Antiferromagnetic behavior in CeCo9_{9}Ge4_{4}

We investigate the novel intermetallic ternary compounds \emph{R}Co$_{9}$Ge$_{4}$ with \emph{R} = La and Ce by means of $X$-ray diffraction, susceptibility and specific heat measurements. CeCo$_{9}$Ge$_{4}$ crystallizes in the space group ${I}$ 4/ ${mcm}$ 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 Ce$^{3+}$-ion, exhibits a magnetically ordered ground state with $T_{\mathrm{N}}=12.5$ K. The Co-based sublattice, however, exhibits magnetic moments due to itinerant 3$d$ 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 ($\Delta_1 = 69$ K, $\Delta_2 = 133$ K).Comment: 4 pages, 3 figures, conference SCES0

Crossover from Single-Ion to Coherent Non-Fermi Liquid Behavior in Ce1−x_{1-x}Lax_xNi9_9Ge4_4

We report specific heat and magneto-resistance studies on the compound Ce${}_{1-x}$La${}_x$Ni${}_9$Ge${}_4$ for various concentrations over the entire stoichiometric range. Our data reveal single-ion scaling with Ce-concentration between $x = 0.1$ and 0.95. Furthermore, CeNi${}_9$Ge${}_4$ turns out to have the largest ever recorded value of the electronic specific heat $\Delta c/T \approx$ 5.5 J $\rm K^{-2}mol^{-1}$ at $T=0.08$ K which was found in Cerium f-electron lattice systems. In the doped samples $\Delta c/T$ increases logarithmically in the temperature range between 3 K and 50 mK typical for non-Fermi liquid (nFl) behavior, while $\rho$ exhibits a Kondo-like minimum around 30 K, followed by a single-ion local nFl behavior. In contrast to this, CeNi${}_9$Ge${}_4$ flattens out in $\Delta c/T$ below 300 mK and displays a pronounced maximum in the resistivity curve at 1.5 K indicating a coherent heavy fermion groundstate. These properties render the compound Ce${}_{1-x}$La${}_x$Ni${}_9$Ge${}_4$ a unique system on the borderline between Fermi liquid and nFl physics.Comment: 2 pages, 3 figures, SCES0

Competing magnetic interactions in CeNi9-xCoxGe4

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 modiffcation 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 modiffcation of the effective spin degeneracy seems to play a crucial role in this system

Possible canted antiferromagnetism in UCu9_9Sn4_4

We report on the new compound UCu${}_9$Sn${}_4$ which crystallizes in the tetragonal structure \emph{I}4/\emph{mcm} with lattice parameters $a = 8.600{\rm\AA}$ and $c = 12.359{\rm\AA}$. This compound is isotyp to the ferromagnetic systems RECu${}_9$Sn${}_4$ (RE = Ce, Pr, Nd) with Curie temperatures $T{}\rm_C$ = 5.5 K, 10.5 K and 15 K, respectively. UCu${}_9$Sn${}_4$ exhibits an uncommon magnetic behavior resulting in three different electronic phase transitions. Below 105 K the sample undergoes a valence transition accompanied by an entropy change of 0.5 Rln2. At 32 K a small hump in the specific heat and a flattening out in the susceptibility curve probably indicate the onset of helical spin order. To lower temperatures a second transition to antiferromagnetic ordering occurs which develops a small ferromagnetic contribution on lowering the temperature further. These results are strongly hinting for canted antiferromagnetism in UCu${}_9$Sn${}_4$.Comment: 2 pages, 3 figures, SCES0

Unusual Non-Fermi Liquid Behavior of Ce1−x_{1-x}Lax_{x}Ni9_{9}Ge4_4 Analyzed in a Single Impurity Anderson Model with Crystal Field Effects

CeNi$_{9}$Ge$_4$ exhibits unusual non-Fermi liquid behavior with the largest ever recorded value of the electronic specific heat $\Delta C/T \cong 5.5$ JK$^{-2}$mol$^{-1}$ without showing any evidence of magnetic order. Specific heat measurements show that the logarithmic increase of the Sommerfeld coefficient flattens off below 200 mK. In marked contrast, the local susceptibility $\Delta\chi$ levels off well above 200 mK and already becomes constant below 1 K. Furthermore, the entropy reaches 2$R$ln2 below 20 K corresponding to a four level system. An analysis of $C$ and $\chi$ was performed in terms of an $SU(N=4)$ single impurity Anderson model with additional crystal electric field (CEF) splitting. Numerical renormalization group calculations point to a possible consistent description of the different low temperature scales in $\Delta c$ and $\Delta \chi$ stemming from the interplay of Kondo effect and crystal field splitting.Comment: 2 pages, 2 figure

Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold degenerate ground state in CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ \chi ~ -ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.Comment: 15 pages, 9 figure

Unusual Single-Ion Non-Fermi Liquid Behavior in Ce_(1-x)La_xNi_9Ge_4

We report on specific heat, magnetic susceptibility and resistivity measurements on the compound Ce_(1-x)La_xNi_9Ge_4 for various concentrations ranging from the stoichiometric system with x=0 to the dilute limit x=0.95. Our data reveal single-ion scaling with the Ce-concentration and the largest ever recorded value of the electronic specific heat c/T approximately 5.5 J K^(-2)mol^(-1) at T=0.08K for the stoichiometric compound x=0 without any trace of magnetic order. While in the doped samples c/T increases logarithmically below 3K down to 50mK, their magnetic susceptibility behaves Fermi liquid like below 1K. These properties make the compound Ce_(1-x)La_xNi_9Ge_4 a unique system on the borderline between Fermi liquid and non-Fermi liquid physics.Comment: 4 pages, 5 figures; v2 contains additional resisitivity measurements; final version to appear in Phys. Rev. Let

Heavy fermion superconductivity and magnetic order in non-centrosymmetric CePt3SiCePt_3Si

$\rm CePt_3Si$ is a novel heavy fermion superconductor, crystallising in the $\rm CePt_3B$ structure as a tetragonally distorted low symmetry variant of the $\rm AuCu_3$ structure type. $\rm CePt_3Si$ exhibits antiferromagnetic order at $T_N \approx 2.2$ K and enters into a heavy fermion superconducting state at $T_c \approx 0.75$ K. Large values of $H_{c2}' \approx -8.5$ T/K and $H_{c2}(0) \approx 5$ T refer to heavy quasiparticles forming Cooper pairs. Hitherto, $\rm CePt_3Si$ is the first heavy fermion superconductor without a center of symmetry.Comment: 4 pages, 4 figure

Spin and orbital frustration in MnSc_2S_4 and FeSc_2S_4

Crystal structure, magnetic susceptibility, and specific heat were measured in the normal cubic spinel compounds MnSc_2S_4 and FeSc_2S_4. Down to the lowest temperatures, both compounds remain cubic and reveal strong magnetic frustration. Specifically the Fe compound is characterized by a Curie-Weiss temperature \Theta_{CW}= -45 K and does not show any indications of order down to 50 mK. In addition, the Jahn-Teller ion Fe^{2+} is orbitally frustrated. Hence, FeSc_2S_4 belongs to the rare class of spin-orbital liquids. MnSc_2S_4 is a spin liquid for temperatures T > T_N \approx 2 K.Comment: 4 pages, to be published in Physical Review Letter