How dipolar interactions shape two-dimensional superfluid behavior

In a recent study, researchers made a significant observation of the Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a 2D dipolar gas of ultracold atoms. This work marks a milestone in understanding how 2D superfluids behave with long-range and anisotropic dipolar interactions. The researchers are an international team of physicists, led by Prof. Jo Gyu-Boong from the Department of Physics at the Hong Kong University of Science and Technology (HKUST).

Their findings are published in the journal Science Advances.

In conventional three-dimensional (3D) systems, phase transitions, such as ice melting into water, are governed by the spontaneous breakdown of symmetries. However, pioneering work in the 1970s predicted that two-dimensional (2D) systems could host a unique topological phase transition known as the BKT transition, where vortex-antivortex pairs drive superfluidity without conventional symmetry breaking, with interaction playing a crucial role. Since then, this phenomenon had primarily been studied in various quantum systems with only short-range isotropic contact interactions.

Unlike contact interactions in conventional ultracold gases, dipolar interactions span the entire system, creating rich collective behaviors. The team’s experiments demonstrated how the dipolar interaction modifies the critical parameters of the BKT transition.

“Dipolar interaction brings a new aspect to quantum many-body phenomena,” says Prof. Jo. “Microscopically, their interactions are directional and far-reaching, meaning that particles ‘feel’ each other even when widely separated. This challenges our intuition about how order emerges in low dimensions.”

Their observation suggests that the 2D superfluid transition in dipolar gases can still be governed by the BKT scenario, while the interaction-dependent transition point is shifted by dipoles’ orientation with respect to the normal direction.

He Yifei, a graduate student led by Prof. Jo and one of the leading authors, added, “The 2D dipolar system is a long-sought platform where exotic phases could exist. We have observed the unique nonlocal effect and anisotropic density-density correlation in 2D dipolar superfluid when aligning all the dipoles in the plane, even with moderate dipolar interaction. It would be interesting to further increase the dipolar strength and see how the system organizes itself in low dimensions.”