For the first time self-consistent band-structure calculations of TI1 single crystals (space group 0:) are performed using nonlocal pseudopotentials. The method is described briefly. The contributions of the different band states to the charge density distributions are estimated. Correlations between the band dispersion, valence electron mobility, and charge density distribution are outlined. The origin of the different bands is defined and the total density of states is calculated.
The influence of uniaxial mechanical stress on the modulated structure which consists of a superstructure sequence is investigated in [N(CH3),],MeCl, (Me = Cu, Fe, Zn) crystals. The peculiarities of the effective piezooptic coefficients n : within the incommensurate phase of these crystals are studied by Senarmont's method. The influence of uniaxial mechanical stress disturbs the conditions of modulated structure interaction with defects what leads to the appearance of the widening of metastable states and to anomalies in the physical characteristics. It causes the change of the region of the localization of the incommensurability wave vector k on the commensurate values of higher order.
IntroductionThe authors of [l to 31 have drawn the conclusion that anomalies of the effective piezooptic coefficients zi"j within the incommensurate (IC) phase are connected with peculiarities of the order parameter and the behaviour of the elastic constants Cij. Anomalies observed in the vicinity of phase transitions were explained by the influence of the term a@/&, proportional to the value aT,laaj, i.e. to the shift of the transition temperature under the influence of uniaxial stress. According to [4] the displacement of the anomaly corresponding to the thermooptical memory effect under the influence of mechanical stresses cj cannot be explained only by the temperature shift of the phase transition at T,. It is necessary to take into account soliton density changes under the action of mechanical stresses o? Besides, the deformation of the soliton structure (the change of the regular distribution along the crystal sample) may proceed in this case, as has been shown theoretically [5]. The experimental confirmation of this fact has been reported in [6]. Besides, the displacement of the ') Kyrylo and Methodiy str. 8, UA-290005 Lviv, Ukraine. , ) Lesna str. 16, PL-25509 Kielce, Poland.
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