Non-magnetic ground state in A$_2$WCl$_6$ (A = Cs, Rb, K): A face-centered cubic system of spin-orbit-entangled $J$ = 2 states

T. Takayama; K. Ishii; S. Bette; J. Nuss; Y. Matsumoto; K. Fürsich; M. Minola; D. P. Sari; I. Watanabe; A. Krajewska; R. Dinnebier; B. Keimer; H. Takagi

arXiv · 2606.08019

Non-magnetic ground state in A$_2$WCl$_6$ (A = Cs, Rb, K): A face-centered cubic system of spin-orbit-entangled $J$ = 2 states

T. Takayama·K. Ishii·S. Bette·J. Nuss·Y. Matsumoto·K. Fürsich·M. Minola·D. P. Sari·I. Watanabe·A. Krajewska·R. Dinnebier·B. Keimer·H. Takagi

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Abstract

Heavy transition metal compounds with strong spin-orbit coupling appeared as a platform for $d$-electron multipolar physics. We report the electronic, magnetic, and structural properties of antifluorite-type tungsten chloride A$_2$WCl$_6$ (A = Cs, Rb, and K), comprising a face-centered cubic lattice of W$^{4+}$ ions. The 5$d^2$ configuration of W$^{4+}$ ions in a cubic environment yields a spin-orbit-entangled $J$ = 2 state, which has been discussed to give rise to multipolar ordering such as charge quadrupolar or magnetic octupolar ordering. We found that K$_2$WCl$_6$ undergoes a cubic-to-tetragonal structural transition which lifts the degeneracy of the $J$ = 2 state, leading to a non-magnetic singlet ground state. By contrast, Rb$_2$WCl$_6$ and Cs$_2$WCl$_6$ show no signs of phase transition and remain non-magnetic down to the lowest temperature measured. At low temperatures, signatures of weak structural anomalies were revealed, which may point to the presence of local distortions of the WCl$_6$ octahedra. We argue that the subtle structural distortion arises from the local quadrupolar component of the $J$ = 2 state but the frustrated quadrupolar interaction, together with chemical disorder, inhibits the formation of long-range quadrupolar ordering.

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T. Takayama, K. Ishii, S. Bette, J. Nuss, Y. Matsumoto, K. Fürsich, M. Minola, D. P. Sari, I. Watanabe, A. Krajewska, R. Dinnebier, B. Keimer, H. Takagi. 2026-06-06. Non-magnetic ground state in A$_2$WCl$_6$ (A = Cs, Rb, K): A face-centered cubic system of spin-orbit-entangled $J$ = 2 states. https://arxiv.org/abs/2606.08019

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