (1x2) reconstructed surfaces are considered. We find that the reconstruction is not driven by a phonon instability. Most of the observed phonon modes for the (1x2) structure can be understood in terms of simple folding of the (1x1) Brillouin zone onto that for the (1x2) surface. Largest changes in the phonon frequencies on surface reconstruction occur close to the zone boundary in the (001) direction. Detailed comparison of atomic force constants for the (1x1) and the (1x2) surfaces and their bulk counterparts show that the bulk value is attained after three layers. Our calculations reproduce nicely the Kohn anomaly observed along the (110) direction in the bulk. We do not find a corresponding effect on the surface

Dynamic polarization of graphene by moving external charges: random phase approximation

We evaluate the stopping and image forces on a charged particle moving parallel to a doped sheet of graphene by using the dielectric response formalism for graphene's $\pi$-electron bands in the random phase approximation (RPA). The forces are presented as functions of the particle speed and the particle distance for a broad range of charge-carrier densities in graphene. A detailed comparison with the results from a kinetic equation model reveal the importance of inter-band single-particle excitations in the RPA model for high particle speeds. We also consider the effects of a finite gap between graphene and a supporting substrate, as well as the effects of a finite damping rate that is included through the use of Mermin's procedure. The damping rate is estimated from a tentative comparison of the Mermin loss function with a HREELS experiment. In the limit of low particle speeds, several analytical results are obtained for the friction coefficient that show an intricate relationship between the charge-carrier density, the damping rate, and the particle distance, which may be relevant to surface processes and electrochemistry involving graphene.Comment: 14 pages, 10 figures, accepted for publication in Phys. Rev.

Twisted-light-induced optical transitions in semiconductors: Free-carrier quantum kinetics

We theoretically investigate the interband transitions and quantum kinetics induced by light carrying orbital angular momentum, or twisted light, in bulk semiconductors. We pose the problem in terms of the Heisenberg equations of motion of the electron populations, and inter- and intra-band coherences. Our theory extends the free-carrier Semiconductor Bloch Equations to the case of photo-excitation by twisted light. The theory is formulated using cylindrical coordinates, which are better suited to describe the interaction with twisted light than the usual cartesian coordinates used to study regular optical excitation. We solve the equations of motion in the low excitation regime, and obtain analytical expressions for the coherences and populations; with these, we calculate the orbital angular momentum transferred from the light to the electrons and the paramagnetic and diamagnetic electric current densities.Comment: 11 pages, 3 figure

Spin waves in ultrathin hexagonal cobalt films on W(110), Cu(111), and Au(111) surfaces

Spin wave spectra of ultrathin epitaxial cobalt films deposited on W(110), Cu(111), and Au(111) surfaces are studied in the wave-vector regime between 0.1Å−1 and 0.7Å−1 using inelastic electron scattering with 6 meV energy resolution. Up to three different spin wave modes are resolved for wave vectors q∥0.35Å−1 and layers thicker than five atom layers the separate modes merge into a single, broad loss feature. Since the shape and position of the loss feature depend on the electron impact energy, a separation into different modes is nevertheless possible for not too large wave vectors. The spin wave dispersion curves of films grown on W(110) agree with those observed on Cu(111) if one takes into account that on copper the cobalt grows in islands so that the mean height of the islands is higher than the nominal coverage. On films grown on Au(111) the low wave vector spin waves are buried in the high elastic diffuse scattering caused by the considerable disorder in the films. The broader appearance of the spectra at higher wave vectors compared to films grown on W(110) and Cu(111) is quantitatively accounted for by disorder-induced kinematic broadening. Because of the granular growth on copper and gold primarily the spin wave spectrum of cobalt films on W(110) is amenable to quantitative theoretical analysis. Such an analysis is not available at present. We show however, that the dispersion curves are incompatible with the Heisenberg model as long as only a single, layer-independent exchange coupling constant is invoked

New systems on northwest Missouri upland farms

November, 1938

Sub-monolayer nucleation and growth of complex oxide heterostructures at high supersaturation and rapid flux modulation

We report on the non-trivial nanoscale kinetics of the deposition of novel complex oxide heterostructures composed of a unit-cell thick correlated metal LaNiO3 and dielectric LaAlO3. The multilayers demonstrate exceptionally good crystallinity and surface morphology maintained over the large number of layers, as confirmed by AFM, RHEED, and synchrotron X-ray diffraction. To elucidate the physics behind the growth, the temperature of the substrate and the deposition rate were varied over a wide range and the results were treated in the framework of a two-layer model. These results are of fundamental importance for synthesis of new phases of complex oxide heterostructures.Comment: 13 pages, 6 figure