Photoelectronic properties of the Zn x Cd1−x In2S4 system

Summary Photoelectrical investigations on the Zn _x Cd_1−x In₂S₄ system have been carried our particularly with respect to the high residual conductivity and the long-relaxation-time properties. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Detection of Mn pinned level in Cd1−x Mn x Se monocrystals by the photoconductivity technique

Summary The photoconductivity of the magnetic semiconductor compound Cd_1−x Mn _x Se has been studied at 300 K and at 77K for the first time for several Bridgman-grown single crystals. With this technique it was possible to detect the Mn 3d level. Experimental data found in this investigation reveal that the Mn 3d level is pinned at 1.9 eV below the bottom of the conduction band,i.e. it is independent of temperature and insensitive to the amount of manganese. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

An optimization model for amorphous solar cells in which optical,electrical and recombination properties are specified

Summary A model for determination and optimization of the fundamental parameters defining the quality of amorphous solar cells in which optical, electrical and recombination properties are specified is developed. It can be applied to any amorphous solar-cell systems. This paper gives theoretical expressions and describes an iterative method of numerical calculation for the current density of different cells. Results of computer calculations based on this model are presented.

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Riassunto In questo lavoro si presenta un modello per la determinazione e l'ottimizzazione dei parametri fondamentali atti a definire la qualità delle celle solari amorfe, specificando le proprietà ottiche, elettriche e di ricombinazione. Esso può essere applicato a qualunque sistema di celle solari amorfe. Sono fornite le espressioni teoriche ed è descritto un metodo iterativo per il calcolo numerico della densità di corrente di alcuni tipi di celle. Sono anche presentati i risultati ottenuti da calcoli eseguiti col computer e basati su questo modello.

     

Photoconductivity of Cu x Ag1−x Cd2InTe4

Summary The prominent photoelectric parameters of the semiconducting solid solutions Cu _x Ag_1−x Cd₂InTe₄ are reported. From the spectral dependence of photoconductivity at room temperature and at 77 K the energy gaps and their thermal coefficients are obtained. The detectivityD ^*, the photocurrent gainG and the carrier lifetime are measured and the carrier mobility is deduced. In order to improve theD ^* characteristics, the analysis of its parameters is also performed. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Microwave control of directed transport in asymmetric antidot structures

It is shown that a polarized microwave radiation creates directed transport in an asymmetric antidot superlattice in two dimensional electron gas. A numerical method is developed that allows to establish the dependence of this ratchet effect on several parameters relevant for real experimental studies. It is applied to the concrete case of a semidisk Galton board where the electron dynamics is chaotic in the absence of microwave driving. The obtained results show that strong currents can be reached at a relatively low microwave power. This effect opens new possibilities for microwave control of transport in asymmetric superlattices.     

Contributions to photodoping in oxygen-deficient YBa2Cu3Ox films

We report our studies on the superconducting and normal-state properties of metallic thin films ( 52 K) exposed to long-term white-light illumination (photodoping). It was observed that the effects of photoexcitation strongly depended on the temperature at which the photodoping was performed. At low temperatures, both the Hall mobility and the Hall number were photoenhanced, whereas, at temperatures slightly below room temperature, the Hall mobility initially showed an abrupt increase followed by a long-term decrease, and the Hall number increased even stronger than at low temperatures. The enhancement of the film's superconducting transition temperature T_c, caused by photodoping, exhibited the same temperature dependence as the enhancement of the Hall number, being largest ( 2.6 K) at high temperatures. From the asynchronous behavior of the Hall quantities, we conclude that both the photoassisted oxygen ordering and charge transfer mechanisms contribute to photodoping. The relative contributions of both mechanisms and, thus, the electronic properties of the photoexcited state are strongly temperature dependent. Studies of the relaxation of the photoexcited state at 290 K showed an unexpectedly short relaxation time of the Hall mobility after termination of the illumination. The relaxation saturated somewhat below the initial, undoped value, similarly to the decrease of the Hall mobility, observed upon long illumination. These latter findings give evidence for a competition between the oxygen ordering and thermal disordering processes during and after the photoexcitation in the high-temperature range. Received: 13 October 1997 / Accepted: 19 November 1997     

The edges and terrace effect of Ag particles on optical resonance absorption property

Alternative Ag and SiO₂ multilayers are prepared by using radio frequency magnetron sputtering. The Ag particles are found to diffuse toward and mostly accumulate near the surface of the Ag—SiO₂ composite film via a rapid thermal treatment. Different shapes of the Ag particles are obtained by changing the thickness of each Ag and SiO₂ layer. The response absorption property of the Ag composite film is also investigated. We relate the resonance absorption to the surface level and the Fermi level. To induce the obvious resonance absorption in an Ag composite film, it is necessary to maintain special shapes with sharp edges and wide terraces and to maintain the particle sizes ranging from 0 nm to 100 nm.     

Photo-induced increase of the superconducting coherence lengths in oxygen-deficient YBa2Cu3Ox thin films

() thin films were photodoped with white light at various temperatures from 70 K to 290 K. Before and after the excitation, the magnetoconductivity was measured in a magnetic field B = 0.5 T, and the experimental results were fitted to the Aslamazov-Larkin theory of superconducting order-parameter fluctuations to determine the superconducting coherence lengths, and . We observed that the photodoping process enhanced and and that the amount increased with the photodoping temperature increase. On the other hand, the superconducting anisotropy / decreased with increasing temperature. The photodoping effect enhances superconducting properties of partially oxygen-deficient samples and is considerably increased by high doping temperatures. Received 15 December 1999 and Received in final form 24 May 2000     

The far-infrared absorption spectra of some alkali halide alloys

A study has been made of the far-infrared transmission spectra of some K_xRb_1−x⁻ Cl_yBr1−y alloys with (x,y) ≈ 1 mole%. and the results are reported in this paper. An absorption band at 94.5cm⁻¹ in KBr:Cl has been observed, in agreement with the results of other workers. New bands at 99 cm⁻¹ in KBr:Rb. 99 cm⁻¹ in RbCl:K and 103 cm⁻¹ in RbCl:Br have also been found. These bands are attributed to impurity-induced modes in the gaps in the frequency distributions of KBr and RbCl. Comparison of these results with the theoretical results of Mitani and Takeno indicates that the ‘force constants’ associated with Rb and Br impurities are enhanced relative to those associated with K and Cl impurities. The relationship between the results presented here and some features in the infrared reflection spectra of non-dilute alloys is discussed.     

The hall effect in some liquid alloys

The Hall coefficient has been measured for (a) liquid CuTe, AgTe and AuTe₂ and (b) liquid Cu-Sn alloy. The results are discussed in terms of the possible semiconducting nature of liquids comprising group (a) and in terms of bound state formation around the Sn atoms for those in group (b).     

Edge absorption and circular photogalvanic effect in 2D topological insulator edges

The electron absorption on the edge states and the edge photocurrent of a 2D topological insulator (TI) are studied. We consider the optical transitions within linear edge branches of the energy spectrum. The interaction with impurities is taken into account. The circular polarization is found to produce the edge photocurrent, the direction of which is determined by light polarization and edge orientation.     

Shape effect on the size and dimension dependent order-disorder transition temperatures of bimetallic alloys

Chemically ordered bimetallic nanocrystals may be promising candidates for the future magnetic-storage applications. In order to theoretically understand the order-disorder transition in nanoscale, a model based on the previous result for the size and dimension dependent melting temperature is developed to describe the effects of sizes, shapes and dimensions on order-disorder transition temperatures (T_OD) of bimetallic alloys. The results show that T_OD drops as size decreases, shape factor increases and dimension decreases. Also, the shape effect on T_OD cannot be neglected. Among these effects on T_OD, size is the strongest, while shape is the weakest. All these conclusions have been compared and confirmed by the recent simulations and experiments.     

Quantitative model of large magnetostrain effect in ferromagnetic shapell memory alloys

A quantitative model describing the large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar types of materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa. Received 29 September 1999     

Lattice dynamics of some binary alloys

Summary The phonon dispersion frequencies for Cu₃Au, Cu₃Zn and Ni₃Fe alloys have been calculated using the model potential approach. The contribution of a short-range three-body interaction to the dynamical matrix has also been included in the present study. The theoretically computed phonon dispersion curves of these alloys are found to be in good agreement with the experimental data.     

X-ray magnetic circular dichroism at rare-earth L23 absorption edges in various compounds and alloys

Theoretical interpretations of X-ray magnetic circular dichroism (XMCD) at rare-earth (called R hereafter) L₂₃ absorption edges are reviewed using differing models, depending on the material under investigation. In the first chapter, we present an overview of recent developments for XMCD in XAS with a few general remarks, especially at R atom absorption edges. In Section 2, we first describe basic mechanism of XMCD at L₂₃ edges of R systems, and then we essentially discuss two examples of XMCD spectra in: (i) R₂Fe₁₄B metallic compounds, with the help of a cluster model, and (ii) RFe₂ Laves-phase compounds, using a tight-binding approximation for R 5d and Fe 3d conducting states. A good agreement between theory and experiment for R₂Fe₁₄B suggests that a cluster model provides a valuable method to quantitatively calculate XMCD spectra of R systems, even with quite complicated atomic arrangements. For RFe₂ systems the XMCD spectral shape, especially for the L₂ edge of heavy R elements, is more complicated than those of R₂Fe₁₄B systems, and this is explained by the competition of some different XMCD mechanisms. In Section 3, we focus on special series of Ce systems, related to XAS and XMCD studies at the Ce L₂₃ edges. Two clearly differing cases are interpreted: (i) A well localized 4f¹ system, i.e. CeRu₂Ge₂; (ii) A less localized 4f¹ system, i.e. CeFe₂, with a 3d partner. Then, from a more experimental point of view, we investigate the influence of substitution on the low temperature properties of CePd₃ compounds, i.e. Ce(_Pd1-xMnx)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Mn}}_x{)}_3$ alloys where x   is about 0.03, giving rise to (_CePd3)8Mn$({\mathrm{CePd}}_3{)}_8\mathrm{Mn}$ ordered structure. We give another example: Ce(_Pd1-xNix)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x{)}_3$ alloys with x taken up to about 0.25. Also the Ce L₂₃ XMCD signal measured in pure CePd₃ demonstrates that in the Ce based dense Kondo materials, only the 4f¹ channel yields a magnetic response.     

Coulomb repulsion effects in driven electron transport

We study numerically the influence of strong Coulomb repulsion on the current through molecular wires that are driven by external electromagnetic fields. The molecule is described by a tight-binding model whose first and last site is coupled to a respective lead. The leads are eliminated within a perturbation theory yielding a master equation for the wire. The decomposition into a Floquet basis enables an efficient treatment of the driving field. For the electronic excitations in bridged molecular wires, we find that strong Coulomb repulsion significantly sharpens resonance peaks which broaden again with increasing temperature. By contrast, it has only a small influence on effects like non-adiabatic electron pumping and coherent current suppression.     

“Forbidden” soft-X-ray absorption edges in the alkali halides

Transitions from s core levels to the conduction-band edge and to excitons derivable therefrom are forbidden by symmetry in the alkali halides. Excepting K spectra, all indications from the literature seem to be that L₁, M₁, N₁, O₁ spectra are in fact unobservable. By untangling some data available for NaBr in the region where the Na⁺L₁ and Br^?M_{4 5} spectra overlap, we suggest that “forbidden” spectra may indeed by observable, and more detailed scans in certain regions of photon energy might be useful. The importance of excitons in X-ray spectra is further elucidated and earlier conclusions are confirmed.