The first unequivocal experimental evidence of a superfluid state in 2D 4He films

Researchers experiment with their setup. Credit: Choi et al.
In order to study the interaction between superfluid phases and supersolid phases, physicists around the world have tried to use the 4He film layer adsorbed on graphite substrates for the past several decades. This layer has been used by some teams to collect torsional oscillator measurements (TO), including P.A. Crowell, F.W. Van Keuls, J.D. Reppy at Cornell University as well as Dr. Jan Nyeki at Royal Holloway and his colleagues.

The Korea Advanced Institute of Science and Technology has recently made improvements to these measurements, obtaining the first unambiguous experimental evidence of a superfluid condition in 2D 4He films on a graphite substrate. The results, published in Physical Review Letters suggest that the second layer 4He films may be a promising candidate for superfluid or supersolid phases.

Eunseong K, one of the researchers involved in the study, stated that it was the first experiment to show unambiguous evidence of a superfluid layer. He used a rigid, two frequency TO to prove the point. This TO is radically improved over the conventional TO.

A reduction in the TO time within 4He films has been interpreted by past studies as an indicator of superfluidity. Studies in the past have shown that TO measurements can also be sensitive to non-superfluid effects like changes in viscoelastic properties. If an experiment isn't carefully planned, TO effects could easily be amplified and overestimated.

Kim stated that there was a TO experiment which examined viscoelastic properties of 4He films adherbed to a disordered substrate. "In several different TOs, the elastic anomaly led to a temperature-dependent period reduction with a slow onset behavior, very similar to what was observed in the second layer of 4He on graphite. This suggests that it is possible that some TOs in previous studies might be sensitive to the unwanted viscoelastic effect. Our new measurement will be crucial in answering the question, "Does superfluid exist at the second layer?" This research will be carried forward.

Kim explained that there were two ways to find the true superfluidity. Kim suggested a TO with a rigid structure to filter out unwanted viscoelastic contributions and Kim suggested a TO with multiple resonance frequencies to determine the frequency dependence of the signal. The TO experiments that met the above criteria were pivotal in reconciling conflicting observations. They led to the conclusion of superfluid-mimicking signals being caused by the viscoelastic change in solid 4He.

Kim and his collaborators collected a two-frequency TO measurement which unambiguously confirmed the existence of superfluidity in the graphite substrate's second layer of 4He film. The researchers claim that this is the first TO measurement of superfluidity on the second layer that has been able to be reliably detected.

Kim stated that "the pioneering work of Crowell & Reppy, Nyeki & Saunders opened a new research avenue and provided new insights." Their TOs did not disentangle superfluid signals from other effects. They often observed a mismatch in the data of the empty-TO periods and the TO period data with nonsuperfluid 4He films (which was called a "composite background").

According to past research, rigid TO measurements are distinguished by the absence of a tilted or composite background. The absence of 'composite backgrounds' in past studies could be due to viscoelastic coupling between samples and TO methods.

Kim and his collaborators measured the temperature dependence on the TO. This TO included a helium movie with two different resonance periods (i.e. frequencies). If there is no superfluid, the temperature dependence will reveal a monotonic increase with decreasing temperature.

"Superfluid Helium decouples from oscillations and reduces the resonance period (or speeds up the frequencyoscillates) due to its reduced mass. Kim stated that superfluid responds independent of driving frequency because it completely decouples with oscillation. Kim said that non-superfluid mechanisms such as the viscoelastic reaction show frequency dependence, because the coupling between the helium and the TO is dependent on the driving frequency.

Kim and his colleagues found that the frequency shift they observed was independent of the measurement frequency. This suggests that the anomaly they discovered is indeed associated with superfluidity. These findings are a significant advance in the understanding of low-dimensional 4He adsorbed onto ordered substrates as well as quantum fluids and solids.

Kim stated, "Another distinctive contribution of our work was that we add new understanding about the relationship between superfluid order and structural order." Kim stated that it was essential to create a phase diagram and accurately determine the film coverage. To accurately measure coverage, we used an in-situ pressure gauge to simultaneously take TO and vapor pressure measurements. We also proposed a region where superfluid order and solid order coexist.

Kim and his coworkers found that the area where the superfluid & solid order coexist was consistent with the most recent diffusive Monte Carlo study. These findings led to the conclusion that two unusual quantum phases emerged in the second layer 4He films.

Other teams had previously measured the vapor pressure of their sample outside the refrigerator. They then set the second-layer promotion to 11.4 atoms/nm2 and defined their coverage scale. It is difficult to determine their absolute coverage scale. Kim and his coworkers were able to identify the areas where superfluid as well as solid orders coexist. This is essential for reliably detecting supersolid phases.

This study provided unambiguous evidence of the superfluid stage, thanks to radical improvements in TO techniques. It is a feat that has never been achieved before. It also confirmed theoretical predictions that the superfluid phase and the solid order coexist within a certain coverage range.

Kim stated that "our work opens up a new era by reconciling different theoretical and experimental studies." Kim stated that even though the second layer was extensively studied, its phase diagram didn't converge to a single conclusion. Crowell & Reppy's coverage of superfluidity did not match the area where heat capacity anomaly was reported. Nyeki and Saunders didn't find the coverage area for the supersolid candidate that corresponded to the region where superfluidity and solid order are co-existing in numerical calculations. Our study does suggest that it is possible to combine all of these results in a coherent fashion.

Coexistence of superfluid phase and solid phase is a key topic of research in many fields. This includes condensed matter physics to AMO Physics. Kim and his colleagues will likely use the two intertwined and exotic quantum phases to inform theoretical and experimental research in different areas.

Kim said, "The next obvious question to ask is whether the helium movie has structural order in its areal density where superfluid appears." Kim said, "This is critical because it is a direct indication of the supersolid state: coexistence of superfluidity & crystalline order." A new mechanical oscillator is being developed that will allow us to probe the structural order at this stage.

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