Spin-Echo Measurements for an Anomalous Quantum Phase of 2D Helium-3

TL;DR

This study uses spin-echo measurements to investigate an anomalous quantum phase in 2D helium-3, revealing non-exponential decay behaviors and supporting the zero-point vacancy model over phase coexistence explanations.

Contribution

It provides microscopic dynamical evidence for the zero-point vacancy phase in 2D helium-3 using pulsed-NMR, challenging previous coexistence models.

Findings

Non-exponential spin echo decay observed

Longer component is small and density-independent

Inverse T2 decreases linearly with density

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 $^{3}$He. The system is monolayer $^{3}$He adsorbed on graphite preplated with monolayer $^{4}$He at densities ($\rho$) just below the 4/7 commensurate phase ($0.8\leq \rho /\rho_{4/7}\leq 1$). 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 $T_{2}$ in 2D systems with respect to the magnetic field direction. The inverse $T_2$ 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.