Search Results (14 total)

The growth anisotropy of different facets has been measured in ³He crystals at 0.55 mK using a low-temperature Fabry-Pérot interferometer and high-resolution pressure measurements. The observed linear dependence of the growth velocity on the driving force shows that facets grow due to the presence of dislocations. The values of the obtained step energies suggest that ³He has stronger coupling of the liquid-solid interface to the lattice than has been expected. The dependence of the step energy versus the step height is consistent with a quartic power law pointing out that the step-step interactions are of elastic origin.

Faceting has been observed on ³He crystals investigated with a low-temperature Fabry-Pérot interferometer. Nine types of facets were clearly identified during growth of a bcc- ³He single crystal at a temperature of 0.55 mK, while previously only three types of facets have been seen. Because of the weak coupling between the liquid-solid interface and the solid lattice in ³He the facets are apparently too small to be observed in equilibrium. The number of facets observed in our experimental conditions is consistent with the theory of dynamic roughening.

We elucidate the melting process of highly magnetized solid ³He by observing the magnetization profile and the liquid-solid interface simultaneously. Clear enhancements of magnetization and magnetization gradients at the interface of both the solid and the liquid were observed during melting. These measurements provide a mesoscopic confirmation of the melting scenario of Castaing and Nozières, and explain the long delay before the instability sets in: The magnetization gradient in the liquid leads to an initial suppression of the melting instability, in accordance with our extension of the stability analysis of Puech et al. This resolves the discrepancy between theory and experiment.

A melting instability has been observed during rapid melting of highly magnetized solid ³He. The instability occurred only if the solid is grown at low initial temperature and in high magnetic field, i.e., with high magnetization, and if the solid is melted sufficiently rapidly. After the instability of the interface occurred, the solid formed many cellular dendrites, directed parallel to the magnetic field. This is the first observation where a clear influence is seen of the magnetic field and the magnetization on a growth and melting process. The instability is attributed to a Mullins-Sekerka–type instability due to the magnetization gradient at the interface in the solid.

We present the first measurements of the viscosity of superfluid ³He in magnetic fields up to 15 T, at temperatures down to 2 mK along the melting curve, using an assumption for the temperature dependence of the normal density. At fields higher than 4 T, the viscosity shows a minimum in the A₁ phase, and reaches a local maximum at the A₂ transition which becomes global at 14.6 T. The sharp decrease just below the A₁ transition can be understood within the Ginzburg-Landau framework and is consistent with both theory and experiments in lower fields. At temperatures below the minimum, the viscosity in the A₁ phase is dominated by a term with T^-2 dependence. Additionally, we have calculated the temperature dependence of the viscosity tensor in the low temperature limit using the method of temperature asymptotes.

Images of ³He crystals at temperatures down to 1 mK have been obtained by means of a new type of optical cryostat, relying on the use of a compact charge-coupled device camera inside a nuclear demagnetization refrigerator. During growth from the superfluid ³He-A phase, some crystals have shown second and third roughening, i.e., the presence of (100) and (211) facets in addition to the (110) facets previously observed. The presence of these new facets is in accordance with the Kosterlitz-Thouless theory of the roughening transition.

We have measured the behavior of a vibrating-wire viscometer in superfluid ³He in the A₁ and A₂ phases in magnetic fields up to 9.2 T and at various pressures. In reduced units the viscosity in the A₁ phase is, within our accuracy, independent of magnetic field and displays a minimum as a function of temperature for sufficiently high magnetic fields. Slightly below the A₂ transition the viscosity shows an anomalous dip, which could indicate a new phase or a textural transformation. We offer a preliminary explanation for the observed anomalies.

The polarization dependence of the viscosity of spin-polarized liquid ³He has been determined in the temperature range 75<T<300 mK and the polarization range Δ<0.35. We find an increase in the viscosity, which can be described as η∝1+αΔ² with α=4.0±1.5, at a pressure of 27 bars. This increase is expected from the ratio of the cross sections for s-wave and p-wave scattering. It is rather surprising, however, that there is no temperature dependence of α.

Magnetic resonance at 1420 MHz in zero magnetic field and for 0.06<T<0.5 K has been used to determine the binding energy of H on liquid ³He, the rate constant for recombination and the frequency shift for H on ³He, and the sticking probability for H on ³He and ⁴He. The binding energy for H on liquid ³He is found to be 0.42±0.05 K, and the sticking probabilities are 0.035 for H on ⁴He and 0.016 for H on ³He.

Magnetic-resonance studies of the 1420-MHz transition in atomic hydrogen have yielded an accurate value for the binding energy of atomic H on liquid ⁴He. A value of 0.93±0.05 K is derived from two independent determinations of the surface binding: one from the shift in the hyperfine frequency, and the other from the recombination rate. The rate constant for H-H recombination in zero field and the hyperfine frequency of an H atom in the bound surface state have also been determined.

High-resolution magnetic-resonance studies are reported for an atomic hydrogen gas confined in a closed glass bulb with superfluid-⁴He-coated walls in zero magnetic field and for 1.0<T<1.3 K. H atoms at low density, 10¹¹<n_H<5×10¹²/cm³, in the equilibrium helium vapor, are found to recombine into molecules in times or order 10³ s. Results are presented for the rate constant for recombination, the diffusion constant and pressure shift for H in ⁴He, and for T₁ due to H-H spin exchange.

The orientational ordering phase transition in solid hydrogen has been studied as a function of para concentration and pressure up to 6 kbar. Measurements are consistent with the Pa3 structure for all densities and high ortho concentration. The density dependence of the critical temperature is consistent with that for the electric quadrupole-quadrupole interaction. When doped with p-H₂, with increasing density the critical concentration decreases anomalously below the zero-pressure value of 0.56.