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Observation of Fermi Polarons
The fate of a single particle interacting with its environment is one of the grand themes of physics. A well-known example is that of the electron moving through the crystal lattice of ions in a solid. The electron attracts positive ions, repels negative ones and thereby distorts the lattice. In other words, it polarizes its surroundings. The electron and the surrounding lattice distortions is best described as a new particle, the lattice polaron. It is a so-called quasiparticle with an energy and mass that differ from that of the bare electron. Polarons are crucial for the understanding of colossal magnetoresistance materials and they are responsible for conduction in fullerenes and polymers. Another famous impurity problem is the Kondo effect: Here, a magnetic impurity interacts with a Fermi sea of electrons, hindering their transport and leading to an increase in the metal's resistance below a certain temperature.
In the present work, we have observed Fermi polarons, dressed "spin down" impurity atoms immersed in a Fermi sea of "spin up" atoms. The interactions between the impurity and the environment can be freely tuned by means of a Feshbach resonance. This allows us to determine the polaron energy as function of interaction strength.
a) For weak interactions, the impurity (blue) can propagate freely through the environment (red), a Fermi sea of atoms. b) As the interaction is increased, the impurity starts to attract its surroundings, "dressing" itself with a cloud of environment atoms. This is the Fermi Polaron. c) For strong attraction, the spin down atom will bind exactly one spin up partner, forming a molecule. The transition from polarons to molecules occurs as soon as the binding can overcome Pauli blocking of the environment.
Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms
Andre Schirotzek, Cheng-Hsun Wu, Ariel Sommer, and Martin W. Zwierlein
| Phys. Rev. Lett. 102, 230402 (2009). paper download |
See accompanying Viewpoint commentary Physics 2, 48 (2009) |
High-Temperature Superfluidity
Vortices in gas clouds Shown at the right are lattices of vortices (mini-tornadoes) in an ultracold gas of sodium atoms (green ball), in a gas of lithium molecules, made out of a "red" and a "blue" lithium atom, and in a strongly interacting Fermi gas, where the lithium atom pairs are only held together by the stabilizing presence of all the other particles in the gas. Those vortices are the direct proof of superfluidity in these systems. The background shows hurricane Isabel in the summer of 2003, NASA image ISS007E14887.
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Vortices and Superfluidity in a Strongly Interacting Fermi Gas |
Fermionic Superfluidity with Imbalanced Spin Populations
Whether it occurs in superconductors, helium-3 or inside a neutron star, fermionic superfluidity requires pairing of fermions, particles with half-integer spin. For an equal mixture of two states of fermions ("spin up" and "spin down"), pairing can be complete and the entire system will become superfluid. When the two populations of fermions are unequal, not every particle can find a partner. Will the system nevertheless stay superfluid?
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Fermionic Superfluidity with Imbalanced Spin Populations |
A condensate emerges in an imbalanced Fermi mixture Image 1 | Image 2 | |
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| Bose-Einstein Condensation of Molecules | Condensation of Fermion Pairs Close to a
Feshbach Resonance ISI Fast breaking comment |
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| Formation Time of a Fermion Pair Condensate |
