Coherent phonons can be generated in solids by ultra-short pulse laser excitation. Here we report observation of coherent phonon oscillations in the time domain using reflection type pump-probe techniques and discuss the phonon dynamics. An A1g phonon mode was observed for Bi, showing an exponentially decaying oscillation. Temperature dependence of the phonon frequency and the decay rate agrees with the result of Raman measurements. This indicates that the decay process of the coherent phonon is dominated by an anharmonic decay route of energy relaxation. The coherence of phonons in time domain was examined using a double-pulse excitation technique. The oscillation amplitude of the A1g mode in Bi enhances when the pulse separation time is equal to the period of the phonon oscillation, and vanishes when the separation time is adjusted to half integral multiples of the period. We present furthermore the results on coherent folded acoustic modes in GaAs/AlAs superlattices.
Ultrafast energy and phase relaxation of photo-excited electrons has been measured by means of interferometric twophoton time-resolved photoemission with 15 fs pulses. The phase relaxation time for the excitation from the occupied surface state on Cu(111) is found to be ∼20 fs, and is longer than for the excitation from the bulk bands. This makes it possible to demonstrate a coherent control of photo-excited electronic populations with phase-locked pulse pair and chirped pulse excitation.
Impulsive stimulated Raman effect is reviewed with a particular emphasis on the principles of the methodology. The key components are the force that the optical pulse exerts on the active mode, motion of the mode after the optical excitation, and the probing the time-evolution. In describing the excitation mechanism, we refer to electrostrictive force as a classical equivalence of the Raman effect. If integrated over the frequency, the intensity of the spontaneous Raman scattering is connected with the strength of the driving force. The motion of the mode following the optical impulse is interrogated with experimental configurations that read out the index change. The typical configurations are described from the viewpoint of optics. The grating method has a unique advantage for the study of the dispersion relation of propagating modes. The estimated degree of the index change is presented to show that the stimulated contribution builds up against the noise of the spontaneous Raman scattering.
A new technique for the production of femtosecond X-rays using the inverse Compton scattering was described. This new method provides high quality monochromatic X-rays short pulse, that is monochromatic and has high brightness and tunable energy performed by using a very compact system. A new electron accelerator and a high brightness laser system, which are under developing, will make 3×107 X-ray per pulse with a lenght of 200 fs. The technological development for the accelerator and the laser system is performed successfully this half year, and we will demonstrate an X-ray source in near future.
O atoms segregate to the surface during Cu homoepitaxial growth on Cu(001)-(2√2×√2)-O to retain the (2√2×√2) surface. The presence of adsorbed O atoms on the Cu surface suppresses the surface diffusivity of Cu adatom and the growth proceeds by site-exchange between Cu adatoms and adsorbed O atoms, which heightens the transition temperature of the growth mode from step-flow to layer-by-layer. There exists a critical Cu deposition rate above which the O atoms can not exchange the site with Cu adatoms. The critical Cu deposition rate obeys an Arrhenius relation and the activation energy for the site-exchange is estimated at 0.66eV.
We applied neural networks to quantitative chemical analysis. The input data corresponding to the spectra of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were prepared by the sum of two or three peaks with a Gaussian distribution. The neural networks with Kohonen's self-organized feature map and with a back propagation algorithm were used. From the results, we found that it was possible to analyze the patterns without determining the number of peaks, the shapes and so on. Therefore, neural networks are thought to be useful in analyzing XPS and AES data quantitatively.
For polymer surface modified by ultraviolet irradiation, plasma treatment, rubbing, ion beam, electron beam or chemical treatment, XPS, Dynamic and Static SIMS, RBS and GIXS were available to estimate the surface and explain formed new functions.
With a variety of surface superstructures formed on silicon surfaces, we have clarified close correlations between the atomic-scale structures on surfaces and surface electrical conduction phenomena. In particular, we have succeeded for the first time in experimentally confirming the electrical conduction via surface-state bands that are inherent in the surface superstructures. An important phenomenon has also been found that atoms adsorbed on the surface donate the carriers into the surface-state band, resulting in remarkable enhancement in conductivity. The ultimate two-dimensional electron systems made up from surface-state bands, which was revealed by our study, are expected to provide a new stage in surface physics as well as a precursory stage leading to atomic-scale electronics devices. This article is an introductory part, followed by a main part on the next issue. Here, two of our sample surface superstructures, Si(111)-7×7 clean and Si(111)-√3×√3-Ag surfaces, are introduced in detail on their atomic and electronic structures. Then, fundamentals on electrical conduction near semiconductor surfaces and our experimental methods are described. Based on these introductions, the article on the next issue will describe our main results on the electrical conduction through surface-state bands.
From a global viewpoint, meteorological phenomena related to the weather during a few days can be considered as phenomena in only a thin layer at the bottom of the atmosphere. This resembles the nature of the surface phenomena of solids which are characterized by a “thin” surface layer distinguished from the bulk. Another feature of meteorological phenomena is that some of the meteorological disturbances have some smaller structure in themselves. This stratified structure of the meteorological disturbances enhances the variety of weather changes. In the rainmaking process in clouds generated in the temperate zone, not only diffusion and condensation of water vapor but also coalescence of raindrops and rapid growth of the ice crystals under the coexistence of ice crystals and supercooled cloud droplets play an important role. A process similar to the ice crystal growth process in clouds is also seen in the growth process of polycrystalline silicon films formed by plasma-enhanced chemical vapor deposition.