Ordering of hidden multipoles in spin-orbital entangled 5$d^{1}$ Ta chlorides

TL;DR

This paper investigates the ordering of hidden multipoles in spin-orbit entangled 5d^1 Ta chlorides, revealing suppressed magnetic moments and phase transitions linked to multipole ordering.

Contribution

It demonstrates the presence of hidden multipole orderings in Ta chlorides with suppressed magnetic dipole moments and identifies two low-temperature phase transitions.

Findings

Suppressed magnetic dipolar moment of ~0.2 μ_B in Ta chlorides.

Observation of two phase transitions with large electronic entropy.

Evidence of hidden multipole ordering at low temperatures.

Abstract

Spin-orbit coupling of as large as a half eV for electrons in 5$d$ orbitals often gives rise to the formation of spin-orbital entangled objects, characterized by the effective total angular momentum $J_{eff}$. Of particular interest are the $J_{eff}$ = 3/2 states realized in 5$d^{1}$ transition metal ions surrounded by an anion octahedron. The pure $J_{eff}$ = 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. Cs$_2$TaCl$_6$ and Rb$_2$TaCl$_6$ are correlated insulators with 5$d^{1}$ 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 ${\mu}_B$. Two phase transitions are observed at low temperatures that are not pronounced in the magnetization but accompanied with large electronic entropy of $R$ln4. We ascribe the two transitions to the ordering of hidden multipoles.