NMR investigation of the low-temperature dynamics of solid4He doped with3He impurities

Abstract

The lattice dynamics of solid 4 He has been explored using pulsed NMR methods to study the motion of 3 He impurities in the temperature range where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid 4 He 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 3 He motion that results from quantum tunneling in the 4 He matrix. The measurements were made for 3 He concentrations 16 < x 3 < 2000 ppm. For 3 He 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 3 He concentrations.

Simulation and understanding of atomic and molecular quantum crystals

“…Several nuclear magnetic resonance studies have shown that at low temperatures 3 He atoms in solid 4 He behave as quantum quasi-particles that can move through the lattice at velocities as high as ∼ 1 mm/s (Allen, Richards, and Schratter, 1982;Kim et al, 2013), that is, significantly higher than 45 µm/s. It has been argued then that near the dislocation the mobility of isotopic impurities could be reduced considerably by the existing local strain (Balibar et al, 2016); however, there is not quantitative evidence showing that such a huge variation of about three orders of magnitude in the mobility of 3 He impurities could be actually possible.…”

Movement of dislocations dressed withHe3impurities inHe4crystals

“…In 4 He crystals, large quantum fluctuations have at least two important consequences. First, 3 He atoms move freely at velocities of order 1 cm/s through the lattice [5,6]. They can also bind to dislocations [7,8], in which case there is a thermodynamic equilibrium between the bound 3 He atoms in the potential well of dislocation cores and a gas of free 3 He quasiparticles in the bulk crystal.…”