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Pyrochlore Iridates display an interesting interplay of geometric frustration, spin-orbit coupling, and Coulomb correlation potentially leading to a wide variety of novel quantum phases[1].  Among the Pyrochlore Iridates, Pr₂Ir₂O₇ is known to be a chiral spin liquid with macroscopically broken time-reversal symmetry but no magnetic long-range ordering down to the lowest temperature. Moreover, unlike many other materials with Pyrochlore lattices, Pr₂Ir₂O₇ is a metal with relatively low resistivity and a positive temperature coefficient. At low temperatures, the resistivity shows a shallow minimum[2]. The local Pr³⁺ 4f² moments have non-Kramer’s J = 4 doublet ground state, separated from the excited Kramer’s doublet by a gap of 160 K. If we consider valence fluctuations to the excited Kramer’s doublet leading to a two-channel Kondo effect instead of the single-channel effect, it should lead to heavy Fermi liquid formation and an associated mean-field like phase transition similar to the famous ‘hastatic order’ transition in URu₂Si₂ [3]. Such ‘hastatic’ order breaks both single and double time-reversal symmetry due to the mixing of integer and half-integer spins[3]. We discuss the experimental evidence of emergent broken symmetry Fermi liquid state in a single-crystalline nanorod of Pr₂Ir₂O₇.

[1] W. Witczak-Krempa, Gang Chen, Yong Baek Kim, and Leon Balents, Annu. Rev. Condens. Matter Phys. 5, 57 (2014).

[2] S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. van Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan, Phys. Rev. Lett.96, 087204 (2006).

[3] P. Chandra, P. Coleman and R. Flint, Nature 493, 621 (2013).