Visual Observations of Disordered Solid Helium

We report visual observations of solid helium from experiments conducted near the melting curve in the vicinity of 1.65 K. Given the recent interest in solid helium and the possible role of disorder, these observations conducted in 1974, but never reported in the literature, may be of some current interest. We show an image and sketches from the laboratory notebook from 1974 that document that solid helium can be milky (i.e. cloudy) in appearance and that this behavior persists for lengths of time longer than thirty minutes. We also report differences in the appearance of hcp and bcc solid ⁴He, with the bcc appearing more lumpy and disturbed by cracks, but also cloudy. These observations suggest that disorder, at least on the length scale of visible light, can be present in solid ⁴He and can persist for extended periods of time near the melting curve in the vicinity of 1.65 K.     

Effect of Crystal Quality on HCP-BCC Phase Transition in Solid 4He

The kinetics of HCP-BCC structure phase transition is studied by precise pressure measurement technique in ⁴He crystals of different quality. An anomalous pressure behavior in bad quality crystals under constant volume conditions is detected just after HCP-BCC structure phase transition. A sharp pressure drop of 0.2 bar is observed at constant temperature. The effect observed can be explained if we suppose that microscopic liquid droplets appear on the HCP-BCC interphase region in bad quality crystals. After the interphase region disappearance, these droplets are crystallized with pressure reduction. It is shown that this effect is absent in high quality thermal-treated crystals.     

Preliminary Report: Heat Pulse Propagation in Solid 4He up to 56 bar Between 40 and 500 mK

Heat pulse propagation in the solid ⁴He has been studied between 40 and 500 mK. Response to heat pulses are detected by a titanium film superconducting transition edge bolometer. Crossover behavior from second sound in normal solid above 500 mK to ballistic propagation below 200 mK is observed. Detailed study is made to search for possible modification of this propagation behavior by the appearance of supersolidity. It is found, that the ballistic phonon propagation velocity remains constant, within 0.3% scatter of data, below 100 mK at all pressures measured between 25 and 56 bar. The temporal evolution of the detected pulse shape has not revealed any anomaly below 200 mK.     

Anharmonic Effects in Neutron Scattering Studies of Lattice Excitations in BCC 4He

We report high resolution measurements of lattice excitations of bcc solid ⁴He using inelastic neutron scattering. The resulting dispersion relations of the phonons agree well with Path Integral Monte Carlo Simulations which we carried out in parallel with the experimental work. In addition, we studied the recently discovered two “optic-like” excitation branches. One of these branches is weakly dispersive, while the second one seems dispersionless. The dispersive optical branch interacts with the phonons. We suggest that this interaction may be responsible for some of the anharmonic effects reported in the past, such as phonon interference. We argue that the agreement of experimental results with the “Self Consistent Phonon Theory” may be improved if the coupling between the phonons and one of the “optic-like” excitation branches is included in the calculations.     

Growth Dynamics and Faceting of 3He Crystals

³He crystals start to show facets on their surface only at about 100 mK, well below the roughening transition temperature. To find out the reason for this discrepancy, we have performed the first quantitative investigation on the growth dynamics of the faceted and rough surfaces of ³He crystals in the temperature range of 60–110 mK. We have applied an original method to obtain the variation of the overpressure on the crystal surface by measuring its curvature and height locally using a Fabry–Pérot interferometer. The growth of the rough surface was found to be limited by the transport of the latent heat which elaborates in the liquid, in accordance with theoretical predictions (Puech L., et al. in J. Low Temp. Phys. 62:315, 1986; Graner F., et al. in J. Low Temp. Phys. 75:69, 1989 and 80:113, 1990) and previous measurements near the minimum of the melting curve (Graner F., et al. in J. Low Temp. Phys. 75:69, 1989 and 80:113, 1990). The mobility of an elementary step on a facet was shown to be limited by the latent heat transport as well. The values obtained for the step free energy are by two orders of magnitude smaller than at ultra low temperatures, which we show to be the result of quantum oscillations of the solid-liquid interface, which quickly become damped when temperature decreases below 100 mK.     

Freezing of Helium-4: Comparison of Different Density Functional Approaches

Solidification of superfluid helium-4 has been addressed within the framework of density functional theory. Early studies used a variational approach, approximating the density distribution in the solid phase by a sum of Gaussians on each lattice site. Recently, we performed an unconstrained minimization of the functional describing the helium system as reported by Ancilotto et al. (Phys. Rev. B, 72, 214522, 2005). At sufficiently high density, we find stable solid like solutions, which exhibit an anisotropic density profile around each lattice site. We compare these results to the previous variational approach, and attempt to improve the family of trial functions by adding a variational parameter to account for anisotropy.     

Homogeneous Nucleation of 4He Crystals by Acoustic Waves

We have recently found experimental evidence for the homogeneous nucleation of crystals in metastable liquid ⁴He at high pressure. For this we combined the focusing of a high intensity spherical acoustic wave with a simple light scattering technique. We discuss the analysis used to distinguish between nucleation of bubbles in the negative pressure swings of the wave from nucleation of crystals in the positive swings. We also discuss the interest of our results and future developments of our experiment in the general context of the study of nucleation and instability limits in phase diagrams.     

Grain Boundary Sliding at Serrated Grain Boundaries

A constitutive model is developed for grain boundary sliding (GBS) at serrated grain boundaries. Based on a previously developed GBS model, using the dynamics of grain boundary dislocation pile-up, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Thus, a geometric factor is introduced to account for the effects of serration wave length and amplitude on the GBS rate, as compared to the GBS rate at planar boundaries. By considering the role of grain boundary shear stress in stress balancing, the proposed model removes the singularity at planar boundaries which exists in the diffusion-controlled GBS model at serrated grain boundaries. The modified model describes very well the transient creep of complex Ni-base superalloys with and without grain boundary serrations and should be suitable for other engineering alloys (with the exception of columnar grained and single crystal alloys).     

Two-Dimensional Faceting Kinetics of Grain Boundaries

Faceting is characteristic of both special and incommensurate grain boundaries. The orientation of a facet is related to the misorientation angle between the grains. The faceting effect is analyzed here in a kinetic model for the faceting phase transition and the formation of pyramids of facets adjacent to one another.     

Wetting of Grain Boundaries in Cuprate Ceramics

The wetting of grain boundaries and the formation and transport properties of liquid-channel grain-boundary structures are considered using Bi₂CuO₄ ceramics as an example.     

Microwave-Frequency Vortex Dynamics in YBCO Grain Boundaries

We report measurements and modeling of microwave-frequency vortex dynamics in YBCO grain boundaries that are modeled as long Josephson junctions by numerically solving the sine-Gordon equation. YBCO bicrystal grain boundaries with misorientation angles from 2 to 24° have been studied experimentally using microwave resonator measurement techniques. Comparison between the measured and calculated microwave impedance and the harmonic generation of the 24° grain boundaries indicates that the 24° grain boundaries are weakly coupled long Josephson junctions. The corresponding results of lower angle grain boundaries are also presented. A transition from strong-coupled single-crystal-like behavior to weak-coupled Josephson-junction-like behavior has been observed in a 10° grain boundary between 55 and 75 K. The physics of Josephson-vortex dynamics and its impact on the microwave properties of superconducting thin films are discussed.     

Supersolidity and the Thermodynamics of Solid Helium

The specific heat of solid helium in the temperature range below about 1.2 K has been found to contain a term varying as T ⁷, in addition to the usual T ³ contribution always found in a crystalline dielectric solid. It has been proposed by Anderson, Brinkman and Huse, (Science 310, 1164 (2005)) that the existence of this T ⁷ term supports their theory of supersolidity. However, in this paper we show that corrections to the phonon specific heat arising from phonon dispersion are much larger than expected based on simple order of magnitude estimates and, as a consequence, it is very unlikely that the existence of this T ⁷ term can be considered as evidence for supersolidity.      

溶质原子在再结晶运动晶界上偏聚的研究

Ｂ~（１０）（ｎ，α）Ｌｉ~７裂变反应的径迹显微照相技术被用来研究含硼Ｆｅ－３０％Ｎｉ合金高温热变形再结晶过程中硼在晶界上的偏聚行为，实验表明，含硼Ｆｅ－３０％Ｎｉ合金１０００℃热变形２０％再结晶过程中，运动晶界前方发生硼偏聚而后方发生贫硼现象，而在静止晶界上看不到硼偏聚，随着再结晶新晶粒的长大，运动晶界上的硼偏聚变弱。本文提出，可以用晶界对溶质原子热运动散射的量子理论来解释溶质原子在运动晶界上的偏聚现象。     

Interaction of Hydrogen Atoms with Grain Boundaries in Palladium Bicrystals

Hydrogen diffusion in palladium bicrystals containing a small-angle twist or tilt boundary or a large-angle boundary similar to a special boundary is investigated using molecular dynamics simulation. We assess the effect of grain boundaries on the hydrogen diffusion process. The types of grain boundaries considered here are shown to differ in their absorption activity for hydrogen. The temporal grain-boundary segregation of hydrogen atoms can be accounted for in terms of their coordination, which differs significantly from that in the grain bulk.     

Microstructure of Carbides at Grain Boundaries in Nickel Based Superalloys

It is well known that carbides at grain boundaries play an important role in affecting mechanical properties of nickel based superalloys. In order to deeply understand the relationship between grain boundary structures and properties, in this work, we have investigated the microstructure of grain boundaries with different misorientation angles in bicrystals of nickel based superalloys. It is found that the bicrystals with smaller misorientation angles contain denser M 23 C 6 but sparse MC particles at grain boundaries, and this kind of bicrystals presents longer stress rupture lives. It was observed that MC carbides were decorated by M 23 C 6 and M 6 C particles at grain boundaries. The formation of these carbide particles, therefore, is likely due to the local fluctuation of chemical composition around MC carbides. In addition, the orientation relationships between MC carbides and γ/γ matrix were also determined.     

Supersolidity and Disorder in Solid Helium 4

Supersolidity is a rather controversial issue which has been revived recently by a number of torsional oscillator (ac-rotation) experiments with solid helium 4. One possibility would be that the ground state of helium 4 crystals contains a Bose-Einstein condensate of mobile vacancies. However, the supersolid signal was shown to depend on sample history. In addition, dc-flow experiments show that superfluid transport of mass does not occur in solid helium, except if it contains appropriate grain boundaries. As a consequence, we believe that supersolidity is not an intrinsic property of helium single crystals, but that it is due to quenched disorder. We report experiments on pressure relaxation in and light scattering from solid samples, that give additional evidence for the existence of disorder in helium crystals. We also describe our recent study of the wetting properties of grain boundaries.