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Register a new userOrdering of hidden multipoles in spin-orbital entangled 5$d^{1}$ Ta chlorides
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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.
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Complex oxides with $4d$ and $5d$ transition-metal ions recently emerged as a new paradigm in correlated electron physics, due to the interplay between spin-orbit coupling and electron interactions. For $4d$ and $5d$ ions, the spin-orbit coupling, $zeta$, can be as large as 0.2-0.4 eV, which is comparable with and often exceeds other relevant parameters such as Hunds coupling $J_{rm H}$, noncubic crystal field splitting $Delta$, and the electron hopping amplitude $t$. This gives rise to a variety of spin-orbit-entangled degrees of freedom and, crucially, non-trivial interactions between them that depend on the $d$-electron configuration, the chemical bonding, and the lattice geometry. Exotic electronic phases often emerge, including spin-orbit assisted Mott insulators, quantum spin liquids, excitonic magnetism, multipolar orderings and correlated topological semimetals. This paper provides a selective overview of some of the most interesting spin-orbit-entangled phases that arise in $4d$ and $5d$ transition-metal compounds.
We have analyzed the experimental evidence of charge and orbital ordering in La0.5Sr1.5MnO4 using first principles band structure calculations. Our results suggest the presence of two types of Mn sites in the system. One of the Mn sites behaves like an Mn(3+) ion, favoring a Jahn-Teller distortion of the surrounding oxygen atoms, while the distortion around the other is not a simple breathing mode kind. Band structure effects are found to dominate the experimental spectrum for orbital and charge ordering, providing an alternate explanation for the experimentally observed results.
The ordered hexagonal perovskite Ba2CuTeO6 hosts weakly coupled S=1/2 spin ladders produced by an orbital ordering of Cu2+. The magnetic susceptibility chi(T) of Ba2CuTeO6 is well described by that expected for isolated spin ladders with exchange coupling of J~86 K but shows a deviation from the expected thermally activated behavior at low temperatures below T*~25 K. An anomaly in chi(T), indicative of magnetic ordering, is observed at T_mag=16 K. No clear signature of long-range ordering, however, is captured in NMR, specific heat or neutron diffraction measurements at and below T_mag. The marginal magnetic transition, indicative of strong quantum fluctuations, is evidence that Ba2CuTeO6 is in very close proximity to a quantum critical point between a magnetically ordered phase and a gapped spin liquid controlled by inter-ladder couplings.
Transition metal oxides exhibit various competing phases and exotic phenomena depending on how their reaction to the rich degeneracy of the $d$-orbital. Large spin-orbit coupling (SOC) reduces this degeneracy in a unique way by providing a spin-orbital-entangled ground state for 4$d$ and 5$d$ transition metal compounds. In particular, the spin-orbital-entangled Kramers doublet, known as the $J_{mathbf{eff}}$=1/2 pseudospin, appears in layered iridates and $alpha$-RuCl$_3$, manifesting a relativistic Mott insulating phase. Such entanglement, however, seems barely attainable in 3$d$ transition metal oxides, where the SOC is small and the orbital angular momentum is easily quenched. From experimental and theoretical evidence, here we report on the CuAl$_2$O$_4$ spinel as the first example of a $J_{mathbf{eff}}$=1/2 Mott insulator in 3$d$ transition metal compounds. Based on the experimental study, including synthesis of the cubic CuAl$_2$O$_4$ single crystal, density functional theory and dynamical mean field theory calculations reveal that the $J_{mathbf{eff}}$=1/2 state survives the competition with an orbital-momentum-quenched $S$=1/2 state. The electron-addition spectra probing unoccupied states are well described by the $j_{mathbf{eff}}$=1/2 hole state, whereas electron-removal spectra have a rich multiplet structure. The fully relativistic entity found in CuAl$_2$O$_4$ provides new insight into the untapped regime where the spin-orbital-entangled Kramers pair coexists with strong electron correlation.
Crystal and magnetic structures of the high-pressure stabilized perovskite phase of TlMnO3 have been studied by neutron powder diffraction. The crystal structure involves two types of primary structural distortions: a+b-b-octahedral tilting and antiferrodistortive type of orbital ordering, whose common action reduces the symmetry down to triclinic P -1. The orbital pattern and the way it is combined with the octahedral tilting are different from the family of LnMnO3 (Ln = lanthanide or Y) manganites who share with TlMnO3 the same tilting scheme. The experimentally determined magnetic structure with the k = (1/2,0,1/2) propagation vector and P_S-1 symmetry implies anisotropic exchange interactions with a ferromagnetic coupling within the (1,0,-1) planes and an antiferromagnetic one between them (A type). The spins in the primary magnetic mode were found to be confined close to the (1,0,-1) plane, which underlines the predominant role of the single ion anisotropy with the local easy axes of Mn3+ following the Jahn-Teller distortions of the octahedra. In spite of the same octahedral tilting scheme in the perovskite structures of both LnMnO3 and TlMnO3 manganites, a coupling of the secondary ferromagnetic component to the primary A-type spin configuration through antisymmetric exchange interaction is allowed in the former and forbidden in the latter cases.
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