In an attempt to clarify conflicting published data, we report new measurements of specific heat, resistivity, magnetic susceptibility, and thermal expansivity up to room temperature for the 6 K superconductor ZrB 12 , using well-characterized single crystals with a residual resistivity ratio Ͼ9. The specific heat gives the bulk result 2⌬͑0͒ / k B T c = 3.7 for the superconducting gap ratio, and excludes multiple gaps and d-wave symmetry for the Cooper pairs. The Sommerfeld constant ␥ n = 0.34 mJ K −2 gat −1 and the magnetic susceptibility = −2.1 ϫ 10 −5 indicate a low density of states at the Fermi level. The Debye temperature D is in the range 1000-1200 K near zero and room temperature, but decreases by a factor of ϳ2 at ϳ35 K. The specific heat and resistivity curves are inverted to yield approximations of the phonon density of states F͑͒ and the spectral electron-phonon scattering function ␣ tr 2 F͑͒, respectively. Both unveil a 15 meV mode, attributed to Zr vibrations in oversized B cages, which gives rise to electron-phonon coupling. The thermal expansivity further shows that this mode is anharmonic, while the vanishingly small discontinuity at T c establishes that the cell volume is nearly optimal with respect to T c .

The superconductor YB 6 has the second highest critical temperature T c among the boride family MB n . We report measurements of the specific heat, resistivity, magnetic susceptibility, and thermal expansion from 2 to 300 K, using a single crystal with T c = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity, and expansivity curves are deconvolved to yield approximations of the phonon density of states F͑͒, the spectral electron-phonon scattering function ␣ tr 2 F͑͒, and the phonon density of states weighted by the frequency-dependent Grüneisen parameter ␥ G ͑͒F͑͒, respectively. Lattice vibrations extend to high frequencies Ͼ100 meV, but a dominant Einstein-like mode at ϳ8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field H c3 is also observed.

The paramagnetic response of mixed-valence samarium hexaboride has been studied by inelastic neutron scattering using a low-absorption double-isotope single crystal of 154Sm11 B6. Measurements were performed for energy transfers 0

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