Mass Flow through Solid ^{3}He in the bcc Phase.

Area of Science:

• Condensed Matter Physics
• Quantum Fluids and Solids
• Low-Temperature Physics

Background:

• Experiments suggest mass transport in solid helium-4 (⁴He) at low temperatures indicates superflow, but its nature is not fully understood.
• Solid helium-3 (³He), a Fermi isotope, offers a system where quantum effects are more pronounced, yet superfluidity is not anticipated, enabling comparative studies.

Purpose of the Study:

• To investigate mass transport in solid body-centered cubic (bcc) ³He using flow measurements.
• To compare the flow behavior of solid ³He with that of solid ⁴He to elucidate the nature of mass transport in solid helium isotopes.

Main Methods:

• Performed flow measurements on high-purity bcc ³He in a cell previously used for observing superfluid-like responses in hexagonal close-packed (hcp) ⁴He.
• Varied temperature conditions, including measurements down to 30 mK and below 100 mK, to observe flow rate dependencies.
• Analyzed flow behavior in relation to temperature, pressure, and sample density, including samples that remelted at low temperatures.

Main Results:

• Observed mass flow in solid ³He, which monotonically decreased with temperature, contrasting sharply with the behavior of ⁴He.
• Identified thermally activated flow near melting with an activation energy of 0.85 K, with residual flow persisting down to 30 mK.
• Found essentially constant flow rates in solid ³He below 100 mK, even in low-density samples, indicating a quantum mechanical origin.

Conclusions:

• The distinct behaviors of solid ³He and ⁴He support the interpretation of superflow in ⁴He.
• The temperature-independent flow in ³He below 100 mK, despite the absence of superfluidity, points to a quantum origin for this transport.
• Concluded that mass flow in both thermal and quantum regimes involves the motion of dislocations, driven by thermally activated or tunneling motion of kinks.