Showing papers in "Physics of the Solid State in 1993"

Gallium arsenide grown by molecular beam epitaxy at low temperatures: crystal structure, properties, superconductivity

Abstract: An investigation was made of the properties of gallium arsenide grown by molecular beam epitaxy at temperatures 150-250 °C (LT-GaAs). This material contains up to 1.5 at. % of excess arsenic, which forms clusters of nanometer size when the material is annealed. The concentration dependences of this excess arsenic and of the clusters and their size on sample preparation conditions were determined. The LT-GaAs films were found to have a high resistivity, but investigations of the microwave absorption as a function of a weak magnetic field revealed a characteristic signal typical of a superconducting phase. This signal was obviously associated with the superconductivity of the arsenic clusters in the LT-GaAs film and not due to the superconductivity of indium clusters in the substrate, as postulated earlier

Model of a sequence of structural phase transitions in a layer TlInS 2 crystal

Abstract: A model is proposed for a sequence of structural phase transitions in a layer TlInS 2 crystal. The transitions observed at 201 and 204 K and TlInS 2 are attributed to splitting of one transition to a commensurate phase into two closely spaced transitions. The experimental results obtained in investigations of dielectric properties of TlInS 2 below the ferroelectric phase transition temperature (T c ) are regarded as an independent confirmation of the appearance of a chaotic state at T < T c

Phase transformations in the Ti-H(D) system at high pressures

Abstract: X-ray diffraction in situ is used to determine the structure of the ξ phase, which forms in the Ti-H system at high pressures and consists of the face-centered tetragonal sublattice of titanium with lattice parameters a = 4.33 A, c = 3.85 A, and c/a = 0.89, and of hydrogen located at the octahedral interstices, as inferred from the ratio of the specific volumes. The results of a study of the conditions of formation of the ξ phase in hydrides and deuterides, and of the process of quenching of TiH ≃0.75 under pressure indicates the occurrence of a ξ → χ phase transformation during removal of pressure after quenching to liquid nitrogen temperature. This transformation is initiated by a modification of the hydrogen sublattice

Electron spin resonance in 2 mm range of transmutation phosphorus impurity in 6H-SiC

Abstract: The method of neutron transmutation doping was used to introduce phosphorus into 6H-SiC single crystals. An investigation carried out at T = 4.2 K revealed two ESR spectra attributed to isolated phosphorus ions in the cubic and hexagonal sites of the lattice. The parameters of these spectra were determined. A study was made of the dynamics of changes in the ESR spectra of phosphorus and nitrogen at temperatures from 4.2 to 73 K. The ionization energy of phosphorus was less than the ionization energy of nitrogen, and the former energy was higher at the hexagonal than at the cubic site

Negative magnetoplastic effect in nonmagnetic crystals

Abstract: An analysis is made of the influence of an external magnetic field on the interaction of a dislocation kink with a paramagnetic impurity. It is shown that the resultant transitions between the states of different multiplicity can account for the observed increase in the plasticity of crystals in a magnetic field. It is predicted that the application of the field should increase strongly the decrement of the amplitude-independent internal friction due to dislocations pinned by paramagnetic impurities

Electron spin resonance of boron in cubic SiC: manifestation of the Jahn-Teller effect

Abstract: A study has been made of the angular and temperature dependences of the ESR spectra of boron in cubic silicon carbide (3C-SiC). It is established that the boron centers in 3C-SiC have the point group symmetry C 3υ ; lowering of the symmetry from cubic to trigonal is due to the Jahn-Teller effect. A detailed investigation is made of the motional effects associated with a reorientation of the unfilled boron-carbon orbital between four directions of the Jahn-Teller displacements of the type. The temperature dependence of the reorientation rate is accounted for by phonon-induced tunneling of boron in the ground vibrational state and by a tunnel-controlled process involving an excited vibrational state

Spectral dependence of the Hanle effect due to diffusion of optically oriented electrons in p-type semiconductors

Abstract: A detailed theoretical and experimental study has been made of the polarization of the luminescence of optically oriented electrons diffusing in films of a direct-gap p-type semiconductor (GaAs) in the presence of self-absorption. The investigation includes a calculation of the degree of circular polarization at different luminescence wavelengths and of the dependence of this degree on a transverse magnetic field (Hanle effect) in the case of a «semiinfinite» sample allowing for surface recombination. The results of this calculation are compared with the experimental data. In the case of thin GaAs films (of thickness of the order of the diffusion length of electrons), a study is made of the spin polarization of the emitted luminescence in transmission and reflection

Internal friction during quasistatic deformation of crystals (review)

Abstract: A review is given of investigations of the internal friction recorded directly during quasistatic deformation of crystals A classification is given of the existing theories and published experimental data Attention is concentrated on the behavior of the internal friction during plastic deformation when the vibrational energy losses are due to both structural changes (structural internal friction) and the deformation process itself (dynamic internal friction) An analysis of the dependences of the absorption of sound on the rate of deformation, frequency, and amplitude of the vibrations makes it possible to separate these mechanisms in the experimental data

Magnetoplastic effect in NaCl, LiF, and Al crystals subjected to an alternating magnetic field

Abstract: The magnetoplastic effect was investigated in NaCl crystals containing different impurity concentrations, and in LiF and Al, subjected to an alternating magnetic field. A critical frequency υ 0 was found and above this frequency the effect disappeared. In all cases this frequency obeyed υ 0 ∞ B 2 . Measurements on NaCl crystals were made at room and liquid-nitrogen temperatures. The value of υ 0 was independent of temperature and of the impurity (Ca) concentration. In the case of LiF crystals, the critical frequency υ 0 for screw dislocations was approximately three times higher than for edge dislocations. The frequency υ 0 was interpreted as a measure of the probability of a spin-dependent transition in a magnetic field in a paramagnetic impurity-dislocation core system

Polarization and resonance effects in optical generation of cylindrical surface polaritons and periodic structures

Abstract: A theory is developed for the elastic scattering of plane polarized optical waves from small-scale insulating inhomogeneities in a layered medium accompanied by excitation of cylindrical polaritons. The renormalization of the polarizability due to the effect of «images» is taken into account in all orders of perturbation theory. The poles of the polarizability are associated with the normal frequencies of the electromagnetic modes of the inhomogeneity. The dependences of the field of the cylindrical surface polaritons on the polarization and frequency of the exciting wave are studied, and estimates of the constant governing the interaction of light with cylindrical surface polaritons are related to a model of an ellipsoidal insulating inclusion

Magnetoresistance of mercury chalcogenides at high pressures up to 30 GPa

Abstract: The behavior of the magnetoresistance of HgX was investigated in the region of semiconductor-metal phase transitions which occurred under the influence of external pressures applied in diamond chambers. A reversal of the spin of the magnetoresistance was observed: it was attributed to a major change in the carrier mobility at phase transitions

Polarized reflection spectra of a new organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl0.5Br0.5

Abstract: The polarized reflection spectra R (ω) of a single crystal of a new organic semiconductor based on the BEDT-TTF donor molecule, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl 0.5 Br 0.5 (T c = 11.3 K), are determined in the spectral ranges 600-5500 and 9000-40000 cm −1 in the polarizations with the electric vector of the optical wave E parallel (E∥) or perpendicular (E⊥) to the layers of the BEDT-TTF molecules. An optical proof of the quasi-two-dimensional nature of the conducting electron system in such a crystal is obtained. It is also shown that a weak maximum observed in the E⊥ polarization may be a consequence of charge transfer between the adjacent layers of the BEDT-TTF molecules

Semiconductor-metal transitions in HgMgTe and HgSeS subjected to ultrahigh pressures

Abstract: High-pressure chambers made of synthetic diamonds were used in an investigation of semiconductor-metal phase transitions due to structural transformations under pressures up to 20 GPa. Measurements were made of the resistivity and thermoelectric power. Replacement of Hg and Mg atoms (x < 0.1) did not influence the pressure of the transition to the semiconductor phase B9 having the cinnabar structure, but replacement of Se with the S atoms (0.1 < x < 0.5) reduced this pressure. Measurements were made of the magnetoresistance of the initial semimetallic, semiconducting, and metallic (at pressures higher than 9 ± 1 GPa) phases of HgMgTe alloys. This made it possible to estimate qualitatively the change in the carrier mobility as a result of these phase transitions

Phase transitions in Ge and Si subjected to shear under pressures up to 12 GPa and the P-T-γ diagrams of these elements

Abstract: The methods of x-ray phase analysis, microcalorimetry, and optical and electrical measurements are used in a study of the effect of shear under pressures up to 12 GPa on the parameters of phase transitions in Ge and Si. It is found that shear not only alters the phase equilibrium conditions, but also the mechanism of phase transitions

Magnetoplastic effect in CsI and LiF crystals

Abstract: An experimental investigation has been made of the mobility of dislocations in CsI and LiF crystals under the influence of a static magnetic field (B = 0.01-1.5 T) in the absence of mechanical loads. The average distance traveled by dislocations l was found to be a linear function of the duration t of application of a magnetic field and a quadratic function of the field intensity. Cooling from 293 to 77 K reduces l by 10-30%. The density of mobile dislocations ρ m increases monotonically with the duration of «magnetization» and with the field intensity. The value of ρ m reaches 80% of the density of the freshly generated dislocations in CsI. A comparison of the experimental dependences l(t,B) and ρ m (t,B) with phenomenological dependences obtained earlier yields the relation between the real and apparent average dislocation velocities

Electron spin resonance spectroscopy of Cr5+ ions in Ca3Ga2Ge3O12:Cr single crystals

Abstract: The ESR spectra of Ca 3 Ga 2 Ge 3 O 12 :Cr (CGGG:Cr) single crystals were investigated at temperatures 4.2-300 K. These crystals were grown by the Czochralski method from a charge with natural abundances of the isotopes and from a charge enriched with the 53 Cr and 69 Ga isotopes to 97.7 and 99.7%, respectively. The chromium impurity ions, present in concentrations of 0.0005-O.02 at. %, were incorporated in the CGGG lattice simultaneously in two different valence states, forming paramagnetic Cr 3+ centers at the octahedral [a] sites (local symmetry C 3i ) and Cr 5+ centers at the tetrahedral (d) sites (local symmetry S 4 )

Many-particle impurity complexes in diamond-like semiconductors

Abstract: A model of two-particle acceptor centers A 1 + and A 2 0 and of exciton-impurity complexes (A 1 0 x) is proposed for cubic semiconductors A study is made of the multiplet structure of the energy levels and it is shown that the particles in such complexes are in different orbital states and the ground state of holes is a fivefold-degenerate level corresponding to the total angular momentum of holes J = 2 A study is made of changes in the energy structure of complexes in the presence of uniaxial strains

Role of an additional extremum in the appearance of superconductivity in IV-VI semiconductors having resonant impurity states

Abstract: An analysis was made of experimental data on superconducting properties of IV-VI semiconductors containing indium and thallium impurities. An important role played by an additional extremum of the valence band of semiconductor compounds based on SnTe:In and PbTe:Tl in the appearance of superconductivity with a relatively high (for semiconductors) critical temperature T c ∼ 1 K. The existence of a superconductivity with c ≥O.4 K in IV-VI semiconductors was found to require not only partial electron occupation of resonant states, but also of the following additional condition: a band of resonant impurity states pinning the Fermi level should overlap, on the energy scale, band states of the additional extremum with a high density of states

Dislocations-dynamic diffusion in crystalline bodies

Abstract: An account is given of the discovery and studies of the phenomenon of dislocation-dynamic diffusion, which is penetration of atoms or molecules from an ambient medium along nucleating and moving dislocations into a surface layer of a crystalline body during its plastic deformation

Investigation of the electrical conductivity of some manganites with the perovskite structure

Abstract: The temperature dependences of the electrical resistivity ρ, and of the magnetization and magnetoresistance of La 0.9 Sr 0.1 MnO 3 and La 0.9 Na 0.1 MnO 3 single crystals and of ceramic samples of La 0.9 Na 0.1 Mn 0.9 Cu 0.1 O 3 were investigated from 77 to 380 K. An analysis was made of the experimental results on the basis of various models of conduction. The temperature dependence of the resistivity ρ(T) of ceramic La 0.9 Na 0.1 Mn 0.9 Cu 0.1 O 3 had two anomalies. This was attributed to a situation in which an exchange-induced modification in the carrier density and an anomaly of the magnetic scattering of carriers occurred at different temperatures

Linear-circular dichroism of drag current due to nonlinear intersubband absorption of light in p-type Ge

Abstract: A study has been made of the effect of the polarization state on linear absorption of radiation in the submillimeter range (λ = 90 μm) in p-type Ge crystals. The dependence of the current due to electron drag by photons on the intensity I of linearly and circularly polarized light has been measured. In both cases, it is observed that inversion of the photocurrent takes place with increasing intensity at I = 114 kW/cm 2 and I = 80 kW/cm 2 (at T = 300 K). The polarization dependences of the drag currents corresponding to two-photon transitions and the nonlinear current due to saturation of one-photon transitions are calculated. The theoretical results are then compared with experimental data, which makes it possible to identify the two contributions

Plasma excitations in low-dimensional systems

Abstract: Explicit dispersion laws of plasmons are obtained for various models of low-dimensional systems

Dislocation structure of epitaxial GaSb films grown on (001) GaAs substrates by molecular beam epitaxy

Abstract: An investigation has been made of the structural quality of heteroepitaxial GaSb films grown by molecular beam epitaxy on (001) GaAs substrates. The investigation was carried out using a range of diffraction methods, including three-crystal x-ray diffractometry, transmission electron microscopy, and x-ray topography. A network of misfit dislocations, elongated along directions and separated by an average distance of 5.5 nm, formed at the GaSb-GaAs interface. They were mainly pure edge Lomer dislocations with the Burgers vectors of the a/2 type and lying in the heteroboundary plane. The dislocation network relieved practically completely the misfit stresses

Pressure-temperature phase diagrams of elpasolites Rb 2 KB 3 + F 6 (B 3 + =Ho, Dy, Tb)

Abstract: The pressure-induced shifts of the temperatures T 0 of the phase transitions in elpasolites Rb 2 KB 3+ Fe 6 (B 3+ = Ho, Dy, Tb) are determined. The nature of the dependences T 0 (p) and the influence of the ordering of the K + and B 3+ atoms on these dependences are considered

Polarized photoluminescence of porous silicon

Abstract: Time and wavelength resolution was investigated to determine the degree of linear polarization of the photoluminescence spectra emitted by porous silicon. The linear polarization was retained for intervals lasting microseconds under conditions of high-energy nonresonant excitation with linearly polarized light. The contribution of the various radiative channels to the spectral dependences of the polarization was identified

Characteristics of the ESR spectrum near coincidence of the positions of transitions in Pb 5 Ge 3 O 11 :Gd 3 +

Abstract: A study is made of the reasons for the appearance of an additional ESR signal between two transitions of a trigonal Gd 3+ center in the direct proximity to a point of coincidence of their resonance fields. The results indicate averaging of some of the spin packets of two inhomogeneously broadened initial ESR signals as a result of interdoublet relaxation transitions

Deformation of polymethylemethacrylate after exposure to 60Co γ rays

Abstract: A study is made of the rate of creep and of the strain corresponding to the forced elasticity limit during compression of polymethylmethacrylate (PMMA) irradiated first with various γ-ray doses. Small doses, characterized by special features of the formation of macroradicals at 300 K, induce a state of PMMA with the most unstable deformation characteristics. A comparison of the effects of large (> 100 kGy) and small (5-50 kGy) γ-ray doses on the mechanical properties of PMMA leads to the conclusion that the critical radiation doses can differ significantly for different characteristics

Temperature dependence of the ac hopping conductivity

Abstract: The temperature dependence of the ac conductivity is calculated on the basis of a Debye analysis for disordered materials with strong spatial fluctuations of the carrier mobility. Charge transport occurs at the percolation level in the region of a density-of-states tail falling exponentially into the band gap. A theoretical law derived for σ(ω, T) is in good agreement with the published experimental results. A parameter T 0 representing the fall of the density of states in the tail increases logarithmically with the ac field frequency