Since 1995, several new quantum states of matter have been created in ultracold gases such as the celebrated Bose-Einstein condensates (BECs) which are atoms or subatomic particles cooled to near absolute zero or - 459.67 °F. This achievement was followed by the creation and detailed exploration of degenerate Fermi gases, Tonks-Girardeau gas of hard-core bosons, and spin-orbit-coupled BEC, all fundamental states of quantum matter. A recent addition to the set is the prediction and experimental demonstration of quantum droplets (QDs), built of coherent atomic waves in binary (two-component) BEC. This is an extension of BEC beyond the limits of the usual mean-field (MF, alias semi-classical) approximation, with the averaged action of quantum fluctuations around the MF states (known as the Lee-Huang-Yang effect) leading to drastic changes in static and dynamical properties of the quantum gas.
A recent review of experimental and theoretical results for QDs, in both 3D and effectively 2D settings, addressed the condensates with contact and dipole-dipole interactions. The theoretical part includes results for QDs with embedded vorticity, which have not yet been created in the experiment.
To further explore the theory, a research team led by Dr. Yong-Yao Li at Foshan University published their new findings in Frontiers of Physics co-published by Higher Education Press and Springer Nature.
Another ingredient of various 2D and 3D models which helps to stabilize zero-vorticity and vortical solitons is a spatially periodic (lattice) potential. In the experiment, such a potential can be readily induced in the form of an optical lattice, i.e., a spatially periodic force exerted onto atoms in BEC by a resonant optical field, created by pairs of laser beams illuminating the condensate in opposite directions. The new findings make an important step forward in the theoretical analysis of 2D QDs, by adding a lattice potential.
"The paper addresses a sufficiently interesting topic, viz., the interplay of the Lee-Huang-Yang (LHY) corrections to the mean-field dynamics of BEC and a spatially periodic 2D potential. Addition of lattice potentials helps to produce new species of stable fundamental and vortical quantum droplets in two dimensions. The analysis is systematic, and produces comprehensive results for the existence and stability of several species of 2D soliton-like modes, both fundamental and vortical ones." Said Boris A. Malomed at Tel Aviv University, Israel.
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This work was supported by the National Natural Science Foundation of China through Grant Nos.11905032 and 11874112, the Key Research Projects of General Colleges in Guangdong Province through Grant No. 2019KZDXM001, the Foundation for Distinguished Young Talents in Higher Education of Guangdong through Grant No. 2018KQNCX279, and the Special Funds for the Cultivation of Guangdong College Students Scientific and Technological Innovation (No. xsjj202005zra01).
NOTE: The study first appeared online on December 02, 2020. Its print version was published in Frontiers of Physics Volume 16, Issue 3, on June 15, 2021.
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Frontiers of Physics