Mass flow has been observed in solid ${}^4\mathrm{He}$ coexisting with superfluid confined in Vycor, but its physical mechanism remains an open question. Here we report observations of flow in experiments in which Vycor has been eliminated, allowing us to study the intrinsic flow in solid ${}^4\mathrm{He}$ without the complications introduced by the presence of superfluid and the associated solid-liquid interfaces. By growing crystals with ${}^3\mathrm{He}$ concentration as low as $x_3=5\times {10}^{-12}$, we also avoided the low temperature flow suppression observed in previous experiments and found that the flow rate continued to increase down to at least 28 mK without saturation. In addition, ${}^3\mathrm{He}$ concentrations of 120 ppb, which suppressed most of the low temperature flow in previous experiments, had no effect in our samples. The larger ${}^3\mathrm{He}$ concentrations needed to block the bulk solid flow suggest that the mass flow involves a larger area, such as disordered liquid layer on solid surface and grain boundaries.

• Received 18 February 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.025301