Quasiparticle recombination time in superconducting tin and normal electronic density of states at the fermi surface from tunnel junction experiments

We have measured the temperature dependence of the effective quasiparticle recombination time in superconducting tin tunnel junctions by current and laser pulse excitation. The experimental times show satisfactory agreement with calculations based on the ray acoustic lifetime model of Eisenmenger et al. taking into account the film thickness dependence of the phonon reabsorption, 2-phonon volume loss processes and phonon transmission from the junction into the substrate and liquid helium. On the basis of the BCS density of thermally excited quasiparticles and simplified rate equations for quasiparticle recombination, and from the analysis of measurements of decaying excess quasiparticle concentrations we obtain a mean valueN ₀=(2.73±0.03) 10²² eV^–1 cm^–3 for the electronic density of states at the Fermi Surface in thin, evaporated tin films. This value differs less than 5% from that obtained from the experimental electronic heat-capacity coefficient of the bulk material.IBM Zürich, Research Laboratory, CH-8803, Rüschlikon, Switzerland     

Phonon scattering at siliconcrystal surfaces

Our observations of the reflection or backscattering of high-frequency phonons (v =280 GHz to 1 THz) at silicon-solid interfaces disagree significantly with predictions from the acoustic mismatch model. Interfaces composed of materials theoretically wellmatched, show high scattering experimentally. In contrast, interfaces theoretically poorly matched, show less phonon scattering than expected. Generally, this is best expressed by the fact that the interface scattering ranges from roughly 30–60% for different phonon modes with little dependence on the material covering the silicon crystal and different techniques of interface preparations. Thus, our experiments indicate that the well-known Kapitza anomaly of the phonon scattering at solid-liquid helium interfaces is not a special case; the same anomaly appears to be present at all tested interfaces. Our experiments are compared with detailed calculations which either assume pure specular or pure diffusive scattering. In these calculations the influence of the crystal anisotropy for the phonon propagation (phonon focussing) is included. This comparison shows, especially for the free silicon surface, that phonons are completely diffuse scattered. Hence, the acoustic mismatched model relying on specular reflection cannot be applied to the real silicon interface. The frequency dependence of phonon scattering at a free silicon interface indicates the existence of at least two different diffusive scattering mechanisms. Within our experimental limits in these two scattering processes the phonons are elastically scattered.     

Refractive index and mass density of quench condensed argon-xenon films

The refractive index and mass density of Ar _x –Xe_100–x films, which are condensed on a silicon substrate held at liquid helium temperature, are determined by ellipsometric measurements in combination, with a quartz microbalance. The mass density is typically about 10% less than the corresponding bulk density. This is consistent with the existence of a porous film structure. No annealing effect, except for pure Xe, can be observed until the Ar evaporates from the layer. Films condensed with a high initial Ar concentration remain in a highly porous sponge structure after the Ar atoms have left the film with a mass density of less than 50% of the Xe bulk density.     

Ions at helium interfaces

The boundaries between different phases of condensed helium provide an interesting testing ground for studying ions in a quantum matter matrix. Here we consider the simplest positive and negative ions in helium — snowballs and electron bubbles, respectively — being trapped at the liquid-liquid interface of phaseseparated³He–⁴He mixtures and at the liquid-solid interface of⁴He. A comparison of experimental results for the trapping with predictions of the snowball and the bubble models shows that the models are in qualitative accord with the observations, but disagree in detail. It is suggested to use such studies for refinements of the ion models. In addition multielectron bubbles (=mesoscopic ions) and electrons on helium films are briefly discussed.     

Phonon emission spectra of thin metallic films

Thin metallic films evaporated on an Al₂O₃-single crystal and cooled to liquid helium temperatures are heated by short electric current pulses. The high frequency part of the emitted phonons is detected by calibrated superconductive tunneling junctions on the opposite surface of the substrate. The observed phonon detector signal amplitude is compared with theoretical models taking account of the boundary conditions for elastic waves in the film. It is found that the phonon spectrum emitted perpendicularly to the substrate-film boundary depends strongly on the thickness of the heater film.     

Tunnel-resonance current in metal-palladium-doped silicon junctions

Results are presented of an investigation of the possibility of realizing the tunnel-resonance current transport mechanism in metal-semiconductor junctions. From an analysis of the temperature dependence of the current-voltage characteristic, it is concluded that the experimental singularities being observed in titanium-palladium-doped silicon junctions correspond to effects predicted by the theory of tunnel-resonance charge transport mechanism.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 76–79, March, 1982.     

A semiempirical formula describing the recoil yield in silicon

Krypton ions in the energy range 20–300 keV are used to generate recoiling atoms in silicon from thin layers evaporated on its surface. The recoil yields and the impurity distributions in the substrate have been measured as a function of several parameters (energy, thickness of the layer, incident dose). The results are used to propose a new formulation of the recoil yield based on the possibility, for both projectiles and recoiling atoms, to remove impurities previously introduced in silicon.The calculation fits very well the experimental results using displacement energies close to the generally admitted values     

Surface dynamics of Mo(1 1 0)–H and Mo(1 1 0)–Li

We compare the surface dynamics of the adsorbate systems Mo(1 1 0)–H and Mo(1 1 0)–Li. In both cases electron energy loss spectroscopy measurements revealed strong substrate surface phonon anomalies. Whereas the phonon anomaly of the hydrogen-covered surface was unequivocally assigned to be of the Kohn type, the anomalous behavior of the surface phonons of the lithium-covered surface remained obscure. In this paper we develop an experimental criterion based on the dispersion of adsorbate phonons, which allows to decide whether the observed substrate surface phonon anomaly is of the Kohn type or not. Employing this criterion we now definitely rule out that the anomaly on Mo(1 1 0)–Li is due to the Kohn effect.     

The influence of thin helium films on the phonon spectrum of optically excited silicon

We have studied the phonon spectra resulting from laser pulse excitation of a silicon surface. We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B-spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusion.     

Carrier transport mechanism in indium tin oxide (ITO)/silicon heterojunctions: effect of chlorine

Transparent-conducting-oxide (TCO)-based photovoltaic junctions have shown complexity in the transport phenomena at the interface. The present study is an attempt to understand the effect of chlorine at the interface between indium tin oxide (ITO) and Si. The ITO/Si junctions have been prepared by depositing transparent and conducting tin-doped indium oxide (ITO) thin films on as-cleaned and chlorine-treated single-crystal p-type and n-type silicon substrates using the reactive electron-beam evaporation technique. ITO/n-Si junctions have shown photovoltaic properties. The photoconversion efficiency of these junctions is observed to increase from 2.3% to 5.5% under chlorine treatment. The transport mechanism across these junctions has been studied by current–voltage (I–V , both dark and illuminated) and capacitance–voltage (C–V ) characterisation techniques. The carrier transport mechanism is found to be dominated by recombination at the depletion region for the junctions prepared with chlorine treatment, whereas for the other junctions, the thermionic process seems to be prominent. The unrealistic barrier heights observed in these junctions by the C–V technique confirms the complex nature of the interface. PACS 73.50.-h; 73.61.-r; 73.50.Pz; 73.40.-c     

Strong visible PL from the nc-Si thin film by Ni silicide mediated crystallization

We studied the growth of nanocrystalline silicon (nc-Si) thin film exhibiting a strong room temperature photoluminescence (PL) at 1.81–2.003 eV. The amorphous silicon was crystallized by Ni silicide mediated crystallization (Ni SMC) and then Secco-etched to exhibit the PL. The PL peak energy and intensity increase with increasing the metal density on the a-Si because of the reduction in the grain size down to 2 nm. The photoluminescence energy and peak intensity depend strongly on the Secco etch time because the grain size is reduced by etching the grain boundaries.     

Heat conductivity and thermomagnetic effects in Ag2Te at low temperatures

The heat conductivity and thermomagnetic effects are investigated in Ag₂Te of stoichiometric composition and compositions with excess Te and Ag in the 5–100 K domain. Experimental results are compared with a theoretical model of the heat conductivity that takes account of intraphonon processes and phonon scattering by the boundaries and point defects.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 41–45, June, 1990.     

Dispersion of nanospheres on large glass substrate by amorphous or polycrystalline silicon deposition for localized surface plasmon resonance

Nanosphere lithography is a cost-effective way to fabricate noble metal nanostructures for plasmonics. However, dispersing nanospheres on a large area of glass substrate is a difficulty encountered when transparent substrate is required in applications such as localized surface plasmon resonance or surface enhanced Raman spectroscopy. Because poly(diallyldimethyl ammonium chloride) (PDDA) on silicon surface introduces a force that can disperse nanospheres on silicon, in this article, we modify the glass surface through amorphous or polycrystalline silicon deposition and thus well disperse polystyrene or silica nanospheres over a glass area of more than 2 cm × 2 cm. Transmission loss of the glass substrate caused by amorphous or polycrystalline silicon deposition is analyzed with good agreement to experimental spectra, and localized surface plasmon resonance signals generated from the gold nanostructures fabricated on these substrates are measured and yield a sensitivity of 317 nm/RIU, which prove the feasibility and effectiveness of our method.     

Optical characterization of laser-induced crystallized silicon films

Optical absorption coefficient spectra of thin silicon films were precisely investigated using a simple reflectance system with total reflectance mirrors placed on the rear side of samples in order to cancel an interference effect in a range between 1.1 eV and 3 eV. The absorption coefficient decreased according to crystallization as the laser energy increased and it got similar to that of single crystalline silicon in the range of 1.7 eV 3 eV. However, the absorption coefficient was higher than 10² cm^–1 in the photon energy lower than 1.3 eV. This probably results from band tail states caused by defect states localized at grain boundaries in the crystallized films. 2.5%-phosphorus doped laser crystallized silicon films had a high optical absorption coefficient ( > 10⁴ cm^–1) in the low photon energy range (1.1 eV 1.7 eV) caused by free carriers produced from the dopant atoms activated in the silicon films. The experimental results gave the carrier density of 1.3 × 10²¹ cm³ and the carrier mobility of 20 cm²/Vs.     

Effect of PbCl2 addition on structure, OH− content, and upconversion luminescence in Yb3+/Er3+-codoped germanate glasses

Yb³⁺/Er³⁺-codoped oxychloride germanate glasses have been synthesized by a conventional melting and quenching method. Structural properties were obtained based on Raman-spectra investigation, indicating that PbCl₂ plays an important role in the formation of the glass network and has an important influence on the phonon density and the maximum phonon energy. The Judd–Ofelt intensity parameters and quantum efficiencies were calculated based on the Judd–Ofelt theory and lifetime measurements. The enhanced upconversion luminescence intensity of Er³⁺ with increasing PbCl₂ content could not be explained only by the maximum phonon-energy change of the host glasses. For the first time, the effect of PbCl₂ addition on phonon density, OH^– content, and upconversion luminescence in oxychloride glasses has been discussed and evaluated. The results show that the effect of phonon density and OH^– content on upconversion luminescence in oxychloride glasses is much stronger than that of the decrease of the maximum phonon energy. The possible upconversion luminescence mechanisms have also been estimated and are discussed.     

Temperature and velocity dependence of the mechanical properties of the intermetallic compound Ni3Al. I

The paper presents the results of theoretical and experimental studies of the deformation mechanism of alloy Ni₃Al, undertaken in order to elucidate the nature of the anomalous temperature dependence of the mechanical properties of that alloy. A study is made of the mechanisms of hardening which do not require diffusion migration of atoms for their occurrence; 1) indeterminacy of dislocation axis; 2) recombination and conservative sliding of jogs in superpartial dislocations (SPD); 3) cross-slip of SPD; 4) intersection of antiphased boundaries introduced by cubic slip; 5) work-hardening due to reactions between dislocations of cubic and octahedral planes. All these mechanisms are effective, even at room and lower temperatures, and could be responsible for the increase of the yield point with increase in the temperature of testing from –180 °C, as observed by Davies and Stoloff.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 10, pp. 39–45, October, 1971.     

X-ray absorption study of light emitting silicon nanocrystals

X-ray absorption spectra obtained by total electron yield (TEY) at the Si absorption K-edge have been measured to have chemical and structural information about Si nanocrystals (Si-nc) produced by plasma-enhanced chemical vapour deposition (PECVD). The TEY technique has been employed to investigate the formation of Si-nc and the modification of the silica matrix as a function of annealing temperature (500–1250°C) and of silicon content in the film (35–46 at%). The amount of silicon present in the Si-nc has been evaluated by TEY. Thanks to Rutherford backscattering spectrometry measurements, the amount of Si atoms bonded to oxygen and to nitrogen, incorporated by PECVD, has been assessed. A compositional model that interprets the experimental findings is presented.     

Raman study of orbital mediated multiphonons in RMnO3 (R = Pr,Sm,Eu,Tb,Y)

We have studied RMnO₃ manganites (R = Pr, Sm, Eu, Tb, Y) Raman excitations in the 200–2800 cm^-1 range as a function of temperature. Combinations of phonon energies are observed up to the fourth order, indicating the presence of electron-phonon coupling. In comparison to Γ-point phonon combinations, double phonon excitations appear to be blue shifted in large size rare earth ion compounds. The phonon combination intensities decrease rapidly with their increasing order, confirming other studies which conclude that the electron-phonon coupling is not as strong as supposed in the localized limit. Moreover, different intensity order dependences are observed between the phonon combination and the so-called Jahn-Teller mode. These effects are better described in the orbiton-phonon coupling scheme.     

Far-infrared spectra of Pb1−xMnxTe alloys

Far-infrared reflectivity spectra of Pb_1−xMn_xTe (0.0001x0.1) single crystals were measured in the 10–250 cm⁻¹ range at room temperature. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed an intermediate one–two mode behavior.