The magnetoresistance of La 2Ϫx Sr x CuO 4 single crystals has been studied extensively over a wide composition range ͑0.07рxр0.28͒ using current parallel ͑in plane͒ and perpendicular ͑out of plane͒ to the CuO 2 plane. In the underdoped superconducting phase (xϳ0.10͒, the in-plane magnetoconductivity above T c is well described as fluctuation conductivity but only with the Aslamasov-Larkin term. The negligibly small MakiThompson contribution is suggestive of anisotropic Cooper pairing. We find a pronounced negative and isotropic out-of-plane magnetoresistance at low temperatures in this composition range. In the optimally doped to the overdoped superconducting phases ͑0.15рxр0.20͒, a substantial normal-state component is observed in the in-plane magnetoresistance. The classical Kohler's rule appears to break down for the normal-state magnetoresistance, which supports the involvement of two distinct scattering rates tr and H . In the out-of-plane magnetoresistance, we find an unconventional scaling ⌬ c / c ϰ(H/ a ) 2 for HЌJ and (H/T) 2 for HʈJ. In contrast to these anomalous behaviors, we find that Kohler's rule holds for both the in-plane and the out-ofplane transverse magnetoresistance in the overdoped normal metal region, implying a conventional anisotropic three-dimensional transport. These findings provide further evidence for the unconventional normal-state transport in the samples which exhibit high-T c superconductivity.

The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV. The low-temperature behavior of intermetallic cerium compounds can be broadly typified in terms of the competition between several interaction channels (intra-atomic couplings, on-site Coulomb repulsion, hybridiza-tion between local f-electron states and itinerant conduction-band states), forming the basis of the well-known and highly successful Anderson model. 1 However, there has also been continued interest in Ce-based materials which do not seem to fit into this general framework. Among those are, for instance, the "Kondo insula-tors", as well as various compounds exhibiting multipole ordering 2 or other types of elusive "hidden order" transitions. One example of such unconventional ordering properties has been discovered very recently by Strydom 3 in the ternary compound CeRu 2 Al 10. CeRu 2 Al 10 is an YbFe 2 Al 10-type orthorhombic compound belonging to the Cmcm space group, with room-temperature lattice constants a = 9.1272Å1272Å, b = 10.282Å 282Å, and c = 9.1902Å1902Å. It has been described as a "cage" crystal structure, in which Ce atoms are separated from each other by an exceptionally large distance of 5.2 ˚ A. From the lattice constants, the Ce valence state was estimated to be close to 3+. The transport properties below room temperature are indicative of a gap in the electronic structure, 3 although the Hall effect still suggests a dominant metallic character. In this regime, the material exhibits considerable magnetic anisotropy (a: easy axis, b: hard axis). 4-6 Upon application of pressure, the system rapidly changes, first to a Kondo insulator, then to a metal above 5 GPa. 4 The striking feature of this compound is the phase transition taking place at T 0 = 27 K, which causes pronounced anomalies in various physical properties. Whereas the origin of this transition remains highly controversial, there is growing evidence that it cannot reduce to a conventional ordering of local Ce magnetic moments. The transition temperature is far too high in view of the large Ce-Ce distance and, more specifically, of the magnetic ordering temperature of 16.5 K found in GdRu 2 Al 10. 4 The drop in the magnetic susceptibility below T 0 occurring for all three magnetic field orientations H a, b, c, with an exponential behavior χ = χ 0 + A exp(−∆/T) and ∆ ∼ 100 K, is also difficult to reconcile with the behavior expected for an antiferromagnet. 4,7 Finally, 27 Al NQR/NMR experiments did not find the splitting of peaks below T 0 expected for a static order of Ce magnetic moments. 8 Alternative mechanisms such as charge-or spin-density-wave formation also have serious shortcomings. 4,8 Recently, Tanida et al. 5,7 sugge...

The nature of the unconventional ordered phase occurring in CeRu2Al10 below T0 = 27 K was investigated by neutron scattering. Powder diffraction patterns show clear superstructure peaks corresponding to forbidden (h + k)-odd reflections of the Cmcm space group. Inelastic neutron scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV.

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A mysterious antiferroquadrupolar ordered phase of CeB6 is considered as originating from the Txyz-type magnetic octupole moment in magnetic fields. By resonant x-ray diffraction, we have verified that the Txyz-type octupole is indeed induced in the 4f orbital of Ce with a propagation vector (1/2,1/2,1/2), thereby supporting the theory. We observed an asymmetric field dependence of the intensity for an electric quadrupole (E2) resonance when the field was reversed and extracted a field dependence of the octupole by utilizing the interference with an electric dipole (E1) resonance. The result is in good agreement with that of the NMR-line splitting, which reflects the transferred hyperfine field at the boron nucleus from the anisotropic spin distribution of Ce with an Oxy-type quadrupole.