title: From spin glass to quantum spin liquid ground states in molybdate pyrochlores.
abstract: We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: S=1 oxide Lu_{2}Mo_{2}O_{7} and S=1/2 oxynitride Lu_{2}Mo_{2}O_{5}N_{2}. Lu_{2}Mo_{2}O_{7} undergoes a transition to an unconventional spin glass ground state at T_{f}∼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 Δ∼0.05  meV or Δ/|θ|∼0.004, in units of its antiferromagnetic Weiss constant θ∼-121  K.
authors.affiliation: Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: L
authors.initials: L
authors.lastname: Clark
authors.affiliation: Institute Laue-Langevin, 6 Rue Jules Horowitz, 38042 Grenoble, France.
authors.forename: G J
authors.initials: GJ
authors.lastname: Nilsen
authors.affiliation: Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: E
authors.initials: E
authors.lastname: Kermarrec
authors.affiliation: Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
authors.forename: G
authors.initials: G
authors.lastname: Ehlers
authors.affiliation: ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, United Kingdom.
authors.forename: K S
authors.initials: KS
authors.lastname: Knight
authors.affiliation: CSEC and School of Chemistry, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom and Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0QX, United Kingdom.
authors.forename: A
authors.initials: A
authors.lastname: Harrison
authors.affiliation: CSEC and School of Chemistry, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom.
authors.forename: J P
authors.initials: JP
authors.lastname: Attfield
authors.affiliation: Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada and Brockhouse Institute for Materials Research, Hamilton, Ontario L8S 4M1, Canada and Canadian Institute for Advanced Research, 180 Dundas Street West, Toronto, Ontario M5G 1Z8, Canada.
authors.forename: B D
authors.initials: BD
authors.lastname: Gaulin