[en] The magnetic and electronic properties of non-Fermi liquid UCu_4Pd depend on annealing conditions. Local structural changes due to this annealing are reported from UL_III- and Pd K-edge x-ray absorption fine-structure measurements. In particular, annealing decreases the fraction of Pd atoms on nominally Cu 16e sites and the U-Cu pair-distance distribution width. This study provides quantitative information on the amount of disorder in UCu_4Pd and allows an assessment of its possible importance to the observed non-Fermi liquid behavior

[en] We present results of the low-temperature specific heat C of samples of the series (U_xLa_1-x)₂Zn₁₇, combined with measurements of the low-temperature magnetic susceptibility (χ) and resistivity (ρ). For x>0.8 we find antiferromagnetic order in coexistence with heavy-fermion behavior. An extrapolation of T_N as a function of the uranium concentration implies that T_N vanishes for x=0.8; at x=0.8, no magnetic order is detected experimentally at temperatures above 0.06 K. The non-Fermi-liquid (NFL) behavior predicted at such a point in the magnetic phase diagram may be observed, but not as clearly as in other systems; some of the behavior is more consistent with spin fluctuations. As the uranium concentration is lowered below x=0.8, C continues to rise in the low-temperature limit, while χ∝χ₀-aT^0.5, but C seems to tend towards the behavior of a Fermi liquid with spin fluctuations at the lowest temperatures (T<0.25 K). First at x=0.3 the temperature dependence of C/T is found to be contrary to Fermi-liquid behavior, while χ∝χ₀-a log T. Thus non-Fermi-liquid behavior is not found so unambiguously at the concentration where T_N vanishes as expected by a quantum critical point theory, but rather at lower uranium concentrations. This presents the possibility that NFL behavior in (U_xLa_1-x)₂Zn₁₇ is not due to nearness to a quantum critical point, but rather to disorder or the presence of spin fluctuations

[en] We present a systematic study of the specific heat in the antiferromagnetic (afm) region of the heavy fermion system UCu_5-xPd_x for annealed and unannealed samples with x=0.5, 0.6, 0.8, 1.0. Annealed UCu₄Pd exhibits a logarithmic temperature dependence of the electronic part of c/T over more than two decades. A careful entropy analysis reveals a nearly constant value of the entropy around 4.0 J mol^-1 K^-1 for all values of x, which supports the scenario of an afm quantum critical phase (QCP) transition with a vanishing transition temperature T_N at x=1

[en] Low-temperature specific-heat measurements on U_1-xTh_xBe₁₃ reveal a linear contribution C_lin = γ_rest(x).T at lowest temperatures which varies systematically with thorium concentration. Size and concentration dependence of this contribution can be explained by resonant defect scattering of the Th impurities in an anisotropic superconductor with gap nodes. The most meaningful observation, however, is that γ_rest(x) shows relatively sharp minima at both critical concentrations (x_C1 ∼ 1.8 at%, x_C2 ∼ 4.5 at%) in the phase diagram of U_1-xTh_xBe₁₃; in between these concentrations a double-maximum structure in the temperature-dependent specific heat is observed. These sharp minima are strong indications of a sudden change in the superconducting ground state at the critical concentrations. This direct information about the gap nodes is important for theory to explain the complex phase diagram of U_1-xTh_xBe₁₃. (orig.)

[en] We report on non-Fermi liquid features observed for CeNi₉Ge₄--a novel stoichiometric compound, which is close to the heavy-fermion quantum critical point. The specific heat measured down to 80 mK reveals an almost logarithmic divergence of the low-temperature contribution yielding an extraordinarily large Sommerfeld value γ exceeding 6 J/mol K². The application of external magnetic fields recovers a conventional Fermi-liquid state

[en] The transition metal carbide Sc₃CoC₄ consists of a quasi-one-dimensional (1D) structure with [CoC₄]_∞ polyanionic chains embedded in a scandium matrix. At ambient temperatures Sc₃CoC₄ displays metallic behavior. At lower temperatures, however, charge density wave formation has been observed around 143 K which is followed by a structural phase transition at 72 K. Below T_c^onset = 4.5 K the polycrystalline sample becomes superconductive. From H_c1(0) and H_c2(0) values we could estimate the London penetration depth (λ_L ≅ 9750 A) and the Ginsburg-Landau (GL) coherence length (ξ_GL ≅ 187 A). The resulting GL-parameter (κ ≅ 52) classifies Sc₃CoC₄ as a type II superconductor. Here we compare the puzzling superconducting features of Sc₃CoC₄, such as the unusual temperature dependence i) of the specific heat anomaly and ii) of the upper critical field H_c2(T) at T_c, and iii) the magnetic hysteresis curve, with various related low dimensional superconductors: e.g., the quasi-1D superconductor (SN)_x or the 2D transition-metal dichalcogenides. Our results identify Sc₃CoC₄ as a new candidate for a quasi-1D superconductor.

[en] We investigate the novel intermetallic ternary compounds RCo₉Ge₄ with R = La and Ce by means of X-ray diffraction, susceptibility and specific heat measurements. CeCo₉Ge₄ crystallizes in the space group I4/mcm and is characterized by the coexistence of two different magnetic sublattices. The Ce-based sublattice, with an effective moment close to the expected value for a Ce³⁺-ion, exhibits a magnetically ordered ground state with T_N = 12.5K. The Co-based sublattice, however, exhibits magnetic moments due to itinerant 3d electrons. The magnetic specific heat contribution of the Ce-sublattice is discussed in terms of a resonance-level model implying the interplay between an antiferromagnetic phase transition and the Kondo-effect and an underlying Schottky-anomaly indicating a crystal field level scheme splitting into three twofold degenerated micro states (Δ₁ = 69K, Δ₂ = 133 K).

[en] CeNi₉Ge₄ exhibits outstanding heavy fermion features with remarkable non-Fermi-liquid behavior which is mainly driven by single-ion effects. The substitution of Ni by Cu causes a reduction of both, the RKKY coupling and Kondo interaction, coming along with a dramatic change of the crystal field (CF) splitting. Thereby a quasi-quartet ground state observed in CeNi₉Ge₄ reduces to a two-fold degenerate one in CeNi₈CuGe₄. This leads to a modification of the effective spin degeneracy of the Kondo lattice ground state and to the appearance of antiferromagnetic (AFM) order. To obtain a better understanding of consequences resulting from a reduction of the effective spin degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and RKKY interactions through the reduction of the effective d-electron count is expected. Accordingly, a paramagnetic Fermi liquid ground state should arise. Our experimental studies, however, reveal AFM order already for small Co concentrations, which becomes even more pronounced with increasing Co content x. Thereby the modification of the effective spin degeneracy seems to play a crucial role in this system.

[en] Ternary skutterudites RE_yTM₄X₁₂ with RE = rare earth, TM = Fe, Co, Rh, Ru, etc. and X = P, As, Sb attracted much interest because of an enormously large figure of merit Z S²/(ρλ ), being a basic requirement for thermoelectric applications. S, ρ, λ are the Seebeck coefficient, the electrical resistivity and the thermal conductivity, respectively. Furthermore, these filled ternary compounds exhibit a rich variety of ground state properties, whereby the magnetic behaviour is dominated by the particular 4f electronic configuration. In the present contribution, the skutterudite PrFe₄Sb₁₂ is investigated with respect to structural, magnetic and transport properties. PrFe₄Sb₁₂ orders magnetically below 5.5 K and exhibits an unusually high electronic contribution to the specific heat C_p/T for T → 0 of about 1000 mJ/molK², thus ranking this compound among strongly correlated electron systems. We attribute some of such extraordinary features of these cubic Pr compounds to crystal field splitting, providing the possibility of non-magnetic singlets, doublets and triplets as ground state and hence the possibility of an occurrence of quadrupolar Kondo interactions. Moreover, the feasibility of a field induced non-Fermi-liquid state is taken into consideration

[en] Superconductivity in the layered yttrium diboride dicarbide, YB₂C₂, has been revisited by means of specific heat, magnetic susceptibility and resistivity measurements down to 50 mK revealing a bulk superconducting transition at 1.0 K. Analysing the low temperature specific heat we obtain a Sommerfeld coefficient of the normal state electronic contribution, γ = 3.1(1) mJ/molK², and a Debye temperature, Θ_D^LT = 680(10) K. The specific heat anomaly at T_c, ΔC ≅ 4.5mJ/mol K, thus, yields a thermodynamic ratio ΔC/γT_c - 1.4 in close agreement with the BCS value. For polycrystalline Y¹¹B₂C₂ prepared by inductive melting we observe a pronounced resistive anomaly near 3.6 K which, however, corresponds to a rather spurious specific heat anomaly and is thus attributed to traces of YC₂ present in this sample.