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We have performed direct measurements of the low-temperature dynamical conductivity and dielectric permittivity of single crystalline SmB 6 in the spectral range from 0.6 to 4.5 meV, i.e., below the hybridization gap. The obtained results together with the data of Hall-effect and infrared reflection measurements give evidence for a 19-meV energy gap in the density of states and an additional narrow donor-type band lying only 3 meV below the bottom of the upper conduction band. It is shown that at temperatures 5 KϽTϽ20 K the electrodynamic response and the dc conductivity of SmB 6 are determined by quasifree carriers thermally excited in the conduction band. We evaluate the microscopic parameters of these carriers: the spectral weight, the concentration, the effective mass, the scattering rate, and the mobility. Below 8 K the concentration of carriers in the conduction band freezes out exponentially and finally the electronic properties of SmB 6 are determined by the localized carriers in the narrow band with the typical signature of hopping conductivity.

We report the temperature-dependent three-dimensional angle-resolved photoemission spectra of the Kondo semiconductor SmB6. We found a difference in the temperature dependence of the peaks at the X and Γ points, due to hybridization between the Sm 5d conduction band and the nearly localized Sm 4f state. The peak intensity at the X point has the same temperature dependence as the valence transition below 120 K, while that at the Γ point is consistent with the magnetic excitation at Q = (0.5, 0.5, 0.5) below 30 K. This suggests that the hybridization with the valence transition mainly occurs near the X point, and the initial state of the magnetic excitation is located near the Γ point.PACS numbers: 71.27.+a, 79.60.-i Materials with strong electron correlation have exotic physical properties that cannot be predicted from firstprinciple band calculations. One example may be seen in a semiconductor with a very small energy gap, which appears in rare-earth compounds such as the Kondo semiconductor/insulator (KI)1 . At high temperatures, KI behaves like a dense Kondo metal, while an energy gap with activation energy of several 10 meV appears at low temperature. The energy gap is believed to originate from hybridization between the nearly localized 4f state near the Fermi level (E F ) and the conduction band (c-f hybridization).Numerous studies have investigated the energy gap of KI, using optical conductivity 2,3 , point contact spectroscopy 4 , angle-integrated photoemission spectroscopy 5,6 , and other methods. However, the momentum dependence of the c-f hybridization gap, as well as the relation of the electronic structure to other physical properties, has yet to be studied. Because the cf hybridization occurs at a specific momentum vector, the most direct method of observing the band dispersion of the c-f hybridization gap is three-dimensional angle-resolved photoemission spectroscopy (3D-ARPES) using a tunable photon source from synchrotron radiation. Thus, we applied the 3D-ARPES method to observe the c-f hybridization gap creation of a typical KI, SmB 6 . SmB 6 is a valence-fluctuation material in between Sm 2+ (4f 6 ) and Sm 3+ (4f 5 ) ions 7 . The electrical resistivity (ρ) decreases on cooling, like a metal, above the temperature of 100 K, but then reveals a semiconductorlike character with activation energy of 15 meV 8 . There are two characteristic temperatures on SmB 6 ; one is valence transition below 120 K, and the other is magnetic excitation below 30 K. The mean valence changes from 2.57 at 120 K to 2.50 at 40 K on cooling 9 . Coincidentally, the lattice constant, which normally shrinks above 120 K on cooling, anomalously expands from 120 K to a few tens K indicating the valence change from Sm 3+ to Sm 2+10 . On the other hand, the magnetic excitation at the scattering vector of Q = (0.5, 0.5, 0.5), observed by inelastic neutron scattering (INS), rapidly increases below 30 K 11 . Then the mean valence slightly recovers from 2.50 to 2.52 below 30 K, and the lattice constant shrinks again. The reason ...

Under zero magnetic field, a quadrupolar order parameter at q Q = ( 1 2 , 1 2 , 1 2 ) in a typical antiferro-quadrupole (AFQ) ordering compound CeB 6 has been observed for the first time by means of a resonant X-ray scattering (RXS) technique. The RXS is observed at the 2p → 5d dipole transition energy of the Ce L 3 -edge. Using this RXS technique to observe the pure order parameter of the AFQ state, the magnetic phase diagram of Phase II is first determined.

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