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arXiv:1303.2212 · Physical Review A

Radio-frequency spectroscopy of a strongly interacting spin-orbit-coupled Fermi gas

Zhengkun Fu, Lianghui Huang, Zengming Meng, Pengjun Wang, Xia-Ji Liu, Han Pu, Hui Hu, Jing Zhang

Published May 28, 2013• 54 citations
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Abstract

We investigate experimentally and theoretically radio-frequency spectroscopy and pairing of a spin-orbit-coupled Fermi gas of ${}^{40}$K atoms near a Feshbach resonance at ${B}_{0}=202.2$ G. Experimentally, the integrated spectroscopy is measured, showing characteristic blue and red shifts in the atomic and molecular responses, respectively, with increasing spin-orbit coupling. Theoretically, a smooth transition from atomic to molecular responses in the momentum-resolved spectroscopy is predicted, with a clear signature of anisotropic pairing at and below resonance. Our many-body prediction agrees qualitatively well with the observed spectroscopy near the Feshbach resonance.

DOI
10.1103/physreva.87.053619

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We theoretically investigate the momentum-resolved radio-frequency spectroscopy of a harmonically trapped atomic Fermi gas near a Feshbach resonance in the presence of equal Rashba and Dresselhaus spin-orbit coupling. The system is qualitatively modeled as an ideal gas mixture of atoms and molecules, in which the properties of molecules, such as the wave function, binding energy, and effective mass, are determined from the two-particle solution of two interacting atoms. We calculate separately the radio-frequency response from atoms and molecules at finite temperatures by using the standard Fermi golden rule and take into account the effect of harmonic traps within local density approximation. The total radio-frequency spectroscopy is discussed as functions of temperature and spin-orbit coupling strength. Our results give a qualitative picture of radio-frequency spectroscopy of a resonantly interacting spin-orbit-coupled Fermi gas and can be directly tested in atomic Fermi gases of 40{}^{40}40K atoms at Shanxi University and 6{}^{6}6Li atoms at the Massachusetts Institute of Technology.
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In this Letter, we report the first experimental realization and investigation of a spin-orbit coupled Fermi gas. Both spin dephasing in spin dynamics and momentum distribution asymmetry of the equilibrium state are observed as hallmarks of spin-orbit coupling in a Fermi gas. The single particle dispersion is mapped out by using momentum-resolved radio-frequency spectroscopy. From momentum distribution and momentum-resolved radio-frequency spectroscopy, we observe the change of fermion population in different helicity branches consistent with a finite temperature calculation, which indicates that a Lifshitz transition of the Fermi surface topology change can be found by further cooling the system.
We investigate theoretically radio-frequency spectroscopy of weakly bound molecules in an ultracold spin-orbit-coupled atomic Fermi gas. We consider two cases with either equal Rashba and Dresselhaus coupling or pure Rashba coupling. The former system has been realized very recently at Shanxi University [Wang et al., Phys. Rev. Lett. 109, 095301 (2012)] and MIT [Cheuk et al., Phys. Rev. Lett. 109, 095302 (2012)]. We predict realistic radio-frequency signals for revealing the unique properties of anisotropic molecules formed by spin-orbit coupling.
We theoretically investigate a Rashba spin-orbit-coupled Fermi gas near Feshbach resonances, by using mean-field theory and a two-channel model that takes into account explicitly Feshbach molecules in the closed channel. In the absence of spin-orbit coupling, when the channel coupling ggg between the closed and the open channels is strong, it is widely accepted that the two-channel model is equivalent to a single-channel model that excludes Feshbach molecules. This is the so-called broad resonance limit, which is well satisfied by ultracold atomic Fermi gases of 6{}^{6}6Li atoms and 40{}^{40}40K atoms in current experiments. Here, with Rashba spin-orbit coupling we find that the condition for equivalence becomes much more stringent. As a result, the single-channel model may already be insufficient to describe properly an atomic Fermi gas of 40{}^{40}40K atoms at a moderate spin-orbit coupling. We determine a characteristic channel coupling strength gc{g}_{c}gc​ as a function of the spin-orbit-coupling strength, above which the single-channel and two-channel models are approximately equivalent. We also find that for narrow resonance with small-channel coupling, the pairing gap and molecular fraction are strongly suppressed by SO coupling. Our results can be readily tested in 40{}^{40}40K atoms by using optical molecular spectroscopy.