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Pauli Paramagnetism of an Ideal Fermi Gas
Using the two lowest hyperfine states of a non-interacting ultracold Fermi gas of 6Li as pseudospin states, we have measured the magnetic susceptibility of such a system as a demonstration of the textbook physics of Pauli paramagnetism [1]. An imbalanced spin mixture of 6Li is trapped in the harmonic confinement potential at the focus of a high power far-off-resonant laser beam, and is made non-interacting by tuning the scattering length to zero around the wide 832G Feshbach resonance. In this system, the effective field producing the magnetization is the normalized chemical potential difference between the two hyperfine states. In the local density approximation, the normalized effective field varies across the trap, as does the corresponding normalized magnetization, meaning that we can determine the magnetic susceptibility from a single trapped cloud. We extract this susceptibility from density profiles of the two hyperfine states, obtained by phase-contrast imaging. Our system realizes the textbook version of Pauli paramagnetism. In contrast to other condensed matter systems, we can reach the fully polarized limit. Reaching the same limit in condensed matter systems requires experimentally unfeasible magnetic fields of 104 T. Our technique for measuring magnetic susceptibility can be extended to strongly interacting Fermi gases, as a probe for magnetic phase transitions.