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Register a new userElectronic tunability of the frustrated triangular-lattice cluster magnet LiZn$_{2-x}$Mo$_3$O$_8$
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LiZn$_2$Mo$_3$O$_8$ is an electrically insulating geometrically frustrated antiferromagnet in which inorganic Mo$_3$O$_{13}$ clusters each behaves as a single $S = 1/2$ unit, with the clusters arranged on a two-dimensional triangular lattice. Prior results have shown that LiZn$_2$Mo$_3$O$_8$ does not exhibit static magnetic order down to at least $T = 0.05,K$, and instead possesses a valence bond ground state. Here, we show that LiZn$_2$Mo$_3$O$_8$ can be hole doped by oxidation with $mathrm{I}_2$ and subsequent removal of $mathrm{Zn}^{2+}$ cations to access the entire range of electron count, from one to zero unpaired electrons per site on the triangular lattice. Contrary to expectations, no metallic state is induced; instead, the primary effect is to suppress the number of sites contributing to the condensed valence-bond state. Further, diffraction and pair-distribution function analysis show no evidence for local Jahn-Teller distortions or other deviations from the parent trigonal symmetry as a function of doping or temperature. Taken together, the data and density functional theory calculations indicate that removal of electrons from the magnetic layers favors Anderson localization of the resulting hole and an increase in the electrical band-gap over the formation of a metallic and superconducting state. These results put strong constraints on the chemical conditions necessary to realize metallic states from parent insulating geometrically frustrated antiferromagnets.
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LiZn$_2$Mo$_3$O$_8$ has been proposed to contain $S~=~1/2$ Mo$_3$O$_{13}$ magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance (ESR), $^7$Li nuclear magnetic resonance (NMR), and muon spin rotation ($mu textrm{SR}$) spectroscopies are used to characterize the local magnetic properties of LiZn$_2$Mo$_3$O$_8$. These results show the magnetism in LiZn$_2$Mo$_3$O$_8$ arises from a single isotropic $S~=~1/2$ electron per cluster and that there is no static long-range magnetic ordering down to $T~=~0.07,textrm{K}$. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn$_2$Mo$_3$O$_8$.
The control of the stoichiometry of Li$_{1+x}$Zn$_{2-y}$Mo$_3$O$_8$ was achieved by the solid-state-reaction. We found that the best sample that has the chemical composition Li$_{0.95(4)}$Zn$_{1.92(8)}$Mo$_3$O$_8$ was obtained from the starting nominal composition with Li : Zn : Mo : O = $(1+w)$ : $(2.8-w)$ : $3$ : $8.6$ with $w = -0.1$, indicating that the stoichiometry is greatly improved compared to those in the earlier reports. For larger $w$ detailed structural analysis indicates that the mixed sites of Li and Zn are preferentially occupied by Li atoms, as well as the fraction of the non-magnetic secondary phase Zn$_2$Mo$_3$O$_8$ decreases. Magnetic susceptibility of the improved stoichiometry powder samples shows a broad hump in the temperature range of 100 $< T <$ 200 K. This suggests that the development of antiferromagnetic correlations at the high temperatures is inherent to the ideal stoichiometric LiZn$_2$Mo$_3$O$_8$.
Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with zig-zag antiferromagnetic (AFM) order. Here we report the discovery of zig-zag AFM order in the layered and non-centrosymmetric honeycomb nickelate Ni$_2$Mo$_3$O$_8$ through a combination of magnetization, specific heat, x-ray and neutron diffraction and electron paramagnetic resonance measurements. It is the first example of such order in an integer-spin non-centrosymmetric structure ($P$$_6$3$mc$). Further, each of the two distinct sites of the bipartite honeycomb lattice has a unique crystal field environment, octahedral and tetrahedral Ni$^{2+}$ respectively, enabling independent substitution on each sublattice. Replacement of Ni by Mg on the octahedral site suppresses the long range magnetic order and results in a weakly ferromagnetic state. Conversely, substitution of Fe for Ni enhances the AFM ordering temperature. Thus Ni$_2$Mo$_3$O$_8$ provides a platform on which to explore the rich physics of $S = 1$ on the honeycomb in the presence of competing magnetic interactions with a non-centrosymmetric, formally piezeo-polar, crystal structure.
The recently discovered material Cs$_3$Fe$_2$Br$_9$ contains Fe$_2$Br$_9$ bi-octahedra forming triangular layers with hexagonal stacking along the $c$ axis. In contrast to isostructural Cr-based compounds, the zero-field ground state is not a nonmagnetic $S=0$ singlet-dimer state. Instead, the Fe$_2$Br$_9$ bi-octahedra host semiclassical $S=5/2$ Fe$^{3+}$ spins with a pronounced easy-axis anisotropy along $c$ and interestingly, the intra-dimer spins are ordered ferromagnetically. The high degree of magnetic frustration due to (various) competing intra- and inter-dimer couplings leads to a surprisingly rich magnetic phase diagram. Already the zero-field ground state is reached via an intermediate phase, and the high-field magnetization and thermal expansion data for $Hparallel c$ identify ten different ordered phases. Among them are phases with constant magnetization of 1/3, respectively 1/2 of the saturation value, and several transitions are strongly hysteretic with pronounced length changes reflecting strong magnetoelastic coupling.
In this paper, we study the spin excitation properties of the frustrated triangular-lattice antiferromagnet Yb(BaBO$_3$)$_3$ with nuclear magnetic resonance. From the spectral analysis, neither magnetic ordering nor spin freezing is observed with temperature down to $T=0.26$ K, far below its Curie-Weiss temperature $|theta_w|sim2.3$ K. From the nuclear relaxation measurement, precise temperature-independent spin-lattice relaxation rates are observed at low temperatures under a weak magnetic field, indicating the gapless spin excitations. Further increasing the field intensity, we observe a spin excitation gap with the gap size proportional to the field intensity. These phenomena suggest a very unusual strongly correlated quantum disordered phase, and the implications for the quantum spin liquid state are further discussed.
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