Effect of isotopic disorder on the thermal conductivity of germanium in the region of the maximum

This paper discusses the question of how isotopic disorder affects the position of the thermal-conductivity maximum of germanium. The discussion is in terms of a Callaway-type model. Experimental data on the thermal conductivity of a natural Ge crystal and of highly enriched Ge⁷⁰ crystals are analyzed. Fiz. Tverd. Tela (St. Petersburg) 41, 1185–1189 (July 1999)     

Low-temperature thermal conductivity of highly enriched and natural germanium

Experimental data on the thermal conductivity K(T) of crystals of natural and highly enriched germanium (99.99%) ⁷⁰Ge with lapped and polished surfaces are analyzed in the temperature range ∼1.5–8 K. In all the samples in the temperature range ∼1.5–4 K the standard boundary mechanism of scattering dominates. As the temperature is raised, an isotopic scattering mechanism is observed in the natural samples. In the highly enriched samples the theoretical values of K(T) turn out to be much smaller than the experimental ones. It is conjectured that a Poiseuille viscous flow regime of the phonon gas emerges in this case. Zh. éksp. Teor. Fiz. 114, 1757–1764 (November 1998)     

Isotope effect in the thermal conductivity of germanium single crystals

The thermal conductivity of chemically, structurally, and isotopically highly pure germanium single crystals is investigated experimentally in the temperature range from 2 to 300 K. It is found that the thermal conductivity of germanium enriched to 99.99% ⁷⁰Ge is 8 times higher at the maximum than the thermal conductivity of germanium with the natural isotopic composition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 463–467 (25 March 1996)     

Observation of dynamical effects in diffuse neutron scattering by the three-crystal spectrometer method

Anomalous angular distributions of the diffuse elastic neutron scattering intensity, which indicate a dynamical character of the scattering near a reciprocal-lattice site, are discovered for low-dislocation-density Ge crystals. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 237–240 (25 February 1996)     

Raman scattering in germanium at high pressure: Isotope effects

The first-order Raman scattering in isotopically enriched samples of germanium ⁷⁰Ge, ⁷²Ge, and ⁷⁴Ge and germanium with the natural isotopic composition is investigated at high pressures. It is found that the isotopic dependence of the frequency of the LTO(Γ) mode in isotopically pure germanium samples can be described in the harmonic approximation (ν∝m ^−1/2). At the same time, the frequency of the LTO(Γ) mode of germanium of natural isotopic composition apparently contains a contribution due to isotopic disorder effects. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 211–214 (10 February 1999)     

Study of the local defect structure of CdTe-Ge crystals by microindentation

This paper demonstrates the effectiveness of the microindentation method as a way to monitor the quality of cadmium telluride crystals. Measurements of microhardness and the length of dislocation rays from the rosette around the indentation pit can clearly identify nonuniform distributions of structural defects, both along the direction of growth and along a diameter of a CdTe boule. The mobility of α dislocations is more sensitive to nonuniformities in the structure than the mobility of β dislocations and the microhardness. A qualitative correspondence is established between the microplasticity characteristics of CdTe:Ge and its germanium content: the hardness of the crystal is insensitive to changes in the Ge content in the range from 3×10¹⁶ to 1×10¹⁷ at/cm³, but increases sharply beginning at a concentration of ∼1×10¹⁷ at/cm³. The correlation between the value of the microhardness and the length of the dislocation rays from the rosette around the imprint of the indentor are analyzed. An estimate is obtained for the hardness of pure CdTe. Fiz. Tverd. Tela (St. Petersburg) 40, 264–268 (February 1998)     

Luminescence properties of calcium-iodide crystals

The effect of the conditions of preparation, temperature, and the action of x rays on the luminescence properties of calcium-iodide scintillation crystals is investigated. On the basis of the results of a study of the spectral characteristics of CaI₂ and CaI₂:H₂ crystals for optical and x-ray excitation in the temperature range 90–400 K, also taking into account the results of a study of the luminescence properties of CaI₂ crystals activated by Cl⁻, Br⁻, OH⁻, and Ca²⁺ impurities, it is suggested that the 236-nm band observed in the excitation spectra of crystals of calcium iodide may be caused by an uncontrollable hydrogen impurity. The luminescence of these crystals with maximum at 395 nm is ascribed to radiative recombination of excitons trapped at H⁻ ions. Zh. Tekh. Fiz. 69, 135–136 (January 1999)     

Evolution of dislocation structures having various orientations in strained single crystals of the alloy Ni3Ge

The dislocation structure of strained single crystals of Ni₃Ge with various orientations is investigated by electron microscopy. The evolution of the dislocation structure parameters is studied as a function of the degree of strain, temperature, and orientation of the single crystals. Analysis of the experimental dependences of the yield stress on the density of dislocations leads to certain conclusions about how various mechanisms for dislocation drag make temperature-dependent contributions to the deforming stress, and about the nature of the thermal hardening of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 40, 672–680 (April 1998)     

Multidetector device for the detection of coincidences of charged particles and γ rays

A 4π position-sensitive, axisymmetrical assembly of Si-Au charged-particle detectors is proposed, implemented, and tested on a beam of heavy ions; the dimensions and structure of the device are conducive to the organization of coincidences of charged reaction products with discrete γ rays emitted by the daughter nucleus and registered by a system of ultrapure Ge detectors. First results are obtained from an investigation of the reaction ⁵⁸Ni(¹⁶O,α2pγ)⁶⁸Ge at E ₀=74.5 MeV. Zh. Tekh. Fiz. 68, 139–142 (April 1998)     

Dielectric relaxation in Bi12SiO20 crystals

Anomalies of the dielectric properties of undoped and aluminum-and gallium-doped crystals of Bi₁₂SiO₂₀ are investigated in the frequency and temperature range ν=10²–10⁸ Hz and T=300–800 K. They are shown to be due to Debye relaxation processes and determined by the relaxor parameters. The mechanism of electron thermal polarization is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1223–1229 (July 1997)     

New approach to achieving the potentially high conductivity of polydiacetylene

It is postulated that only the surface layers of the structural formations are doped in single crystals of various polydiacetylenes because of the high packing density of their macromolecules and that this is the reason for the low level of conductivity attained in these conjugated polymers. For the purpose of raising the efficacy of the doping process by increasing the surface-to-volume ratio of the polymer, single crystals of the polydiacetylene poly-1,1,6,6-tetraphenylhexadiinediamine (poly-THD) are ground by various methods. A correlation between the particle size of the ground single crystals and their dc conductivity in the doped state is discovered. A record value of the conductivity (3×10⁻² S/cm) for polydiacetylenes, which is more than two orders of magnitude higher than published values (10⁻³–10⁻⁵ S/cm), is achieved. Fiz. Tverd. Tela (St. Petersburg) 39, 774–777 (April 1997)     

Nonlinear response of photorefractive lithium tantalate and niobate at acoustic frequencies

An amplification of the intensity of pump oscillations is observed experimentally at frequencies from 100 Hz to 1 kHz during photoinduced light scattering and holographic-type parametric scattering in photorefractive lithium tantalate and niobate. Possible ways are analyzed for explaining the existence of a photorefractive response in these crystals over times of 10⁻²–10⁻³ s, which are five orders of magnitude shorter than the Maxwell time. Zh. éksp. Teor. Fiz. 112, 1490–1498 (October 1997)     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

Luminescence of short-lived color centers induced in LiF crystals by a pulsed microwave discharge

A new phenomenon — intense luminescence of noncolored lithium fluoride (LiF) crystals excited by an electrodeless pulsed microwave discharge at the prebreakdown stage of development — is observed. This luminescence consists of the luminescence of short-lived aggregate F₂ and F ₃ ⁺ color centers at room temperature. It is shown that the density of short-lived color centers induced in the surface layer of LiF crystals by a microsecond microwave discharge reaches values of ∼10¹⁹−10²⁰ cm⁻³. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 163–167 (10 August 1997)     

Propagation of nonequilibrium phonons in ferroelectric ceramics and single crystals

Phonon transport in ferroelectric ceramics and single crystals has been experimentally investigated. Our measurements indicate that, in the temperature range 1.7–3.8 K studied, the effective phonon diffusion coefficient behaves as D _eff∞T ⁻⁵ in ferroelectrics with a broadened phase transition. This experimental dependence is in accord with the presence of a plateau in the thermal conductivity of such materials. The scattering by domain walls in BaTiO₃ single crystals has been identified, and our results are in quantitative agreement with calculations. Zh. éksp. Teor. Fiz. 115, 624–631 (February 1999)     

Micro-channel fabrication by femtosecond laser to arrange neuronal cells on multi-electrode arrays

The surface of a single-crystal germanium wafer was transformed to crystals of germanium fluorides and oxides upon exposure to a vapor of HF and HNO₃ chemical mixture. Structure analysis indicates that the transformation results in a germanate polycrystalline layer consisting of germanium oxide and ammonium fluogermanate with preferential crystal growth orientation in 〈101〉 direction. Local vibrational mode analysis confirms the presence of N–H and Ge–F vibrational modes in addition to Ge–O stretching modes. Energy dispersive studies reveal the presence of hexagonal α-phase GeO₂ crystal clusters and ammonium fluogermanates around these clusters in addition to a surface oxide layer. Electronic band structure as probed by ellipsometry has been associated with the germanium oxide crystals and disorder-induced band tailing effects at the interface of the germanate layer and the bulk Ge wafer. The acid vapor exposure causes Ge surface to emit yellow photoluminescence at room temperature.     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Low thermal expansion behavior and transport properties of Ni and Ge co-doped manganese nitride materials at cryogenic temperatures

A series of Ni and Ge co-doped manganese nitride materials were fabricated by mechanical ball milling followed by solid-state sintering. Their thermal expansion properties and electrical and thermal conductivities were investigated in the temperature range of 77–300 K. The results show that Ni and Ge co-doped manganese nitride materials have negative thermal expansion (NTE), and the operation-temperature window of NTE shifts toward the lower temperature region and the variation of linear thermal expansion (ΔL/L _(300K)) in the operation-temperature window of NTE decreases with increasing Ni content. The combination of these two factors results in a low coefficient of thermal expansion (CTE) at cryogenic temperatures. The average CTE of Mn₃(Cu_0.2Ni_0.4Ge_0.4)N drops to ‘zero’ in the temperature range of 190–77 K. The values of electrical and thermal conductivities of the Ni and Ge co-doped manganese nitride materials are in the ranges of 2–3×10³ (ohm cm)⁻¹ and 1.6–3.4 W (m K)⁻¹, respectively.     

Valence instability of uranium in U(Al1−x Gex)3

The electronic structure of U and Ge in the solid solutions U(Al_1−x Ge_x)₃ is investigated by measuring x-ray line shifts. It is shown that uranium has the mixed valence U³⁺ [Rn](5f ³)-U⁴⁺ [Rn](5f ²) over the entire composition range (0⩽x⩽1) and that the population of the uranium 5f shell increases by ∼0.28 5f electrons/U atom from UAl₃ (x=0) to UGe₃ (x=1). The electronic structure of Ge is close to the electronic structure of Ge metal over the entire composition range 0<x⩽1. No variation of the population of the Ge 4p shell is detected to within the experimental error (∼0.1 4p electrons/Ge atom) as the composition varies from x=0.2 to 1. It is established that the delocalization of a U 5f electron occurs as a result of its transition to the s or d band of the same uranium atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1505–1508 (September 1997)     

Physical properties of n-CdGeAs2 single crystals prepared by low-temperature crystallization

The first results obtained in studies of the temperature dependences of electrical conductivity and Hall constant of n-CdGeAs₂ single crystals prepared by low-temperature crystallization are reported. It has been established that the method developed permits growing single crystals with a free-electron concentration ⋍(1−2)×10¹⁸ cm⁻³ and a Hall mobility ⋍10000 cm²/(Vs) at T=300 K. It is shown that the temperature dependence of Hall mobility exhibits a behavior characteristic of electron scattering by lattice vibrations, whereas below 150 K a deviation from this law is observed to occur evidencing an increasing contribution of static lattice defects to scattering. The Hall mobility in the crystals prepared was found to reach ⋍36000 cm²/(Vs) at 77 K. Photosensitive heterojunctions based on n-CdGeAs₂ single crystals were prepared. The spectral response of the photosensitivity of these structures is analyzed. It is concluded that this method is promising for preparation of perfect CdGeAs₂ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 1190–1193 (July 1999)