177 research outputs found

    Heavy fermion superconductivity in the filled skutterudite compound PrOs4_4Sb12_{12}

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    The filled skutterudite compound \PrOsSb{} has been found to exhibit superconductivity with a critical temperature Tc=1.85T_\mathrm{c} = 1.85 K that develops out of a heavy Fermi liquid with an effective mass m50mem^{*} \approx 50 m_\mathrm{e}. The current experimental situation regarding the heavy fermion state, the superconducting state, and a high field, low temperature phase that is apparently associated with magnetic or quadrupolar order in \PrOsSb{} is briefly reviewed herein.Comment: 4 pages, 2 figures, presented at the LT23 23rd International Conference on Low Temperature Physics, Hiroshima, Japa

    Microwave properties of (PrxY1x)Ba2Cu3O7δ(Pr_xY_{1-x})Ba_2Cu_3O_{7-\delta} : Influence of magnetic scattering

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    We report measurements of the surface impedance Zs=Rs+iXsZ_s=R_s+iX_s of (PrxY1x)Ba2Cu3O7δ(Pr_xY_{1-x})Ba_2Cu_3O_{7-\delta}, (x=0,0.15,0.23,0.3,0.4,0.5)(x=0,0.15,0.23,0.3,0.4,0.5). Increasing PrPr concentration leads to some striking results not observed in samples doped by non-magnetic constituents. The three principal features of the Rs(T)R_s(T) data - multiple structure in the transition, a high residual resistance and, at high PrPr concentrations, an upturn of the low TT data, are all characteristic of the influence of magnetic scattering on superconductivity, and appear to be common to materials where magnetism and superconductivity coexist. The low TT behavior of λ(T)\lambda (T) appears to change from TT to T4T^4 at large PrPr doping, and provides evidence of the influence of magnetic pairbreaking of the PrPr.Comment: 5 pages, 3 eps figures, Revtex, 2-column format, uses graphicx. To appear in Physica C. Postscript version also available at http://sagar.physics.neu.edu/preprints.htm

    Hybridization gap in some ternary f-electron and d-electron intermetallics

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    In this work we have discussed electronic structure, structural and magnetic properties of the CeMX-type compounds, where M is a transition metal and X is an sp element. The main goal of this presentation is to find the influence of metal M on the coherent gap formation at the Fermi level in the Ce-Kondo insulators. We also discuss a similar semiconductor-like resistance anomaly of Fe2TiSn and CeNiSn-type Kondo insulators

    Nonlinear paramagnetic magnetization in the mixed state of CeCoIn_5

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    Torque and magnetization measurements in magnetic fields HH up to 14 T were performed on CeCoIn5_5 single crystals. The amplitude of the paramagnetic torque shows an H2.3H^{2.3} dependence in the mixed state and an H2H^{2} dependence in the normal state. In addition, the mixed-state magnetizations for both HcH\parallel c and HabH\parallel ab axes show anomalous behavior after the subtraction of the corresponding paramagnetic contributions as linear extrapolations of the normal-state magnetization. These experimental results point towards a nonlinear paramagnetic magnetization in the mixed state of CeCoIn5_5, which is a result of the fact that both orbital and Pauli limiting effects dominate in the mixed state.Comment: 2 pages, 2 figures, conferenc

    Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics

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    A scaling consideration of the Kondo lattices is performed with account of singularities in the spin excitation spectral function. It is shown that a non-Fermi-liquid (NFL) behavior between two critical values of the bare sfs-f coupling constant occurs naturally for complicated magnetic structures with several magnon branches. This may explain the fact that a NFL behavior takes place often in the heavy-fermion systems with peculiar spin dynamics. Another kind of a NFL-like state (with different critical exponents) can occur for simple antiferromagnets with account of magnon damping, and for paramagnets, especially with two-dimensional character of spin fluctuations. The mechanisms proposed lead to some predictions about behavior of specific heat, resistivity, magnetic susceptibility, and anisotropy parameter, which can be verified experimentally.Comment: 16 pages, RevTeX, 4 Postscript figures. Extended versio

    Improved superconducting properties of MgB2

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    We present electrical transport, magnetization, and specific heat measurements on bulk MgB2 samples (T_{c} = 38.5 K) synthesized under 200 MPa pressure using a process based on hot isostatic pressing with cooling under pressure. The samples are fully dense and display excellent superconducting properties, including a narrow superconducting transition width (\Delta T_{c} = 0.75 K), a high upper critical field H_{c2} (H_{c2}(0) ~ 155 kOe), and a critical current density J_{c} that is the largest yet measured for bulk samples of MgB2 (J_{c}(0) ~ 1.4 MA/cm^{2}). Specific heat measurements yielded a jump \Delta C at T_{c} of 92 mJ/mol K. These superconducting properties are comparable to those obtained with techniques that are not so well suited to industrial scale fabrication.Comment: 7 text pages, 5 figures, submitted to Physica

    Glassy Spin Dynamics in Non-Fermi-Liquid UCu_{5-x}Pd_x, x = 1.0 and 1.5

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    Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^\gamma) expected from glassy dynamics. At 0.05 K \gamma(x=1.0) = 0.35 \pm 0.1, but \gamma(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic neutron scattering results, which yield \gamma = 0.33 for both concentrations. There is no sign of static magnetism \gtrsim 10^{-3} \mu_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.Comment: 4 pages, 4 figures, to be published in Physical Review Letter

    Superconductivity and crystalline electric field effects in the filled skutterudite series Pr(Os1x_{1-x}Rux_x)4_4Sb12_{12}

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    X-ray powder diffraction, magnetic susceptibility χ(T)\chi(T), and electrical resistivity ρ(T)\rho(T) measurements were made on single crystals of the filled skutterudite series Pr(Os1x_{1-x}Rux_x)4_4Sb12_{12}. One end of the series (x=0x = 0) is a heavy fermion superconductor with a superconducting critical temperature Tc=1.85T_{c} = 1.85 K, while the other end (x=1x = 1) is a conventional superconductor with Tc1T_{c} \approx 1 K. The lattice constant aa decreases approximately linearly with increasing Ru concentration xx. As Ru (Os) is substituted for Os (Ru), TcT_{c} decreases nearly linearly with substituent concentration and exhibits a minimum with a value of Tc=0.75T_{c} = 0.75 K at x=0.6x = 0.6, suggesting that the two types of superconductivity compete with one another. Crystalline electric field (CEF) effects in χdc(T)\chi_\mathrm{dc}(T) and ρ(T)\rho(T) due to the splitting of the Pr3+^{3+} nine-fold degenerate Hund's rule J=4J = 4 multiplet are observed throughout the series, with the splitting between the ground state and the first excited state increasing monotonically as xx increases. The fits to the χdc(T)\chi_\mathrm{dc}(T) and ρ(T)\rho(T) data are consistent with a Γ3\Gamma_{3} doublet ground state for all values of x, although reasonable fits can be obtained for a Γ1\Gamma_{1} ground state for xx values near the end member compounds (x=0x = 0 or x=1x = 1).Comment: 10 pages, 8 figures, submitted to Phys. Rev.

    Susceptibility inhomogeneity and non-Fermi liquid behavior in UCu_{5-x}Pt_x

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    Transverse-field muSR shifts and relaxation rates have been measured in the non-Fermi liquid (NFL) alloy system UCu_{5-x}Pt_x, x = 1.0, 1.5, and 2.5. At low temperatures the fractional spread in Knight shifts delta K/K approx deltachi/chi is gtrsim 2 for x = 1, but is only half this value for x = 1.5 and 2.5. In a disorder-driven scenario where the NFL behavior is due to a broadly distributed (Kondo or Griffiths-phase cluster) characteristic energy E, our results indicate that delta E/E_{rm av} approx (delta K/K)_{T=0} is similar for UCu_{5-x}Pd_x (x = 1 and 1.5) and UCu_4Pt, but is reduced for UCu_{5-x}Pt_x, x = 1.5 and 2.5. This reduction is due to a marked increase of E with increasing x; the spread delta E is found to be roughly independent of x. Our results correlate with the observed suppression of other NFL anomalies for x > 1 in UCu_{5-x}Pt_x but not in UCu_{5-x}Pd_x, and are further evidence for the importance of disorder in the NFL behavior of both these alloy systems.Comment: 4 pages, 2 figures, submitted to 10th International Conference on Muon Spin Rotation, Relaxation, and Resonance, Oxford, UK, August 200

    Hylleraas-Configuration Interaction calculations on the 1 ^1S ground state of helium atom

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    Hylleraas-Configuration Interaction (Hy-CI) calculations on the ground 111^1S state of helium atom are presented using s-, p-, d-, and f-Slater orbitals of both real and complex form. Techniques of construction of adapted configurations, optimization of the orbital exponents and structure of the wave function expansion are explored. A new method to evaluate the two-electron kinetic energy integrals occurring in the Hy-CI method has been tested here and compared with other methods. The non-relativistic Hy-CI values are approximately 10 picohartree accurate, about 2.2 x 106^{-6} cm1^{-1}. The Hy-CI calculations are compared with Configuration Interaction (CI) and Hylleraas (Hy) calculations employing the same orbital basis set, same computer code and same computer machines. The computational required times are reported.Comment: 33 pages, 9 tables, 38 references and 0 figure
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