BEC-BCS crossover with exciton-polaritons and ultracold fermions at finite interaction range

胡辉 教授
Swinburne University of Technology
2019-12-20 (周五) 10:00

Microcavity electron-hole-photon systems are long anticipated to exhibit a crossover from Bose-Einstein condensate (BEC) to Bardeen-Cooper-Schrieffer (BCS) superfluid, when carrier density is tuned to reach the Mott transition density. Yet, theoretical understanding of such a BEC-BCS crossover largely relies on the mean-field framework and the nature of the carriers at the crossover remains unclear to some extent. The BEC-BCS crossover is also known to occur in ultracold atoms confined in optical traps. In this talk, I will introduce some recent results on the universal BEC-BCS crossover physics from the theory group at Swinburne University.

In the first part of the talk, motivated by the recent demonstration of a BCS polariton laser [1], I will discuss the properties of a strongly interacting BCS polariton condensate at thermal equilibrium [2]. Based on a simplified short-range description of the electron-hole attraction, we examine the role of quantum fluctuations in an exciton-polariton condensate and determine the number of different type carriers at the crossover beyond mean-field. Near Mott density and with ultra-strong light-matter coupling, we find an unexpectedly large phase window for the strongly correlated BCS polariton condensate, where both fermionic Bogoliubov quasi-particles and bosonic excitons are significantly populated and strongly couple to photons. We predict its absorption spectrum and show that the upper polariton energy gets notably renormalized, giving rise to a high-energy side-peak at large carrier density, as observed in recent experiments.

In the second part, I will talk about spin-1/2 ultracold Fermi gases confined in two dimensions with a tight harmonic trapping potential along the z-direction. Here, virtual excitations along the tightly-confined direction play the same role of photons in microcavities. In the case of an additional harmonic trap added on the xy-plane, the Fermi cloud shows an interesting violation of a classical scale invariance (i.e., the so-called quantum anomaly), as indicated by its breathing mode frequency. We present theoretical understanding of such a quantum anomaly from both few-body [3] and many-body perspective [4]. We also find the formation of cluster states at a positive interaction range [5], similar to biexcitons, triexcitons and bipolaritons observed in microcavities.



[1] J. Hu, Z. Wang, S. Kim, H. Deng, S. Brodbeck, C. Schneider, S. Höfling, N. H. Kwong, and R. Binder, Signatures of a Bardeen-Cooper-Schrieffer Polariton Laser, arXiv:1902.00142v1.

[2] H. Hu and X.-J. Liu, Quantum fluctuations in a strongly interacting BCS polariton condensate at thermal equilibrium, arXiv:1910.06494v1.

[3] X.-Y. Yin, H. Hu, and X.-J. Liu, Few-body perspective of quantum anomaly in two-dimensional interacting Fermi gases, arXiv:1907.10195v1.

[4]  H. Hu, B. C. Mulkerin, U. Toniolo, L. He, and X.-J. Liu,  Phys. Rev. Lett. 122, 070401 (2019).

[5]  X.-Y. Yin, H. Hu, and X.-J. Liu, Phys. Rev. Lett. 123, 073401 (2019).