NMR Investigation of the Low Temperature Dynamics of solid 4He doped with 3He impurities

TL;DR

This study uses pulsed NMR to investigate the low-temperature quantum tunneling dynamics of 3He impurities in solid 4He, revealing concentration-dependent relaxation behaviors linked to observed anomalies in other physical properties.

Contribution

It provides new insights into impurity motion in solid 4He at low temperatures through detailed NMR relaxation measurements across different 3He concentrations.

Findings

Significant changes in T_1 and T_2 near anomaly temperatures for low 3He concentrations.

No notable relaxation changes observed at higher 3He concentrations.

Results suggest impurity tunneling influences low-temperature anomalies in solid 4He.

Abstract

The lattice dynamics of solid 4He has been explored using pulsed NMR methods to study the motion of 3He impurities in the temperature range where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid 4He lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the 3He motion that results from quantum tunneling in the 4He matrix. The measurements were made for 3He concentrations 16<x_3<2000 ppm. For 3He concentrations x_3 = 16 ppm and 24 ppm, large changes are observed for both the spin-lattice relaxation time T_1 and the spin-spin relaxation time T_2 at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher 3He concentrations.