Nanoresonator and Superfluid Helium: Breakthrough in Quantum Fluids express
A frictionless liquid that flows forever: that's superfluidity. Helium-3 only achieves it at thousandths of a degree above absolute zero. The theory of this state was developed by Lev Landau and Richard Feynman. Here, quantum order is protected by an energy gap — an invisible armor, much like a taut string: as long as the pluck is gentle, the tone stays pure. In a new experiment, helium was confined in channels hundreds of nanometers wide and made to vibrate like a tiny string. At low amplitude, superfluidity held, but a stronger 'pluck' sent the frequency sliding downward. The string went out of tune: the flow breached the defense, suppressed the gap, and the system's entropy, a measure of disorder, grew.
By measuring the frequency drop, scientists calculated the critical speed at which the protection vanishes. The results matched the theory. This approach paves the way for ultra-sensitive sensors and quantum devices based on superfluid liquids.
Original abstract
as written by the authors arXiv:2603.27131
Superfluidity in $$^3$$He exhibits many unique properties that are of interest to modern condensed matter research, including multiple superfluid phase transitions, topological defects, and exotic classes of excitations like Majorana and Weyl fermions. Many of the most interesting theoretical proposals, which remain underexplored, are realized in highly confined geometries, where surface effects play a dominant role in the thermodynamic and hydrodynamic properties. We have developed nanofluidic resonators capable of exciting a fourth-sound acoustic mode in thin channels with a highly confined dimension ($$750-1800$$ nm) that is only $$1-2$$ orders of magnitude larger than the superfluid coherence length. When a sufficiently large drive force is applied, we observe a non-linear softening of the resonance that we interpret as due to the flow suppression of the superfluid gap. We have developed a model of the device that allows the resonance amplitude to be calibrated into a superfluid velocity, which exhibits critical behavior at particular velocities. We identify one of the observed critical velocities as being the velocity at which the gap component parallel to the flow is suppressed to zero. We compare the calibrated velocity to the prediction of a Ginzburg-Landau model, and find reasonable agreement. This measurement represents an ongoing effort to link the hydrodynamic measurements of these nanofluidic devices to theoretical predictions regarding surface gap suppression and surface-bound states.
In all ordinary helium, helium-3 is the rarest impurity: just one particle in a million. Its superfluidity 'switches on' at only 0.0025 degrees above absolute zero.