Compression-Driven Mass Flow in Bulk Solid ^{4}He.

Area of Science:

• Condensed Matter Physics
• Quantum Fluids and Solids

Background:

• Mass flow in solid helium-4 (⁴He) coexisting with superfluid helium-4 (⁴He) in Vycor has been observed, but its underlying mechanism is not fully understood.
• Previous studies were complicated by the presence of superfluid and solid-liquid interfaces within the Vycor porous glass.

Purpose of the Study:

• To investigate the intrinsic mass flow in solid ⁴He by eliminating the Vycor confinement and superfluid interactions.
• To understand the physical mechanism of mass flow in solid ⁴He at ultra-low temperatures.

Main Methods:

• Experiments were conducted using solid ⁴He crystals grown with extremely low concentrations of helium-3 (³He), down to x₃=5×10⁻¹².
• The absence of Vycor allowed for the study of bulk solid ⁴He flow.
• Measurements were performed down to 28 mK to observe flow behavior at ultra-low temperatures.

Main Results:

• Mass flow rate in solid ⁴He continued to increase down to at least 28 mK without showing signs of saturation.
• Ultra-low ³He concentrations (x₃=5×10⁻¹²) did not suppress the observed low-temperature flow.
• Even higher ³He concentrations (120 ppb), which previously suppressed flow, had no effect in these experiments, suggesting a different flow mechanism.

Conclusions:

• The results indicate that the mass flow in solid ⁴He is not suppressed at ultra-low temperatures as previously thought.
• The observed flow likely involves a larger area, such as a disordered liquid layer on the solid surface or grain boundaries, rather than being solely dependent on ³He impurities.
• This study provides new insights into the intrinsic properties of solid ⁴He and the mechanisms governing mass transport.