G. M. Ribeiro

Temperature dependence of the equilibrium Hall concentration in silicon planar-doped GaAs samples

Abstract Hall concentration and mobility were measured on nine MBE grown silicon planar-doped GaAs samples as a function of the temperature in the range of 20 to 300K. All planar-doped layers were buried in undoped GaAs layers, and all the GaAs layers were 0.20μm thick, except one that was 0.50μm. A 60A or 80A n+-GaAs cap layer was used in all samples. The nominal silicon concentration varied in the range from 1.4 × 1012cm−2 to 8.1 × 1013cm−2. We found that, for measurements carried out in darkness and for temperatures below about 200K, the Hall carrier concentration nHall decreases for increasing temperature. We propose a two-subband model that allows an excellent fitting to the Hall measurements. According to the model, the decrease of nHall results from thermally induced changes of the relative occupation of the subbands in the planar-doped well.

EPR Study of the Ferroelastic Transition in KD3(SeO3)2

Electron paramagnetic resonance spectra of the SeO 2 - centers in KD 3 (SeO 3 ) 2 demonstrate that the phase transition in this system is associated with an ordering of the O–H(2)–O hydrogens and a small rotation of the SeO 3 groups around the a -axis with the angle of rotation being proportional to the order parameter. The ferroelastic transition is thus triggered by a soft optic deuteron-lattice mode of B 3g symmetry inducing the acoustic C 44 instability.

Phase Transition Structural Changes of Rb 2Cd 2(SO 4) 3 Investigated by Electron Paramagnetic Resonance of the Tl 2+ Ion

The electron paramagnetic resonance (EPR) of the Tl 2+ center in Rb 2 Cd 2 (SO 4 ) 3 was investigated in the temperature range 10 K to 140 K. Special attention was paid to the Rb + site surrounded by nine oxygens. The studies show that at the cubic →monoclinic phase transition, at 130 K, that block of oxygens rotates almost rigidly around one of the crystallographic axes. The angle of rotation changes abruptly at the transition temperature and increases linearly with decreasing temperature in the monoclinic phase. The spectra show no evidence for the cell multiplication observed in X-ray diffraction. Additional changes of the EPR spectra were observed at 101 K and 68 K. Between those two temperatures the spectra seem to be consistent with a triclinic symmetry. Below 68 K the spectra become more complex, which contrasts with the higher symmetry of the orthorhombic phase previously attributed to the crystal.

Symmetry analysis of KD3(SeO3)2:SeO2− ESR spectra

Abstract The symmetry analysis of KD 3 (SeO 3 ) 2 :SeO 2 - ESR spectra, in previous publications, is incompatible with C 2h symmetry of the low temperature phase. Such incompatibility motivated a suggestion that KD 3 (SeO 3 ) 2 might go to a pyroelectric phase at low temperature. The present work demonstrates that the correlated rotations of SeO 2 groups, induced in the phase transition, transform according to the representation B 3g of group D 2h , the same symmetry of the order parameter. Therefore, the ESR spectra are in harmony with the symmetry change in the transition D 2h → C 2h and the crystal cannot be pyroelectric.

Proton-Thallium Coupling in Thallium Doped Crystals of the KDP family

A previous model for proton-thallium coupling in thallium doped crystals of the KDP family, is corrected and extended in order to explain new ESR experiments in the temperature range 20–120 K. According to the reviewed model, two kind of centers, a symmetry-preserving and a symmetry-breaking, appear in those crystals and coexist in a temperature range. At low temperatures only the symmetry-breaking center is observed, whereas above about 77 K one can observe only the symmetry-preserving center. A semi-quantitative analysis of the exprimental results is presented, based on the new model.