74 research outputs found

    Realization of Strong Coupling Fixed Point in Multilevel Kondo Models

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    Impurity four- and six-level Kondo model, in which an ion is tunneling among four- and six-stable points and interacting with surrounding conduction electrons, are investigated by using the perturbative and numerical renormalization group methods. It is shown that purely orbital Kondo effects occur at low temperatures in these systems which are direct generalizations of the Kondo effect in the so-called two-level system. This result offers a good explanation for the enhanced and magnetically robust Sommerfeld coefficient observed in SmOs_4Sb_12 and some other filled-skutterudites.Comment: 3 pages, 3 figures, for proceedings of ASR-WYP-2005. To be published in Journal of Physical Society Japan supplemen

    Limits on Superconductivity-Related Magnetization in Sr2_2RuO4_4 and PrOs4_4Sb12_{12} from Scanning SQUID Microscopy

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    We present scanning SQUID microscopy data on the superconductors Sr2RuO4 (Tc = 1.5 K) and PrOs4_4Sb12_{12} (Tc = 1.8 K). In both of these materials, superconductivity-related time-reversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However in neither Sr2_2RuO4_4 nor PrOs4_4Sb12_{12} do we observe spontaneous superconductivity-related magnetization. In Sr2_2RuO4_4, many experimental results have been interpreted on the basis of a px±ipypx \pm ipy superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than ~0.1% and ~0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to ~30 nm for random domains, or ~500 nm for periodic domains.Comment: 8 pages, 7 figures. Submitted to Phys. Rev.

    Superconductivity, magnetic order, and quadrupolar order in the filled skutterudite system Pr1−x_{1-x}Ndx_{x}Os4_4Sb12_{12}

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    Superconductivity, magnetic order, and quadrupolar order have been investigated in the filled skutterudite system Pr1−x_{1-x}Ndx_{x}Os4_4Sb12_{12} as a function of composition xx in magnetic fields up to 9 tesla and at temperatures between 50 mK and 10 K. Electrical resistivity measurements indicate that the high field ordered phase (HFOP), which has been identified with antiferroquadruoplar order, persists to xx ∼\sim 0.5. The superconducting critical temperature TcT_c of PrOs4_4Sb12_{12} is depressed linearly with Nd concentration to xx ∼\sim 0.55, whereas the Curie temperature TFMT_{FM} of NdOs4_4Sb12_{12} is depressed linearly with Pr composition to (1−x1-x) ∼\sim 0.45. In the superconducting region, the upper critical field Hc2(x,0)H_{c2}(x,0) is depressed quadratically with xx in the range 0 << xx ≲\lesssim 0.3, exhibits a kink at xx ≈\approx 0.3, and then decreases linearly with xx in the range 0.3 ≲\lesssim xx ≲\lesssim 0.6. The behavior of Hc2(x,0)H_{c2}(x,0) appears to be due to pair breaking caused by the applied magnetic field and the exhange field associated with the polarization of the Nd magnetic moments, in the superconducting state. From magnetic susceptibility measurements, the correlations between the Nd moments in the superconducting state appear to change from ferromagnetic in the range 0.3 ≲\lesssim xx ≲\lesssim 0.6 to antiferromagnetic in the range 0 << xx ≲\lesssim 0.3. Specific heat measurements on a sample with xx == 0.45 indicate that magnetic order occurs in the superconducting state, as is also inferred from the depression of Hc2(x,0)H_{c2}(x,0) with xx.Comment: 7 pages, 7 figures, currently submitted to Phys. Rev.

    31P-NMR and muSR Studies of Filled Skutterudite Compound SmFe4P12: Evidence for Heavy Fermion Behavior with Ferromagnetic Ground State

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    The 31P-NMR (nuclear magnetic resonance) and muSR (muon spin relaxation) measurements on the filled skutterudite system SmFe4P12 have been carried out. The temperature T dependence of the 31P-NMR spectra indicates the existence of the crystalline electric field effect splitting of the Sm3+$ (J = 5/2) multiplet into a ground state and an excited state of about 70 K. The spin-lattice relaxation rate 1/T1 shows the typical behavior of the Kondo system, i.e., 1/T1 is nearly T independent above 30 K, and varies in proportion to T (the Korringa behavior, 1/T1 \propto T) between 7.5 K and 30 K. The T dependence deviated from the Korringa behavior below 7 K, which is independent of T in the applied magnetic field of 1 kOe, and suppressed strongly in higher fields. The behavior is explained as 1/T1is determined by ferromagnetic fluctuations of the uncovered Sm3+ magnetic moments by conduction electrons. The muSR measurements in zero field show the appearance of a static internal field associated with the ferromagnetic order below 1.6 K.Comment: 6 pages, 9 figures, to be published in J. Phys. Soc. Jpn. 75 (2006

    Penetration depth, multiband superconductivity, and absence of muon-induced perturbation in superconducting PrOs4_{4}Sb12_{12}

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    Transverse-field muon spin rotation (μ\muSR) experiments in the heavy-fermion superconductor PrOs4_{4}Sb12_{12} (Tc=1.85T_{c}=1.85 K) suggest that the superconducting penetration depth λ(T)\lambda(T) is temperature-independent at low temperatures, consistent with a gapped quasiparticle excitation spectrum. In contrast, radiofrequency (rf) inductive measurements yield a stronger temperature dependence of λ(T)\lambda(T), indicative of point nodes in the gap. This discrepancy appears to be related to the multiband structure of PrOs4_{4}Sb12_{12}. Muon Knight shift measurements in PrOs4_{4}Sb12_{12} suggest that the perturbing effect of the muon charge on the neighboring Pr3+^{3+} crystalline electric field is negligibly small, and therefore is unlikely to cause the difference between the μ\muSR and rf results.Comment: 10 pages, 7 figure
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