Antiferro-Quadrupolar Ordering at the Lowest Temperature and Anisotropic Magnetic Field--Temperature Phase Diagram in the Cage Compound PrIr2Zn20

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The cage compound PrIr2Zn20 with the non-Kramers doublet ground state shows a phase transition at T_{\text{Q}}\sim 0.11 K. To investigate the origin of the phase transition, we have carried out ultrasonic measurements on single-crystalline samples. From the disappearance of electric quadrupole degrees of freedom at T_{\text{Q}} and the negative sign of a quadrupole--quadrupole coupling constant, we clarified that the phase transition is antiferro-quadrupolar ordering at the lowest temperature ever measured. We also found anisotropic magnetic field H dependence and the re-entrant behavior of T_{\text{Q}}(H) from the measurements in H parallel to the [100], [110], and [111] axes. The quadrupole Kondo effect interrupts the long-range quadrupole interaction at very low temperatures and thus lowers T_{\text{Q}}. We first reported the rattling motion of Zn atoms at the 16c site in the RT_{2}Zn20 system.
2011-09-15