National Institute Of Materials Physics - Romania

We consider semiconductor devices that are composed of two parts: first, a small quantum structure constituting the active region, and second, a classical environment with larger typical length scales, While in general the classical environment should be represented by a drift-diffusion model, we consider here only simple contacts which we take as ideal metals with infinite conductivity, The transport through the quantum structure is described as in the Landauer-Buttiker formation through electronic scattering wave functions which define the electron density in the quantum system. Further sources of the self-consistent Coulomb field are layers of classical charges in the contacts at each of the interfaces to the quantum system. We present further a capacitance model that takes into account the openness of the quantum structure and the existence of finite contacts embedding the system. As particular quantum structures we study simple tunneling barriers. (C) 2000 Elsevier Science Ltd. All rights reserved.

We report experiments on conducting polymer polypyrrole leading to sensor applications. The resonance frequency, stability and sensitivity of AT-cut quartz crystals, electrochemically covered with polypyrrole tosylate: were tasted under various operating conditions, The interaction of this organic semiconductor with thin metallic films was analysed by photoelectron investigations. Shifts of the valence band spectra during silver deposition are explained by the neutralization of positive charged surface defects. Thus the existence of a space charge region in polypyrrole is demonstrated spectroscopically. (C) 2000 Elsevier Science Ltd. All rights reserved.

The molecular dynamics of the liquid crystals 8CB and 5CB confined to the mesopores of the molecular sieve Al-MCM-41 with pore diameter less than 2.5 nm was investigated by broadband dielectric spectroscopy. The phase transitions of the liquid crystal cannot be detected for the molecules confined to the pores. Moreover it has been found that even about 20 K below the melting temperature of the bulk material a relaxation process occurs for molecules confined to the mesoporous material which does not exist in the bulk liquid crystal state. The temperature dependence of the relaxation rate of that process obeys a Vogel-Fulcher-Tammann law.

p-type lead sulphide (PbS) thin films were chemically deposited onto Si3N4/n-Si substrates. A three-contact pseudo-metal-oxide-semiconductor structure was designed in order to investigate the possibility of enhancing the signal-to-noise ratio in the PbS film by field effect. It was proved that on the entire sensitivity domain of PbS (1-3 mu m at room temperature), the magnitude of the photoconductive signal generated by the PbS film depends on the polarity and value of the gate voltage. It is shown that the signal-to-noise ratio can be enhanced in this way with by at least 25% compared with the case when the gate electrode is in air (standard configuration for photoconductive measurements). (C) 2000 American Institute of Physics. [S0003-6951(00)01514-X].

We develop a theory of the kinetics of the Bose-Einstein condensation of bosonic excitons interacting with a thermal bath of acoustic phonons. We emphasize several delicate aspects of the condensation kinetics within the framework of rate equations. We give detailed proofs about the existence and uniqueness of the solution with a condensate and illustrate details within exactly or almost exactly solvable models. In particular, we predict an exciton condensation time in Cu2O which is shorter than the lifetime of paraexcitons.

Quasicrystalline icosahedral and amorphous phases prepared as thin films or bulk samples in the systems Al-Pd-Re, Al-Cu-Fe and Ti-Zr-Ni, have been investigated with respect to their temperature dependence of conductance and thermopower, in the range between 10 and 300 K. Various conduction mechanisms yield different exponent values in the power-law temperature dependence of conductance, Tt. A semi-metal behaviour (zeta congruent to 1.5) is found in the amorphous phase and in the disordered icosahedral Al-Pd-Rt: film and Ti-Zr-Ni ribbons, as well as in icosahedral Al-Cu-Fe films. Lower zeta values found in the quasi-perfect icosahedral Al-Pd-Re bulk sample and in two of the icosahedral Al-Cu-Fe films suggest that electron localization in a hierarchy of clusters is the main conduction mechanism in these samples. The thermopower data support the existence of a pseudo-gap at the Fermi level and suggest a similar short-range order in both amorphous and icosahedral phases. The special sensitivity of the thermopower to the DOS details at E-F is responsible for the strong variation of its magnitude with the degree of icosahedral order.

The Ca0.45CU0.55O phase was successfully produced from a mixture of Ca and Cu nitrate powders with cation ratio Ca:Cu = 1:1. The heat treatment for decomposition and the synthesis of the indicated phase were pet-formed at 710 degrees C in air and flowing oxygen, respectively. The Ca0.45Cu0.55O phase concentration increases with synthesis time. After 209 h of heat treatment, the concentration of the Ca0.45Cu0.55O phase determined by X-ray diffraction (XRD) analysis has reached 87% and the kinetic data have indicated that the material enters the saturation stage of the reaction, The secondary phases are CaO and CuO. (C) 2000 Elsevier Science B.V. All rights reserved.

A series of compounds YbxM4Sb12, M = Fe, Co, FeCo, Rh,Ir, were synthesised by reaction sintering. From Rietveld refinements isotypism was determined in all cases with the LaFe4P12-(skutterudite)-type, space group Im (3) over bar - No. 204. These refinements also served to derive the Yb-content in the samples. There is a systematic trend for the Yb-occupancy in the parent lattice M4Sb12; revealing a gradual decrease of the Yb-content from x = 0.8 (M = Fe), x = 0.5 (FeCo), x = 0.2 (Co); x = 0.1 (Rh) to x approximate to 0.02 (Ir). This dependency seems to correlate with the thermal stability of the ternary compounds: a true ternary compound forms for M = Fe, whilst for M = Co, Rh, Ir stable binary skutterudite compounds MSb3 already exist. Measurements of various bulk properties revealed the absence of any long range magnetic order in this series of compounds. While the samples rich in Yb behave metallic like, the Rh and Ir based skutterudites show a semiconducting-like resistivity which at lower temperatures is characterised by variable range hopping in the presence of strong Coulomb interaction. Although Yb0.1Rh4Sb12 exhibits a Seebeck coefficient up to about 150 mu V/K, figures of merit ZT generally are below 0.1 near room temperature, primarily due to the large resistivities of the sintered material.

We calculate the ground state of a two-dimensional electron gas in a shea-period lateral potential in magnetic field, with the Coulomb electron-electron interaction included in the Hartree-Fock approximation. For a sufficiently short period the dominant Coulomb effects are determined by the exchange interaction. We find numerical solutions of the self-consistent equations that have hysteresis properties when the magnetic field is tilted and increased, such that the perpendicular component is always constant. This behavior is a result of the interplay of the exchange interaction with the energy dispersion and the spin splitting. We suggest that hysteresis effects of this type could be observable in magnetotransport and magnetization experiments on quantum-wire and quantum-dot superlattices.