“…These jogs are annealed out at K thanks to the thermal activation of vacancies, after which the network length is only limited by the network nodes. Annealing effects around K have been observed in other experiments, for example by Suhel and Beamish [20], when studying the relaxation of pressure gradients.Fig. 5Crystal T2 with a negligible He concentration of is an opportunity to study annealing in helium crystals.…”
Section: Resultsmentioning
confidence: 75%
“…However, the stresses involved should not exceed few millimeters of hydrostatic pressure difference. That value is small compared to the yield stress of helium crystals that is larger than a few millibars [20, 21], and is unlikely to create plastic deformations. Consequently, pinning and the associated jumps should not create dislocations, which could have explained why our dislocation densities are so much larger than Ruutu’s.…”
The giant plasticity of He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996–1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm, sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10–10 per cm, that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood.
“…These jogs are annealed out at K thanks to the thermal activation of vacancies, after which the network length is only limited by the network nodes. Annealing effects around K have been observed in other experiments, for example by Suhel and Beamish [20], when studying the relaxation of pressure gradients.Fig. 5Crystal T2 with a negligible He concentration of is an opportunity to study annealing in helium crystals.…”
Section: Resultsmentioning
confidence: 75%
“…However, the stresses involved should not exceed few millimeters of hydrostatic pressure difference. That value is small compared to the yield stress of helium crystals that is larger than a few millibars [20, 21], and is unlikely to create plastic deformations. Consequently, pinning and the associated jumps should not create dislocations, which could have explained why our dislocation densities are so much larger than Ruutu’s.…”
The giant plasticity of He crystals has been explained as a consequence of the large mobility of their dislocations. Thus, the mechanical properties of dislocation free crystals should be quite different from those of usual ones. In 1996–1998, Ruutu et al. published crystal growth studies showing that, in their helium 4 crystals, the density of screw dislocations along the c-axis was less than 100 per cm, sometimes zero. We have grown helium 4 crystals using similar growth speeds and temperatures, and extracted their dislocation density from their mechanical properties. We found dislocation densities that are in the range of 10–10 per cm, that is several orders of magnitude larger than Ruutu et al. Our tentative interpretation of this apparent contradiction is that the two types of measurements are somewhat indirect and concern different types of dislocations. As for the dislocation nucleation mechanism, it remains to be understood.
“…with P L at 28.5 and P R at 27.3 bar) is found to persist for the duration of observation (more than 56 hours) provided the temperatures of the solid is kept below 0.8 K. This indicates a complete absence of flow across the solid. Above 0.8 K, thermal diffusion induced mass flow is found [21,22]. A total of five attempts were made and no evidence of mass flow was found in any of these attempts below 0.8 K. More details on the growth of solid in aerogel and search of mass flow can be found in Supplementary Materials V [16].…”
We studied the transport of 4 He atoms through 2.5 mm thick solid 4 He samples sandwiched between two superfluid leads with five different tailored made sample cells. Measurements in a cell with a barrier at the center of the solid samples and in a cell filled with silica aerogel establish the causal relation between the observed mass flow and the dislocation network in the solid sample. Comparing the results from these cells and prior measurements on solid samples with thicknesses of 2 cm and 8 µm reveals that the mass flow rate decreases logarithmically with the thickness of the solid 4 He. Interestingly the mass flow exhibit both superfluid and Bosonic Luttinger liquid characteristics at low temperature.
“…At low temperatures, solid helium can sustain pressure gradients if the shear stresses are smaller than the critical stress for plastic flow, σ c ∼ 40 mbar [22]. This is the case for the compressions in our measurements [21], so only helium near the Vycor end is involved in the low temperature mass transfer; the solid further away can remain at the pressure generated by the initial compression.…”
mentioning
confidence: 94%
“…This cannot be used for calibration (since liquid leaves the cell during compression), but the rapid response guarantees that the Vycor is not a flow bottleneck when the 4 He is superfluid. For the solid sample, at high temperature (1.45 K) [21] where thermally activated vacancy diffusion ensures pressure equilibrium throughout the cell [16,22], the same 150 V squeeze generated a 100 mbar increase. This implies a 0.04% reduction of the cell volume, which corresponds to a displacement at the center of the diaphragm Δd s ≈ 0.5 μm [21].…”
We report the results of flow experiments in which two chambers containing solid ^{4}He are connected by a superfluid Vycor channel. At low temperatures and pressures, mechanically squeezing the solid in one chamber produced a pressure increase in the second chamber, a measure of mass transport through our solid-superfluid-solid junction. This pressure response is very similar to the flow seen in recent experiments at the University of Massachusetts: it began around 600 mK, increased as the temperature was reduced, then decreased dramatically at a temperature, T_{d}, which depended on the ^{3}He impurity concentration. Our experiments indicate that the flow is limited by mass transfer across the solid-liquid interface near the Vycor ends, where the ^{3}He collects at low temperature, rather than by flow paths within the solid ^{4}He.
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