Showing papers by "Jinwu Ye published in 2014"

Coherence lengths in attractively interacting Fermi gases with spin-orbit coupling

Abstract: Extensive research has been lavished on the effects of spin-orbit couplings (SOCs) in attractively interacting Fermi systems in both neutral cold-atom systems and condensed-matter systems. Recently, it was suggested that a SOC drives a class of BCS to Bose-Einstein condensate (BEC) crossover that is different from the conventional one without a SOC. Here, we explore what are the most relevant physical quantities to describe such a BCS to BEC crossover and their experimental detections. We extend the concepts of the coherence length and ``Cooper-pair size'' in the absence of SOC to Fermi systems with SOC. We investigate the dependence of chemical potential, coherence length, and Cooper-pair size on the SOC strength and the scattering length at three dimensions (3D) (the bound-state energy at 2D) for three attractively interacting Fermi gases with 3D Rashba, 3D Weyl, and 2D Rashba SOC, respectively. We show that only the coherence length can be used to characterize this BCS to BEC crossover. Furthermore, it is the only length which can be directly measured by radio-frequency dissociation spectra type of experiments. We stress crucial differences among the coherence length, Cooper-pair size, and the two-body bound-state size. Our results provide the fundamental and global picture of the BCS to BEC crossover and its experimental detections in various cold-atom and condensed-matter systems.

Itinerant ferromagnetism in repulsively interacting spin-orbit coupled Fermi gas

Abstract: We investigate magnetic transitions of repulsively interacting Fermi gases with spin-orbit coupling (SOC). We find that the SOC provides a new and efficient mechanism to realize the itinerant ferromagnetism (FM) which is a long sought state in material science at relatively weak repulsive interactions. The coupling between the density and the spin fluctuations due to the SOC leads to new collective and particle-hole excitations in both paramagnet and FM state, also new quantum to classical crossover regimes near the paramagnet to the FM transition. All these new phenomena are dramatically different from those in conventional Fermi gas without SOC and can be probed by various current experimental techniques.

Itinerant magnetism in Weyl spin-orbit coupled Fermi gas

Abstract: Magnetic ordering of itinerant fermionic systems is at the forefront of condensed matter physics dating back to Stoner's instability. Spin-orbit coupling (SOC) which couples two essential ingredients of an itinerant fermionic system, namely spin and orbital motion, opens up new horizons to this long-standing problem. Here we report that the itinerant ferromagnetism is absent in 3D Fermi gas with a Weyl SOC and various itinerant spin density waves emerge instead, which is deeply rooted in the unique symmetry and spin-momentum locking effect in spin-orbit coupled systems. What is more appealing is that, the strong SOC provides a new and efficient mechanism to realize itinerant spin density waves at extremely weak repulsion---a significant benefit for present ultra-cold atom experiment. These novel phenomena can be probed by Bragg spectroscopy, time of flight imaging and {\sl In-Situ } measurements in ultra-cold atom experiment.