Quadrupole Ordering in Clathrate Compound Ce_3Pd_<20>Si_6(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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Low-temperature elastic properties of the clathrate compound Ce_3Pd_<20>Si_6 were investigated using ultrasonic measurements. Pronounced softening of elastic constants (C_<11>-C_<12>)/2 and C_<44> with decreasing temperature indicates a precursor of electric quadrupole ordering due to a Γ_8 quartet ground sate with an orbital degeneracy for 8c site of a Ce ion. A minimum of (C_<11>-C_<12>)/2 at T_C=170mK indicates a transition from paramagnetic phase I to an ordered phase III. Applied magnetic fields induce an antiferro-quadrupole (AFQ) ordered phase II with highly anisotropic character depending on the field direction, which resembles the AFQ phase with O_<xy>-type order parameter in CeB_6 with the Γ_8 ground sate. The order paramater of the phase III may be characterized by either magnetic dipole or magnetic octupole. The Γ_7 ground state at 4a site may fall into a singlet screened by conduction electrons at low temperatures without long-range magnetic ordering.
2009-02-15