Intrinsic quantum disorder in Yb2Ti2O7 and the quantum S=1/2 pyrochlore phase diagram

We present an experimental and theoretical study of the anisotropic pyrochlore phase diagram. Inelastic field-dependent neutron scattering on Yb${}_2$Ti${}_2$O${}_7$ shows intrinsic broadening and a flat low-energy magnon mode which is partially captured by interacting magnon models. Exact diagonalization reveals the existence of an emergent quantum phase between ferromagnetism and antiferromagnetism, in which Yb${}_2$Ti${}_2$O${}_7$ Hamiltonian potentially resides. This behavior matches the phenomenology of quantum criticality in heavy fermion systems, and shows Yb${}_2$Ti${}_2$O${}_7$ is a clean system which can be field-tuned from well-defined magnons to a nontrivial quantum ground state. This suggests that quantum criticality is a generic feature of the dipolar phase diagram.