Bound-State Band Reconstruction and Resonance in Spin-1/2 Bose Gas with 1D Spin-Orbit Coupling
Qi Gu, Yuncheng Xiong, Lan Yin

TL;DR
This paper investigates how 1D spin-orbit coupling in a two-component Bose gas leads to the reconstruction of bound-state bands and the emergence of multiple resonances, with potential experimental observability.
Contribution
It reveals the effects of finite Raman coupling on bound-state band structure and resonance phenomena in spin-orbit coupled Bose gases, providing new insights into their many-body physics.
Findings
Reconstruction of three bound-state bands due to Raman coupling.
Multiple resonances can be induced at finite scattering lengths.
Tuning intra-species interactions can control bound-state bands and resonances.
Abstract
In this work, we study two-body bound states in two-component Bose gas with a one-dimensional (1D) spin-orbit coupling (SOC) induced by Raman lasers. The finite Raman coupling strength generates coupling among three spin channels, resulting in the reconstruction of three bound-state bands. In addition, multiple resonances can be induced at finite scattering lengths. By tuning the interaction in one intra-species channel, one bound-state band can be lifted and three resonances can be achieved at different center-of-mass momenta, which can be observable under current experimental conditions in Rb atoms.
| 2 | -4.682 | -2 | 0.995 | 0.00003 | 0.00234 |
|---|---|---|---|---|---|
| 2 | -2.695 | 0 | 0.987 | 0.0004 | 0.0063 |
| 6.9 | -1.08 | -2 | 0.924 | 0.00548 | 0.0354 |
| 6.9 | -0.811 | -1 | 0.855 | 0.0186 | 0.0633 |
| 6.9 | -0.0123 | 0 | 0.373 | 0.196 | 0.216 |
| 7.09 | 0 | 0 | 0.25 | 0.25 | 0.25 |
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Bound-State Band Reconstruction and Resonance in Spin-1/2 Bose Gas with 1D Spin-Orbit Coupling
Qi Gu, Yuncheng Xiong, Lan Yin
School of Physics, Peking University, Beijing 100871, China
Abstract
In this work, we study two-body bound states in two-component Bose gas with a one-dimensional (1D) spin-orbit coupling (SOC) induced by Raman lasers. The finite Raman coupling strength generates coupling among three spin channels, resulting in the reconstruction of three bound-state bands. In addition, multiple resonances can be induced at finite scattering lengths. By tuning the interaction in one intra-species channel, one bound-state band can be lifted and three resonances can be achieved at different center-of-mass momenta (COMM), which can be observable under current experimental conditions in 87Rb atoms.
pacs:
Introduction. Synthetic spin-orbit coupling (SOC) is an important tool in the study ultracold quantum gases. There have been a lot of experimental and theoretical studies on spin-orbit coupled quantum gases during the past decade Galitski and Spielman (2013); Goldman et al. (2014); Zhai (2015); Zhang and Zhou (2017). The 1D SOC was first generated by dressing two hyperfine spin states with a pair of lasers in a 87Rb Bose-Einstein condensate Lin et al. (2011). The similar scheme was also used to generate 1D SOC in 40K Wang et al. (2012) and 6Li Cheuk et al. (2012) Fermi gases. 2D SOC has been experimentally realized in 40K gases with three Raman lasers Huang et al. (2016) and in 87Rb atoms with an optical Raman lattice Wu et al. (2016). Bose gases with 1D SOC can condense into stripe phase, magnetized phase, or non-magnetized phase for different SOC parameters Lin et al. (2011); Ji et al. (2014); Li et al. (2012). Fermi gases with SOC were predicted to be unconventional superfluid at low temperatures Hu et al. (2011); Seo et al. (2012); Wu et al. (2013). Condensation of two-body bound states were predicted not only in Fermi gases Yu and Zhai (2011); Vyasanakere et al. (2011), but also in Bose gases with SOC Li and Yin (2014).
How SOC affects bound-state formation and atom scattering generally in a Bose gas with SOC is an important and unanswered problem. Interactions between atoms can be strongly altered by the light dressed with Williams et al. (2012); Vyasanakere and Shenoy (2011). Previous studies showed that in the case of fermions with 1D SOC, finite Raman strength can shift the location of the Feshbach resonance Vyasanakere and Shenoy (2011); Zhang et al. (2013); Williams et al. (2013); Wang and Greene (2016). In a Bose gas with anisotropic SOC, one can induce resonance by tuning the anisotropy of SOC strengths Gu and Yin (2018). In the case with Rashba SOC, the resonance position can only be shifted in intra-species channels Li and Yin (2014). In the case with Wely SOC, resonance positions of all three scattering channels are shifted Luo and Yin (2017). Nonetheless, in the case of 1D SOC with vanishing Raman coupling, the SOC does not change either the resonance position or bound-state binding energy.
In this work, we study two-body bound states in spin-1/2 Bose gas with 1D SOC at finite Raman coupling. The Raman coupling can be viewed as the effective zeeman field that causes spin-flipping processes. We find that three scattering channels are coupled together resulting in the formation of three new bound-state bands. The finite Raman coupling also induces resonances at finite scattering lengths. By tuning the scattering length in one intra-species channel, one bound state band can be lifted up and the resonance locations can be shifted. We propose a scheme to observe this resonance in 87Rb system.
Model. We consider a two-component homogeneous Bose gas with a Raman-induced SOC, described by the Hamiltonian . The single-particle term is given by
[TABLE]
where is the Raman coupling strength, is the SOC strength, and is the detuning energy, and are Pauli matrices, and . The recoil energy is defined as . The single-particle Hamiltonian can be diagonalized, yielding two helical excitation branches . For simplicity, we just consider the case with zero detuning. The energy minimum of the lower branch is given by
[TABLE]
where and . The spin-dependent -wave interactions between bosons are given by
[TABLE]
where is the creation operator of a boson with momentum and spin component or . The -wave coupling constant is related to the scattering length in the absence of SOC by the renormalization relation with .
Two-body bound state. The eigenequation of a two-body bound state is given by , where and are bound state eigenenergy and eigenstate with COMM . From the eigenequation, we obtain a set of linear equations for the coefficient
[TABLE]
where and the matrix is given by
[TABLE]
with , , and the coupling matrix is given by . Eq. (4) can be further written as
[TABLE]
where . The eigenenergy can be determined from the secular equation
[TABLE]
Eigenenergies of both two-boson and two-fermion bound states satisfy Eq. (6). For two-boson bound state, Eq. (6) can be further written as
[TABLE]
where and are functions of energy , momentum , and , as given below,
[TABLE]
When the Raman strength is finite, all the off-diagonal matrix elements in Eq. (7) are finite, indicating that in the presence of Raman field, the three spin channels and mix together.
In the zero Raman strength limit, , Eq. (7) is reduced to three independent equations. Three spin channels are decoupled, and the bound-state energy depends only on the scattering length of its spin channel, in the intra-species channel
[TABLE]
and in the inter-species channel
[TABLE]
As shown in Fig. 1, the minimum of the bound-state band in the intra-species () channel is located at COMM (), while that in the inter-species channel is located at zero COMM. Bound states composed of two spin-1/2 atoms behave as a single spin-1 particle with a pure 1D SOC, , where is the -component spin operator for the spin-1 bound state.
When Raman coupling strength is finite, the three parabolic bands are reconstructed to three new disjoint energy bands. As shown in Fig. 1(a), with symmetric interactions , the lowest band has three minimum points located near and [math] when is very small. In such case, to the first order of , bound states can be approximated as a spin-1 single particle with SOC described in Ref. Lan and Öhberg (2014). As increases, two minimum points in the lowest band disappear and only the one at zero is left, as shown in Fig. 1(b). When , two minimum points in the middle band merge into one, as shown in Fig. 1(c). When the scattering length increases, the bound states have less binding energy and all the bands are lifted up, as shown in Fig. 1(d).
The bound-state bands also change with the asymmetry of interactions. In an extreme case with , one band is lifted up with the band minimum located near COMM , as shown in Fig. 2(a). In another case with with , one band is raised up with the band minimum located at zero COMM , as shown in Fig. 2(b). In these two cases, the middle band has only one minimum while the bottom band has one or two minima depending on the Raman strength . The minimum energies of these two lower bands are almost the same as the case of symmetric interactions with the same .
The bound-state wavefunctions can also be solved 111One can obtain the wavefunction from , where is the -th eigenvector of with eigenvalue . . In the case with asymmetric interactions , we find that at the bottom of top band, the bound state is largely made of atom pairs with spin- when the Raman strength is weak, as shown in Table 1. When the Raman strength increases to the resonance point where the bound state energy equals to twice the lowest atom energy, the bound state consists of atom pairs with all the spin configurations.
Induced resonance. At zero Raman strength, the resonance condition is the same as that without SOC, i.e. when the scattering length diverges. At finite Raman strength, the resonance condition changes due to the reconstruction of bound-state bands. In experiments, atoms are often condensed in the two single-particle states with the lowest energy. Since the different spin channels are coupled at finite Raman strength, the resonance occurs whenever the bound state energy satisfies or . As a result, multiple resonances can be induced at finite scattering lengths with finite Raman strength.
For symmetric interactions , as shown in Fig. 3(a) and (b) , resonance condition and are satisfied in the top band at two different scattering lengths for . There are totally three different resonances that one can induce by tuning Raman strength for each band, but due to symmetry , they are located at only two different scattering lengths. When , there is only one induced resonance for each band as .
With asymmetric interactions , for fixed and , the bound-state energy displays different behavior as a functions of in different bands. When increases, only the top bound-state band can reach the lowest scattering energy . The other two bands are insensitive to the change in as shown in Fig. 2(a). When , as shown in Fig. 4, there are three induced resonances with COMM . When , there is only one induced resonance.
Effective interactions near Resonance. With finite Raman strength, the effective interactions between atoms are no longer described by the bare coupling constants, but given by the -matrix which satisfying Bethe-Salpeter equation
[TABLE]
where is the 3 by 3 coupling matrix and is the pair susceptibility function. The solution of Eq. (11) is given by
[TABLE]
With symmetric interactions , the coupling matrix is proportional to the identity matrix and Eq. (12) can be rewritten as,
[TABLE]
where is the identity matrix and is the modified pair susceptibility function. From Eq. (13), the -matrix can be solved explicitly,
[TABLE]
where and are eigenvalue and unitary transformation matrices of , , and is the -th eigenvalue of . A resonance occurs whenever the scattering length is near . The effective interactions near a resonance can be approximated by
[TABLE]
where refer to the scattering channels , and .
With asymmetric interactions , we obtain each matrix element of in the leading order of as given by
[TABLE]
where refers to either the scattering channel or . To the leading order of , the dominant interaction near resonance is in the channel, and the resonance occurs at where as given in Eq. (Bound-State Band Reconstruction and Resonance in Spin-1/2 Bose Gas with 1D Spin-Orbit Coupling).
Discussion and Conclusion. In experiments, 87Rb atom gas with Raman-induced SOC is a common platform for studying spin-orbit coupled bosons Wei and Mueller (2013). Usually, a pair of Raman laser beams couple two hyperfine states, and . The scattering lengths of different channels are almost equal , with for Raman laser wavelength 804.1 nm van Kempen et al. (2002); Lin et al. (2011). The Raman strength can be tuned up to the order of . At , we find for symmetric interactions the resonance positions is at , much larger than the scattering lengths. It is difficult to observe the induced resonance by adjusting SOC alone. However the scattering length is tunable by Feshbach resonance Marte et al. (2002). It is possible to observe the induced resonance in the region . For example, at , the resonance position is given by , for shown in Fig. 4(b), available by Feshbach resonance in experiments. When , three resonances can be observed instead of one.
The resonance induced by SOC has important effects in a Bose gas. Near the resonance, severe particle loss is expected to occur which may be used as a tool to locate the resonance. Over the resonance, the effective interaction turns into an attractive interaction, and the system is expected to collapse at low temperatures. The effective interaction is strongly modified by SOC near the resonance, and interaction-determined many-body properties are expected to be different from predictions of the simple mean-field theory, which will be the subject of our future work.
In summary, we study bound state bands and resonances in a Bose gas with SOC. We find that finite Raman strength generates coupling among different scattering channels, leading to the reconstruction of bound-state bands. The resonance positions are also shifted due to finite Raman coupling strength, and the effective interactions near these resonance are obtained. We predict that by tuning the scattering length in one intra-species channel, the resonance induced by SOC can be observed in rubidium-87 systems.
Acknowledgements.
We would like to thank X.-J. Zhou, Z.-Q. Yu, P. Zhang, and T.-L. Ho for helpful discussions. This work is supported by the National Key Research and Development Project of China under Grant No. 2016YFA0301501.
The reference list from the paper itself. Each links out to its DOI / PubMed record.
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