Spin-imbalance in a one-dimensional Fermi gas.

Superconductivity and magnetism generally do not coexist. Changing the relative number of up and down spin electrons disrupts the basic mechanism of superconductivity, where atoms of opposite momentum and spin form Cooper pairs. Nearly forty years ago Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) proposed an exotic pairing mechanism in which magnetism is accommodated by the formation of pairs with finite momentum. Despite intense theoretical and experimental efforts, however, polarized superconductivity remains largely elusive. Unlike the three-dimensional (3D) case, theories predict that in one dimension (1D) a state with FFLO correlations occupies a major part of the phase diagram. Here we report experimental measurements of density profiles of a two-spin mixture of ultracold (6)Li atoms trapped in an array of 1D tubes (a system analogous to electrons in 1D wires). At finite spin imbalance, the system phase separates with an inverted phase profile, as compared to the 3D case. In 1D, we find a partially polarized core surrounded by wings which, depending on the degree of polarization, are composed of either a completely paired or a fully polarized Fermi gas. Our work paves the way to direct observation and characterization of FFLO pairing.

Pairing and phase separation in a polarized Fermi gas.

We report the observation of pairing in a gas of atomic fermions with unequal numbers of two components. Beyond a critical polarization, the gas separates into a phase that is consistent with a superfluid paired core surrounded by a shell of normal unpaired fermions. The critical polarization diminishes with decreasing attractive interaction. For near-zero polarization, we measured the parameter beta = -0.54 +/- 0.05, describing the universal energy of a strongly interacting paired Fermi gas, and found good agreement with recent theory. These results are relevant to predictions of exotic new phases of quark matter and of strongly magnetized superconductors.

The 4051-Angstroms band of C3 (A 1Piu-X 1Sigmag +, 000-000): perturbed low-J lines and lifetime measurements.

Rotational analyses have been carried out at high resolution for the 000-000 and 000-100 bands of the A (1)Pi(u)-X (1)Sigma(g) (+) transition of supersonic jet-cooled C(3). Two different spectra have been recorded for each band, using time gatings of 20-150 and 800-2300 ns. At the shorter time delay the spectra show only the lines observed by many previous workers. At the longer time delay many extra lines appear, some of which have been observed previously by [McCall et al.Chem. Phys. Lett. 374, 583 (2003)] in cavity ring-down spectra of jet-cooled C(3). Detailed analysis of these extra lines shows that at least two long-lived states perturb the A (1)Pi(u), 000 state. One of these appears to be a (3)Sigma(u) (-) vibronic state, which may possibly be a high vibrational level of the b (3)Pi(g) state, and the other appears to be a P = 1 state with a low rotational constant B. Our spectra also confirm the reassignment by McCall et al. of the R(0) line of the 000-000 band, which is consistent with the spectra recorded towards a number of stars that indicate the presence of C(3) in the interstellar medium. Fluorescence lifetimes have been measured for a number of upper-state rotational levels. The rotational levels of the A (1)Pi(u) state have lifetimes in the range of 230-190 ns, decreasing slightly with J; the levels of the perturbing states have much longer lifetimes, with some of them showing biexponential decays. An improved value has been obtained for the nu(1) vibrational frequency of the ground state, nu(1) = 1224.4933 +/- 0.0029 cm(-1).