Investigation of quantum systems with surface plasmons and surface state electrons

We report on investigations of thin films of the quantum systems hydrogen and helium both in thermodynamic equilibrium and in metastable states. Information about the film thickness and surface roughness is obtained from the excitation of surface plasmons and mobility measurements of surface state electrons. The equilibrium studies show triple-point wetting for H ₂ on Au substrates, in agreement with earlier results for similar systems. Unexpectedly a distinct hysteretic behavior of the saturated film thickness is found near the triple-point. Superfluid ⁴ He films on as prepared Ag display complete wetting. Investigations of H ₂ films in metastable states prepared by quench-condensation at 1.5K reveal pronounced changes in the film structure upon annealing. These changes set in far below the desorption temperature and are ascribed to surface diffusion.     

The Nonwetting Behavior of Dilute 3He-4He Mixtures on Cs

We present evidence for ³ He interfacial bound states at the Cs-liquid He interface and show their influence on the interfacial free energies and the nonwetting behaviour of dilute ³ He- ⁴ He mixtures on Cs. The bound state energy and effective mass for the ³ He atoms at this interface are determined and found to be in good agreement with theoretical predictions. From the temperature dependence of the contact angle, direct evidence for a first order wetting transition at the lower wetting temperature is found. A detailed model is presented for the Cs-He mixture interface free energy and the contact angle and this model is shown to be in excellent agreement with the measurements. There is a short discussion on hysteresis and the thin film state. Finally it is emphasised that the large measured contact angles are only consistent with ³ He bound states together with ripplons at the Cs-He mixture interface     

A possible observation of quantum nucleation in superfluid He4 near crystallization

The overpressurization of superfluid He⁴ under crystallization is studied experimentally. The effect of a substrate and He³ impurities on the nucleation rate W is observed. The dependence of the metastable state lifetime versus the overpressurization and temperature is measured in the range 10^–7–1 s and 0.52–1.75 K. The results are compared with the theoretical predictions for the classic and quantum nucleation. The surface relief effect is considered. It is shown that relative to a flat surface, a cone-shaped pit is a site corresponding to a smaller nucleation energy. In this case a change of the nucleation rate versus the metastability degree and temperature in the classic region is analyzed. As a result, the most reliable qualitative manifestation of quantum nucleation is the independence of the metastable degree under the constant rate W upon temperature, which is observed below 1 K in the experiment.     

Partition of solute elements during solidification of iron-carbon-chromium alloys

The partition coefficients of chromium between austenite and liquid iron,k _Cr ^A/L , were determined from the experiment of rapid cooling of iron-carbon hypo-eutectic alloys containing a small amount of chromium from coexisting solid—liquid states; the partition coefficients between eutectic and its liquid,k _0,Cr, andk _0,Cr, for the stable and metastable eutectic solidifications were obtained from the zone-melting experiment of iron—carbon eutectic alloys containing a small amount of chromium. Chromium was rejected to liquid iron on the crystallization of primary austenite,k _Cr ^A/L <1. On the eutectic solidification, chromium was enriched in eutectic liquid for the stable system,k _0,Cr<1, and was conversely diluted in the liquid for the metastable system,k _0,Cr>1. The relationship between effective and equilibrium partition coefficients given by Burtonet al. was observed for the results of the zone melting experiment and, from the relationship, the thickness of boundary layer in the liquid ahead of the solid—liquid interface was found to be 0.17 mm for the stable system and 0.11 mm for the metastable system. Thermodynamic calculation of the partition coefficients of chromium and carbon proved to represent the observed partition coefficients well.     

Emission of core-excited autoionizing sodium atoms from NaCl induced by electron bombardment

In the time-of-flight spectrum of positive ions produced by electron bombardment of NaCl surface a delayed signal of Na⁺ is detected. This observation is attributed to production of core-excited autoionizing metastable Na^** atoms which have their characteristic lifetimes in the 100 ns range. Moreover, it is noticed that the populations of the various metastable Na^** states depend on the NaCl layer thickness. The lifetimes of some quartet metastable Na^** states have been determined as well.     

Stable and metastable phase equilibria in the liquid-state and solid-state PbO-GeO<Subscript>2</Subscript> system

A literature review on phase equilibria in the PbO-GeO₂ system is reported and the main sources of discrepancies in the results of different authors are considered. Uncontrolled metastable phase formation is shown to be the most probable cause for disagreements about composition and structures of phases formed in the system. The authors’ results of investigations of physicochemical properties for stable and metastable phases obtained by various routes, including mechanochemical and thermal (crystallization of a metastable melt) activation, are presented. Similar phenomena are shown to take place in the binary PbO-SiO₂ system and under partial substitution Ge → Si in the ternary oxide system PbO-GeO₂-SiO₂. The particular role played by the 6s² electron pair a Pb²⁺ ion in stabilization of intermediate energy-saturated states and meta-stable phase formation is discussed.     

Surfactants for Exploring the Liquid–Solid Surface Free Energy

Helium-3 and helium-4 can be used as surfactants to modify the observed wetting transitions of hydrogen and deuterium on rubidium. The effect of these surfactants on the wetting temperature is calculated. Prewetting induced by ⁴ He in H ₂ on Rb is studied, and found to be re-entrant. Experiments can use the effect to measure the difference in temperature and concentration dependence of a free solid surface and a liquid/solid interface. This may help to resolve some unresolved questions pertaining to the wetting behavior of helium isotopic mixtures on cesium.     

The effect of the wetting droplets size on power consumption during drum granulation

Changes of torque during fine material (foundry dolomite) granulation in a horizontal drum granulator at changing wetting parameters were studied. The variable parameters were droplet diameter and wetting of granular material. The bed of loose material was sprayed during feeding, at a constant liquid flow rate V_l = 0.012 m³/h. The size of wetting liquid droplets was changed using different rates of air flow through pneumatic spraying nozzles in the range from V_p = 1.0 to 3.0 m³/h and applying a sprinkler which supplied (drop-wise) the liquid uniformly along the entire drum length. In each test, instantaneous values of torque on the granulator shaft were measured in 1 s time intervals. The effect of droplet size and moisture content of the bed of granular material on torque in the whole granulation cycle was estimated. It was found that bed wetting conditions had a significant influence on the nucleation and growth of agglomerates. A minimum (boundary) size of liquid droplets, at which the bed is not transformed into granulated material, depends on the total amount of liquid supplied at the wetting stage. Changes of torque during the drum granulation are a good representation of the phenomena related to the transformation of wetted feed into granulated product and can be an easy indicator of the process.The work was carried out within the project no. 4 T09C 023 22. financed by the State Committee for Scientific Research in the years 2002–2005.     

Dimple instability in a metastable superfluid film

It is shown that the surface dimple in a metastable rotating superfluid film becomes unstable as the surface spinodal of the thick film is approached. The dimple ceases to exist when its free energy E _d equals the excess free energy E _c of a critical hole in the film, which indicates the breakdown of the metastable film. The lifetime of the metastable film upon approach of the instability is calculated. Possibilities of enhancing the instability are briefly addressed.     

Dehydration products of gypsum: positron annihilation and dielectric measurements

Bassanite (CaSO₄·1/2 H₂O) and anhydrite (CaSO₄) are the low-temperature products of gypsum (CaSO₄·2H₂O) dehydration, which are obtained at about 373 and 433 K, respectively. These sulphates have non-centrosymmetric crystallographic point groups, but dielectric measurements do not reveal any piezo- or ferroelectric characteristic, and they practically behave like linear dielectrics. Positron lifetime spectra exhibit the existence of two different positron states, besides a free positron state. There is positronium formation in the three sulphates, and there is also evidence for the presence of a highly populated positron bound state which may be a complex state associated with positrons bound to SO ₄ ^2– ions. Parameter S estimated from the Doppler curve and the average positron lifetime show unquestionably the sensitivity of positrons to the phase transitions gypsum-bassanite and bassanite-anhydrite.     

Theoretical study of photoresponse to pulsed radiation in n-InSb in the temperature range 77–300 K

The results of a theoretical simulation of a transient experiment in n-InSb in the temperature range 77–300 K, are reported. Minority carrier lifetimes for the three recombination processes, radiative, SRH and Auger, have been calculated at different temperatures using the temperature dependence of intrinsic carrier concentration,n _i , and density of states effective mass of heavy holes,m _d. It was found that around room temperature the lifetimes for the three processes become comparable and at higher temperatures the Auger lifetime becomes dominant. This fact was ignored in previous work where only SRH and radiative processes were considered in the calculation of effective lifetime. The present results of effective lifetime in n-InSb are in reasonably good agreement with the results obtained previously. The effect of higher time modes on the decay of photoresponse to pulsed radiation is discussed. An instantaneous time constant has been defined and its variation with time at different temperatures has been studied.On leave from Atma Ram Sanatan Dharma College, University of Delhi, New Delhi 110021, India.     

Prediction of reentrant wetting of3He-4He mixtures on cesium

We examine the effect of ³ He impurities on the wetting behavior of⁴He on cesium, predicting a phase diagram which includes reentrant wetting transitions. This phase diagram is shown to be very sensitive to effects such as a theoretically predicted bound state of³He at the liquid-cesium interface, and the contact angle may be sensitive to interesting temperature dependences of the helium-cesium surface tension resulting from surface rotons or Rayleigh waves.     

The influence of3He on the wetting of4He to a cesiated substrate

We report preliminary measurements of the influence of ³ He in the adsorption of ⁴ He to a cesiated substrate by the use of quartz crystal resonance techniques. When compared with the case of pure ⁴ He, the presence of ³ He in the apparatus induces the wetting of the mixture film to Cesium at low temperatures. The wetting is hysteretic, beginning at a concentration-dependent T_c on cooling with continued wetting for T>T_c on warming. The temperatures T_c are given as a function of the total concentration of ³ He in the apparatus.     

Characterisation of pore structure by combining mercury porosimetry and micrography

Savonnières, a French layered oolithic limestone, shows important ink-bottle effects. As a consequence, a great discrepancy is observed between the results of different techniques to determine the pore volume distribution. It is shown that mercury intrusion porosimetry results can be considered as the main drainage curve and are, for this kind of materials, not convenient to determine the apparent pore volume distribution due to hysteresis phenomena. The main wetting curve is obtained using image analysing techniques on SEM-micrographs, combined with pressure membrane apparatus results on capillary saturated samples. A simple structural hysteresis model is developed to predict the mercury intrusion porosimetry results, starting from the main wetting curve. The good agreement between theory and experiment verifies that the large difference between main wetting and main drainage curve can indeed be attributed to structural hysteresis. In this way, the presented technique gives, in addition to a better knowledge of the pore volume distribution, insight in the pore geometry and connectivity, which can serve as input for microstructure-based transport models.     

Abnormal Synergetic Effect of Organic and Halide Ions on the Stability and Optoelectronic Properties of a Mixed Perovskite via In Situ Characterizations

The mixed cation lead mixed halide perovskite (MLMP) Cs_xFA_1‐xPbI_yBr_3‐y is one of the most promising candidates for both single‐junction and tandem solar cells due to its high efficiency and remarkable stability. However, the composition effect on thermal stability and photovoltaic performances has not yet been comprehensively investigated. Therefore, the interplay between composition, crystal structure, morphology, and optoelectronic properties under heat stress, is systematically elucidated here through a series of in situ characterizations. It is revealed for the first time that the FA⁺ and Br^? release synchronously at first even under mild annealing. This leads to a serious FA‐ and Br‐deficiency issue, with only 88.3% of Br and 90.2% of FA retained after annealing at 100 °C, which significantly magnifies the hysteresis, phase segregation, and instability issues. Finally, a trace amount of FA⁺ and Br^? is introduced onto the post‐annealed MLMP surface to compensate for the deficiency through vacancy filling. The degradation lifetime to 80% of the initial efficiency (t₈₀) is improved from 504 to 1056 h and the hysteresis issue is also well resolved. This work highlights the importance of the synergetic composition effect of the organic cation and halide anion on stability and efficiency optimization for long‐term applications.     

Hysteresis in the phase-slip state of superconducting filaments

Recently some papers on measurements of the I-V characteristics (where V is the time-averaged voltage) of superconducting indium microbridges ¹ as well as tin and zinc whiskers ^2,3 driven by a dc current into the phase-slip state have appeared. Special emphasis was placed on a discussion of the hysteresis, which is well-known in such experiments (see, e.g., Refs.1–18 in Kramer and Rangel ⁴ ). The hysteresis was compared with the predictions of the generalized time-dependent Ginzburg-Landau (GTDGL) equations for dirty superconductors in local equilibrium. ^4,5 Unfortunately these predictions represent the only results in this context derived ultimately in a rigorous fashion from the standard microscopic theory of superconductivity. Comparison was also made with a model by Kadin, Smith, and Skocpol (KSS), ^6,7 which gives a much smaller hysteresis. The authors of Ref. 1 found good agreement with the KSS model. The authors of Refs. 2 and 3 found a hysteresis which is larger than that of the KSS model, but still considerably smaller than predicted by GTDGL theory. They proposed a generalization of KSS which can be fitted to the data.     

Effect of drawing on the dielectric properties and polarization of pressed solution cast β-PVDF films

Poly(vinylidene fluoride), PVDF, samples containing exclusively the polar β phase were obtained by crystallization from N,N-dimethylformamide (DMF) solution at 60 °C and subsequent pressing. Some of these samples were uniaxially drawn at 120 °C at draw ratio of 4, resulting in oriented films. Oriented and unoriented samples were characterized as to relative fraction of β phase, degree of crystallinity and orientation by infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The dependence on frequency of the real ( e_\textr^￠ \varepsilon_{\text{r}}^{\prime} ) and imaginary ( e_\textr^￠￠ \varepsilon_{\text{r}}^{\prime\prime} ) components of the relative permittivity of the samples was determined between 10⁴ and 1.7 × 10⁷ Hz. The coercive field and stable and metastable remanent ferroelectric polarization were determined from the hysteresis loop obtained by the ramp voltage technique, described in detail in this work. The results allowed verification of the effect of drawing on structure and of the resulting structure on the dielectric properties, remanent polarization, and coercive field of β-PVDF. A possible effect of the crystal–amorphous interphase region on the metastable remanent polarization is suggested. The results obtained with the oriented and unoriented samples were compared with those obtained for films commercialized by Piezotech S.A.     

The solid-liquid surface tension at a helium/cesium interface

The existence of a wetting transition for helium adsorbed on cesium provides an experimental opportunity to investigate the solid-liquid surface tension. We have extracted the temperature dependence of the solid-liquid surface tension at the⁴He/Cs interface from our earlier measurements of the wetting temperature of pure⁴He on thin layers of cesium overlaying gold. More recently we have measured the wetting temperature as a function of the³He concentration for³He-⁴He mixtures. An analysis of these experiments paralleling Andreev's investigation of the free surface reveals³He bound states at the helium/cesium interface.This work was supported by NSF grant DMR-9223775.     

Studying the Normal-Fluid Flow in Helium-II Using Metastable Helium Molecules

We demonstrate that metastable helium molecules can be used as tracers to visualize the flow of the normal fluid in superfluid ⁴He using a laser-induced-fluorescence technique. The flow pattern of a normal-fluid jet impinging on the center of a copper disc is imaged. A ring-shaped circulation structure of the normal fluid is observed as the jet passes across the disc surface. The fluorescence signal for the molecules trapped in the circulation structure is measured as a function of time after we turn off the molecule source. The radiative lifetime and density of the molecules can be determined by fitting the measured data using a simple analytic model. We also discuss a proposed experiment on using a previously developed molecule tagging-imaging technique to visualize the normal-fluid velocity profile during the transition of quantum turbulence in a thermal counterflow channel.