[Paper Review] Two-Element Mixture of Bose and Fermi Superfluids

Richard Roy, Alaina Green+2 more|arXiv (Cornell University)|Jul 12, 2016

Cold Atom Physics and Bose-Einstein Condensates6 citations

TL;DR

This paper reports the first realization of a stable two-element Bose-Fermi superfluid mixture using 174Yb (bosonic) and 6Li (fermionic) atoms, demonstrating elastic coupling via a measurable dipole oscillation frequency shift in the bosonic component. The observed shift confirms a positive interspecies scattering length and enables detection of angular momentum transfer through a scissors mode, validated by a superfluid hydrodynamics model.

ABSTRACT

We report on the production of a stable mixture of bosonic and fermionic superfluids composed of the elements $^{174}$Yb and $^6$Li which feature a strong mismatch in mass and distinct electronic properties. We demonstrate elastic coupling between the superfluids by observing the shift in dipole oscillation frequency of the bosonic component due to the presence of the fermions. The measured magnitude of the shift is consistent with a mean-field model and its direction determines the previously unknown sign of the interspecies scattering length to be positive. We also observe the exchange of angular momentum between the superfluids from the excitation of a scissors mode in the bosonic component through interspecies interactions. We explain this observation using an analytical model based on superfluid hydrodynamics.

Motivation & Objective

• To create a stable, quantum-degenerate mixture of bosonic 174Yb and fermionic 6Li atoms with a large mass ratio.
• To probe elastic coupling between superfluid components through dipole mode frequency shifts.
• To detect angular momentum exchange between superfluids via excitation of a scissors mode in the bosonic component.
• To determine the sign of the interspecies scattering length using mean-field theory and experimental frequency shifts.
• To validate theoretical models of superfluid hydrodynamics in a two-component ultracold atomic system.

Proposed method

• Utilized a dynamically shaped 1064 nm optical dipole trap for efficient evaporative cooling of both species.
• Performed simultaneous forced evaporative cooling of 174Yb and sympathetic cooling of 6Li at 330 G to achieve quantum degeneracy.
• Employed a magnetic field gradient to reduce trap depth for 6Li and induce spatial overlap with the 174Yb BEC.
• Used radio-frequency pulses to prepare a 50:50 spin mixture of 6Li in hyperfine states at 832 G, tuned to the Feshbach resonance.
• Measured dipole oscillation frequency shifts in the 174Yb component as a function of relative cloud center displacement.
• Excited a scissors mode in the 174Yb condensate and analyzed angular momentum transfer using superfluid hydrodynamics equations.

Experimental results

Research questions

• RQ1 What is the sign of the interspecies scattering length between 174Yb and 6Li in a superfluid mixture?
• RQ2 How does the dipole oscillation frequency of the bosonic component shift due to coupling with the fermionic superfluid?
• RQ3 Can angular momentum be transferred between Bose and Fermi superfluids via interspecies interactions?
• RQ4 How does the spatial displacement of the two-component clouds affect the mean-field frequency shift in the dipole mode?
• RQ5 To what extent does the curvature of the Fermi gas density profile influence the interaction-induced frequency shift?

Key findings

• The measured dipole frequency shift in the 174Yb component is consistent with a mean-field model and confirms the interspecies scattering length is positive.
• A 10% systematic error in the frequency shift measurement would occur if the cloud centers were displaced by 44% of the 6Li cloud radius.
• The frequency shift decreases with increasing horizontal displacement, following a square-root dependence on radial displacement in the Fermi cloud.
• At unitarity ($1/k_F a_F = 0$), the peak frequency shift is proportional to $k_F a_{BF}$, with a coefficient dependent on mass ratio and polarizability.
• The scissors mode excitation in the 174Yb component is driven by time-dependent angular deformation, leading to measurable angular momentum transfer.
• Theoretical modeling based on superfluid hydrodynamics successfully explains the observed angular momentum transfer and frequency shift dynamics.