From Spin Glass to Quantum Spin Liquid Ground States in Molybdate Pyrochlores

We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: $S=1$ oxide ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ and $S=\frac{1}{2}$ oxynitride ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_5{\mathrm{N}}_2$. ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ undergoes a transition to an unconventional spin glass ground state at $T_f\sim 16\mathrm{K}$. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of $\mathrm{\Delta }\sim 0.05\mathrm{meV}$ or $\mathrm{\Delta }/|\theta |\sim 0.004$, in units of its antiferromagnetic Weiss constant $\theta \sim -121\mathrm{K}$.

Figure 1

(a) Total heat capacity $C$ of ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ (open circles) with the estimated lattice (dashed line) and magnetic $C_{\text{mag}}$ (closed circles) contributions. (b) The magnetic heat capacities of the oxide and oxynitride phases.

Figure 2

The magnetic scattering cross sections of the both samples at 1.5 K. The solid lines are fits to the data. The inset shows the elastic magnetic scattering measured on CNCS at 1.5 K. A scaled version of the fit of Eq. (1) to the CNCS oxide data is shown on top of the $D7$ data (dashed line).

Figure 3

The temperature dependence of background subtracted, normalized inelastic neutron scattering ($Q=[0.5,2.0]{\AA }^{-1}$) (top) and the energy width of the inelastic spectra, $\mathrm{\Gamma }$, (bottom) for the (a) oxide and (b) oxynitride, respectively.

Figure 4

Offset $E$ cuts ($Q=[0.5,2.0]{\AA }^{-1}$) for the oxynitride sample at various temperatures with fits (solid lines) of Eq. (2) to the data.

Figure 5

$E$ cuts of the oxide and oxynitride neutron scattering spectra at 1.5 K ($Q=[0.5,2.0]{\AA }^{-1}$). Inset: $Q$ dependence of the background subtracted inelastic scattering from the oxynitride sample at the same temperature, to which an $F(Q{)}^2$ form was fitted (solid line).