Spin-Echo Measurements for an Anomalous Quantum Phase of 2D Helium-3
Abstract
Previous heat-capacity measurements of our group had shown the possible existence of an anomalous quantum phase containing the zero-point vacancies (ZPVs) in 2D 3He. The system is monolayer 3He adsorbed on graphite preplated with monolayer 4He at densities () just below the 4/7 commensurate phase (0.8≤ /4/7≤ 1). We carried out pulsed-NMR measurements in order to examine the microscopic and dynamical nature of this phase. The measured decay of spin echo signals shows the non-exponential behaviour. The decay curve can be fitted with the double exponential function, but the relative intensity of the component with a longer time constant is small (5%) and does not depend on density and temperature, which contradicts the macroscopic fluid and 4/7 phase coexistence model. This slowdown is likely due to the mosaic angle spread of Grafoil substrate and the anisotropic spin-spin relaxation time T2 in 2D systems with respect to the magnetic field direction. The inverse T2 value deduced from the major echo signal with a shorter time constant, which obeys the single exponential function, decreases linearly with decreasing density from n=1, supporting the ZPV model.