Principal Investigator Martin Zwierlein
Collision of two spin states of an ultracold Fermi gas. Although each spin cloud is a million times thinner than air, the two spin states essentially completely repel each other. The interactions between unlike spins are as strong as quantum mechanics allows. A spin down atom scatters with spin up atoms at every encounter, i.e. the mean free path for collisions is just one interparticle spacing - the shortest possible in a gas. This leads to the minimum diffusivity and the smallest spin conductivity ever possible. This leads to the interesting fact that an almost perfect fluid, i.e. the best conductor of mass, is the worst conductor for spin.
Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators. Transport of electron spin, rather than of charge, is being explored as a new way to carry information. Neutrino transport energizes supernova explosions following the collapse of a dying star, and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity when interactions are tuned to a scattering resonance. However, in this work we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coefficient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by h/m, the quantum limit of diffusion, where h is Planck's constant and m the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state.
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.