Ordering of hidden multipoles in spin-orbit entangled 5d1 Ta chlorides

Abstract

Spin-orbit coupling for an electron in 5d orbitals is as large as 0.5 eV, and gives rise to the formation of spin-orbital entangled objects, characterized by the effective total angular momentum Jeff. Of particular interest are the Jeff = 3/2 states realized in 5d 1 transition metal ions surrounded by an anion octahedron. The pure Jeff = 3/2 quartet does not have any magnetic dipolar moment (<M> = 0) but hosts hidden pseudo-dipolar moments accompanied by charge quadrupoles and magnetic octupoles. Promising materials to realize such physics include Cs2TaCl6 and Rb2TaCl6, which are correlated insulators with 5d 1 Ta 4+ ions in a regular Cl octahedron. Here we demonstrate that these Ta chlorides have a substantially suppressed effective magnetic dipolar moment of ~ 0.2 B and behave almost like a non-magnetic insulator. The four-fold degeneracy of Jeff = 3/2 is lifted only at low temperatures due to the ordering of charge quadrupolar and pseudo-dipolar moments. While the transition is not magnetically pronounced, it can be observed due to recovery of the full Rln4 entropy.

Jahn-Teller Effect and Spin-Orbit Coupling: Friends or Foes?

“…That is, depending on the parameters of the system such as SOC λ and lattice anharmonicity, one may expect for d 1 with strong SOC both the JT contraction and JT elongation. This is nearly unthinkable for the JT effect in the case of t 1 2g configuration without SOC, but apparently this is exactly what is observed experimentally for systems A 2 TaCl 6 , with A ¼ K, Rb, Cs [18]. The JT distortion of the "usual" sign, c=a < 1, was observed for larger ions Rb, Cs, but the opposite distortion was found for the smaller A ion K. First, in these results we see that indeed the JT distortions are preserved even for the 5d ion Ta 4þ (d 1 ) with strong SOC.…”

“…One of the few known statements in this field is that the appearance of the j eff ¼ 1=2 state in Ir 4þ ions "kills" the JT distortions: strong SOC lifts the threefold t 2g degeneracy of the t 5 2g levels and leads to the formation of the j eff ¼ 1=2 Kramers doublet, without any extra degeneracy. Only rarely is the JT physics mentioned in the present literature devoted to such systems; notable exceptions are the discussions of the JT effect in excited states of Ir compounds [15], the discovery of the role of pseudo-JT effect on lattice distortions in Sr 2 IrO 4 and orbital order in Ca 2 RuO 4 [16], possible appearance of SU(4) "Kugel-Khomskii-type" interaction in d 1 systems with strong SOC on a honeycomb lattice [17], and the experimental study of unusual type of distortion in the 5d 1 system K 2 TaCl 6 [18]. But the general answer to the question formulated in our title-are the JT effect and SOC friends or foes-seems to remain unanswered.…”

Spin-orbital-lattice entangled states in cubic d1 double perovskites

“…The relevance of the spin-orbital-lattice entanglement is expected in other cubic d 1 [4,12,15] and d 2 double per- ovskites [55][56][57][58][59], and also in other types of cubic crystals such as 5d 1 Ta chlorides persisting cubic symmetry down to low temperature [60]. For the complete understanding of the unconventional magnetic phases of the family of cubic double perovskites containing heavy transition metal, concomitant treatment of the vibronic and magnetic interactions is found to be crucial.…”

Magnetoelectronic properties of ferromagnetic compounds Rb2TaZ6 (Z = Cl, Br) for possible spintronic applications

“…They observed purple color for Rb 2 TaCl 6 and blue color for Rb 2 TaBr 6 . Ishikawa et al [ 31 ] experimentally confirmed the cubic structure of Rb 2 TaCl 6 with a lattice constant of 10.0686 Å. This shows that the existing literature about these compounds is only limited to structure exploration.…”