659 research outputs found

    Itinerant and local-moment magnetism in EuCr2As2 single crystals

    Full text link
    We report on the crystal structure, physical properties, and electronic structure calculations for the ternary pnictide compound EuCr2As2. X-ray diffraction studies confirmed that EuCr2As2 crystalizes in the ThCr2Si2-type tetragonal structure (space group I4/mmm). The Eu ions are in a stable divalent state in this compound. Eu moments in EuCr2As2 order magnetically below Tm = 21 K. A sharp increase in the magnetic susceptibility below Tm and the positive value of the paramagnetic Curie temperature obtained from the Curie-Weiss fit suggest dominant ferromagnetic interactions. The heat capacity exhibits a sharp {\lambda}-shape anomaly at Tm, confirming the bulk nature of the magnetic transition. The extracted magnetic entropy at the magnetic transition temperature is consistent with the theoretical value Rln(2S+1) for S = 7/2 of the Eu2+ ion. The temperature dependence of the electrical resistivity \r{ho}(T) shows metallic behavior along with an anomaly at 21 K. In addition, we observe a reasonably large negative magneto-resistance (~ -24%) at lower temperature. Electronic structure calculations for EuCr2As2 reveal a moderately high density of states of Cr-3d orbitals at the Fermi energy, indicating that the nonmagnetic state of Cr is unstable against magnetic order. Our density functional calculations for EuCr2As2 predict a G-type AFM order in the Cr sublattice. The electronic structure calculations suggest a weak interlayer coupling of the Eu moments.Comment: 9 pages, 7 figure

    Valence Fluctuation in CeMo2Si2C

    Full text link
    We report on the valence fluctuation of Ce in CeMo2_{2}Si2_{2}C as studied by means of magnetic susceptibility χ(T)\chi(T), specific heat C(T)C(T), electrical resistivity ρ(T)\rho(T) and x-ray absorption spectroscopy. Powder x-ray diffraction revealed that CeMo2_{2}Si2_{2}C crystallizes in CeCr2_{2}Si2_{2}C-type layered tetragonal crystal structure (space group \textit{P4/mmm}). The unit cell volume of CeMo2_{2}Si2_{2}C deviates from the expected lanthanide contraction, indicating non-trivalent state of Ce ions in this compound. The observed weak temperature dependence of the magnetic susceptibility and its low value indicate that Ce ions are in valence fluctuating state. The formal LIIIL_{III} Ce valence in CeMo2_{2}Si2_{2}C = 3.11 as determined from x-ray absorption spectroscopy measurement is well bellow the value \simeq 3.4 in tetravalent Ce compound CeO2_{2}. The temperature dependence of specific heat does not show any anomaly down to 1.8 K which rules out any magnetic ordering in the system. The Sommerfeld coefficient obtained from the specific heat data is γ\gamma = 23.4 mJ/mol\,K2^{2}. The electrical resistivity follows the T2T{^2} behavior in the low temperature range below 35 K confirming a Fermi liquid behavior. Accordingly both the Kadowaki Wood ratio A/γ2A/\gamma^{2} and the Sommerfeld Wilson ratio χ(0)/γ\chi(0)/\gamma are in the range expected for Fermi-liquid systems. In order to get some information on the electronic states, we calculated the band structure within the density functional theory, eventhough this approach is not able to treat 4f electrons accurately. The non-ff electron states crossing the Fermi level have mostly Mo 4d character. They provide the states with which the 4f sates are strongly hybridized, leading to the intermediate valent state.Comment: 18 pages, 10 figures Submitted to Journal of Alloys and Compound

    Unconventional magnetism in multivalent charge-ordered YbPtGe2_2 probed by 195^{195}Pt- and 171^{171}Yb-NMR

    Full text link
    Detailed 195^{195}Pt- and 171^{171}Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe2_2 are reported. The temperature dependence of the 195^{195}Pt-NMR shift 195K(T)^{195}K(T) indicates the opening of an unusual magnetic gap below 200\,K. 195K(T)^{195}K(T) was analyzed by a thermal activation model which yields an isotropic gap Δ/kB200\Delta/k_B \approx 200\,K. In contrast, the spin-lattice relaxation rate 195^{195}(1/T11/T_1) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreover, 195^{195}(1/T11/T_1) follows a simple metallic behavior, similar to the reference compound YPtGe2_2. A well resolved NMR line with small shift is assigned to divalent 171^{171}Yb. This finding supports the proposed model with two sub-sets of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the YbPtGe2_2 lattice.Comment: Submitted in Physical Review B (Rapid Communication

    Self-supervised automated wrapper generation for weblog data extraction

    Get PDF
    Data extraction from the web is notoriously hard. Of the types of resources available on the web, weblogs are becoming increasingly important due to the continued growth of the blogosphere, but remain poorly explored. Past approaches to data extraction from weblogs have often involved manual intervention and suffer from low scalability. This paper proposes a fully automated information extraction methodology based on the use of web feeds and processing of HTML. The approach includes a model for generating a wrapper that exploits web feeds for deriving a set of extraction rules automatically. Instead of performing a pairwise comparison between posts, the model matches the values of the web feeds against their corresponding HTML elements retrieved from multiple weblog posts. It adopts a probabilistic approach for deriving a set of rules and automating the process of wrapper generation. An evaluation of the model is conducted on a dataset of 2,393 posts and the results (92% accuracy) show that the proposed technique enables robust extraction of weblog properties and can be applied across the blogosphere for applications such as improved information retrieval and more robust web preservation initiatives

    Lifshitz transitions and quasiparticle de-renormalization in YbRh2_2Si2_2

    Full text link
    We study the effect of magnetic fields up to 15 T on the heavy fermion state of YbRh2_2Si2_2 via Hall effect and magnetoresistance measurements down to 50 mK. Our data show anomalies at three different characteristic fields. We compare our data to renormalized band structure calculations through which we identify Lifshitz transitions associated with the heavy fermion bands. The Hall measurements indicate that the de-renormalization of the quasiparticles, {\it i.e} the destruction of the local Kondo singlets, occurs smoothly while the Lifshitz transitions occur within rather confined regions of the magnetic field.Comment: 7 pages, 5 figure

    Valence fuctuation and magnetic ordering in EuNi2(P1-xGex)2 single crystals

    Full text link
    Unusual phases and phase transitions are seen at the magnetic-nonmagnetic boundary in Ce, Eu and Yb-based compounds. EuNi2_2P2_{2} is a very unusual valence fluctuating Eu system, because at low temperatures the Eu valence stays close to 2.5 instead of approaching an integer value. Eu valence and thus the magnetic property in this system can be tuned by Ge substitution in P site as EuNi2_2Ge2_{2} is known to exhibit antiferromagnetc (AFM) ordering of divalent Eu moments with TNT_N = 30 K. We have grown EuNi2_2(P1x_{1-x}Gex_x)2_2 (0.0 \leq xx \leq 0.5) single crystals and studied their magnetic, thermodynamic and transport properties. Increasing Ge doping to x>x > 0.4 results in a well-defined AFM ordered state with TNT_N = 12 K for xx = 0.5. Moreover, the reduced value of magnetic entropy for xx = 0.5 at TNT_N suggests the presence of valance fluctuation/ Kondo effect in this compound. Interestingly, the specific heat exhibits an enhanced Sommerfeld coefficient upon Ge doping. Subsequently, electronic structure calculations lead to a non-integral valence in EuNi2_2P2_{2} but a stable divalent Eu state in EuNi2_2Ge2_{2} which is in good agreement with experimental results.Comment: 7 pages, 8 figure

    Quasiquartet CEF ground state with possible quadrupolar ordering in the tetragonal compound YbRu2_{2}Ge2_{2}

    Full text link
    e have investigated the magnetic properties of YbRu2_{2}Ge2_{2} by means of magnetic susceptibility χ\chi(T), specific heat C(T) and electrical resistivity ρ\rho(T) measurements performed on flux grown single crystals. The Curie-Weiss behavior of χ\chi(T) along the easy plane, the large magnetic entropy at low temperatures and the weak Kondo like increase in ρ\rho(T) proves a stable trivalent Yb state. Anomalies in C(T), ρ\rho(T) and χ\chi(T) at T0_{0} = 10.2 K, T1_{1} = 6.5 K and T2_{2} = 5.7 K evidence complex ordering phenomena, T0_{0} being larger than the highest Yb magnetic ordering temperature found up to now. The magnetic entropy just above T0_{0} amounts to almost Rln4, indicating that the crystal electric field (CEF) ground state is a quasiquartet instead of the expected doublet. The behavior at T0_{0} is rather unusual and suggest that this transition is related to quadrupolar ordering, being a consequence of the CEF quasiquartet ground state. The combination of a quasiquartet CEF ground state, a high ordering temperature, and the relevance of quadrupolar interactions makes YbRu2_{2}Ge2_{2} a rather unique system among Yb based compounds.Comment: 11 pages, 5 figure, submitted to PRB rapi

    Quantum criticality in the cubic heavy-fermion system CeIn_{3-x}Sn_x

    Full text link
    We report a comprehensive study of CeIn3x_{3-x}Snx_x (0.55x0.8)(0.55 \leq x \leq 0.8) single crystals close to the antiferromagnetic (AF) quantum critical point (QCP) at xc0.67x_c\approx 0.67 by means of the low-temperature thermal expansion and Gr\"uneisen parameter. This system represents the first example for a {\it cubic} heavy fermion (HF) in which TNT_{\rm N} can be suppressed {\it continuously} down to T=0. A characteristic sign change of the Gr\"uneisen parameter between the AF and paramagnetic state indicates the accumulation of entropy close to the QCP. The observed quantum critical behavior is compatible with the predictions of the itinerant theory for three-dimensional critical spinfluctuations. This has important implications for the role of the dimensionality in HF QCPs.Comment: Physical Review Letters, to be publishe

    Superconductivity in SrFe_(2-x)Co_xAs_2: Internal Doping of the Iron Arsenide Layers

    Full text link
    In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination, magnetic susceptibility, electrical resistivity, and specific heat are reported. The observation of bulk superconductivity in all thermodynamic properties -- despite strong disorder in the Fe-As layer -- favors an itinerant picture in contrast to the cuprates and renders a p- or d-wave scenario unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3) leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of states.Comment: 5 pages, 3 figure
    corecore