Finding solid ground – Experimental evidence for the existence of supersolids

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Supersolids and superfluids rank among the most exotic of quantum mechanical phenomena.
Superfluids can flow without any viscosity, and experience no friction as they flow along the walls of
a container, because their atoms ‘condense’ into a highly coherent state of matter. Supersolids are
also characterized by coherent effects, but between vacancies in a crystal lattice rather than between
the solid’s atoms themselves.
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The reduction in the rotational inertia of a bar of solid helium-4 as it was cooled to very low temperatures
provided the first experimental evidence for supersolids. Physicists interpreted the reduction to mean
that some amount of supersolid helium had formed and decoupled from the remainder of the bar, affecting
its rotational inertia and frequency. Others argued that the reduction in inertia resulted from a change in the
helium’s viscosity and elasticity with temperature, rather than from the onset of supersolidity.
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Kimitoshi Kono from the RIKEN Advanced Science Institute in Wako, Eunseong Kim from KAIST in Korea, and
their colleagues from these institutes have now disproved the alternative interpretation by simultaneously
measuring the shear modulus (a measure of viscosity and elasticity) and the rotational inertia of a solid helium-4
cell as its temperature dropped from 1 kelvin to 15 thousandths of a kelvin1. The cell was made to rotate clockwise
and then counterclockwise periodically, as well as to rotate clockwise or counterclockwise continuously .
The continuous rotation affected the inertial mass of the helium but its shear modulus, allowing these quantities
to be monitored independently.
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Under continuous rotation, the degree of change in the rotational inertia had a clear dependence on rotation velocity
, while the shear modulus did not. In addition, the energy dissipated by the rotation increased at high speeds. Both of
these observations contrast to what would be expected if viscoelastic effects were at play, rather than supersolidity.
The researchers also found that periodic rotation and continuous rotation affected the rotation differently, raising new
questions about the experimental system.
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The data support the interpretation that changes in the rotational inertia of helium-4 at low temperature result from
supersolidity. This is important because of the novel and surprising nature of the phenomenon itself, says Kono.
“Superfluidity in a solid is a very radical concept which, if proven, is certainly a good candidate for the Nobel Prize
” he adds. “Therefore the first priority is to determine whether it can be proven in a fashion that will convince the
low-temperature physics community.”
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