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Register a new userHigh temperature series expansion study of the Heisenberg antiferromagnet on the hyperkagome lattice: Comparison with Na$_4$Ir$_3$O$_8$
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We develop high temperature series expansions for $ln{Z}$ and the uniform structure factor of the spin-half Heisenberg model on the hyperkagome lattice to order $beta^{16}$. These expansions are used to calculate the uniform susceptibility ($chi$), the entropy ($S$), and the heat capacity ($C$) of the model as a function of temperature. Series extrapolations of the expansions converge well down to a temperature of approximately $J/4$. A comparison with the experimental data for Na$_4$Ir$_3$O$_8$ shows that its magnetic susceptibility is reasonably well described by the model with an exchange constant $Japprox 300 K$, but there are also additional smaller terms present in the system. The specific heat of the model has two peaks. The lower temperature peak, which is just below our range of convergence contains about 40 percent of the total entropy. Despite being a 3-dimensional lattice, this model shares many features with the kagome lattice Heisenberg model and the material must be considered a strong candidate for a quantum spin-liquid.
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Recent experiments indicate that Na$_4$Ir$_3$O$_8$, a material in which s=1/2 iridium local moments form a three dimensional network of corner-sharing triangles, may have a quantum spin liquid ground state with gapless spin excitations. Using a combination of exact diagonalization, symmetry analysis of fermionic mean field ground states and Gutzwiller projected variational wavefunction studies, we propose a quantum spin liquid with spinon Fermi surfaces as a favorable candidate for the ground state of the Heisenberg model on the hyper-kagome lattice of Na$_4$Ir$_3$O$_8$. We present a renormalized mean field theory of the specific heat of this spin liquid and also discuss possible low temperature instabilities of the spinon Fermi surfaces.
We report on the effects of introducing magnetic and non-magnetic disorder in the hyperkagome iridate quantum spin liquid (QSL) candidate Na$_4$Ir$_3$O$_8$ by partially replacing Ir$^{4+}$ ($S = 1/2$) with Ru$^{4+}$ ($S = 1$) or Ti$^{4+}$ ($S = 0$). Specifically, we synthesized Na$_4$(Ir$_{1-x}$Ru$_x$)$_3$O$_8 (x = 0.05, 0.10, 0.2, 0.3)$ and Na$_4$Ir$_{2.7}$Ti$_{0.3}$O$_8$ samples and measured electrical transport, AC and DC magnetization, and heat capacity down to $T = 1.8$ K. Na$_4$Ir$_3$O$_8$ is associated with a large Weiss temperature $theta = -650$ K, a broad anomaly in magnetic heat capacity C$_{mag}$ at T $approx25$ K, low temperature power-law heat capacity, and spin glass freezing below $T_f approx 6$ K. We track the change in these characteristic features as Ir is partially substituted by Ru or Ti. We find that for Ru substitution, $theta$ increases and stays negative, the anomaly in C$_{mag}$ is suppressed in magnitude and pushed to lower temperatures, low temperature $C sim T^alpha$ with $alpha$ between $2$ and $3$ and decreasing towards $2$ with increasing $x$, and $T_f$ increases with increase in Ru concentration $x$. For Ti substitution we find that $theta$ and T$_f$ become smaller and the anomaly in $C_{mag}$ is completely suppressed. In addition, introducing non-magnetic Ti leads to the creation of orphan spins which show up in the low temperature magnetic susceptibility.
The hyperkagome antiferromagnet Na$_{4}$Ir$_3$O$_8$ represents the first genuine candidate for the realisation of a three-dimensional quantum spin-liquid. It can also be doped towards a metallic state, thus offering a rare opportunity to explore the nature of the metal-insulator transition in correlated, frustrated magnets. Here we report thermodynamic and transport measurements in both metallic and weakly insulating single crystals down to 150 mK. While in the metallic sample the phonon thermal conductivity ($kappa^{ph}$) is almost in the boundary scattering regime, in the insulating sample we find a large reduction $kappa^{ph}$ over a very wide temperature range. This result can be ascribed to the scattering of phonons off nanoscale disorder or off the gapless magnetic excitations that are seen in the low-temperature specific heat. This works highlights the peculiarity of the metal-insulator transition in Na$_{3+x}$Ir$_3$O$_8$ and demonstrates the importance of the coupling between lattice and spin degrees of freedom in the presence of strong spin-orbit coupling.
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$_8$.
We study the many-body electronic structure of the stoichiometric and electron-doped trilayer nickelate Pr$_4$Ni$_3$O$_8$ in comparison to that of the stoichiometric and hole-doped infinite layer nickelate NdNiO$_2$ within the framework of density functional plus dynamical mean field theory, noting that Pr$_4$Ni$_3$O$_8$ has the same nominal carrier concentration as NdNiO$_2$ doped to a level of 1/3 holes/Ni. We find that the correlated Ni-$3d$ shells of both of these low valence nickelates have similar many-body configurations with correlations dominated by the $d_{x^2-y^2}$ orbital. Additionally, when compared at the same nominal carrier concentration, the materials exhibit similar many-body electronic structures, self energies, and correlation strengths. Compared to cuprates, these materials are closer to the Mott-Hubbard regime due to their larger charge transfer energies. Moreover, doping involves the charge reservoir provided by the rare earth $5d$ electrons, as opposed to cuprates where it is realized via the oxygen $2p$ electrons.
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