Quadrupolar Singlet Ground State of Praseodymium in a Modulated Pyrochlore
Abstract
The complex structure and magnetism of Pr$_{2x}$Bi$_x$Ru$_2$O$_7$ was investigated by neutron scattering and EXAFS. Pr has an approximate doublet groundstate and the first excited state is a singlet. This overall crystal field level scheme is similar to metallic Pr$_2$Ir$_2$O$_7$, which is also reported here. While the Bsite (Ru) is well ordered throughout, this is not the case for the Asite (Pr/Bi). A distribution of the PrO2 bond length indicates the Pr environment is not uniform even for $x=0$. The Bi environment is highly disordered ostensibly due to the 6s lone pairs on Bi$^{3+}$. Correspondingly we find the nonKramers doublet ground state degeneracy otherwise anticipated for Pr in the pyrochlore structure is lifted so as to produce a quadrupolar singlet ground state with a spatially varying energy gap. For $x=0$, below T$_N$, the Ru sublattice orders antiferromagnetically, with propagation vector \textbf{k}= (0,0,0), as for Y$_2$Ru$_2$O$_7$. No ordering associated with the Pr sublattice is observed down to 100 mK. The low energy magnetic response of Pr$_{2x}$Bi$_x$Ru$_2$O$_7$ features a broad spectrum of magnetic excitations associated with inhomogeneous splitting of the Pr quasidoublet ground state. For $x=0$ ($x=0.97$) the spectrum is temperature dependent (independent). It appears disorder associated with Bi alloying enhances the inhomogeneous Pr crystal field level splitting so that intersite interactions become irrelevant for $x=0.97$. The structural complexity for the Asite may be reflected in the hysteretic uniform magnetization of Bsite ruthenium in the Néel phase.
 Publication:

arXiv eprints
 Pub Date:
 July 2014
 arXiv:
 arXiv:1407.0661
 Bibcode:
 2014arXiv1407.0661V
 Keywords:

 Condensed Matter  Strongly Correlated Electrons
 EPrint:
 20 pages, 21 figures, submitted to PRB