States with Unmatched Spins Lead to Novel Superfluids

A strange new form of superfluid with unequal numbers of spin up and spin down atoms has been created in a lab by two independent research groups. Though theorists have speculated for years about what would happen in states with unmatched spins, this is the first experimental observation of such a state. At the APS March Meeting, two research groups described their observations of cold gases of lithium atoms with unequal numbers of spin up and spin down atoms.

Normally in a superconductor, each spin up electron pairs with a spin down electron, and the pairs can flow with no resistance. Similarly, in a superfluid, atoms pair up and can flow with zero viscosity.

States with unequal numbers of spin up and down atoms do not fit the conventional model of superfluidity. There have been attempts to observe such phases, but until recently nothing definitive had been seen. The two experimental groups, one from Harvard and one from Rice, have now measured the effects of unmatched spins.

Both groups chill a cloud of lithium-6 atoms, which are fermions, in a laser trap to within billionths of a degree of absolute zero. The spin up atoms form pairs with spin down atoms. The researchers have been able to tune the pair interaction strength by adjusting a magnetic field. At one extreme, the atoms pair up into tightly bound molecules and condense into a molecular BEC. At the other extreme, the atoms form loosely coupled Cooper pairs, as in a BCS superconductor. This BEC-BCS crossover region has been explored by a number of research groups in the past several years.

Those studies involved gases with an equal number of spin up and down atoms. More recently, researchers have found they can use radio waves to control the number of spin up and spin down atoms.

“The beauty of these experiments is that the essential physics can be revealed in a very controllable, clear way,” said Randy Hulet of Rice in a press conference at the March Meeting.

Above a critical mismatch of spin up and spin down atoms, Hulet observed a phase separation, in which the excess unpaired atoms were expelled from the superfluid gas in the center and collected at the edges of the traps, while the gas in the center remained paired.

Below the critical mismatch, the system behaved in an unexpected way–it incorporated the extra unmatched spins as if there were no mismatch. There was no phase separation. Although his group did not directly measure superfluidity, based on previous experiments, Hulet believes the gas was a superfluid. This behavior suggests some new form of exotic pairing is going on, said Hulet. “This is completely unexpected.”

Martin Zwierlein, a graduate student in Wolfgang Ketterle’s group at MIT, described his group’s experiments on a gas of lithium-6 atoms with unequal numbers of spin up and spin down atoms. By rotating the condensate, they observed the formation of vortices that only occur in rotating superfluids, and found that in these experiments, some superfluidity persists even up to a 70 percent mismatch in some cases. The critical mismatch depends on the pair interaction strength.

Theorist Eugene Demler of Harvard commented that these experiments are intriguing, and even though we don’t fully understand what’s going on in these strange new states, we are in a new era of cold atom research.

Scientists believe these studies could provide insights into the extremely dense quark matter in the cores of neutron stars, where there may be unpaired quarks. The cold atom studies could also help explain the peculiar heavy-fermion materials that exhibit both magnetism and superconductivity. In addition, they could shed light on high temperature superconductivity.