On September 17th, Dr. Li Yun from Universita di Trento and INO-CNR BEC Center(Italy) was invited by Prof. Jiang Kaijun to visit Wuhan Institute of Physics and Mathematics (WIPM, CAS) and delivered a lecture entitled "Bose-Einstein condensed gas with equal Rashba and Dresselhaus spin-orbit coupling". Dr. Li Yun obtained her doctoral degree from East China Normal University and Ecole Normale Superieure Paris., and has been carrying out postdoctoral research with Sandro Stringari, a prominent theoretist in the field of ultracold atomic physics, at the Universita di Trento and the BEC Center since then. She had done outstanding jobs on both atomic spin squeezing, universal thermodynamic behaviors of degenerate Fermi gases and spin-orbit coupled BEC, and some of them were published on Nature and PRL.
In the research of ultracold atomic physics, simulating complex phenomena in condensed matter physics has always been a frontier topic, of which the realization of artificial gauge field in atomic gases is recognized as a landmark. Recently, by two-photon Raman processes, spin-orbit coupling has been implemented in BEC and degenerate Fermi gases, which correlates the spin and orbital degree of freedom of the neutral atoms. The dynamical and thermodynamical properties of this novel system represent a hot topic in current theoretical and experimental research.
Dr. Li’s lecture was based on three recent papers and one of them appeared on PRL. Her lecture gave a detailed discussion on the ground state and excited state properties of the spin-1/2 BEC system with equal Rashba and Dresselhaus spin-orbit coupling. First, in the mean-field approximation, the ground state can be solved using variational method by employing plane-wave spinon trial wavefunctions, and it exhibits three different phases with variable spin-orbit coupling strength, and by calculating the energy and spin-polarization per particle near the phase transition points, the order of these phaser transitions were determined. Interestingly, Dr. Li showed that there exists a tricritical point in this system, which represents a novel property in such systems. Since most experiments concerning spin-orbit coupling effects were carried out in optical lattices or optical dipole traps, she invesigated the case with external harmonic confinement through numerically solving Gross-Pitaevskii Equations, and found that there exists no foundamental differences between the uniform and the confined systems, and particularly, quantitative agreements were reached when Local Density Approximations were taken into accounted. Then, Dr. Li presented the dynamic behaviors of the system in various ground state phases, and variational methods were used combined with several sum rules to obtain the dipole oscillation frequencies of the harmonically trapped system. By comparing the calculation results with the newly experimental data acquisted by Shuai Chen et al at USTC, agreement between theory and experiment was demonstrated and good correspondence was found far away from the phase transitions. Furthermore, She gave a concise review on their work on the excited state properties of this system, the excitation spectrum and sound velocity across vastly variable spin-orbit coupling strength regime were analyzed employing time-dependent variational methods and hydrodynamic equations simultaneously. At the end of the lecture, Dr. Li also made some remarks on the experimental implications of their theoretical results.
The lecture had attracted experts and students both in experimental and theoretical research, and Dr. Li carried out productive communication and discussion with research staff and students on the validity, mathematical derivation, physical essence of the model and possible experimental investigations.This is her second visit to our institute since her first one in 2010, and she has established close collaboration with several research fellows in our institute to continue theoretical research in combination with the ongoing experiments.