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)     

Electrical conductivity of germanium with dislocation grids

Samples of n-type germanium with a donor concentration N _d=2.4×10¹⁶ cm⁻³ are plastically deformed to a degree of strain equal to 18–40% to detect static conduction by electrons trapped on dislocations in a system of dislocation grids. In samples with 20%<δ<31%, which retain an electronic type of conductivity, the conductivity for T<8 K, which is weakly temperature-dependent, is associated with conduction by electrons trapped on dislocations. The nonmonotonic dependence of the conductivity at 4.2 K on the degree of strain as the latter increases from 18% to 40% attests to the existence of an energy gap between the donor and acceptor dislocation states in strongly plastically deformed germanium. Zh. éksp. Teor. Fiz. 115, 115–125 (January 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)     

Effect of normal processes on thermal conductivity of germanium,silicon and diamond

The effect of normal scattering processes is considered to redistribute the phonon momentum in (a) the same phonon branch — KK-S model and (b) between different phonon branches — KK-H model. Simplified thermal conductivity relations are used to estimate the thermal conductivity of germanium, silicon and diamond with natural isotopes and highly enriched isotopes. It is observed that the consideration of the normal scattering processes involving different phonon branches gives better results for the temperature dependence of the thermal conductivity of germanium, silicon and diamond with natural and highly enriched isotopes. Also, the estimation of the lattice thermal conductivity of germanium and silicon for these models with the consideration of quadratic form of frequency dependences of phonon wave vector leads to the conclusion that the splitting of longitudinal and transverse phonon modes, as suggested by Holland, is not an essential requirement to explain the entire temperature dependence of lattice thermal conductivity whereas KK-H model gives a better estimation of the thermal conductivity without the splitting of the acoustic phonon modes due to the dispersive nature of the phonon dispersion curves.      

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)     

Thermal conductivity of <Emphasis Type="Italic">n</Emphasis>-type Ge monocrystals

The pyroelectric method is used to demonstrate the dependence of the thermal conductivity and thermal diffusivity coefficients of n-type germanium monocrystals on the concentration of antimony impurities (6 × 10¹³, 1.3 × 10¹⁴, 1.7 × 10¹⁴, 3.7 × 10¹⁴, 6 × 10¹⁴ cm^?3). The investigated samples are cut from germanium crystals grown from a melt using the Czochralski method with crystallographic orientation [111].     

Thermal conductivity of nonlinear waves in disordered chains

We present computational data on the thermal conductivity of nonlinear waves in disordered chains. Disorder induces Anderson localization for linear waves and results in a vanishing conductivity. Cubic nonlinearity restores normal conductivity, but with a strongly temperature-dependent conductivity κ(T). We find indications for an asymptotic low-temperature κ ∼ T ⁴ and intermediate temperature κ ∼ T ² laws. These findings are in accord with theoretical studies of wave packet spreading, where a regime of strong chaos is found to be intermediate, followed by an asymptotic regime of weak chaos (Laptyeva et al, Europhys. Lett. 91, 30001 (2010)).     

Monitoring of the thermal neutron flux in the EDELWEISS II dark matter direct search experiment

The results from measurements of thermal neutron flux in the EDELWEISS II experiment aimed at the direct detection of WIMPs (weakly interacting massive particles) by means of cryogenic germanium bolometers are described. Detailed knowledge of the neutron background is of crucial importance for the experiment, since neutrons with the MeV energy range of scattering seem to be hard to distinguish from the expected WIMP signal within the bolometers. Monitoring of the thermal neutron flux is performed using a mobile detection system with a low background proportional ³He counter. The neutron flux measurements were performed both outside and inside the device’s shielding, in the direct proximity of a cryostat with built-in germanium detectors. The sensitivity of the created thermal neutron detection system is on the level of 10⁻⁹ neutron (cm² s)⁻¹.     

Low-temperature diffusion of gold in germanium under the influence of atomic hydrogen

An investigation is made of the diffusion of gold in germanium under the influence of the energy released by the recombination of hydrogen atoms to form molecules. Crystals of n-type germanium with gold films (d=1×10⁻⁷ m) are exposed to atomic hydrogen for various times (up to 10⁴ s) at temperatures close to room temperature. The diffusion of gold in the germanium is analyzed by laser mass spectrometry, and also by measuring the surface resistance, the minority carrier lifetime, and the infrared transmission spectra. Mechanisms are proposed for the stimulation of heterodiffusion and accompanying processes. Zh. Tekh. Fiz. 69, 73–76 (July 1999)     

Thermal conductivity of the diamond-paraffin wax composite

The thermal conductivity of diamond-paraffin wax composites prepared by infiltration of a hydrocarbon binder with the thermal conductivity λ _m = 0.2 W m⁻¹ K⁻¹ into a dense bed of diamond particles (λ _f ∼ 1500 W m⁻¹ K⁻¹) with sizes of 400 and 180 μm has been investigated. The calculations using universally accepted models considering isolated inclusions in a matrix have demonstrated that the best agreement with the measured values of the thermal conductivity of the composite λ = 10–12 W m⁻¹ K⁻¹ is achieved with the use of the differential effective medium model, the Maxwell mean field scheme gives a very underestimated calculated value of λ, and the effective medium theory leads to a very overestimated value. An agreement between the calculation and the experiment can be provided by constructing thermal conductivity functions. The calculation of the thermal conductivity at the percolation threshold has shown that the experimental thermal conductivity of the composites is higher than this critical value. It has been established that, for the composites with closely packed diamond particles (the volume fraction is ∼0.63 for a monodisperse binder), the use of the isolated particle model (Hasselman-Johnson and differential effective medium models) for calculating the thermal conductivity is not quite correct, because the model does not take into account the percolation component of the thermal conductivity. In particular, this holds true for the calculation of the heat conductance of diamond-matrix interfaces in diamond-metal composites with a high thermal conductivity.     

Molecular-dynamics calculation of the thermal conductivity coefficient of the germanium single crystal

The thermal conductivity coefficient of the germanium crystal lattice has been calculated by molecular dynamics simulation. Calculations have been performed for both the perfect crystal lattice and the crystal lattice with defects such as monovacancies. For the perfect germanium single crystal, the dependence of the thermal conductivity coefficient on the lattice temperature has been obtained in the temperature range of 150–1000 K. The thermal conductivity coefficient of the germanium lattice has been calculated as a function of the monovacancy concentration.     

Subsurface layers with degenerate hole gas arising in germanium immersed into liquid helium under 60Co-gamma irradiation

Abstract

The subsurface layers with quasi-metal hole conductivity have been observed in n- and p-Ge immersed into liquid He under the ⁶⁰Co-gamma-irradiation. three conditions are to be fulfilled simultaneously for the production of the layers: low temperature, direct contact between helium and germanium and gammairradiation.     

Thermal expansion of crystal lattices of germanium with different isotopic compositions

The linear thermal expansion coefficient of crystal lattices of germanium with different isotopic compositions is analyzed. Fiz. Tverd. Tela (St. Petersburg) 40, 1829–1831 (October 1998)     

Normal-metal hot-electron bolometer with Andreev reflection from superconductor boundaries

This paper describes the design and experimental testing of a high-sensitivity hot-electron bolometer based a film of normal metal, exploiting the Andreev reflection from superconductor boundaries, and cooled with the help of a superconductor-insulator-normal metal junction. At the measured thermal conductivity, G≈6×10⁻¹² W/K, and a time constant of τ=0.2 μs, and a temperature of 300 mK, the estimated noise-equivalent power NEP=5×10⁻¹⁸ W/Hz^1/2, assuming that temperature fluctuations are the major source of noise. At a temperature of 100 mK, the thermal conductivity drops to G≈7×10⁻¹⁴ W/K, which yields NEP=2×10⁻¹⁹ W/Hz^1/2 at a time constant of τ=5 μs. The microbolometer has been designed to serve as a detector of millimeter and FIR waves in space-based radio telescopes. Zh. éksp. Teor. Fiz. 115, 1085–1092 (March 1999)     

Thermal conductivity and thermal Hall effect in Bi- and Y-based high-T c superconductors

Measurements of the thermal conductivity (k_xx) and the thermal Hall effect (k_xy) in high magnetic fields in Y- and Bi-based high-T _c superconductors are presented. We describe the experimental technique and test measurements on a simple metal (niobium). In the high-T _c superconductors k_xx and k_xy increase below T _c and show a maximum in their temperature dependence. k_xx has contributions from phonons and quasiparticle (QP) excitations, whereas k_xy is purely electronic. The strong increase of k_xy below T _c gives direct evidence for a strong enhancement of the QP contribution to the heat current and thus for a strong increase of the QP mean free path. Using k_xy and the magnetic field dependence of k_xx we separate the electronic thermal conductivity ( k _xx ^el ) of the CuO ₂ -planes from the phononic thermal conductivity ( k _xx ^ph ). In YBa₂Cu₃O _{7 - δ} k _xx ^el shows a pronounced maximum in the superconducting state. This maximum is much weaker in Bi₂Sr₂CaCu₂O _{8 + δ} , due to stronger impurity scattering. The maximum of k _xx ^el is strongly suppressed by a magnetic field, which we attribute to the scattering of QPs on vortices. An additional magnetic field independent contribution to the maximum of k_xx occurs in YBa₂Cu₃O _{7 - δ} , reminiscent of the contribution of the CuO-chains, as determined from the anisotropy in untwined single crystals. Our data analysis reveals that below T _c as in the normal state a transport (τ) and a Hall ( ) relaxation time must be distinguished: The inelastic (i.e. temperature dependent) contribution to τ is strongly enhanced in the superconducting state, whereas displays the same temperature dependence as above T _c . We determine also the electronic thermal conductivity in the normal state from k_xy and the electrical Hall angle. It shows an unusual linear increase with temperature. Received 23 August 2000     

Filler-matrix thermal boundary resistance of diamond-copper composite with high thermal conductivity

A composite material with a high thermal conductivity is obtained by capillary infiltration of copper into a bed of diamond particles of 400 μm size, the particles having been pre-coated with tungsten. The measured thermal conductivity of the composite decreases from 910 to 480 W m⁻¹ K⁻¹ when the coating thickness is increased from 110 to 470 nm. Calculations of the filler/matrix thermal boundary resistance R and the thermal conductivity of the coating layer λ _i using differential effective medium, Lichtenecker’s and Hashin’s models give similar numerical values of R and λ _i ≈ 1.5 W m⁻¹ K⁻¹. The minimal thickness of the coating h ∼ 100 nm necessary for ensuring production of a composite while maximizing its thermal conductivity, is of the same order as the free path of the heat carriers in diamond (phonons) and in copper (electrons). The heat conductance of the diamond/tungsten carbide coating/copper interface when h is of this thickness is estimated as (0.8–1) × 10⁸ W m⁻² K⁻¹ and is at the upper level of values characteristic for perfect dielectric/metal boundaries.     

Effect of isotopic composition on the linear thermal expansion coefficient of a germanium crystal lattice

Features of the thermal expansion coefficient α(T) of crystal lattices with different isotopic compositions have been analyzed. The case of germanium lattices has been studied in detail. Zh. éksp. Teor. Fiz. 114, 654–668 (August 1998)     

Thermal conductivity of solid cyclohexane in orientationally ordered and disordered phases

Thermal conductivity Λ _P of solid cyclohexane is measured at a pressure P = 0.1 MPa in the temperature range from 80 K to the melting point, which covers the ranges of low-temperature orientationally ordered phase II and high-temperature orientationally disordered phase I. Thermal conductivity Λ _V is measured at a constant volume in orientationally disordered phase I. The thermal conductivity measured at atmospheric pressure decreases with increasing temperature as Λ _P ∝ T ^−1.15 in phase II, whereas Λ _P ∝ T ^−0.3 in phase I. As temperature increases, isochoric thermal conductivity Λ _V in phase I increases gradually. The experimental data are described in terms of a modified Debye model of thermal conductivity with allowance for heat transfer by both phonons and “diffuse” modes.     

Isotope effect for the thermal expansion coefficient of germanium

The first experimental and theoretical investigation of the difference in the temperature behavior of the linear expansion coefficients of single crystals grown from isotopically highly enriched and natural germanium is reported. A comparison of the data for ⁷⁰Ge and ^natGe crystals reveals the significant influence of isotopic composition over a wide range of temperatures 30–230 K. Zh. éksp. Teor. Fiz. 115, 243–248 (January 1999)     

Nonlinear percolation conductivity and negative differential resistivity in microcrystalline PbTe layers with an adjustable potential well

The effect of electric fields on the electrical conductivity of PbTe films with block sizes smaller than the Debye screening length is studied. As the temperature is varied, a readjustment of the potential well is observed due to thermal spread of barriers with height ϕkT and the expansion of higher barriers. Spatial ensembles, which consist of several blocks that increase rapidly with temperature, are established for each T. This process leads to an increase in the height of the potential barriers as the linear size of these ensembles increases. This determines the potential well in these films and their nonlinear properties, which originate in the nonlinear percolation conductivity of a microscopic crystalline system with intergranular barriers. A comparison with the experimental data of Shklovskii shows that the scale length of the spatial inhomogeneity a=3.7×10⁻⁶ cm at T=4.2 K corresponds to the average block size. The value of a increases with temperature, reaching 5×10⁻⁴ cm at T=240 K. This mechanism for electrical conductivity is compared with the hopping conductivity with a variable hopping length. The negative differential resistance in the structures examined here is found to be electrothermal in nature. Zh. éksp. Teor. Fiz. 116, 276–298 (July 1999)