A lightly doped manganite La 0.88 Sr 0.12 MnO 3 exhibits a phase transition at T OO 145 K from a ferromagnetic metal (T C 172 K) to a novel ferromagnetic insulator. We identify that the key parameter in the transition is the orbital degree of freedom in e g electrons. By utilizing the resonant x-ray scattering, orbital ordering is directly detected below T OO , in spite of a significant diminution of the cooperative Jahn-Teller distortion. The experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present studies uncover a crucial role of the orbital degree of freedom in the metal-insulator transition in lightly doped manganites.[S0031-9007(99)09213-3]

Neutron scattering experiments have been carried out on the heavy fermion antiferromagnetic (AFM) superconductor CePt 3 Si with T N = 2.2 K and T SC = 0.75 K. We observed clear AFM Bragg reflections with Q 0 = (001/2) below and above T SC , indicating microscopic coexistence of AFM order and heavy fermion superconductivity. The AFM structure, of two interleaved ferromagnetic sublattices of local Ce 4f moments, has inversion symmetry under simultaneous space-time reversal. However, hybridization with Pt and Si breaks this degeneracy and a combination of these two competing effects may be relevant to an understanding of the simultaneous occurrence of superconductivity and AFM order. The observed magnetic moment 0.16(1) µ B /Ce is strongly reduced from the Curie-Weiss effective moment 2.54 µ B /Ce. Clear crystal field excitations at 1 and 24 meV were observed. The magnetic susceptibility can be well explained in a level scheme assuming the 7 ground state, 6 and 7 first and second excited states, respectively.

X-ray and Neutron diffraction as well as muon spin relaxation and Mössbauer experiments performed on SrFe 2 As 2 polycrystalls confirm a sharp first order transition at T 0 = 205 K corresponding to an orthorhombic phase distortion and to a columnar antiferromagnetic Fe ordering with a propagation vector (1,0,1), and a larger distortion and larger size of the ordered moment than reported for BaFe 2 As 2 . The structural and the magnetic order parameters present an remarkable similarity in their temperature dependence from T 0 down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scale with the lattice distortion when going from SrFe 2 As 2 to BaFe 2 As 2 . Full-potential band structure calculations confirm that the columnar magnetic order and the orthorhombic lattice distortion are intrinsically tied to each other.

We have succeeded in realizing a single ferroelectric phase in CuFe 0.963 Ga 0.037 O 2 by substituting nonmagnetic Ga 3+ for Fe 3+ sites in CuFeO 2 . Ferroelectric polarization P in CuFe 0.963 Ga 0.037 O 2 is observed below 7.5 K, and has the relatively large value of ϳ250 C / m 2 , which is comparable to P = 300ϳ 400 C / m 2 in the magnetic-field-induced ferroelectric phase of CuFeO 2 . In neutron-diffraction measurements, a single magnetic diffraction peak with an incommensurate wave number was observed below 7.5 K in CuFe 0.963 Ga 0.037 O 2 , indicating that the ferroelectric-incommensurate ͑FEIC͒ phase is realized as a single phase. Therefore, CuFe 0.963 Ga 0.037 O 2 with a single FEIC phase is strongly expected to provide the best opportunity to investigate unresolved problems regarding the ferroelectric mechanism in CuFeO 2 . In this paper, we report measurements of magnetic susceptibility, specific heat, pyroelectric, dielectric constant, and neutron diffraction of a single crystal of CuFe 0.963 Ga 0.037 O 2 .

We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs(4)Sb(12). The observed excitation clearly softens at a wave vector Q=(1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at Q=(1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.

Thermoelectric devices convert heat flow to charge flow, providing electricity. Materials for highly efficient devices must satisfy conflicting requirements of high electrical conductivity and low thermal conductivity. Thermal conductivity in caged compounds is known to be suppressed by a large vibration of guest atoms, so-called rattling, which effectively scatters phonons. Here, the crystal structure and phonon dynamics of tetrahedrites (Cu,Zn) (Sb,As) S are studied. The results reveal that the Cu atoms in a planar coordination are rattling. In contrast to caged compounds, chemical pressure enlarges the amplitude of the rattling vibration in the tetrahedrites so that the rattling atom is squeezed out of the planar coordination. Furthermore, the rattling vibration shakes neighbors through lone pairs of the metalloids, Sb and As, which is responsible for the low thermal conductivity of tetrahedrites. These findings provide a new strategy for the development of highly efficient thermoelectric materials with planar coordination.

From neutron diffraction measurements on a quasi-1D Ising-like Co2+ spin compound BaCo2V2O8, we observed an appearance of a novel type of incommensurate ordering in magnetic fields. This ordering is essentially different from the Néel-type ordering, which is expected for the classical system, and the peculiar spin structure is caused by quantum fluctuation inherent in the quantum spin chain. A Tomonaga-Luttinger liquid nature characteristic of the gapless quantum 1D system is responsible for the realization of the incommensurate ordering.

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