Quantum-well resonances and image states in the Ar/Cu(1 0 0) system

A theoretical study of the effect of an atomically thin rare gas layer on the dynamics of excited electronic states at metal surfaces is presented for the case of a few mono-layers of Ar on a Cu(1 0 0) surface. We develop a 3D-microscopic model with predictive capabilities of the interaction of an electron with an Ar layer physisorbed on a metal surface. It takes into account the 3D structure of the Ar layer as well as its dielectric character. The dynamics of the excited electron on the surface is treated within a wave-packet propagation approach. The calculations show that two different types of excited states are present at the Ar/Cu(1 0 0) surface. (i) Image states that are repelled into vacuum as compared to their position on clean Cu(1 0 0) surfaces, leading to a decrease of their binding energies and to an increase of their lifetimes. (ii) Quantum-well resonances, corresponding to quasi-stationary states localised inside the Ar layer; they are associated with the quantisation of the conduction band in the finite size Ar layer. The present results on image states nicely agree with very recent time-resolved two-photon-photo-emission experiments by Berthold, Feulner and Höfer.     

Anisotropy of electron structure at InAs(1 1 1) surfaces by laser pump-and-probe photoemission spectroscopy

The electronic structure and the electron dynamics of the clean InAs(1 1 1)A 2 × 2 and the InAs(1 1 1)B 1 × 1 surfaces have been studied by laser pump-and-probe photoemission spectroscopy. Normally unpopulated electron states above the valence band maximum (VBM) are filled on the InAs(1 1 1)A surface due to the conduction band pinning above the Fermi level (E_F). Accompanied by the downward band banding alignment, a charge accumulation layer is confined to the surface region creating a two dimensional electron gas (2DEG). The decay of the photoexcited carriers above the conduction band minimum (CBM) is originated by bulk states affected by the presence of the surface. No occupied states were found on the InAs(1 1 1)B 1 × 1 surface. This fact is suggested to be due to the surface stabilisation by the charge removal from the surface into the bulk. The weak photoemission intensity above the VBM on the (1 1 1)B surface is attributed to electron states trapped by surface defects. The fast decay of the photoexcited electron states on the (1 1 1)A and the (1 1 1)B surfaces was found to be τ_1 1 1 A ? 5 ps and τ_1 1 1 B ?  4 ps, respectively. We suggest the diffusion of the hot electrons into the bulk is the decay mechanism.     

Determining the structure of Ru(0 0 0 1) from low-energy electron diffraction of a single terrace

While a perfect hcp (0 0 0 1) surface has threefold symmetry, the diffraction patterns commonly obtained are sixfold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180°. Here we use a low-energy electron microscope to acquire the threefold diffraction pattern from a single hcp Ru terrace and measure the intensity vs. energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(±0.9)% at 456 K.     

Kinetics of residual gas adsorption on Ge(1 1 1) surface in low-energy electron backscattering

Low-energy (0.4-1.2 eV) electron backscattering is applied for the investigation of kinetics of residual gas adsorption effect on the concentration and energy positions of surface electron states of Ge(1 1 1) surface. Chemosorption of residual gas molecules on Ge(1 1 1) at P ∼ 10⁻⁷ Pa and room temperature is shown to be most active during the first 48 h. Low concentration of dangling valence bonds on the reconstructed Ge(1 1 1) (2 × 8) surface is shown to determine its low activity to chemosorption.     

Surface excitation effects in elastic peak electron spectroscopy

Surface electron inelastic excitations, a consequence of electron-surface interaction, effect the measured intensities in surface-sensitive electron spectroscopic methods and distort the quantitative information. This phenomenon is more pronounced at low electron energy and glancing emission angles. In this work we investigate quantitatively the influence of the surface excitation effects on the measured electron elastic backscattering probability. As a model system we used Si, Cu and Al, i.e. materials with different surface excitation properties. Results obtained show that properly corrected measured elastic electron backscattering probabilities lead to inelastic mean free path values which compare well with the theory.     

电子束蒸发氧化锆薄膜的粗糙度和光散射特性

利用电子束蒸发工艺，以Ag层为衬底，沉积了中心波长为632.8nm的氧化锆(ZrO₂)薄膜，膜层厚度在80—480nm范围内变化.研究了不同厚度样品的粗糙度变化规律和表面散射特性.结果发现，随着膜层厚度的逐渐增加，其表面均方根(RMS)粗糙度和总积分散射(TIS)均呈现出先减小后增大的趋势.利用非相关表面粗糙度的散射模型对样品的TIS特性进行了理论计算，所得结果与测量结果相一致. 关键词： 氧化锆 表面粗糙度 标量散射 电子束蒸发     

Resonant channel coupling in electron scattering by pyrazine

Detailed investigation of the three low-energy resonances seen in electron scattering by the diazabenzene molecule pyrazine reveals that the first two are nearly pure single-channel shape resonances, but the third is, as long suspected, heavily mixed with core-excited resonances built on low-lying triplet states. Such resonant channel coupling is likely to be widespread in pi-ring molecules, including the nucleobases of DNA and RNA, where it may form a pathway for radiation damage.     

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for Al_xGa_1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13 K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.     

Dephasing times in a nondegenerate two-dimensional electron Gas

Studies of weak localization by scattering from vapor atoms for electrons on a liquid helium surface are reported. There are three contributions to the dephasing time. Dephasing by the motion of vapor atoms perpendicular to the surface is studied by varying the holding field to change the characteristic width of the electron layer at the surface. A change in vapor density alters the quasielastic scattering length and the contribution to dephasing due to the motion of atoms both perpendicular and parallel to the surface. Dephasing due to the electron-electron interaction is dependent on the electron density.     

Interaction of CO molecules with surface state electrons on Ag(1 1 1)

Like other close-packed noble metal surfaces, Ag(1 1 1) exhibits an occupied Shockley-type surface state that is believed to influence the adsorption of atoms and molecules. Using low-temperature scanning tunneling microscopy, we have directly probed this interaction by investigating the local CO distribution dependent on the Ag(1 1 1) surface state standing wave pattern forming in the neighborhood of strong scattering centers such as step edges or hexagonal holes. A quantitative analysis of the STM data reveals that the CO molecules are not arbitrarily distributed upon adsorption at 5 K; they adsorb preferentially near the minima of the standing wave pattern.     

Temperature dependence of the coupled electron–ripplon resonance spectrum of electron crystal over liquid helium

Spectra of coupled electron–ripplon oscillation of two-dimensional (2D) Wigner crystal with the surface electron density n_s=6.4×10⁸ cm⁻² (melting temperature of 0.58 K) are measured under conditions of both complete and noncomplete screening of the holding electric field in temperature range 0.08–0.4 K. The frequencies of the resonances are compared with the results of calculation in the frameworks of a self-consistent theory by Monarkha and Shikin. It is shown that the amplitude of the (0,1) resonance mode tends to 0 as temperature reaches the melting temperature.     

Mapping of the electron transmission through the wall of a quantum corral

We report on a theoretical study of the escape of confined surface states electrons from quantum corrals made of Cu adatoms on a Cu(1 1 1) surface. This study maps electron transmission through the corral wall and provides an extension of our earlier work focused on confinement in Cu corrals [S. Díaz-Tendero, F.E. Olsson, A.G. Borisov, J.P. Gauyacq, Phys. Rev. B 77 (2008) 205403]. The existence of two decay modes for the confined surface state is stressed: (i) non-resonant tunnelling through the corral wall concentrated on the Cu adatoms and (ii) a resonant-induced decay involving the transient formation of a resonant state localized on top of the corral wall. The present mapping of the electron transmission reveals how the interference between the two decay modes works: there exist regions where the electron leaves the corral, balanced by regions where it enters the corral, though the global behaviour of the quasi-stationary states is electron escape from the corral.     

The nanoparticle shape's effect on the light scattering cross-section

In the framework of kinetic approach we develop a theory for light scattering by ellipsoidal metallic nanoparticles whose dimensions are less than those of a free electron path. In this case, the surface of the particle starts to play a dominant role in electron scattering. When the size of the particle decreases below the free electron path at least in one direction, the optical conductivity becomes a tensor quantity, and the diagonal components of this tensor define the half-widths of the plasmon resonances peaks. Thus, the effect of the particles' shape both on the frequencies of plasmon resonances and on their half-widths is considered. Additionally, the expression for a cross-section of the light scattering by a collection of chaotically oriented spheroidal nanoparticles is obtained and averaged over different directions of particles in the collection. Our results highlight the plasmonic properties of metallic nanospheroids, notably, the spectrum of the light scattering has two peaks at the frequencies of plasmon resonances even if there is no preferential direction in the collection of particles.     

Band structure of surface states and resonances for clean Cu(1 1 0) surface

We have used the layer KKR method to calculate the Shockley and Rydberg surface states and resonances for Cu(1 1 0) for a given model of the surface potentials. This method has not been used before to predict all of the surface band structure for the energy range from the bottom of the conduction band to ∼7 eV above the vacuum level. The previous methods that used only local electron interactions in ab initio calculations could not produce the Rydberg surface barrier bands while those relying on nearly-free-electron parameterisation of bands could not deal with d-bands.     

Determination of the transport lifetime limiting scattering rate in InSb/AlxIn1−x Sb quantum wells using optical surface microscopy

We report magnetotransport measurements of InSb/Al_1−xIn_xSb quantum well structures at low temperature (3 K), with evidence for 3 characteristic regimes of electron carrier density and mobility. We observe characteristic surface structure using differential interference contrast DIC (Nomarski) optical imaging, and through use of image analysis techniques, we are able to extract a representative average grain feature size for this surface structure. From this we deduce a limiting low temperature scattering mechanism not previously incorporated in transport lifetime modelling of this system, with this improved model giving strong agreement with standard low temperature Hall measurements. We have demonstrated that the mobility in such a material is critically limited by quality from the buffer layer growth, as opposed to fundamental material scattering mechanisms. This suggests that the material has immense potential for mobility improvement over that reported to date.     

Adsorption of Pb on NiAl(1 1 0): energetics and structure

The adsorption of Pb onto a NiAl(1 1 0) single crystal surface at 300 K has been studied by Auger electron spectroscopy (AES), Low energy electron diffraction (LEED), molecular beam/surface scattering and single crystal adsorption calorimetry (SCAC). AES indicates a Stranski-Krastanov growth mode, i.e., Pb initially grows on NiAl(1 1 0) two-dimensionally until the first layer completes at 0.89 ML, where a superstructure is observed by LEED, followed by 3D islanding. Measurements of the Pb gas that does not stick indicate that Pb sticks on NiAl(1 1 0) with an initial probability of 0.99. The initial heat of adsorption of Pb on NiAl(1 1 0) is 249 ± 10 kJ/mol. Due to the repulsive interactions between Pb adatoms, the heat of adsorption decreases within the first layer to a value identical to the heat of sublimation of bulk Pb (195 kJ/mol), where it remains at higher coverages. This first application of adsorption calorimetry on such a thick sample (75 μm versus 0.2-8 μm previously) demonstrates that adsorption calorimetry can be extended to a wider range of surfaces, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning.     

Effect of various adsorbates in dephasing and population decay of the Cu(1 0 0) image potential states

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of adsorbates is presented. Scattering of the image state electron by an adsorbate induces inter-band and intra-band transitions leading respectively to the population decay and to the dephasing of the image state. We compare results obtained with low coverage (typically 1 adsorbate atom per 1000 surface atoms) Cs, Ar, and a model electronegative adsorbates. As follows from our results, Cs adsorbates lead to both appreciable dephasing and decay, while electronegative adsorbates mostly affect the dephasing rate. The effect of low coverage Ar adsorbates is small, consistent with their neutrality.     

Image states and excitons at insulator surfaces with negative electron affinity

We discuss electronic excitation processes at two ionic insulator surfaces, LiF(001)-(1x1) and MgO(001)-(1x1), within ab initio many-body perturbation theory. Because of the negative electron affinity of the surfaces, the lowest unoccupied electronic states are image states located in the vacuum outside the surface. Excitations of electrons from the surface layer into these image states are much lower in energy than the bulk excitons. They are responsible for characteristic surface features in the electron energy-loss spectra of LiF(001) and MgO(001).     

Evidence for long-range surface state mediated interaction between sodium atoms on copper (0 0 1)

Inelastic helium atom scattering from sodium atoms on the Cu(0 0 1) surface at 50 K reveals a remarkable 15% increase in the frequency of the frustrated translational vibrations (T-mode) from ?ω=5.56 to 6.34 meV with increasing coverage from Θ_Na=0.008 to 0.125. The coverage dependence and the negligible dispersion of the frequency cannot be explained by the direct dipole-dipole coupling but are well-understood in terms of the Lau-Kohn effective long-range interaction via intrinsic surfaces states.