On the effective Debye temperatures of the C60 fullerite

The effective Debye temperatures Θ_eff determined for solids by different physical methods have been analyzed and compared. Attention has been focused on the original parameter of the Debye theory of heat capacity, i.e., the translational calorimetric Debye temperature Θ _c ^t (0), and the X-ray Debye temperature Θ _x in the framework of the Debye-Waller theory for the C₆₀ fullerite. It has been established that the true Debye law T ³ is satisfied for the C₆₀ fullerite over a very narrow range of temperatures: 0.4 K ≤ T ≤ 1.8 K. For this reason, the experimental Debye temperatures Θ _c ^t (0) obtained for the C₆₀ fullerite by different authors in the range T > 4.2 K are characterized by a large scatter (by a factor of ∼5). It has been revealed that the value Θ _c ^t (0) = 77.12 K calculated in this paper with the use of the six-term Betts formula from the harmonic elastic constants $ \tilde C_{ijkl} $ \tilde C_{ijkl} of the C₆₀ single crystal in the limit T = 0 K is closest to the true Debye temperature. It has been demonstrated using the method of equivalent moments that the real spectral frequency distribution of translational lattice vibrations g(ω) for the C₆₀ fullerite deviates from a parabolic distribution. The effective Debye temperatures Θ_eff involved in applied problems of thermodynamics of crystals and elastic scattering of different radiations from lattice vibrations have been determined. The quantitative measure of anharmonicity of translational and librational lattice vibrations of the C₆₀ fullerite has been determined. This has made it possible to empirically evaluate the lattice thermal conductivity κ of the C₆₀ fullerite at T ≈ 300 K: κ(300) = 0.80 W (m/K), which is in good agreement with the experimental thermal conductivity κ_exp = 0.78 W (m/K) at T ≈ 250 K.     

Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity κ and electrical resistivity ρ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400°C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at T _carb = 1000 and 2400°C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 κ(T) are determined for the samples cut along and across the tree growth direction. The thermal conductivity κ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to T, to transfer subsequently to ∼T ^1.5 scaling. The results obtained are analyzed.     

Magnetic and thermal properties of CuFeS<Subscript>2</Subscript> at low temperatures

The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS₂, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS₂ sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.     

Thermal conductivity of the spin-Peierls compound CuGeO3

The thermal conductivity of the pure and zinc-doped spin-Peierls compound CuGeO₃ is investigated for the first time. Characteristic features reflecting the changes in the phonon spectrum of the crystals are observed in the k(T) curves at a transition into the dimerized state in CuGeO₃ at T _sp =14.2 K and in Cu_0.98Zn_0.02GeO₃ at T _sp =10.6 K. Near the spin-Peierls transition temperature T _sp the thermal conductivity of Cu_0.98Zn_0.02GeO₃ (k=3 W/m·K) is much less than the thermal conductivity of CuGeO₃ (k=21.5 W/m·K). Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 823–826 (25 December 1997)     

Characteristic features of the low-temperature thermal conductivity of highly-enriched and natural germanium

Experimental data on the thermal conductivity K(T) of natural and highly enriched (99.99%) Ge⁷⁰ crystals with ground and polished surfaces are analyzed in the temperature interval ∼2–8 K. In all samples, the boundary scattering mechanism predominates in the interval from 2 to 4.0 K. As temperature increases, in highly enriched samples N processes start to contribute to phonon transport and the behavior of K(T) corresponds to viscous Poiseuille flow of a phonon gas. The isotopic scattering mechanism plays a large role in isotopically nonideal samples. Fiz. Tverd. Tela (St. Petersburg) 40, 1604–1607 (September 1998)     

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)     

Thermal conductivity of (VO)2P2O7 single crystals

Thermal conductivity ϰ of single-crystal (VO)₂P₂O₇ has been studied within the 4–300 K range. A break was found in the ϰ(T) relation about 200 K, in the region of the transition from diffuse antiferromagnetic ordering (200–4 K) to a classical paramagnet (T=200–300 K). In the low-temperature domain (4–200 K), one may expect an additional contribution to ϰ(T) from the magnon component of thermal conductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 2093–2094 (November 1998)     

Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

Temperature dependence of surface impedance of Tl2Ba2CaCu2O8−δ and YBa2Cu3O6.95 single crystals measured in the microwave band

The real part R _s and the imaginary part X _s of the surface impedance Z _s=R _s+ iX _s of Tl₂Ba₂CaCu₂O_8−δ and YBa₂Cu₃O_6.95 single crystals have been measured with high precision at frequency ω/2π=9.4 GHz in the temperature range 0<T<140 K. In the Tl₂Ba₂CaCu₂O_8−δ crystal a linear temperature dependence R _s(T) has been found for T⩽50 K, and the magnetic field penetration depth λ(4.2 K)=X _s(4.2 K)/ω μ ₀≈3760 Å has been measured. Along with well known features of the function Z _s(T) in high-quality YBa₂Cu₃O_6.95 single crystals, such as the linearity of λ(T) and R _s(T) for T<T _c/3 and a maximum of R _s(T) at T∼T _c/2, the linearity range of λ(T) extends to T≃50 K, and this curve has a plateau in the range 60<T<85 K. The curve of R _s(T) in both the superconducting and normal states of YBa₂Cu₃O_6.95 is well described by a two-fluid model with the electron-phonon mechanism of quasiparticle relaxation. A formula describing the curve of λ ²(0)/λ ²(T) throughout the studied temperature range is also given. Zh. éksp. Teor. Fiz. 112, 2210–2222 (December 1997)     

Magnetism,exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

FePS₃ is a layered antiferromagnet (T _N=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate⁵ T _2g ground state of the Fe²⁺ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σ_i [δ(L _iz ² −2)+|λ|L _i .S _i ]−Σ _ij J _ij S _i .S _j . The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,J _ij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;J _nn k=27.7 K; andJ _nnn k=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aT _N=122 K for FePS₃, which is remarkably close to the observed value of theT _N. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS₃, for which the high temperature series expansion susceptibility is available.     

Cathodoluminescence of ZnSe(Bi):Al crystals

The cathodoluminescence (CL) in ZnSe crystals annealed at T=1200 K in a Bi melt containing an aluminum impurity is investigated. The spectra are recorded for different excitation levels, temperatures, and detection delay times t ₀. As t ₀ is increased, the intensity of the orange band at λ _max=630 nm (1.968 eV) in the CL spectrum decreases in comparison to the intensity of the dominant yellow-green band at λ _max=550 nm (2.254 eV), whose half-width increases in the temperature range 6–120 K and then decreases as the temperature increases further. It is shown that such behavior of the yellow-green band is caused by the competition between two processes: recombination of donor-acceptor pairs and of free electrons with holes trapped on acceptors. The former mechanism is dominant at low temperatures, and the latter mechanism is dominant at high temperatures. At T∼120 mK the contributions of the two mechanisms to the luminescence are comparable. The resultant structureless band then achieves its greatest half-width, which is dictated by the interaction of the recombining charge carriers with longitudinal-optical and longitudinal-acoustic phonons and with the free-electron plasma. The mean number of longitudinal-optical phonons emitted per photon is determined mainly by their interaction with holes trapped on deep acceptors in the form of Al atoms replacing Se. The donor in the pair under consideration is an interstitial Al atom. Fiz. Tverd. Tela (St. Petersburg) 39, 1526–1531 (September 1997)     

Temperature dependences of the high-field magnetization of dilute frustrated ferrimagnetic spinels

Results are presented of an investigation of the magnetic properties of dilute frustrated ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄ (x=0.8–1.2), which characterize the main parameters of the ferrimagnetic state and provide evidence of local violation of collinear spin ordering and frustrations. In particular, measurements were made of the concentration dependences of the magnetic moment n ₀(x) and the Curie point T _c (x), the magnetization isotherms σ _T (H) at T=4.2 K and H⩽10 kOe, and also the low-and high-field magnetization polytherms σ _H (T). It was established that for x⩾0.8 in fields exceeding the technical saturation field H _s ∼2 kOe, the temperature dependences of the high-field magnetization σ _H (T) between 4.2 and 230 K cannot be described by the Bloch T ^3/2 law whereas this law is satisfied for undiluted Li spinel (x=0). Over the entire temperature range (4.2–230 K) the experimental curves σ _H (T) may be approximated by σ _H (T)=σ ₀(1−AT ^3/2 − BT ^5/2) for x=0.8–1.0 and σ _H (T)=σ ₀[1−CT ^3/2exp(μ(H−H ₀)/k _B T)] for x=1.1, 1.2, where μH ₀∼15 K is the internal field produced by competition between exchange interactions and frustrations. Fiz. Tverd. Tela (St. Petersburg) 40, 1075–1079 (June 1998)     

Paramagnetic and spin-glass properties of pyrochlore-like oxides Ln2Mn2/3Mo4/3O7 (Ln=Sm, Gd, Tb, or Y)

The magnetic properties of complex oxides Ln ₂Mn_2/3Mo_4/3O₇ (Ln=Sm, Gd, Tb, or Y) with a pyrochlore-type structure are studied in the temperature range 2–300 K. For all compounds in the paramagnetic state, the temperature dependence of the magnetic susceptibility is described by a generalized Curie-Weiss law with a temperature-independent component of ∼10⁻⁶ cm³/g and with a Weiss constant Θ<0 and |Θ|<16 K. At low temperatures (T<10–12 K), the compounds have spin-glass properties; they exhibit magnetic and temperature hysteresis and the typical dependences of the imaginary and real parts of the dynamic magnetic susceptibility on temperature and the frequency of an ac magnetic field in a wide range of magnetization relaxation times. The data obtained suggest that d electrons are responsible for the formation of frustrated exchange interactions in the compounds and that 4f electrons in the compounds with Sm or Tb provide strong magnetic-anisotropy effects. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 2, 2004, pp. 287–295. Original Russian Text Copyright ? 2004 by Korolev, Bazuev.     

Low-temperature transport properties of UCu5

The electrical resistivity (T) and the thermal conductivity (T) have been measured for UCu₅ in the temperature range between 0.02 and 20 K. Two distinct anomalies in (T) are due to previously established phase transitions at approximately 15 and 1 K, respectively. They indicate considerable changes in the electronic structure of this compound, implying sizeable truncations of the Fermi surface with decreasing temperature at both transitions. In almost the entire covered temperature range the thermal conductivity is dominated by phonon contributions. Its temperature dependence is fairly well reproduced by a calculation considering phonon scattering by electrons and by point defects. At very low temperatures, asT approaches 0 K, the Wiedemann-Franz law _e L ₀ T, where _e is the electronic part of (T) andL ₀ is the Lorenz number, is almost perfectly fulfilled.     

Thermopower in quasi-two-dimensional (BEDT-TTF) m X n organic conductors

Thermopower of (BEDT-TTF) _m X _n organic conductors has been studied using a dedicated measurement technique in the temperature range of 4.2 to 300 K. It turned out that some features of the thermopower in quasi-two-dimensional metals, namely the presence of a peak in the thermopower of α-(BEDT-TTF)₂MHg(SCN)₄ and a plateau in κ-(BEDT-TTF)₂Cu(NCS)₂ in the temperature interval between 10 and 50 K, are probably due to the phonon drag effect. Similar temperature dependences of the Seebeck coefficient can be satisfactorily interpreted in terms of a simple model taking into account the real experimental curve of the phonon heat capacity versus temperature, C ∝ T ², which is not described by the Debye formula. One feature distinguishing organic superconductors from magnetically ordered metals is a stronger temperature dependence of the characteristic electron-phonon scattering time τ _e-ph(T). Phonon drag effects also determine the behavior of the thermopower in the (BEDT-TTF)₃Cl₂·2H₂O organic conductor, which is characterized by a metal-insulator transition at T∼150 K. An analysis of measurements of the conductivity and thermopower vs. temperature taken together indicates that the transition in this compound has a complex nature: first (at T∼150 K) a metal-insulator transition occurs, which produces an energy gap in the band spectrum, then at a lower temperature (T∼20 K) a transition to a charge-density wave state takes place. Zh. éksp. Teor. Fiz. 113, 323–338 (January 1998)     

Effect of γ irradiation on the low-temperature electrical conductivity and dielectric properties of TlGaSe2 crystals

Electrical conductivity and dielectric properties of single-crystal TlGaSe₂ have been studied as a function of γ irradiation dose in the 100–280 K range including the existence of an incommensurate phase. Anomalies in the form of maxima have been observed in the σ=f(T), tan δ=f(T), and ɛ=f(T) curves at the points of transition from the paraphase to incommensurate (IC) phase, T _i, and from the IC to commensurate phase, T _c. The increase in the quantities σ, tan δ, and ɛ observed initially with increasing irradiation dose is followed by their strong decrease and disappearance of the anomalies. It has been established that γ irradiation does not affect the phase transition temperatures T _i and T _c. Fiz. Tverd. Tela (St. Petersburg) 40, 1328–1331 (July 1998)     

Anomalies in the microwave conductivity of a polycrystalline C60 membrane

A study is reported of an anomaly in the microwave conductivity of a polycrystalline C₆₀ membrane at T _c=260 K (the transition width is 30 K). Raman scattering measurements indicate that the sample is the C₆₀ fullerite without any signs of graphitization, amorphous phase, or the presence of C₇₀, and that the detected microwave conductivity jump can be unambiguously identified as due to the C₆₀ phase. Fiz. Tverd. Tela (St. Petersburg) 40, 577–579 (March 1998)     

Theoretical analysis of the thermal conductivity of high-temperature superconductors

Summary The thermal conductivity of YBa₂Cu₃O_7−δ high-T _c superconductor is analysed self-consistently on both normal and superconducting states on the base of the Bardeen-Rickayzen-Tewordt extended theory to take into account the effects of magnetic field and superconducting fluctuations. It is shown that experimental data are in a quantitative agreement with theory even if the number and variation intervals of adjustable parameters are substantially reduced in comparison with previous works. Phonon relaxation rates due to different mechanisms of phonon scattering as well as the parameters of electron-phonon interaction are estimated. It is shown that thermal conductivity in YBa₂Cu₃O_7−δ is consistent with the BCS model with intermediate electron-phonon coupling λ=1–3 the phonon-electron and electron-phonon relaxation times near critical temperature are evaluated to be 10⁻¹⁰s and 10⁻¹²s, respectively.     

Excess 1/f noise in systems with an exponentially wide spectrum of resistances and dual universality of the percolation-like noise exponent

The excess 1/f noise in a random lattice with bond resistances r∼exp(−λx), where x is a random variable and λ≪1, is studied theoretically. It is shown that if the correlation function {δr ²}∼r r ^θ+2, then the relative spectral density of the noise in the system is expressed as C _e∼λ^m exp(−λ(1−p _c)), where p _c is the percolation threshold and m=νd (ν is the critical exponent of the correlation length and d is the dimensionality of the problem). It is hypothesized that the exponent m possesses a dual universality: It is independent of 1) the geometry of the lattice and 2) the θ-mechanism responsible for the generation of the local noise. Numerical modeling in a three-dimensional lattice gives m=52.3 for θ=1 and θ=0, in agreement with the hypothesis. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 614–618 (25 April 1996)     

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.