- Takahiro Onimaru: Strongly correlated electronic phenomena arising from quadrupolar degrees of freedom in non-Kramers 4f2 systems PrT2Zn20 (T = Rh and Ir)
- 5. 12. 2014, 14:00
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
There has been considerable interest in praseodymium-based intermetallic compounds with 4f 2 electronic configurations, because an abundance of unusual phenomena arising from strong hybridization between 4f and conduction electrons has been discovered in Pr-filled skutterudite; heavy-fermion superconductivity in PrOs4Sb12, a metal-insulator transition in PrRu4P12, and scalar-type multipole order in PrFe4P12 [1,2]. We have recently focused on a novel family of Pr-based compounds with a caged structure, PrT2Zn20 (T: Transition metal), where large coordination number of the Pr ion leads to weak crystalline electric field (CEF) effect, whereas hybridization of the 4f 2 electrons with conduction electrons of cage atoms is strengthened. Such conditions may enable the compounds to show strongly correlated electronic phenomena.
Keeping this in mind, we have synthesized and studied the caged compounds PrT2Zn20 (T = Ru, Rh, and Ir) crystallizing in the cubic CeCr2Al20-type structure, where the Pr ion is encapsulated in a highly symmetric Zn cage [3]. The three compounds show the Van-Vleck paramagnetic behavior in χ(T), indicating the nonmagnetic CEF ground states [4,5]. The magnetic specific heat Cmag shows a Schottky-type peak at around 14 K, which can be reproduced by a doublet-triplet model [6,7]. These facts corroborate that the CEF ground states of the Pr ions are the nonmagnetic Γ3 doublet with quadrupolar degrees of freedom.For T = Rh and Ir, the AFQ order of the non-magnetic Γ3 doublet takes place at TQ=0.06 and 0.11 K, respectively [6,7]. Furthermore, below TQ, superconducting transitions occur at Tc=0.05 and 0.06 K. The entropy at TQ’s is only 10 and 20% of Rln2 which is expected for the twofold degeneracy of the Γ3 doublet, indicating that fluctuations of quadrupole probably remain active below TQ. The facts suggest that the superconducting Copper pair is possibly mediated by the quadrupole fluctuations. Furthermore, in Cmag for T = Rh and Ir, broad peaks appear at around 0.3 K, where the electrical ρ shows temperature dependence of ρ(T)∞√T. The anomalous temperature dependence of Cmag and ρ possibly suggests emergence of a quadrupole Kondo lattice formed by the strong hybridization of the 4f 2 electrons with the conduction electrons [8].
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[8] A. Tsuruta and K. Miyake, unpublished.