67 research outputs found

    Low-Temperature X-ray Crystal Structure Analysis of the Cage-Structured Compounds MBe13 (M = La, Sm, and U)

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    The beryllides MBe13 (M = rare earths and actinides) crystallize in a NaZn13-type cubic structure, which can be categorized as a cage-structured compound. In this study, powder X-ray diffraction measurements have been performed on LaBe13, SmBe13, and UBe13 in the temperature range between 7 and 300 K in order to investigate their crystallographic characteristics systematically. They keep the NaZn13-type cubic structure down to the lowest temperature. We estimated their Debye temperature to be 600 - 750 K from analyses of the temperature dependence of a lattice parameter, being in good agreement with the values reported previously. Rietveld refinements on the obtained powder patterns revealed that the M atom in the 8a site is located in an almost ideal snub cube formed by 24 Be atoms in the 96i site, whose caged structure is unchanged even at the low temperatures. In addition, it is argued from the temperature variation of an isotropic mean-square displacement parameter that the MBe13 compounds commonly have a low-energy phonon mode, which can be described by a model assuming an Einstein oscillation of the M atom with a characteristic temperature of ~ 160 K.Comment: 8 pages with 6 figures and 2 table

    Magnetic Properties of YbBe13 Probed by Neutron Scattering and Thermodynamic Measurements

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    We examined the magnetic properties of YbBe13, which exhibits an antiferromagnetic order below TN = 1.2 K. Unlike other MBe13 compounds (M = rare earth/actinide elements), based on elastic neutron scattering, we observed an incommensurate magnetic propagation vector {\tau} = (0,0,{\tau}z), where {\tau}z = 0.5385 is in the reciprocal lattice unit. Additionally, we constructed a precise magnetic phase diagram for YbBe13. We observed non-trivial magnetic anomalies in YbBe13, which cannot be understood based on a simple helical order. Our results for YbBe13 provide an opportunity to reconsider the electron state of UBe13 and present an important step toward a comprehensive understanding of magnetic correlations in MBe13 series.Comment: 5 pages, 5 figure

    Unconventional Strong Spin-Fluctuation Effects around the Critical Pressure of the Itinerant Ising-Type Ferromagnet URhAl

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    Resistivity measurements were performed for the itinerant Ising-type ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5 GPa, using single crystals. We found that the critical pressure of the Curie temperature exists at around PcP_c ~ 5.2 GPa. Near PcP_c, the AA-coefficient of the AT2AT^{2} Fermi-liquid resistivity term below TT^* is largely enhanced with a maximum around 5.2-5.5 GPa. Above PcP_c, the exponent of the resistivity ρ(T)\rho(T) deviates from 2. At PcP_c, it is close to n=5/3n = 5/3, which is expected by the theory of three-dimensional ferromagnetic spin fluctuations for a 2nd-order quantum-critical point (QCP). However, TC(P)T_C(P) disappears as a 1st-order phase transition, and the critical behavior of resistivity in URhAl cannot be explained by the theory of a 2nd-order QCP. The 1st-order nature of the phase transition is weak, and the critical behavior is still dominated by the spin fluctuation at low temperature. With increasing pressure, the non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies point out a ferromagnetic wing structure with a tri-critical point (TCP) at ~ 4.8-4.9 GPa in URhAl. One open possibility is that the switch from the ferromagnetic to the paramagnetic states does not occur simply but an intermediate state arises below the TCP as suggested theoretically recently. Quite generally, if a drastic Fermi-surface change occurs through PcP_c, the nature of the interaction itself may change and lead to the observed unconventional behavior.Comment: 9 pages, 9 figure

    Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

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    The gap structure of a novel uranium-based superconductor UTe2_2, situated in the vicinity of ferromagnetic quantum criticality, has been investigated via specific-heat C(T,H,Ω)C(T,H,\Omega) measurements in various field orientations. Its angular Ω(ϕ,θ)\Omega(\phi,\theta) variation shows a characteristic shoulder anomaly with a local minimum in HaH \parallel a at moderate fields rotated within the abab and acac planes. Based on the theoretical calculations, these features can be attributed to the presence of point nodes in the superconducting gap along the aa direction. Under the field orientation along the easy-magnetization aa axis, an unusual temperature dependence of the upper critical field at low fields together with a convex downward curvature in C(H)C(H) were observed. These anomalous behaviors can be explained on the basis of a nonunitary triplet state model with equal-spin pairing whose TcT_{\rm c} is tuned by the magnetization along the aa axis. From these results, the gap symmetry of UTe2_2 is most likely described by a vector order parameter of d(k)=(b+ic)(kb+ikc)d(k)=(b + ic)(k_b + ik_c).Comment: 6 pages, 4 figures (main text) + 7 pages, 5 figures (Supplementary Material), accepted for publication in Phys. Rev. Researc
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