The thermal and electrical transport properties of single-crystalline LaBe 13 have been investigated by specific-heat (C) and electrical-resistivity (ρ) measurements. The specific-heat measurements in a wide temperature range revealed the presence of a hump anomaly near 40 K in the C(T )/T curve, indicating that LaBe 13 has a low-energy Einstein-like-phonon mode with a characteristic temperature of ∼ 177 K. In addition, a superconducting transition was observed in the ρ measurements at the transition temperature of 0.53 K, which is higher than the value of 0.27 K reported previously by Bonville et al. Furthermore, an unusual T 3 dependence was found in ρ(T ) below ∼ 50 K, in contrast to the behavior expected from the electron-electron scattering or the electron-Debye phonon scattering.The beryllides MBe 13 (M = rare earths and actinides) show several novel physical properties depending on the M atom, such as unconventional superconductivity (SC) in UBe 13 , Recently, such cage-structured systems, as represented by filled skutterudites and β-pyrochlores, have been attracting much attention because of the presence of a low-energy

We have performed ultrasonic measurements on single-crystalline URu 2 Si 2 with pulsed magnetic fields, in order to check for possible lattice instabilities due to the hybridized state and the hidden-order state of this compound. The elastic constant ðC 11 À C 12 Þ=2, which is associated with a response to the À 3 -type symmetry-breaking (orthorhombic) strain field, shows a three-step increase at H ! 35 T for H k c at low temperatures, where successive meta-magnetic transitions are observed in the magnetization. We discovered a new fact that the absolute change of the softening of ðC 11 À C 12 Þ=2 in the temperature dependence is quantitatively recovered at the suppression of hybridized-electronic state and the hidden order in high-magnetic field for H k c associated with the successive transitions. The present results suggest that the À 3 -type lattice instability, is related to both the emergence of the hybridized electronic state and the hidden-order parameter of URu 2 Si 2 . On the other hand, magnetic fields H k ½100 and [110] enhance the softening of ðC 11 À C 12 Þ=2 in the hidden order phase, while no step-like anomaly is observed up to 68.7 T. We discuss the limitation of the localized-electron picture for describing these features of URu 2 Si 2 by examination of a crystalline electric field model in terms of mean-field theory.KEYWORDS: URu 2 Si 2 , hidden order, elastic constant, ultrasound, pulsed-magnetic field ''What is the primary order parameter and its ordering vector for the hidden order of URu 2 Si 2 ?'' is still an open and controversial question and a longstanding issue of condensed matter physics. 1) URu 2 Si 2 , which crystallizes in the ThCr 2 Si 2 -type tetragonal structure (space group No. 139, I4=mmm), shows an unknown second-order transition at T o ¼ 17:5 K, known as the hidden order (HO) phase, and also a transition into an unconventional superconducting state below T c $ 1:4 K. 1-3) Many theoretical models have been proposed from both localized and itinerant electron pictures in attempts to identify the order parameter of the HO phase. Since 29 Si-NMR and zero-magnetic-field SR measurements have detected little ( 1 G) or no significant internal-magnetic field in the HO phase, 4-6) non-dipolar-type order parameters, e.g., electric quadrupole (rank 2), hexadecapole (rank 4), À 3 -type magnetic octupole (rank 3), or dotriacontapole (rank 5) order parameters, are recently attracting attention. 7-13) Thus far, none of the characteristic signatures of the electric-quadrupole or magnetic-octupole order has been identified in resonant X-ray scattering (under magnetic fields) and neutron scattering under uniaxial stress. 14,15) On the other hand, a nematicity of the electron state in the HO is pointed out, since an in-plane rotational four-fold symmetry breaking is observed in the HO by the magnetic-torque measurement by use of cantilever technique. 16) The interpretation of these experimental results remains controversial.URu 2 Si 2 is also considered a heavy-fermion compound since several ph...

We have measured the elastic constant (C 11 -C 12 )/2 in URu 2 Si 2 by means of high-frequency ultrasonic measurements in pulsed magnetic fields H [001] up to 61.8 T in a wide temperature range from 1.5 to 116 K. We found a reduction of (C 11 -C 12 )/2 that appears only in the temperature and magnetic field region in which URu 2 Si 2 exhibits a heavy-electron state and hidden order. This change in (C 11 -C 12 )/2 appears to be a response of the 5f electrons to an orthorhombic and volume conservative strain field xx -yy with 3 symmetry. This lattice instability is likely related to a symmetry-breaking band instability that arises due to the hybridization of the localized f electrons with the conduction electrons and is probably linked to the hidden-order parameter of this compound. DOI: 10.1103/PhysRevB.88.195150 PACS number(s): 71.27.+a, 62.20.de, 62.65.+k The heavy-fermion compound URu 2 Si 2 exhibits a secondorder phase transition involving uranium's 5f -electron state at T o = 17.5 K, and also exhibits unconventional superconductivity at T c ∼ 1.4 K. Though the transition at T o shows clear anomalies in several thermodynamic quantities, 1-3 the order parameter has not been identified microscopically for more than a quarter of a century; thus, the unknown transition has been called "hidden order (HO)." 4In high magnetic fields (using static or pulsed magnetic field) for H [001] at low temperatures, URu 2 Si 2 undergoes three metamagnetic transitions in the range 35 and 39 T with a collapse of the HO phase, [5][6][7] where the c-axis magnetization increases in three steps and reaches a value of ∼1.5 μ B /U at 60 T and 1.5 K, which is approximately half of the value expected from the localized 5f -electron state of U 3+ (3.62 μ B ) or U 4+ (3.58 μ B ). In addition, the electrical resistivity shows an abrupt decrease above 40 T. This suggests that tuning URu 2 Si 2 by means of a magnetic field decreases the hybridization between 5f and conduction electrons, and, therefore, leads to a reduced effective electron mass. 6,7 Given this situation, it is plausible that the hybridized-electron state observed at low-temperature and in low-magnetic-field regions will change to a relatively light Fermi-liquid state when a high magnetic field is applied along the [001] axis as indicated by the disappearance of the heavy band.7 Thus, the collapse of the HO and the drastic change of the 5f electronic state appear to be strongly connected and suggest that the order parameter of the HO phase is veiled in the screening of the localized 5f electronic state via their strong hybridization with conduction electrons states.In contrast, several recent theoretical models predict that the order parameter of the HO phase is of local nature and based on higher-rank electric and magnetic multipoles. [8][9][10] In particular, the xy(x 2 -y 2 ) electric hexadecapole model, which has been proposed by Kusunose and Harima and also partly related to the theory of Haue and Kotliar, 11 suggests that electric quadrupole moments O xy (=J x J y +...

The elastic properties of URu2Si2 in the high-magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare 5f -electron properties, since the unidentified electronic thermodynamic phase of unknown origin, so called the 'hidden order'(HO) and associated hybridization of conduction and f -electron (c-f hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the Γ4(B2g) and Γ5(Eg) symmetry modes C66 and C44 show elastic softening that is enhanced above 30 T, while the characteristic softening of the Γ3(B1g) symmetry mode (C11 − C12)/2 is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven Γ3(B1g) lattice instability in URu2Si2. However, the results from this work cannot be explained by using existing crystalline electric field schemes applied to the quadrupolar susceptibility in a local 5f 2 configuration. Instead, we present an analysis based on a band Jahn-Teller effect.

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