Exotic Skyrmion Lattice Phase in Spin-Orbit-Coupled Bose-Einstein Condensates
May, 2012
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)]
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