T. Mizusaki

Application of nuclear magnetic resonance pore structure analysis to porous silica glass

The internal pore structure of a series of porous silica glasses has been investigated using nuclear magnetic resonance (NMR). Longitudinal and transverse relaxation of protons in deionized water confined to the pores of the samples has been measured and interpreted to provide pore size distributions. Two sets of leached, phase‐separated porous glasses were used: the first nominally bimodal, the second with a more uniform pore space with pore sizes varying from 32 to 1350 A. Our results provide a stringent test of NMR relaxation as a method for pore size analysis. NMR relaxation experiments were performed on one sample as a function of the amount of water imbibed confirming the validity of the theoretical model used to interpret the magnetic resonance data.

Orientation effect on superfluid 3He in anisotropic aerogel

The orientation of the order parameter in the A-like and B-like phases of superfluid 3He immersed in uniaxially compressed aerogel is reported. With the use of NMR methods, it is found that the orbital momentum of the A-and B-like phases is oriented along the deformation. In the A-like phase, a relatively narrow NMR line with an anomalously large negative frequency shift is observed. The Leggett frequency in the A-like phase, which shows the same energy gap suppression as in the B-like phase, is measured.

Coherent precession of magnetization in the superfluid 3He A-phase.

We report the first observation of coherent precession of magnetization in superfluid 3He A-like phase (CP-A) in aerogel. The coherent precession in bulk 3He A-phase is unstable due to the positive feedback of spin supercurrent to the gradient of phase of precession. It was predicted that the homogeneous precession will be stable if the orbital momentum of the 3He A-phase can be oriented along the magnetic field. We have succeeded to prepare this configuration by emerging 3He in uniaxially deformed anisotropic aerogel. The dissipation rate of coherent precession states in aerogel is much larger than that in bulk 3He B-phase. We propose a mechanism of this dissipation.

NMR study of the solidification and melting of3He in porous glasses

TheP-T phase diagrams of the liquid-solid phase transition of3He in three porous glasses with different pore sizes have been determined from spin-lattice relaxation measurements in the temperature range 0.5–4.2 K. The onset of solidification of3He in the pores occurs at excess pressure over the bulk phase transition. The excess pressure depends on the pore size. A model of the phase transition in small pores which takes into account the contribution of the surface energy to the free energy is described and compared with experimental results. TheT1 relaxation mechanism of3He in the pores is found to be due to the surface relaxation when3He is in the liquid phase and due to the relaxation of bulk solid3He when it is in the solid phase.

Crystal growth of U2D2 solid3He

We have measured the growth rate of single crystal U2D2 solid3He in the superfluid B phase as a function of chemical potential difference Δμ between the solid and liquid for temperatures between 0.4 and 0.9 TN. The growth rate was not proportional to Δμ, with very slow growth rates for Δμ less than about 10 erg/cm3. For larger values of Δμ the growth rate depended weakly on temperature and the growth was attributed to a quantum growth mechanism due to screw dislocations. The melting rate was too fast to be measured quantitatively, but was slower at higher temperatures.

Comments on Heterogeneous Nucleation in Helium

We first recall some basic ideas on homogeneous and heterogeneous nucleation. We then reconsider the heterogeneous nucleation of solid helium from liquid helium in the presence of walls. Recent experiments by Ruutu et al. demonstrate that the presence of walls drastically reduces the energy barrier for such “heterogeneous” nucleation events, although walls are usually more favorable to the liquid phase. In order to explain this reduction, we propose a simple model based on the unpinning of an already existing interface. In the light of such results, we then reconsider the nucleation of the B-phase of superfluid helium 3 from its A-phase. We propose a model where the B-phase nucleation is thermally activated on walls, without need of cosmic rays.

Spin dynamics and onset of Suhl instability in bcc solid3He in the nuclear-ordered U2D2 phase

Pulsed NMR experiments have been performed on U2D2 solid3He with a single domain in high fields where the Larmor frequency was much larger than the zero-field antiferromagnetic resonance frequency. The free induction signal decayed rapidly under certain conditions. The rapid decay is attributed to the onset of the instability of the uniform precession. We propose a model for the instability due to the self-induced emission from the upper-mode magnon to the lower-mode magnon which is similar to the Suhl instability in electronic ferromagnets. A large negative frequency shift was observed during the rapid decay, which is consistently explained by the model. Under stable conditions of the spin motion outside of the instability region, we observed the tipping-angle-dependent frequency shift and multiple spin echoes, both of which agree well with Namaizawas theory.

Spin dynamics of superfluid3He-B in a slab geometry

The spin dynamics and the spin relaxation mechanisms of the superfluid3He-B were studied by using the NMR method in a slab geometry, where the superfluid3He-B was confined between narrow parallel plates with a gap smaller than the healing length of then-texture and the magnetic field was applied parallel to the plates. The relaxation parameter in the Leggett-Takagi (LT) equations was determined from a line width measurement of the transverse CW NMR. By using the pulsed NMR method, spin dynamics were studied in the nonlinear region. The observed spin dynamics were in good agreement with a numerical calculation of the LT equations together with the relaxation parameter determined by the CW NMR. When the tipping angle became larger than a certain critical value, the superfluid3He-B entered the Brinkman-Smith (BS) state. In this case, we observed the slow relaxation process in the BS state and then the rapid recovery process from the BS state to the initial “non-Leggett configuration.” The slow process in the BS state was attributed to the surface relaxation mechanism due to the torque from the surface-field energy.

Visualization of phase-separated boundary in liquid 3He–4He mixtures by MRI

A magnetic resonance imaging technique (MRI) for ultra low-temperature physics was developed. The MRI was applied to visualize the shape of a phase-separated boundary of liquid 3He}4He mixtures as a function of temperature near the tricritical point. We determined the capillary length and the contact angle of the boundary against a sample cell wall. The interfacial tension calculated from the capillary length agreed with reported values and we estimated the spatial resolution of the MRI to be a few 10 lm. ( 2000 Elsevier Science B.V. All rights reserved.

The second order recombination and spin-exchange relaxation of atomic deuterium at low temperatures in a low magnetic field

We measured the 2nd order recombination rates and spin-exchange relaxation of atomic deuterium (D) in a4He coated sample cell, using the hyperfine resonance of β(F = 1/2, mF= −1/2) − δ(F = 3/2, mF= −1/2) transition in a low magnetic field (3.9 mT) at temperatures between 0.6 K and 1.2 K. At lower temperatures below 0.9 K, the density decay of D atoms was dominated by D-D recombination on the liquid He surface. We found that the surface recombination cross length was 1DD= (5.5 ± 1.3) × 10−9cm and the adsorption energy of D on4He surface was εa= 3.97 ± 0.07 K. Compared with prior measurements at high magnetic fields by other groups, 1DDat low field was orders of magnitude smaller than what was expected when the scaling of 1/B2dependence of the direct recombination mechanism was used, and in addition, εawas significantly larger. This was attributed to the onset of the resonant recombination mechanism for the D-D surface recombination at high fields. Above 0.9 K, D-D volume recombination and recombination of D with hydrogen impurity became dominant processes of the density decay of D. The transverse relaxation times were measured and we determined the D-D spin-exchange relaxation rates, GDD= (1.4 ± 0.6) × 10−10cm3sec−1. It was smaller than theoretical calculations.