Anomalous Anisotropic Magnetoresistance in Heavy-Fermion PrFe4P12

We have investigated the anisotropy of the magnetoresistance in the Pr-based HF compound PrFe4P12. The large anisotropy of effective mass and its strong field dependence have been confirmed by resistivity measurements. Particularly for H||[111], where the effective mass is most strongly enhanced, the non-Fermi liquid behavior has been observed. Also, we have found the angular dependence of the magnetoresistance sharply enhanced at H||[111], which is evidently correlated with both the non-Fermi liquid behavior and the high-field ordered state (B-phase).Comment: 3 pages, 3 figures. J. Phys. Soc. Jpn. Vol.77, No.8, in pres

Evolution of superconducting order in Pr(Os1−x_{1-x}Rux_{x})4_{4}Sb12_{12}

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density $\rho_{s}$ exhibit an exponential behavior for the $x$$\geq$0.4 samples, going from weak ($x$=0.4,0.6), to moderate, coupling ($x$=0.8). For the $x$$\leq$0.2 samples, both $\lambda$ and $\rho_{s}$ vary as $T^{2}$ at low temperatures, but $\rho_{s}$ is s-wave-like at intermediate to high temperatures. Our data are consistent with a three-phase scenario, where a fully-gapped phase at $T_{c1}$ undergoes two transitions: first to an unconventional phase at $T_{c2}$$\lesssim$$T_{c1}$, then to a nodal low-$T$ phase at $T_{c3}$$<$$T_{c2}$, for small values of $x$.Comment: Changed title, enlarged numbering in figures 5 pages, 4 figures, 1 tabl

Anomalous Metal-Insulator Transition in Filled Skutterudite CeOs4_4Sb12_{12}

Anomalous metal-insulator transition observed in filled skutterudite CeOs$_4$Sb$_{12}$ is investigated by constructing the effective tight-binding model with the Coulomb repulsion between f electrons. By using the mean field approximation, magnetic susceptibilities are calculated and the phase diagram is obtained. When the band structure has a semimetallic character with small electron and hole pockets at $\Gamma$ and H points, a spin density wave transition with the ordering vector $\mathbf{Q}=(1,0,0)$ occurs due to the nesting property of the Fermi surfaces. Magnetic field enhances this phase in accord with the experiments.Comment: 4 pages, 4 figure

High magnetic field phase diagram of PrOs4Sb12

The magnetic phase diagram of PrOs$_4$Sb$_{12}$ has been investigated by specific heat measurements between 8 and 32 T. A new Schottky anomaly due to excitations between two lowest crystalline-electric-field (CEF) singlets, has been found for both $H \parallel (100)$ and $H \parallel (110)$ above the field where the field-induced ordered phase (FIOP) is suppressed. The constructed $H-T$ phase diagram shows weak magnetic anisotropy and implies a crossing of the two CEF levels at about 8 - 9 T for both field directions. These results provide an unambiguous evidence for the $\Gamma_1$ singlet being the CEF ground state and suggest the level crossing (involving lowest CEF levels) as the driving mechanism of FIOP.Comment: Submitted to Phys. Rev. Let

Thermodynamics of Superstring on Near-extremal NS5 and Effective Hagedorn Behavior

We study the thermodynamical torus partition function of superstring on the near-extremal black NS5-brane background. The exact partition function has been computed with the helps of our previous works:[arXiv:1012.5721 [hep-th]], [arXiv:1109.3365 [hep-th]], and naturally decomposed into two parts. The first part is contributed from strings freely propagating in the asymptotic region, which are identified as the superstring gas at the Hawking temperature on the linear-dilaton background. The second part includes the contribution localized around the `tip of cigar', which characterizes the non-extremality. Remarkably, the latter part includes massless excitations with non-vanishing thermal winding, which signifies that the Hagedorn-like behavior effectively appears, even though the Hawking temperature is much lower than the Hagedorn temperature. We also explore the high-temperature backgrounds defined by the orbifolding along the Euclidean time direction. In those cases, the thermal winding modes localized around the tip are found to be tachyonic, reflecting the singularities of Euclidean backgrounds caused by orbifolding.Comment: 1+29 pages, no figure; v2 the footnote 1 is enhanced, to appear in JHE

Penetration depth study of LaOs4_4Sb12_{12}: Multiband s-wave superconductivity

We measured the magnetic penetration depth $\lambda(T)$ in single crystals of LaOs$_{4}$Sb$_{12}$ ($T_c$=0.74 K) down to 85 mK using a tunnel diode oscillator technique. The observed low-temperature exponential dependence indicates a s-wave gap. Fitting the low temperature data to BCS s-wave expression gives the zero temperature gap value $\Delta (0)= (1.34 \pm 0.07) k_B T_c$ which is significantly smaller than the BCS value of 1.76$k_B T_c$. In addition, the normalized superfluid density $\rho(T)$ shows an unusually long suppression near $T_c$, and are best fit by a two-band s-wave model.Comment: 5 pages, 2 figure

High-Field de Haas-van Alphen Effect in non-centrosymmetric CeCoGe3 and LaCoGe3

We report on de Haas-van Alphen effect measurements in the non-centrosymmetric systems CeCoGe3 and LaCoGe3 in magnetic field up to 28 Tesla. In both compounds, two new high frequencies were observed in high fields. The frequencies were not detected in previous lower field measurements. The frequencies do not originate from magnetic breakdown, and, therefore, are likely to be intrinsic features of the compounds. In CeCoGe3, the corresponding effective masses are strongly enhanced, being of the order of 30 bare electron masses.Comment: 3 pages, 4 figures, to be published in Proc. Int. Conf. Heavy Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011