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Breakthrough in Understanding 2D Dipolar Gases: The BKT Phase Transition

A team of international physicists, headed by Prof. JO Gyu-Boong from the Department of Physics at the Hong Kong University of Science and Technology (HKUST), has achieved a noteworthy milestone in the study of two-dimensional (2D) systems. Their recent research observed the Berezinskii-Kosterlitz-Thouless (BKT) phase transition within a 2D dipolar gas composed of ultracold atoms, significantly enhancing our comprehension of superfluid dynamics under long-range and anisotropic dipolar interactions.

Traditionally, phase transitions in three-dimensional (3D) systems, such as the melting of ice into water, are understood through the lens of symmetry breaking. However, groundbreaking theoretical work from the 1970s suggested that 2D systems could exhibit distinct topological transitions, like the BKT transition, which is characterized by vortex-antivortex pairs facilitating superfluidity without typical symmetry breaking. Interaction among particles plays a key role in this phenomenon. Until now, most research on the BKT transition had primarily focused on quantum systems that utilized short-range isotropic contact interactions.

In contrast, the dipolar interactions in this research extend throughout the entire system, fostering a wealth of collective phenomena. The team’s experimental findings illustrate how these dipolar interactions influence the critical parameters associated with the BKT transition, opening new avenues for exploration in the field.

According to Prof. Jo, “Dipolar interaction introduces a novel dimension to quantum many-body physics. These interactions, which are both directional and extensive, allow particles to ‘sense’ each other over considerable distances. This poses intriguing questions about how order emerges in lower-dimensional systems.” Their results indicate that the BKT scenario still governs the transition in 2D superfluid states, although the orientation of the dipoles relative to the normal direction alters the interaction-dependent transition point.

HE Yifei, a graduate student under Prof. Jo and one of the principal authors of the study, remarked, “The 2D dipolar system is a long-desired experimental platform where we could expect the emergence of exotic phases. We have detected unique non-local effects and anisotropic density-density correlations in our 2D dipolar superfluid when all dipoles are aligned within the plane, even with moderate dipolar interactions. It will be fascinating to further augment the dipolar strength and investigate how the system organizes itself in lower dimensions.”

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