Investigation of the morphology of the van der Waals surface of the InSe single crystal

The morphology of the (0001) van der Waals surfaces of the layered single crystal In_1.03Se_0.97, which were prepared using different techniques, has been investigated by scanning probe microscopy methods. It has been assumed that the van der Waals surface prepared with the use of an adhesive tape oxidizes in air due to the chemisorption of acid agents on dangling bonds of the metal and selenium. An analysis of the current-voltage characteristics of the tunneling current has shown that the composition of natural oxides represents a mixture of phases of the In₂O₃ oxide and wide-band-gap selenium oxides. In the InSe surface prepared by cleavage with subsequent exposure in air for approximately 2 min, the scanning with a tunneling microscope has revealed a surface ordering in the form of a corrugation of a complex profile with a fine structure. The last fact reflects the charge density redistribution after the chemisorption of gas molecules from air on this surface and its relaxation to the state with a minimum energy. Atoms of the basal plane are observed on the InSe(0001) van der Waals surface prepared by cleavage in an oxygen-free medium. The surface corrugation is absent. Point defects cause a disturbance of the periodic potential of the single crystal, which extends over a distance equal to four lattice spacings and appears as a shadowing. A technique has been proposed for producing In₂O₃ oxide nanostructures on the surface of the single crystal of the layered semiconductor InSe with the use of an atomic-force microscope probe as a nanoindenter. The ability of the probe to operate in gaseous and liquid media significantly extends the capabilities of the method.     

Trends in the kinetic and magnetic properties of Pb0.8Sn0.2Te single crystals of high structural perfection

In the temperature interval 4.2–300K and magnetic fields up to 4 T, we have investigated the electrical, thermal-electrical, thermal-magnetic, and magnetic properties of single crystals of the solid solution Pb_0.8Sn_0.2Te. It is shown that an enhancement of its level of structural perfection leads to the appearance of a second structural phase transition (SPT), and also to an increase in the temperature of the first structural phase transition. Fiz. Tverd. Tela (St. Petersburg) 41, 1750–1752 (October 1999)     

Spin splitting in narrow-gap Pb1−xSnxTe semiconductors

The g factor in the solid solutions Pb_1–xSn_xTe of the p and n type (x = 0, 0.1, 0.19, 0.2, 0.28, and 0.38), doped with germanium, cadmium, and bismuth, is determined from investigations of the quantum oscillations of the magnetoresistance. Comparison of the results obtained with theoretical calculations, taking into account the dependence of the matrix element of the momentum operator on the composition x, shows that the experiment agrees well with the theory within the framework of Dimmok's model. Analysis of the experimental results indicate that doping with nonmagnetic impurities does not affect the magnitude of the spin splitting.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 78–83, August, 1987.We are grateful to Doctor of Physical-Mathematical Sciences K. D. Tovstyuk and Candidate of Physical-Mathematical Sciences V. B. Orletskii for providing the possibility of formulating and successfully carrying out these studies, their constant interest in this work, and for valuable discussions of the results.     

Transport phenomena in metal nanocomposites

Ferromagnetic nanocomposites are the special case of metal composites; they are of practical interest for spintronics. Temperature dependences of resistivity ρ(T) and thermoelectric power α(T) of ferromagnetic nanocomposites with the composition Co _x (Al₂O₃)_{100 ? x} (36.6 ≤ x ≤ 52.5 at %) are investigated near the percolation threshold (x _p ≈ 43.3 at %) in a temperature range of 77–300 K. Sizes of Co nanoparticles are no larger than 25 nm. Specific features are observed in the dependences α(T) in the form of a kink at T _b ≈ 170 K. The analysis of the structural and electrical schematics as well as energy diagrams of percolation channels of electrons shows that only the diffusion thermoelectric power appears in Co nanoparticles, at which α(T) is the linear function. No mechanisms of the thermoelectric power caused by nanosizes of Co particles or by electron tunneling between them are found. The kink of the α(T) linear dependence is explained by the existence of the oxide shell of Co nanoparticles. It is assumed that the temperature dependences of energy barriers of oxide shells of metal nanoparticles (including ferromagnetic ones) in oxygen-containing dielectric matrices determine the features of α(T) and ρ(T) dependences of such nanocomposites.     

Peculiarities of electron field emission from ZnO nanocrystals and nanostructured films

ZnO microcrystals and nanocrystals were grown on silicon substrates by condensation from vapour phase. Nanostructured ZnO films were deposited by plasma enhanced metal organic chemical vapour deposition (PEMOCVD). The parameters of field emission, namely form-factor β and work function , were calculated for ZnO structures by the help of the Fowler–Nordheim equation. The work functions from ZnO nanostructured films were evaluated by a comparison method. The density of emission current from ZnO nanostructures reaches 0.6 mA/cm² at electric force F=2.110⁵ V/cm. During repeatable measurements β changes from 5.810⁴ to 2.310⁶ cm⁻¹, indicating improvement of field emission. Obtained values of work functions were 3.7±0.37 eV and 2.9–3.2 eV for ZnO nanostructures and ZnO films respectively.     

Ferromagnetic semiconductor Ge1−x−ySnxMnyTe with phase transformation of ferroelectric type

It is expected that joint existence of ferromagnetic properties and ferroelectric structural phase transition in diluted magnetic semiconductors IV-VI leads to new possibilities of these materials. Temperature of ferroelectric transition for such crystals can be tuned by the change of Sn/Ge ratio. Magnetic susceptibility, Hall effect, resistivity and thermoelectric power of Ge_1−x−ySn_xMn_yTe single crystals grown by Bridgeman method (x=0.083-0.115; y=0.025-0.124) were investigated within 4.2-300 K. An existence of FM ordering at T_C∼50 K probably due to indirect exchange interaction between Mn ions via degenerated hole gas was revealed. A divergence of magnetic moment temperature dependences at T?T_C in field-cooled and zero-field-cooled regimes is obliged to magnetic clusters which are responsible for superparamagnetism at T>T_C≈T_f (freezing temperature) and become ferromagnetic at T_C arranging spin glass state at T<T_f≈T_C. Phase transition of ferroelectric type at T≈46 K was revealed. Anomalous Hall effect which allows to determine magnetic moment was observed.     

Neutron diffraction studies of the negative thermal expansion in a layered indium selenide crystal

The neutron diffraction patterns have been analyzed for a layered single crystal and a powder of the γ-polytype of indium selenide in the temperature range 10–300 K. In the temperature range 10–50 K, the excitation of bending vibrations due to the charge density waves changes the phonon spectrum and gives rise to a negative thermal expansion in the plane of layers, i.e., α_‖c = −2.2 × 10⁻⁶ K⁻¹, which is characteristic of two-dimensional structures. The average (over the range T = 50–300 K) coefficients of thermal expansion along the principal crystallographic directions have been calculated: $ \bar \alpha _{ \bot c} $ \bar \alpha _{ \bot c} = 10.48 × 10⁻⁶ K⁻¹ and $ \bar \alpha _{\parallel c} $ \bar \alpha _{\parallel c} = 12.97 × 10⁻⁶ K⁻¹, which agree with the X-ray diffraction data previously obtained by the authors at T = 290 K.     

Luminescence anisotropy of ZnO microrods

The local features of light emission from ZnO microrods were studied: it is revealed that ZnO luminescence spectra are significantly influenced by the crystal morphology. It is shown that the near and edge ultraviolet emission occurs primarily from the top (0001) planes of ZnO microrods; while the defect related visible emission was found to occur dominantly from the side facets. The room temperature cathodoluminescence analysis revealed that visible emission consists of a few overlapping peaks, arising due to recombination on common points and surface defects (Zn_i, V_o, V_o⁰/V_o^?*_, V_o^** and surface defects.). While at low temperature, only the luminescence due to neutral donor bound exciton (D⁰X) emission is observed. The data obtained suggest that the light emission spectra of ZnO material of diverse morphology cannot be directly compared, although some common spectral features are present.     

Effect of elastic stresses on the thermoelectric properties of Pb1−x GexTe epitaxial layers

The temperature dependence of the thermopower of Pb_1?x Ge_xTe (x = 0.01?0.05) epitaxial films grown on BaF₂(111) substrates was measured. The temperature of the ferroelectric phase transition was found to disagree with that obtained for bulk single crystals of the same composition. This disagreement was explained as resulting from the effect of elastic stresses arising in the crystal lattice of the Pb_1?x Ge_xTe compound due to the difference between the thermal expansion coefficients of the film and the substrate during cooling of the layers from the growth temperature to the temperature of measurement.