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Imbalanced Fermi Mixtures

 

Fermionic pairing requires symmetry between components with different internal states, for example a system with equal number of spin-up and spin-down electrons. But what if the number of these two components is not equal? Imbalanced Fermi systems have been discussed since the ‘60s, but it was experimentally difficult to realize.

The superconducting state in solid state systems, where electrons form Cooper pairs is a potential testing ground. Imbalance in electron states can be achieved by applying a magnetic field, but this tends to destroy the superfluid state. The degenerate Fermi gas in which the atomic number in each state is individually tunable provides us an opportunity to experimentally explore the exotic phenomena the imbalanced Fermi system exhibits.

Fermionic 6Li atomic gas in the F = 1/2, mF = ± 1/2 states is a good candidate to perform the imbalanced Fermi mixture experiment. The system is electron-like and the result can be compared with theories in electronic systems. Furthermore, the interaction between these two states is tunable by Feshbach resonances. This enables us to study imbalancing under different pairing conditions.

Experimentally, we transfer the 6Li atoms from the magnetic trap where the atoms are in the magnetically trappable state F = 3/2, mF = 3/2 into a single laser beam far off-resonant optical trap. Then a first rf sweep drives the transition from the F = 3/2, mF = 3/2 state into the F = 1/2, mF = 1/2 state. An incoherent mixture of the mF = ±1/2 states is necessary if the atoms are to interact. This is accomplished by a subsequent sawtooth ramp, repeated up to 100 times, that drives mF = 1/2 to mF = - 1/2. By adjusting the rf power, we can control the relative population of the mF =±1/2 states.

In-situ Images of Phase Separation, T< 0.05 TF.Critical features: uniformly paired cores, deformation, sharp boundaries. [Ref: G. B. Partridge et. at. PRL 2006]

 

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