Exotic Skyrmion Lattice Phase in Spin-Orbit-Coupled Bose-Einstein Condensates

Ultracold atoms - atoms at incredibly low temperatures that are only one billionth degree above absolute zero temperature - have been proven to be an ideal table-top system to reveal novel states of quantum matter. At such nano-Kelvin temperatures, bosonic atoms, such as ⁷Li, ²³Na, ⁴¹K and ⁸⁷Rb, accumulate into a single lowest-energy quantum state, forming highly coherent matter known as Bose-Einstein condensate (BEC). First observed in ⁸⁷Rb atoms in 1995, which won its discoverer a Nobel Prize, BEC in atomic vapours opened new ways to explore the fundamental physics of the intriguing quantum world, as well as to build atom lasers and other extraordinary technologies. The latest development in ultracold atoms concerns the engineering of a synthetic non-Abelian gauge field in ⁸⁷Rb BEC [Nature 471, 83 (2011)], which leads to the coupling between the spin and the orbital degrees of freedom of the atom. Such a spin-orbit coupling (SOC) creates a new frontier that is endowed with a strong interdisciplinary character and a close connection to other research fields, including condensed matter physics, quantum computation and astrophysics. SOC is the key ingredient underlying topological insulators and quantum Hall materials which are new types of functional materials that may lead to novel quantum devices. This explains the recent tremendous interest in studying the physics of SOC in the context of condensed matter physics and material science.

The particle that one needs to deal with in solid state materials is the electron, which is a fermion. Spin-orbit coupled BEC represents a brand new many-body quantum system that has no analogies in solids. Cold atoms form an ideal platform to explore the physics of such a system. In a paper appeared recently in Physical Review Letters, Hui Hu and Xia-Ji Liu from Swinburne University of Technology, Australia, and B. Ramachandhran and Han Pu from Rice University, the USA, reported theoretically the discovery of novel exotic quantum phases in a two-dimensional spin-orbit-coupled BEC in optical dipole traps. In this system, the interplay between the SOC and the inter-atomic interaction leads to a very rich phase diagram, with each phase featuring a distinct spin-texture pattern and symmetry class. The intricate spin texture, an example of which is shown in the figure, can arise purely from the SOC even when the interaction is completely spin-independent. This study will lead to critical understanding of spin-orbit coupled quantum systems and deepen our knowledge on such forefront concepts as topological order and strong correlation. In the longer term, it may contribute to the design and exploration of new functional materials.

This work was highlighted in the first issue of Asia Pacific Physics Newsletter (APPN) and was selected as the cover story. For more details, see, http://www.worldscinet.com/appn/

The cover of APPN features a diagram showing the spin texture representing a Skyrmion lattice phase in a trapped 2D two-component Bose-Einstein condensate in the presence of Rashba spin-orbit coupling. The arrows represent the transverse spin, while the background color represents the axial spin. [Hui Hu, B. Ramachandhran, Han Pu, and Xia-Ji Liu, Phys. Rev. Lett. 108, 010402 (2012)]