Thermal conductivity of a moderate heavy-fermion compound YbZnCu4

The thermal conductivity κ and electrical resistivity ρ of a cast polycrystalline sample of YbZnCu₄, which belongs to the class of moderately heavy-fermion compounds, are measured and studied in the temperature range 5–300 K. It is shown that the phonon thermal conductivity of the sample follows an amorphous-like pattern throughout the temperature range under investigation, which should be assigned to the presence of Yb ions with a homogeneous mixed valence in this compound. The temperature dependence ρ(T) has two specific portions: a high-temperature portion (T > 220 K) characteristic of conventional metals and a moderate-temperature portion (14–35 K) typical of Kondo compounds.     

Thermal conductivity of the YbMgCu<Subscript>4</Subscript> “light” heavy-fermion system

The thermal conductivity and electrical resistivity of a sample of YbMgCu₄ belonging to “light” heavy-fermion compounds have been measured in the temperature range 5–300 K. The sample studied was in the region of homogeneity of this compound. It is shown that, throughout the temperature range studied, the phonon thermal conductivity of the sample has an amorphous-like character, which should be assigned to the homogeneous mixed valence of the Yb ion in YbMgCu₄.     

Heat conductivity of the heavy-fermion compound YbAgCu4

The heat conductivity and electrical resistivity of a polycrystalline YbAgCu₄ sample were measured in the 4.2–300-K temperature range. It is shown that at low temperatures (in the region corresponding to the coherent Kondo lattice) the Lorenz number behaves in accordance with a theoretical model developed for heavy-fermion materials.     

Thermopower of a moderate heavy-fermion compound YbZnCu4

The thermopower coefficients S of samples of a moderate heavy-fermion compound YbZnCu₄ and metallic LuZnCu₄ are measured in the temperature range 5–300 K. Data on the temperature dependence of the thermopower coefficient S of YbZnCu₄ suggest that this material is a heavy-fermion compound with a Kondo temperature of ～50 K.     

Thermopower of the light heavy-fermion compound YbMgCu4 in the region of its homogeneity

The thermopower coefficient S of the light heavy-fermion system YbMgCu₄ and, for comparison, the thermopower coefficient of metallic LuMgCu₄ are measured in the temperature range 5–300 K. It is shown that the YbMgCu₄ compound has a fairly broad region of homogeneity. The obtained data on the temperature dependence of the thermopower coefficient S for the YbMgCu₄ compound confirm that this compound belongs to heavy-fermion systems. The Kondo temperature of YbMgCu₄ is shown to depend on the unit cell parameter in the region of its homogeneity.     

Specific heat and velocity of sound in a moderate heavy-fermion compound YbZnCu4

The specific heat at a constant pressure (C _p) and the velocity of sound (v) are measured for a moderate heavy-fermion compound YbZnCu₄ in the temperature range 3.5–250 K and at 77 K, respectively. The experimental values of C _p and v obtained in this study and the phonon thermal conductivity previously measured in the temperature range 5–300 K are used to calculate the phonon mean free path l for this compound. The temperature dependence of the phonon mean free path l thus determined is characteristic of classical amorphous materials.     

Behavior of the Lorenz number in the light heavy-fermion system YbInCu4

The electrical resistivity and thermal conductivity of two polycrystalline YbInCu₄ samples prepared by different techniques at the Ioffe Physicotechnical Institute, RAS (St. Petersburg, Russia), and the Goethe University (Frankfurt-am-Main, Germany) are studied within the temperature range 4.2–300 K. At T _v～75–78 K, these samples exhibited an isostructural phase transition from a state with an integer valence (at T>T _v) to a state with an intermediate valence (at T<T _v) of the Yb ions. It is shown that at T<T _v; i.e., in the temperature range where YbInCu₄ is assumed to be a light heavy-fermion compound, the Lorenz number behaves as it should in a classical heavy-fermion system. At T>T _v, where YbInCu₄ is a semimetal, the Lorenz number has a value characteristic of standard metals.     

Thermoelectric properties of SnzCo8Sb24 skutterudites

Sn-filled CoSb₃ skutterudite compounds were synthesized by the induction melting process. Formation of a single δ-phase of the synthesized materials was confirmed by X-ray diffraction analysis. The temperature dependences of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined in the temperature range of 300-700 K. Positive Seebeck and Hall coefficients confirmed p-type conductivity. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled CoSb₃ skutterudite is a degenerate semiconductor. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The lowest thermal conductivity was achieved in the composition of Sn_0.25Co₈Sb₂₄.     

Unusual behavior of the lattice thermal conductivity and of the Lorenz number in the YbIn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>4+<Emphasis Type="Italic">x</Emphasis></Subscript> system

Samples of various compositions were obtained in the homogeneity range of the Yb-In-Cu system (YbIn_1?xCu_4+x), from stoichiometric (YbInCu₄) to YbIn_0.905Cu_4.095. Their lattice constant (at 300 K and in the range 20–100 K), total thermal conductivity, and electrical resistivity (from 4 to 300 K) were measured. All the compositions studied exhibited an isostructural phase transition at T _v ?40–80 K driven by a change in the Yb ion valence state. It was shown that within the YbIn_1?xCu_4+x homogeneity range, the lattice thermal conductivity κ_ph decreases with increasing x; at T>T _v , κ_ph grows with temperature and the Lorenz number (which enters the Wiedemann-Franz law for the electronic component of thermal conductivity) of the light heavy-fermion system, to which YbIn_1?xCu_4+x belongs for T<T _v , behaves as it does in classical heavy-fermion systems. Thermal cycling performed through T _v generates stresses in the YbIn_1?xCu_4+x lattice, which entails an increase in the electrical resistivity and a decrease in the thermal conductivity. “Soft anneal” (prolonged room-temperature aging of samples) makes the effect disappear. A conclusion is drawn as to the nature of the effects observed.     

Yb-Based Kondo Lattices: Insight from 170Yb Mössbauer, Positive Muon and γ–γ Perturbed Angular Correlations Spectroscopies

The usefulness of local hyperfine techniques in the investigation of the physical properties of Kondo lattices is illustrated through three examples. First, we show how Mössbauer spectroscopy on ¹⁷⁰Yb down to very low temperature (0.025 K) provides evidence for the existence of an incommensurate modulated magnetic structure close to T=0 in YbPtAl; a modulated structure at T=0 is in principle forbidden for Kramers ions, but it is made possible in YbPtAl due to the presence of the Kondo coupling. Second, we present a μSR study of the Kondo insulator YbB₁₂, and we evidence the persistence of fluctuations of very small correlated Yb moments close to T=0. Third, we report on a study by Perturbed Angular Correlation spectroscopy on the isotope ¹⁷²Yb of the Kondo lattice Yb₂Co₃Ga₉, which is characterised by a high Kondo temperature (T ₀?260 K). Our aim was to compare, on a large temperature range, the thermal variations of the 4f quadrupole moment and of the magnetic susceptibility, to check whether the scaling property predicted by theory could be observed.     

Thermal conductivity and heat capacity of LuMgCu<Subscript>4</Subscript>

This paper reports on the measurements of the thermal conductivity κ and electrical resistivity ρ in the temperature range 5–300 K and the heat capacity at constant pressure C _p in the range 80–300 K for the metallic nonmagnetic compound LuMgCu₄. The experimental values of κ and C _p for the LuMgCu₄ compound are compared with the corresponding data available in the literature for the light heavy-fermion compound YbMgCu₄. It is shown that, in the low-temperature range (5–20 K), the phonon thermal conductivity κ_ph of YbMgCu₄ is lower than κ_ph of LuMgCu₄ as a result of phonon scattering from magnetic moment fluctuations of the Yb 4f electrons and, conversely, the heat capacity of LuMgCu₄ in the range 80–300 K is lower than that of YbMgCu₄ because the heat capacity of the latter compound has an additional magnetic component.     

C-axis thermoelectric transport in low stage SbCl5 graphite intercalation compounds

c-axis thermal conductivity, electrical resistivity and thermopower measurements performed on stages-2, 3 and 4 SbCl₅-graphite intercalation compounds in the temperature range 3 < T < 300 K are reported. Contrary to the electrical resistivity and thermopower data, the temperature variation of the thermal conductivity is qualitatively different from that previously observed on other intercalation compounds.     

Heat capacity and velocity of sound in the YbMgCu<Subscript>4</Subscript> “light” heavy-fermion system

This paper reports on a measurement of the heat capacity at constant pressure (C _p ) in the temperature range 3–320 K and the sound velocity (v) at 77 K for the “light” heavy-fermion compound YbMgCu₄. The present experimental data on C _p and v of YbMgCu₄, combined with our earlier phonon thermal conductivity data for YbMgCu₄ in the range 5–300 K, have been used to calculate the phonon mean free path l in this compound. The temperature dependence of l obtained is found to be characteristic of classical amorphous materials.     

Transport phenomena in superionic Na<Subscript><Emphasis Type="Italic">х</Emphasis></Subscript>Cu<Subscript>2 − <Emphasis Type="Italic">х</Emphasis></Subscript>S (<Emphasis Type="Italic">х</Emphasis> = 0.05; 0.1; 0.15; 0.2) compounds

The electronic and ionic conductivity, the electronic and ionic Seebeck coefficients, and the thermal conductivity of Na _x Cu_2 ? x S (x = 0.05, 0.1, 0.15, 0.2) compounds were measured in the temperature range of 20–450 °С. The total cationic conductivity of Na_0.2Cu_1.8S is about 2 S/cm at 400 °С (the activation energy ≈ 0.21 eV). Over the studied compounds, the composition Na_0.2Cu_1.8S has the highest electronic conductivity (500–800 S/cm) in the temperature range from 20 to 300 °С, and the highest electronic Seebeck coefficient (about 0.2 mV/K) in the same temperature range is observed for Na_0.15Cu_1.85S composition; the electronic Seebeck coefficient increases abruptly above 300 °С for all compounds. The thermal conductivity of superionic Na_0.2Cu_1.8S is low, which causes high values of the dimensionless thermoelectric figure of merit ZT from 0.4 to 1 at temperatures from 150 to 340 °С.     

Non-Fermi liquid behavior in polycrystalline Ce2PdIn8

We report on the formation of a novel ternary compound Ce₂PdIn₈ that is isostructural with the heavy-fermion superconductors Ce₂CoIn₈ and Ce₂RhIn₈. Its magnetic, electrical transport and thermodynamic properties were studied on polycrystalline samples in wide ranges of temperature and magnetic field strength. The results revealed Ce₂PdIn₈ to be a paramagnetic Kondo lattice with a coherence temperature of about 12 K. The C/T ratio of the specific heat reaches at 350 mK a strongly enhanced magnitude of about per Ce-atom, thus clearly indicating a heavy-fermion nature of this material. Moreover, a logarithmic divergence of C/T vs. T, observed below 3 K, which is accompanied by a linear temperature dependence of the electrical resistivity below 6 K, hint at a non-Fermi liquid character of the electronic ground state in the new compound reported.     

Effect of substitution of Ce by La in the CeNixPt1 − x dense Kondo ferromagnets

The effects of substitution of Ce by La in the orthorhombic CeNi_{1 − x}Pt_x dense Kondo ferromagnets are studied by means of magnetization and electrical resistivity measurements. A decrease of the exchange RKKY interactions leads to a decrease of the Curie temperature T_c as a function of the La content and hence to an enhancement of the Kondo character in the thermal dependence of the resisitivity. However, the Ce moment is almost independent of the La amount. The Kondo temperature being also independent, this surprising result seems in contradiction with the available Kondo lattice models.     

Volume collapse in the Kondo lattice

The α-γ transition of Ce and its compounds are explained within a compressible Kondo lattice model where the variation of |J|/D with volume is taken into account. We show that, contrary to the valence change model, the Kondo contribution is sufficient to induce a first order transition at low temperature from a magnetic to a Kondo phase. The disappearance of magnetism is then related to an extremely high Kondo temperature. Applications to Ce and CeAl₂ cases are given.     

Effect of pressure on the temperature dependence of the thermal conductivity of InSb

The dependence of the thermal conductivity of indium antimonide on temperature (in the range 300–450 K) and hydrostatic pressure (up to 0.4 GPa) has been investigated. It is shown that the phonon thermal conductivity λ_ph obeys the law T ^?n (n ≥ 1). Hydrostatic pressure affects the magnitude and temperature dependence of the thermal conductivity of InSb: with an increase in pressure, the thermal conductivity increases, while the parameter n in the dependence λ_ph ≈ T ^?n decreases.     

Anomalous ferromagnetism and non-Fermi-liquid behavior in the Kondo lattice CeRuSi<Subscript>2</Subscript>

The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi₂ are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and μSR measurements. Data on specific heat, resistivity, heat conductivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, a non-Fermi-liquid behavior is observed at low temperatures, which is unusual for a ferromagnetic Kondo system. A comparison with other magnetic Kondo lattices is performed.