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Dark–bright solitons in a superfluid Bose–Fermi mixture

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Dark–bright solitons in a superfluid Bose–Fermi mixture
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The recent experimental realisation of Bose–Fermi superfluid mixtures of dilute ultracold atomic gases has opened new perspectives in the study of quantum many-body systems. Depending on the values of the scattering lengths and the amount of bosons and fermions, a uniform Bose–Fermi mixture is predicted to exhibit a fully mixed phase, a fully separated phase or, in addition, a purely fermionic phase coexisting with a mixed phase. The occurrence of this intermediate configuration has interesting consequences when the system is nonuniform. In this work we theoretically investigate the case of solitonic solutions of coupled Bogoliubov–de Gennes and Gross–Pitaevskii equations for the fermionic and bosonic components, respectively. We show that, in the partially separated phase, a dark soliton in Fermi superfluid is accompanied by a broad bosonic component in the soliton, forming a dark–bright soliton which keeps full spatial coherence.
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Transcript: English(auto-generated)
Ultracold atomic gases are well-suited to study the phenomenon of superfluidity. In the case of bosons, the gas forms a Bose-Einstein condensate where most of atoms occupy the
same single particle state and they behave in a particularly coherent way. In the case of fermions, the gas forms Cooper pearls, which are pearls of atoms of opposite spin which can form also a coherent configuration giving rise to superfluidity.
One of the most prominent features of superfluids is the existence of solitons and quantized vortices, which are a consequence of phase coherence. In one dimension, a soliton corresponds to a well-defined deformation of the modulus
and the phase of the order parameter of the system, which can propagate without dispersion. Now, since it became experimentally possible to realize mixtures of Bose and Fermi superfluids
with ultracold gases, we decided to study the properties of such systems theoretically, focusing precisely on the solitonic solutions. We use Bogoliubov-Dazhen equations in order to describe the Fermi superfluid and the Gross-Pitesky equation for the condensate of bosons.
In the Bogoliubov-Dazhen equations we have to solve an eigenvalue problem shown here. We have to solve these equations together with the equation for the superfluid gap delta and the density. Instead, the Gross-Pitesky equation is a wave equation for an order parameter psi of the Bose-Einstein condensate.
The phase diagram of the mixture of superfluid bosons and an ideal Fermi gas has already been calculated and features three phases. A uniform mixture, where both gases coexist and occupy the same space. A complete phase separation, where bosons separate in space from fermions.
And the least obvious phase, where some fraction of fermions separates in space from the rest while the remaining region is filled by a mixture of bosons and fermions. Using our equations, we indeed reproduce all three phases by varying the strength of the interaction between bosons and fermions.
Between the coexistence phase and the fully separated phase, a narrow range of interaction parameter exists, where only a fraction of fermions separates from the mixture. As the next step, we study a dark soliton in the Fermi superfluid and how it is modified by bosons in all those three regimes.
In the coexistence regime, the bosons merely fill the Fermi soliton as an external potential, which attracts them, but the effect is weak and does not change the overall properties of the soliton. In the phase separated regime, the solitonic depletion is completely filled by bosons and the two components of the gas are spatially separated.
The partial separated regime brings the most interesting result. Bosons localize around the depletion in the Fermi gas, which in turn becomes very wide and deep, maintaining the phase coherence between the two sides. This is a novel structure in the form of a dark-bright soliton. Our results may have some interesting implications in the search for the elusive Fulda-Ferrale
Larkin or Chinnikov phase. In such a phase, due to the imbalance between spin-up and spin-down fermions, the Fermi radius of the two spin components is different. As a consequence, Cooper pairs have a finite center of mass momentum and the other parameter
exhibits a partial modulation with the period set by this momentum. Our results suggest that the presence of a small bosonic component can be used as a way to stabilize density modulations of the fermionic order parameter and also to
make them more directly observable.