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 Lu2Mo2O7 and S = 1/2 oxynitride Lu2Mo2O5N2. Lu2Mo2O7 undergoesa transition to an unconventional spin glass ground state at T-f similar to 16 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 Delta similar to 0.05 meV or Delta/vertical bar theta vertical bar similar to 0.004, in units of its antiferromagnetic Weiss constant. theta similar to -121 K.