Low-frequency spin dynamics in the XY quantum spin ice Yb2Pt2O7

The XY pyrochlore Yb2Ti2O7, with pseudospin-1/2 at the Yb3+ site, has been celebrated as a potential host for the quantum spin ice state. The substitution of nonmagnetic Ti with Pt gives Yb2Pt2O7, a system with remarkably similar magnetic properties. The large nuclear gyromagnetic ratio (γn/2π=9.15 MHz/T) of Pt195 makes Yb2Pt2O7 an ideal material for NMR investigation of its unconventional magnetic properties. Based on the Pt195 nuclear spin-lattice relaxation rate 1/T1 and the magnetic specific heat Cp measured in a broad range of magnetic field Bext, we demonstrate that the field-induced magnon gap linearly decreases with Bext but an additional low-energy mode of spin excitations emerges below ∼0.5 T.