High-field electrical conductivity in disordered systems

The electric field dependence of the tunnelling probability between localized states in a disordered solid is presented. The dc conductivity has been determined as a function of the dimension d of the system. We find that, at high fields ( E U 106Vcm-1), the conductivity decreases with increasing field as ln - E 1/2/ T, independently of the dimension of the system.     

Thermal-equilibrium processes and electronic transport in undoped hydrogenated amorphous silicon

Abstract

The temperature and time dependence of the d.c. conductivity of undoped hydrogenated amorphous silicon is presented. Measurements of the electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behaviour are observed. When samples are rapidly cooled from 200°C below a temperature T _E～145°C a non-equilibrium dark conductivity, higher than that corresponding to slow cooling, is observed. The electronic and atomic structure then slowly relax and the time dependence of the excess conductivity is well described by a stretched exponential function. The second regime above T _E corresponds to a relaxation time short compared to experimental times and the conductivity is independent of which order the annealing temperature is chosen. Thus the thermal equilibrium processes observed in undoped samples are qualitatively very similar to those observed in doped samples as recently reported in the literature.     

Solid-phase epitaxy of implanted silicon at liquid nitrogen and room temperature induced by electron irradiation in the electron microscope

Abstract

Electron-beam-induced solid-phase epitaxy (SPE) has been obtained on cross-sections of implanted Si layers, by in situ irradiation in the electron microscope, with electrons of energies of 200, 250 and 300 keV, at both room and liquid-nitrogen temperature. The, absence of a transition from SPE to layer-by-layer amorphization (which is observed during ion-beam irradiation on decreasing the temperature below a certain critical value) and the athemal nature of the electron-induced crystallization process below room temperature, indicate that, although elastic displacement is the basic mechanism of both processes, the models which describe ion-beam-induced epitaxy in the temperature range 200≤T≤400°C cannot be extrapolated to explain the results of electron irradiation below room temperature.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

Deformation-induced dislocations in 15R-SiC grown by sublimation

Single-crystal 15R-SiC boules have been successfully grown by sublimation. The Vickers hardness of a Si-terminated (0001) face has been measured in the temperature range 25-1300C. As expected, the hardness decreases with increasing temperature from about 30GPa at room temperature to about 10GPa at 1300C. The fracture toughness is estimated to be about 1.0MPam 1/2 at room temperature. Transmission electron microscopy investigation of the dislocations introduced by indentation at 900 and 1300C shows that they are activated predominantly on the basal plane. Most of them consist of a single leading partial without the corresponding trailing partial.     

Effect of spin-orbit interaction on the metal-insulator transition in doped silicon

Abstract

We show that in disordered metallic systems, spin-orbit interactions lead to the existence of two metallic phases separated in temperature. The low-temperature metallic phase arises from spin-orbit interactions. The crossover temperature is proportional to (Z-Z′)² (Z is the atomic number of the donor and Z′ that of the matrix) and is expected to be observable for donors with large atomic numbers. The presence of two metallic phases implies two mobility edges, existing at different temperatures, and affecting the conductivity when the Fermi energy E _F lies below the mobility edge E _c We thus predict anomalies in the conductivity in this range of concentration, consistent with the measurements of Long and Pepper on Si:Sb.     

Microstructure of icosahedral Al-Pd-Mn quasicrystals deformed at room temperature in an anisotropic confining medium

Plastic deformation of Al-Pd-Mn icosahedral quasicrystals has been achieved at room temperature using a high-confining-pressure medium. The deformation microstructure, investigated by transmission electron microscopy, is characterized by long straight bands of dislocations. A detailed analysis of the dislocation configurations indicates that the plastic deformation is controlled by dislocation glide.     

Thermosensory reversal effect quantified

At room temperature, some materials feel colder than others due to differences in thermal conductivity, heat capacity and geometry. When the ambient temperature is well above skin temperature, the roles of ‘cold’ and ‘warm’ materials are reversed. In this paper, this effect is quantified by measuring discrimination thresholds for subjective coldness at different ambient temperatures using stimuli of different thicknesses. The reversal point was found to be at 34 °C, somewhat above skin temperature. At this reversal point, discrimination is quite impossible. At room temperature, subjects were able to discriminate between stimuli of different thickness based on subjective coldness, showing that the sense of touch, unlike vision, can penetrate solid objects. Furthermore, somewhat surprisingly, at ambient temperatures well below normal room temperature, discrimination is worse than at room temperature.     

Dependence of the persistent photoconductivity in doping-modulated a-Si: H multilayers on the exposure temperature

Abstract

We find that the photo-induced excess conductivity (persistent photoconductivity, PPC) in doping-modulated hydrogenated amorphous silicon (a-Si: H) is thermally activated for exposure temperatures above 220 K in agreement with Kakalios and with Hundhausen and Ley. However, we find a temperature regime between 80 and 220 Kin which the PPC measured at 300 K is independent of excitation temperature. Whether the PPC is excited in the thermally activated regime above 220 K or in the temperature-independent regime, the annealing occurs at 410 K. The pre-exponential factor and the activation energy of PPC are related by the Meyer-Neldel rule independent of the excitation temperature.     

Transmission electron microscopy observations of low-load indents in GaAs

Nanoindentations have been made on (001) surfaces of GaAs single crystals at room temperature and the indents observed by transmission electron microscopy. The permanent deformation, generated by a Berkovitch indentor submitted to maximum loads ranging between 600 to 1700 muN, was analysed and interpreted as a function of the loading curves. The plastic zone size was measured as a function of the maximum load to determine the yield strength of GaAs at room temperature. Finally, the fine structure of the dislocations generated by the indenter has been analysed and this is compared with previously reported structures for higher indenting loads.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

The alpha -> beta polytypic transformation in SiC induced by ball milling

The alpha -> beta polytypic transformation in SiC was observed during ball milling at room temperature. Microstructural evolution was characterized on an atomic scale by high-resolution electron microscopy (HREM). HREM observations showed that partial dislocations play an important role in the transformation under the complicated conditions during ball milling.     

Sb 2 Te 3 and In 0.2 Sb 1.8 Te 3 : A comparative study of thermoelectric and related properties

Sb 2 Te 3 and its solid solutions with group III elements such as In are materials of current interest for their potential thermoelectric applications. Single crystals of Sb 2 Te 3 and In 0.2 Sb 1.8 Te 3 have been grown by the Bridgman method and their powder X-ray diffractograms obtained and indexed. The transport properties, namely electrical conductivity, Seebeck coefficient and thermal conductivity, as well as the optical bandgap of the alloys have been measured and are compared. The ternary alloy containing In has a smaller optical bandgap and lower electrical and thermal conductivity than Sb 2 Te 3, but a higher Seebeck coefficient.     

Defect luminescence in undoped p-type GaSe

Photoluminescence (PL) spectra of undoped single crystals of the layered semiconductor GaSe have been measured in the temperature range from 10K to room temperature and in the wavelength range from 635 to 750nm. Two wide bands centred at 644 and 695nm have been observed at T = 10K. A detailed analysis of the spectra obtained by varying the excitation intensity and temperature resulted in the identification of the levels involved. A simple model is proposed to account for the observed data.     

High-resolution electron microscopy observations of microplastic fracture in SiC under ball milling at room temperature

Nanometre-sized kinks and cracks formed in 6H SiC under ball milling (BM) at room temperature have been observed and characterized on the atomic scale using high-resolution electron microscopy (HREM). Observations of the kinks show that numerous positive and negative partials are aligned at either of the kink boundaries, and the stacking sequences in the kink band are considerably different from those in the other areas. It was also observed that the (0001) lattice planes in the kink band are kinked, indicating that microplasticity occurs in the normally brittle material SiC under BM even at room temperature. HREM observations of cracks show that cracks previously observed by transmision electron microscopy are not completely open but are at the initiation stage of fracture. Inside a crack, one residual kink region can be clearly observed, which indicates a correlation between kink and crack, that is a crack evolves from a kink.     

Toughening of a brittle material by means of dislocation subboundaries

A novel technique to toughen brittle materials has been developed. Surfaces of a brittle material were damaged at room temperature, followed by annealing at a high temperature. During annealing, cracks, which were introduced during surface damage, disappear and dislocation subboundaries are formed. This treatment improves the fracture toughness K_Ic by a factor of two.     

Does domain-specific time perspective predict accelerometer assessed physical activity? An examination of ecological moderators

ObjectivesThe primary objective of this study was to examine the association between time perspective (domain-general or domain-specific) and accelerometer-assessed physical activity behavior in ecological conditions (i.e., ambient temperature and precipitation levels) that vary in the implementational opportunities that they provide.DesignProspective observational study.MethodsThe study sample consisted of 208 community dwelling residents, free of functional impairments, stratified by age. Physical activity was assessed using a hip mounted tri-axial accelerometer worn during waking hours for 7 consecutive days. Participants completed the time perspective questionnaire, exercise version (TPQ-E) at baseline, to assess tendencies toward present versus future-oriented thinking about physical activity behavior. Ecological support for physical activity was assessed as average weekly temperature and precipitation patterns for the week that the accelerometer was worn by each participant.ResultsFindings revealed main effects of both temperature and time perspective on accelerometer-assessed physical activity. Most importantly, there was also a significant two way interaction between domain-specific time perspective and temperature, such that future-oriented individuals were more likely to increase their activity level when temperatures were warmer (and therefore more activity-supportive) than their less future-oriented counterparts. Precipitation level was not associated with activity level alone or in combination with time perspective.ConclusionsThe effect of domain specific-time perspective on accelerometer assessed physical activity is moderated by ambient temperature. In the current study, present and future-oriented participants were active at similar levels when temperatures were cool; when conditions were warmer (i.e., more supportive of physical activity) those with more future-oriented time perspectives were more active than their present-oriented counterparts. These effects were invariant by age group.     

Formation of prismatic loop alignments in crystals deformed in single slip

In γ-TiAl deformed at room temperature in single slip, prismatic loops ar often organized in a staircase-like configuration, called strings whose generation by cross-slip is facilitated by the dislocation tendency to form screw locks as described recently by Grégori and Veyssière. The present letter is aimed at showing that strings may be encountered in virtually any crystal provided that the two impinging dislocations exhibit significantly different velocities. The crossslip annihilation mechanism of crossed dislocations considered by Tetelman is revisited and shown to evolve into a configuration significantly distinct from that originally predicted.     

Secondary ageing in Al-Cu-Mg

Secondary ageing, that is microstructural evolution occurring at room temperature after short heating at temperatures above the metastable phase boundary of Guinier-Preston zones, has been studied for an Al-Cu-Mg alloy with a high Cu-to-Mg ratio. Combined data from positron annihilation spectroscopy, Vickers microhardness measurements and differential scanning calorimetry show that, on secondary ageing after 5 or 7min at 190°C, firstly, hardening takes place at a rate nine to 16 times slower than natural age hardening; secondly, vacancies slowly released by Cu-rich aggregates formed during the heat treatment at 190°C promote further formation of solute aggregates, with a time-dependent chemical composition; thirdly, the thermal stability of the structures formed during secondary ageing increases with increasing dwell time at room temperature; and, fourthly, solute aggregates formed at 190°C undergo a structural reorganization and possibly a change in the composition, leading to species with a different thermal stability. The slow release of vacancies from Cu-rich aggregates is proposed as one of the limiting factors of the hardening rate.     

Quasi-two-dimensional or hcp palladium nanoparticles with host-guest interaction prepared at room temperature

Quasi-two-dimensional palladium nanoparticles with an average lateral dimension of 7 nm have been prepared by reduction of a PdCl₂ graphite intercalation compound precursor by lithium-diphenylide in tetrahydrofuran at room temperature. Selected-area electron diffraction patterns provide evidence that the palladium nanoparticles are hcp single-crystal particles. Owing to the template effect of the graphite lattice, the lattice parameter of palladium was found to have a strong relation with the graphite, and a 3a _graphite superstructure was inferred. The palladium structure is rotated by 30^o with regard to the carbon host lattice. Raman spectroscopy on this sample showed that a charge transfer between carbon and palladium occurs. The sample can be considered as a common Pd-graphite intercalation compound with palladium nanoparticles as guest. This behaviour is different from palladium nanoparticles prepared by hydrogen reduction at higher temperatures from the same precursor material. These particles may represent an early stage of nanoparticle formation.