An international team led by chemists at the University of British Columbia (UBC), Canada, has reported strong experimental evidence for a superfluid phase in molecular hydrogen at 0.4 K. This phase, theoretically predicted in 1972, had only been observed in helium and ultracold atomic gases until now, and never in molecules. The work could give scientists a better understanding of quantum phase transitions and collective phenomena. More speculatively, it could advance the field of hydrogen storage and transportation.
Superfluidity is a quantum mechanical effect that occurs at temperatures near absolute zero. As the temperatures of certain fluids approach this value, they undergo a transition to a zero-viscosity state and begin to flow without resistance – behaviour that is fundamentally different to that of ordinary liquids.
Previously, superfluidity had been observed in helium (3He and 4He) and in clusters of ultracold atoms known as Bose-Einstein condensates. In principle, molecular hydrogen (H2), which is the simplest and lightest of all molecules, should also become superfluid at ultracold temperatures. Like 4He, H2 is a boson, so it is theoretically capable of condensing into a superfluid phase. The problem is that it is only predicted to enter this superfluid state at a temperature between 1 and 2 K, which is lower than its freezing point of 13.8 K.
A new twist on a spinning experiment
To keep their molecular hydrogen liquid below its freezing point, team leader Takamasa Momose and colleagues at UBC confined small clusters of hydrogen molecules inside helium nanodroplets at 0.4 K. They then embedded a methane molecule in the hydrogen cluster and observed its rotation with laser spectroscopy.
Momose describes this set-up as a miniature version of an experiment performed by the Georgian physicist Elephter Andronikashvili in 1946, which showed that disks inside superfluid helium could rotate without resistance. They chose methane as their “disk”, Momose explains, because it rotates quickly and interacts only very weakly with H2, meaning it does not disturb the behaviour of the medium in which it spins.
Onset of superfluidity
In clusters containing less than six hydrogen molecules, they observed some evidence of friction affecting the methane’s rotation. As the clusters grew to 10 molecules, this friction began to disappear and the spinning methane molecule rotated faster, without resistance. This implies that most of the hydrogen molecules around it are behaving as a single quantum entity, which is a signature of superfluidity. “For clusters larger than N = 10, the hydrogen acted like a perfect superfluid, confirming that it flows with zero resistance,” Momose tells Physics World.
The researchers, who have been working on this project for nearly 20 years, say they took it on because detecting superfluidity in H2 is “one of the most intriguing unanswered questions in physics – debated for 50 years”. As well as working out how to keep hydrogen in a liquid state at extremely low temperatures, they also had to find a way to detect the onset of superfluidity with high enough precision. “By using methane as a probe, we were finally able to measure how hydrogen affects its motion,” Momose says.
A deeper understanding
The team say the discovery opens new avenues for exploring quantum fluids beyond helium. This could lead scientists to a deeper understanding of quantum phase transitions and collective quantum phenomena, Momose adds.
The researchers now plan to study larger hydrogen clusters (ranging from N = 20 to over a million) to understand how superfluidity evolves with size and whether the clusters eventually freeze or remain fluid. “This will help us explore the boundary between quantum and classical matter,” Momose explains.
They also want to test how superfluid hydrogen responds to external stimuli such as electric and magnetic fields. Such experiments could reveal even more fascinating quantum behaviours and deepen our understanding of molecular superfluidity, Momose says. They could also have practical applications, he adds.
“From a practical standpoint, hydrogen is a crucial element in clean energy technologies, and understanding its quantum properties could inspire new approaches for hydrogen storage and transportation,” he says. “The results from these [experiments] may also provide critical insights into achieving superfluidity in bulk liquid hydrogen – an essential step toward harnessing frictionless flow for more efficient energy transport systems.”
The researchers report their work in Science Advances.
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