Terahertz surfoluminescence

The cleaving of a solid to form two new surfaces may result in the emission of light. Conventional mechanoluminescence involves the transfer of charge between the two surfaces. We now demonstrate that the ultra-fast separation of charge within a newly-formed surface will lead to the emission of electromagnetic radiation. In contrast to the visible light previously observed and modeled, the intra-surface radiation contains terahertz frequencies. This newmechanism – named here surfoluminescence – introduces a new class of terahertz-frequency emitters. It also may in part explain the recent observation of terahertz emission from peeling adhesive tape

The importance of scattering, surface potential, and vanguard counter-potential in terahertz emission from gallium arsenide

It is well established that under excitation by short (\u3c1 ps), above-band-gap optical pulses, semiconductor surfaces may emit terahertz-frequency electromagnetic radiation via photocarrier diffusion (the dominant mechanism in InAs) or photocarrier drift (dominant in GaAs). Our three-dimensional ensemble Monte Carlo simulations allow multiple physical parameters to vary over wide ranges and provide unique direct insight into the factors controlling terahertz emission. We find for GaAs (in contrast to InAs), scattering and the surface potential are key factors. We further delineate in GaAs (as in InAs) the role of a vanguard counter-potential. The effects of varying dielectric constant, band-gap, and effective mass are similar in both emitter type

How reliable are scanning tunneling microscopy measurements of electron transport in molecules ?

Scanning tunneling microscopy measurements of tunneling through molecules adsorbed on a surface have been simulated using a standard empirical model based upon the Wentzel-Kramer-Brillouin method applied to tunneling through a barrier. The Gaussian noise inherent in these experiments has been added to the model data using a Monte Carlo technique. By generating multiple sets of current-voltage curves and fitting these to the model we have evaluated how reliably barrier height can be determined as a function of noise level. The results suggest that for constant percentage standard deviation in the noise greater than 5% the barrier height cannot be determined reliably. At this level, the standard deviation in the estimate of the barrier height is about 10%. Weighted fits give more reliable estimates of the barrier height. If the height of the tip above the molecule is known, so that the fit is only a single parameter the barrier height can be determined reliably even at percentage noise levels as high as 20%. However, in this case unweighted fits must be used otherwise the estimated value deviates by up to 15% from the true value. Data with constant absolute noise give similar results. The effects of experimental resolution have been evaluated in a similar manner and are shown to have a significant influence on the reliability. At a resolution of about 0.1% of full scale the standard deviation in the estimate of barrier height is only about 2% but increases rapidly to 10% for a resolution of about 1%

A 500 °C isothermal section for the Al-Au-Cu system

The Al-Au-Cu system and its associated ternary alloys and intermetallic compounds is surprisingly poorly known, and the authors could find no phase diagram for it in the literature. This article addresses this omission by presenting an isothermal section at 500 °C, derived with the aid of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), metallography, and hardness measurements. The samples studied had generally received an anneal of 2 hours at 500 °C, primarily in order to complete any transformations that occurred during solidification and cooling of the castings. The possibility of further changes on protracted annealing at 500 °C is not ruled out, and the diagram presented is, therefore, applicable only to material prepared by thermal processing of an industrial nature. The presence of a ternary Β phase with a nominal stoichiometry of AlAu2-xCu1+x (0 ≤ x ≤ 1) was confirmed, and its phase field at 500 °C was determined. A number of the binary intermetallic phases were found to exhibit some solid solubility of the ternary element. In particular, the γ-Al4Cu9 phase extends deep into the ternary and, in the vicinity of the commercially interesting 18-carat line, appears to exist in a ternary ordered form, designated here as γ2

South Africa, now at 2600 Natta Blvd., Bellmore, NY 11710, USA. § Physical Metallurgy Division, Mintek, Private Bag X3015, Randburg, 2125, South Africa. # School of Process and Materials Engineering

Abstract. Colour and hardness were measured on a series of alloys along the 76 wt % Au line of the Au-Cu-Al system. Complex, non-monotonic behaviour was observed, which is shown to be correlated with microstructural changes. The available colours include reddish, yellow, 'apricot', white and purple. The hardness of as-cast material varies from 150 to 500 Vickers

Spin depolarization of muonium in mesoporous silica

We report muon spin rotation/relaxation measurements of muonium in mesoporous silica (SBA-15) with a high specific surface area of 600 m2/g. Up to 70 percent of the incoming muons form muonium and escape efficiently into the open pores at all temperatures between 3 and 300K. We present evidence that the interaction with the silica surfaces involves both spin exchange and a transition to a diamagnetic state, possibly due to dangling bonds on the surface. At very low temperatures, below 20K, the interaction between muonium and the silica surfaces is suppressed due to a He film coating the surfaces. These results indicate that it should be possible to use muonium to probe the surfaces of uncapped nanoparticles supported in silica

B-NMR of 8Li+ in rutile TiO2

We report preliminary low-energy B-NMR measurements of 8Li+ implanted in single crystal rutile TiO2 at an applied field of 6.55 T and 300 K. We observe a broad 12 kHz wide quadrupole split resonance with unresolved features and a sharp component at the Larmor frequency. The line broadening may be caused by overlapping multi-quantum transitions or motion of 8Li+ on the scale of its lifetime (1.21 s). We also find spin-lattice relaxation that is relatively fast compared to other wide band gap insulators. The origin of this fast relaxation is also likely quadrupolar and may be due to anisotropic 8Li+ diffusion