[en] Low-temperature electrical-resistivity, ρ(T), measurements on single crystals of the structurally disordered ThAsSe reveal an anomalous scattering mechanism, which is apparently derived from two-level systems. For the ThAsSe specimen displaying a resistivity saturation at millikelvin temperatures, a crossover from a logarithmic to a non-Fermi-liquid behavior Δρ∝1-aT^1/2 is observed upon cooling below ≅4 K. A comparison of experimental results with the theoretical ones yields a characteristic energy scale of the order of a few K for ThAsSe

[en] We report on low-T (T≥0.02 K) measurements of the electrical resistivity, ρ(T,B), and the magnetic AC-susceptibility, χ_ac(T,B), on the low-carrier density, one-dimensional S=((1)/(2)) antiferromagnet Yb₄As₃ in magnetic fields B≤19 T. For 2 K≤T≤20 K we find ρ-ρ₀=AT² (ρ₀: residual resistivity), with a large coefficient A∼0.75 μΩ cm/K² followed by a minimum around 1 K and a 0.1% increase for T→0. In finite fields and below about 5 K, ρ(T,B) shows a history-dependent hysteretic behavior. The oscillatory behavior superimposed is attributed to the Shubnikov-de Haas effect arising from a low density of mobile As-p holes. For T≥0.4 K, χ_ac(T) follows the prediction of the quantum sine-Gordon model. A cusp-like anomaly is found at 0.15 K

[en] We discuss recent results obtained for the heavy-fermion metals UPd₂Al₃ and YbRh₂Si₂. UPd₂Al₃ is the first among all superconductors for which tunneling and inelastic neutron-scattering data highlight a non-phononic, i.e., magnetic-exciton mediated, pair state. YbRh₂Si₂ represents a model system exhibiting pronounced non-Fermi liquid effects above a weak antiferromagnetic phase transition at T_N=70 mK. Upon approaching the quantum critical point (T_N→0), by low doping with Ge, one observes for T<0.3 K disparate behavior in the temperature dependences of both the electrical resistivity and the electronic specific heat as well as a Curie-Weiss law in the uniform magnetic susceptibility, implying uncompensated large 4f moments. These observations indicate a break up of the composite quasiparticles into their local f-spin and itinerant conduction-electron parts

[en] We present DC-magnetization, M(B), magnetostriction, ΔL(B)/L, and magnetoresistance ρ(T,B) measurements on high-quality single crystals of YbRh₂Si₂ in magnetic fields up to 18 T and at temperatures down to 15 mK. At B^starf∼9.5 T, both M(B) and ΔL(B)/L show pronounced changes of their slopes, indicative for a broadened phase transition. For fields above B^starf, the coefficient A of the Fermi liquid behavior Δρ=ρ₀+A(B)T² is reduced to very small values, indicating the suppression of the Heavy Fermion state

[en] We discuss the results of thermodynamic and transport measurements at ambient and under hydrostatic pressure of the recently discovered heavy fermion superconductor Ce_3PtIn_1_1. At p = 0 superconductivity emerges in the complex antiferromagnetic state below T_c = 0.32 K. Both phenomena coexist in a wide range of the p — T phase diagram. The critical pressure where T_N → 0 and T_c is maximum equals p_c = 1.3 GPa. Here, the resistivity follows ρ(T) α T"n with n = 0.90±0.05 suggesting a local moment-type of quantum critical point. The presence of a maximum structure in the susceptibility and the unusual pressure dependence of T_c hint to an understanding of properties of Ce_3PtIn_1_1 in the view of distinct different Kondo screened sublattices. Therefore, we analyze the magnetic entropy S_m_a_g within the spin-1/2 Kondo model while accounting for the fact that Ce_3PtIn_1_1 unit cell possesses two inequivalent Ce-sites, Ce1 and Ce2. We employ an extreme case scenario of non-interacting sublattices where the Ce1-sublattice is paramagnetic and only Ce2-sublattice orders magnetically and discussed its consequences. (paper)

[en] We report low temperature specific heat, C, magnetization, M, susceptibility, χ , and electrical resistivity, ρ , measurements on high-quality single crystals of the heavy-fermion system YbRh₂(Si_1-xGe_x)₂ (x=0 and 0.05). The undoped compound shows weak antiferromagnetic (AF) order at T_N=70 mK which is suppressed to below 20 mK by a tiny volume expansion in the x=0.05 system. In the latter pronounced deviations from Landau Fermi liquid (LFL) behavior occur, e.g. Δρ ∼ T over three decades in T. Both thermodynamic and magnetic properties show a crossover at about 0.3 K. At 0.3 K ≤ T ≤ 10 K we observe C/T ∼ log (T₀/T) and a ''non-Curie'' behavior χ^-1 ∼ T^α with α <1 similar to what was found for the prototypical system CeCu_5.9Au_0.1. Below 0.3 K, χ turns into a Curie--Weiss dependence χ^-1 ∼ (T-Θ) indicating large unscreened Yb³⁺ moments whereas in C(T)/T a pronounced upturn occurs. In the undoped compound the AF order is suppressed continuously by critical fields B_c0∼ 0.06 T and 0.7 T applied perpendicular and parallel to the c-axis, respectively. For B>B_c0 a LFL state with Δρ =A(B)T² and C(T)/T=γ₀(B) is induced, that fulfills the Kadowaki-Woods scaling A∼γ₀². Upon reducing the magnetic field to B_c0 a 1/(B-B_c0) dependence of A(B) and γ₀²(B) indicates singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. (author)

[en] The heavy fermion compound Ce₃Pd₂₀Si₆ is one of the rare examples of a cubic system with a readily accessible quantum critical point. Ce atoms at the two different sites 4a and 8c of the crystal structure have recently been shown to have different crystal field ground states (Γ₇ and Γ₈, respectively) and are assumed to be responsible for the two different low-temperature phase transitions at T_L and T_U, which have been tentatively attributed to antiferromagnetic and antiferroquadrupolar order, respectively. Here we present electrical resistivity measurements in a wide temperature range (50 mK-300 K) on two new representatives of the La substitution series Ce_3-xLa_xPd₂₀Si₆, x= 1/3 and 2/3 . Put in the context of previously published data our results indicate that La preferentially occupies the 4a site and that Ce ions at the 8c site have a sizably larger Kondo temperature. Low-temperature resistivity measurements in applied magnetic fields suggest that (disorder smeared) quantum critical points exist in the x=1/3 and 2/3 samples at fields below 1 T.

[en] High-quality single crystals of YbRh₂(Si_1-xGe_x)₂ ( x = 0, 0,05 ) have been studied by low-temperature (T) and high-magnetic-field (B) measurements of the electrical resistivity ρ(T,B), dc-magnetization M(T,B) and magnetic ac-susceptibility χ_ac(T,B). For the undoped compound (x=0) a weak Anti Ferromagnetic (AF) transition at T_N≅70 mK is observed which can be suppressed for small fields B_c≅0.06 T and 0.7 T applied along the crystallographic a - and c -direction, respectively. Below T_N a ρ(T)=ρ₀ + aT² behavior is found with very low residual resistivity ρ₀≅μΩcm. Above T_N pronounced Non-Fermi-Liquid (NFL) effects are observed with a linear T-dependence of the electrical resistivity. A small volume expansion of ΔV∼+0.3% is sufficient to tune the system YbRh₂(Si_1-xGe_x)₂ to its Quantum Critical Point (QCP) at x_c=(0.06±0.01). The x=0.05 compound shows a ρ(T)=ρ₀+bT dependence from below 10 mK up to above 10 K. The low ρ₀≅5μΩcm proves that this NFL behavior is intrinsic due to the proximity to the QCP and not related to the disorder induced by Ge-alloying. No evidence for a metamagnetic phase transition in fields up to 56 T applied along the magnetic hard direction (BIIc) has been observed. (author)

[en] We report on detailed low-temperature measurements of the magnetization, the specific heat and the electrical resistivity on high-quality CeRuSn single crystals. The compound orders antiferromagnetically at T_N = 2.8 K with the Ce³⁺ ions locked within the a–c plane of the monoclinic structure. Magnetization shows that below T_N CeRuSn undergoes a metamagnetic transition when applying a magnetic field of 1.5 and 0.8 T along the a- and c-axis, respectively. This transition manifests in a tremendous negative jump of ∼25% in the magnetoresistance. The value of the saturated magnetization along the easy magnetization direction (c-axis) and the magnetic entropy above T_N derived from specific heat data correspond to the scenario of only one third of the Ce ions in the compound being trivalent and carrying a stable Ce³⁺ magnetic moment, whereas the other two thirds of the Ce ions are in a nonmagnetic tetravalent and/or mixed valence state. This is consistent with the low-temperature CeRuSn crystal structure i.e., a superstructure consisting of three unit cells of the CeCoAl type piled up along the c-axis, and in which the Ce³⁺ ions are characterized by large distances from the Ru ligands while the Ce–Ru distances of the other Ce ions are much shorter causing a strong 4f-ligand hybridization and hence leading to tetravalent and/or mixed valence Ce ions. (paper)

[en] Heavy electron superconductors mostly have anisotropic gap function which vanishes (has nodes) for certain directions in the momentum space. Since the nodal structure is closely related to the pairing mechanism, its experimental determination is very important. In anisotropic superconductors, low energy spectrum of the quasiparticles in the vortex state much depends on the nodal structure. In particular, the electronic specific heat has been demonstrated to exhibit the characteristic dependence on the angle between the field and the nodal direction, thereby the nodal structure can be probed experimentally. In this contribution, we present our recent experimental results on the field-orientation dependence of the specific heat on f electron superconductors CeRu₂, CeCoIn₅, PrOs₄Sb₁₂, and URu₂Si₂, and discuss their gap structures. (author)