A Solid with a Superflow

Scientists at the Max Planck Institute for Dynamics and Self-Organization in Göttingen provide the first evidence of vacancy  induced superflow in solid helium

March 21, 2016

Experiments conducted by researchers at the Max Planck Institute for Dynamics and Self-Organization provide the first evidence that a large concentration of vacancies can produce a superflow in solid helium and consider this may be related to a Bose-Einstein condensation. The experiments are based on a prediction of this phenomena  by Galli and Reatto in 2001. The reason for their approach was that even though it was actually predicted several decades ago that solid Helium at zero Kelvin could exhibit superfluidity, the various experimental attempts trying to prove the existence of this new state of matter also dubbed “supersolidity” have failed.

Quantum effects in solid helium

In 1969-1970 Andreev and Lifshitz, and independently Chester and Leggett, predicted that solid helium at zero Kelvin could exhibit superfluidity resulting from Bose-Einstein condensation. A Bose-Einstein condensate is the state of matter of a dilute gas of bosons cooled to temperatures close to absolute zero (0 K or -273.15 C). In this state, the bosons exhibit macroscopic quantum phenomena including superfluidity – a flow without friction or viscosity. Then, in 2004, Kim and Chan at Pennsylvania State University provided the first experimental evidence for the effect, but later on discovered that, in an improved apparatus, the effect disappeared. Although their first experiment was validated by 9 other groups worldwide and triggered much theoretical and experimental activity to understand quantum effects in solid helium, presently there is no conclusive evidence for supersolidity.

Vacancy induced superfluidity

On a slightly different note, in 2001 Galli and Reatto predicted that solid helium with a large concentration of vacancies of several percent, an order of magnitude greater than at equilibrium, could exhibit a superfluid-like behavior at much higher temperatures of about one Kelvin. Their prediction could not be tested until recently since large vacancy concentrations are not accessible under equilibrium conditions and had to be created in a dynamic solid flow system with a cell exposed to vacuum through an orifice. In 2003 the research group in Göttingen, discovered that such a system exhibits periodic pressure pulses resulting from the sudden collapse of an upstream solid plug in the feed line due to the facile intrusion of vacancies propagating from the orifice region.

Evidence of superflow in solid helium with high vacancy concentrations

In the apparatus a high pressure pulse pushes solid helium through a capillary situated in a cell which is inside a liquid helium bath at temperatures of 1.3-2.6 K. Pressure gauges Phigh and Plow measure the pressure drop across the channel. Contrary to what is expected for a classical fluid, the flow rate measured externally is constant and does not depend on the pressure drop. The flow velocity, obtained from the flow rate divided by the cross section of the capillary, is orders of magnitude larger than that expected for the flow of a normal solid. As ilustrated in the greatly magnified view at the bottom, the exceptionally large flow of helium atoms is attributed to a frictionless upstream motion of a vacancy superfluid.

In the present experiments, published in the March 1 issue Physical Review B, (Vol. 93, Art. No. 104505) researchers at the Max Planck Institute inserted a micro-channel between the orifice and the upstream plug to study the pulsed downstream vacancy-induced flow of the solid. In the apparatus the pressure drop across and the flux through the channel are measured simultaneously. At pressures above about 30 bar and temperatures between 1.64 and 2.66 K, solid helium is observed to flow with a nearly constant and unusually large flow velocity of 20 cm/sec, which, surprisingly, is independent of the pressure drop across the capillary. Such a large constant flow velocity is unprecedented in ordinary solids, which at high concentrations of defects flow with velocities of the order of only 10-3 cm/sec. As Prof. Toennies mentioned, “Our research showed that solid helium has unusual flow properties, which no other solid has showed so far.”

The new observations are consistent with the predictions of Galli and Reatto and other more recent theories and provide the first evidence that a large concentration of vacancies can indeed produce a superflow in solid helium.

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