Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systems Yb2Ti2O7 and Yb2Sn2O7

International audience ; We report a positive muon spin relaxation and rotation (mu SR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature T-c at which the specific heat displays an intense peak, namely T-c = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above T-c up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the mu SR spectra recorded above T-c. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T-c. Considering previously reported ac susceptibility, neutron scattering, and mu SR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.