Yb$_{2}$Sn$_{2}$O$_{7}$: a quantum critical point approaching the ferromagnetic ordering from the quantum spin liquid side*

The neutron scattering measurements on pyrochlore Yb$_{2}$Sn$_{2}$O$_{7}$ show no long range ordering down to 0.05 K but appearance of diffuse scattering, low energy spin wave excitations, and temperature-independent relaxation time below 2 K, which indicate the system enters a quantum dynamics region with ferromagnetic interactions. The AC susceptibility further shows that Yb$_{2}$Sn$_{2}$O$_{7}$ enters a ``spin freezing'' region below 0.14 K. Our results suggest that Yb$_{2}$Sn$_{2}$O$_{7}$ sits on a quantum critical point by approaching the ferromagnetic ordering from the spin liquid side.