R. Wawryk

Correlated Electron Phenomena in Ce- and Pr-based Filled Skutterudite Arsenides and Antimonides

In order to elucidate the variety of correlated electron phenomena found in filled skutterudite compounds, experiments were performed on single crystals of ternary Ce- and Pr-based filled skutterudite arsenides LnT 4 As 12 (Ln = Ce, Pr; T = Fe, Ru, Os) and pseudoternary Pr-based skutterudite antimonides Pr(Os 1- x Ru x ) 4 Sb 12 and Pr 1- x Nd x Os 4 Sb 12 . For example, the intermediate valence compound CeRu 4 As 12 exhibits non-Fermi liquid characteristics, while the compounds PrT 4 As 12 display ferromagnetic order (18 K) for T = Fe, BCS superconductivity (2.4 K) for T = Ru, unknown order (2.3 K) and antiferromagnetic order (2.2 K) for T = Os. PrOs 4 As 12 appears to be a Kondo lattice system with a small Kondo temperature (∼ 1 K) and an electronic specific heat coefficient γ∼1 J/mol K 2 . The T – x phase diagrams of Pr(Os 1- x Ru x ) 4 Sb 12 and Pr 1- x Nd x Os 4 Sb 12 reveal regions of superconductivity with different order parameter symmetry and magnetic order.

Crystal Growth and Properties of the Filled Skutterudite Arsenides

Single crystals of LaOs 4 As 12 , LaOs 4 As 12 , LaOs 4 As 12 , LaOs 4 As 12 PrFe 4 As 12 , PrRu 4 As 12 , PrOs 4 As 12 , and ThFe 4 As 12 were grown from high purity elemental specimens by mineralization in a Cd : As flux. The process was carried out over 3–4 weeks in a sealed quartz ampoule with pressures of 15–40 atm. The elevated pressures were produced by As 4 vapor and Ar gas at the mineralization temperatures between 750–825 °C. To avoid disintegration, the ampoule was heated in a custom built pressure cell. After the synthesis and mineralization were completed, the Cd : As flux components were removed by sublimation and repeatedly used for subsequent crystallizations, thus avoiding dangerous environmental contamination. The morphology and various physical properties of the single crystal filled skutterudite arsenides are presented.

Non-Fermi liquid behavior in the filled skutterudite compound CeRu4As12

Electrical resistivity ρ, specific heat C, and magnetic susceptibility χ measurements made on the filled skutterudite CeRu4As12 reveal non-Fermi liquid (NFL) T dependences at low T, i.e., ρ(T) ∼ T and weak power law or logarithmic divergences in C(T)/T and χ(T). Measurements also show that the T dependence of the thermoelectric power S(T) deviates from that seen in other Ce systems. The NFL behavior appears to be associated with fluctuations of the Ce valence between 3 and 4 rather than a typical Kondo lattice scenario that would be appropriate for an integral Ce valence of 3. PACS numbers: 71.10.Hf, 71.28.+dElectrical resistivity ρ, specific heat C, and magnetic M measurements made on the filled skutterudite CeRu4As12 reveal non-Fermi liquid (NFL) temperature dependences at low T, i.e. ρ(T)~T1.4 and weak power law or logarithmic divergences in C(T)/T and M/H = χ(T). Measurements also show that the temperature dependence of the thermoelectric power S(T) deviates from that seen in other Ce systems. The NFL behavior appears to be associated with a nonmagnetic ground state, as revealed by magnetization M(H,T) measurements.

Low-temperature resistivity and thermoelectric power controlled by defects in the USb antiferromagnet

Abstract The resistivity ρ(T), magnetoresistivity, magnetization and thermoelectric power S(T) of several USb crystals have been examined in the temperature range 0.03–300K, in magnetic fields up to 14T. The resistivity ratio (RR)(=ρ(300K)/ρ(4.2K)) of examined crystals ranged from 1.6 to 13. It was found that defects, which decrease the RR, contribute both to ρ(T) in the shape of a temperature dependent Kondo-like component and to S(T) by means of a peak-like component. The ρ(T) curve shows a maximum below 2K and a minimum between 8 and 14K, depending on the RR. S(T) reaches a maximum at 40 K, which is as large as 40μVK−1 for the sample with RR = 13. The low-temperature ρ (T) behaviour indicates a Kondo temperature of 37 K, fairly close to the temperature of the S(T) maximum. The origin of the Kondo-like behaviour of the resistivity and the thermoelectric power of USb crystals at low temperatures is discussed.

The filled skutterudite CeOs4As12: A hybridization gap semiconductor

X-ray diffraction, electrical resistivity, magnetization, specific heat, and thermoelectric power measurements are presented for single crystals of the new filled skutterudite compound CeOs4As12, which reveal phenomena that are associated with f-electron-conduction electron hybridization. Valence fluctuations or Kondo behavior dominates the physics down to T ∼ 135 K. The correlated electron behavior is manifested at low temperatures as a hybridization gap-insulating state. The small energy gap Δ1/kB ∼ 73 K, taken from fits to electrical resistivity data, correlates with the evolution of a weakly magnetic or nonmagnetic ground state, which is evident in the magnetization data below a coherence temperature Tcoh ∼ 45 K. Additionally, the low-temperature electronic specific heat coefficient is small, γ ∼ 19 mJ/mol K2. Some results for the nonmagnetic analogue compound LaOs4As12 are also presented for comparison purposes.

Magnetic and transport properties of UBi2 and USb2 single crystals

The thermoelectric power, S(T), of USb2 and UBi2, which are tetragonal, uniaxial antiferromagnets below T N = 202 K and 180.8 K, respectively, have been examined between 0.4 K and 300 K. The values of S(T), up to now known above 70 K for USb2 and unknown for UBi2, are positive along the a-axis for both compounds in the whole examined temperature range. The S(T) data for the c-axis (the easy magnetization axis) are positive near room temperature for USb2 and UBi2 but becomes negative below 120 K and 170 K, respectively, with two very deep minima in S(T) dependence for USb2. In the latter compound the Fermi surface, known from literature, is composed of the only cylindrical sheets that are slightly corrugated and parallel to the c-axis. UBi2, the Fermi surface of which is composed of one spherical and two cylindrical sheets, shows corresponding minima although less pronounced than those in USb2. Having at disposal the highest purity single crystals in comparison to those for which the resistivity, ρ(T), has been reported in literature, the ρ(T) anisotropy was re-examined for these two systems. Magnon and phonon contributions to their total electrical resistivity have been determined and the critical fluctuation behaviour of the resistivity near T N for both dipnictides has been analysed. Although the magnetic susceptibilities of UBi2 and USb2 reveal a similarity, their transport properties are significantly different due to the difference in the Fermi surface topology.

Thermal conductivity of polycrystalline and amorphous Se–Te–Cu system

Abstract The thermal conductivity of chalcogenide binary Se–Te and ternary Se–Te–Cu alloys (polycrystalline and amorphous) has been investigated in stationary conditions within the temperature range 7–315 K. Non-typical low-temperature sharp maximum at about 14 K in the crystalline samples has been observed. A hysteresis has appeared in the amorphous sample with 5% Cu content. Qualitative analysis of the measurement results based on the Boltzmann equation has been presented.

The influence of microsphere diameter on the coefficient of thermal conductivity of microsphere insulation

Abstract Measurements of thermal conductivity versus temperature for three microsphere samples of different diameter are presented. The measurements were conducted between 80 and 300 K. Using this data an analytical equation for the apparent coefficient of thermal conductivity was estimated. The coefficient was then divided into the conduction and radiation components and the solid fraction for all microsphere samples was estimated. For one sample the measurement of k(T) was repeated after some special treatment and a drop in the thermal conductivity was observed. The results confirm the effect of microsphere diameter on the radiation contribution of the thermal conductivity.

Physical properties of polycrystalline Sm2PdGe6 and Sm2PtGe6

The compounds Sm₂TGe₆ (T = Pd, Pt,) were synthesized and characterized by x-ray diffraction, magnetization, electrical resistivity, thermoelectric power, and specific heat measurements performed in the temperature range 2-300 K. Additional resistivity and thermoelectric power measurements performed down to 0.35 K have not indicated superconductivity. These compounds crystallize in the orthorhombic structure of Ce₂NiGe₆ type and order antiferromagnetically at 23(1) and 30(1) K, respectively, showing localized magnetism of a Sm(3+) ion with a crystal field doublet level being the ground state. Below T(N), the electrical resistivity, the thermoelectric power, and the specific heat are dominated by electron-magnon scattering with an antiferromagnetic spin-wave spectrum typical of anisotropic antiferromagnetic systems. The thermoelectric power, S, achieves medium positive values at high temperatures, indicating a hole domination in electrical transport in both samples. At low temperatures, S changes its sign and becomes negative. At about 10 K a small negative maximum in S(T) occurs for both studied compounds. All the measurements carried out point to well-localized 4f-electrons in these two compounds, being strongly influenced by the crystal-electric-field effect with a significant admixture of two J-multiplets (5/2 and 7/2), typical for Sm-containing compounds.

Thermal conductivity of YBCO and thermal conductance at YBCO/ruby boundary Part 1: Joint experimental set-up for simultaneous measurements and experimental study in the temperature range 10–260 K

Abstract A joint experimental set-up has been constructed for simultaneous low temperature measurements of thermal conductivity, k , of two solid samples and thermal conductance, R b −1 , at a solid/solid boundary using the steady state method. The thermal measurements presented here were carried out in the temperature range 10–260 K. A discussion of the peculiarities of the thermal conductivity behaviour of YBCO superconducting samples during the cooling cycle has been presented. Initial thermal boundary conductance measurements at the high temperature superconductor/solid (YBCO/ruby) boundary have been performed.