Research with ultracold atomic gases has greatly expanded by rapid advancement in the ability to trap, cool, and manipulate atomic gases, so that they may be applied to gain better understanding of complex phenomena in condensed matter, material, nuclear, and particle physics. Atomic gases can be cooled to the nano-Kelvin regime where atoms behave more as waves than as classical particles. In this "quantum regime," the atoms exhibit some of the most startling phenomenon known in the physical world, including superfluidity, an analog of superconductivity but with neutral atoms taking the place of electrons. Unlike real materials that possess defects, impurities, and random disorder, the atomic analogs are inherently clean and extraordinarily tunable enabling highly controllable tests of fundamental models.
The experiments performed under this grant will use ultra-cold Fermionic lithium atoms, Li-6, to simulate the properties and characteristics of high-temperature superconductors, which are materials of great technological significance, but remain only partially understood. The group will also continue their studies of pairing of mixtures with unequal spin numbers to map out the phase diagram of this incredibly rich system. A particular focus is the "FFLO" phase, where pairing occurs with non-zero center of mass momentum. The FFLO phase has been long studied theoretically, but as of yet there are no clear experimental observations. These experiments will hopefully clarify differences in observations of phase separation with other experiments. Finally using a Bosonic form of lithium, Li-7, the group will investigate Anderson localization, the transition from a superfluid or conductor to an insulator due to disorder. A second experiment in the Boson system will explore a recently proposed dynamical stabilization scheme for creating two-dimensional solitons for the first time.
The broader impact of this work is two-fold: firstly, greater understanding of fundamental models of materials may enable the design of real materials with improved performance, such as higher temperature superconductors; and secondly, this research will provide a broad scientific training for the students and post-doctoral scientists engaged in it.
One of the most challenging problems facing physicists in the 21st century is to understand a broad class of materials, such as superconductors, that involve many particles (millions or more) acting in concert. Superconductors are a remarkable material, that when cooled below a certain critical temperature conduct electricity without electrical resistivity. Superconducting wires are employed in MRI machines to generate high magnetic fields that provide the resolution needed for doctors to image parts of the human body. These machines must be cooled to nearly absolute zero, or approximately -270 Celsius in order that the wires forming the magnets transition to the superconducting state. There is a class of "high-temperature" (-200 Celsius!) superconductors that would enable many more applications, including lossless electrical power transmission, if their critical temperatures could be raised to room temperature. The mechanisms responsible for bestowing the superconducting property in these materials is still largely unknown, however, and the lack of a fundamental understanding has impeded efforts to design new materials with higher critical temperatures. We employ a new approach using ultra-cold atoms to gain insight into how collections of many particles can act together to create a collective behavior, such as superconductivity. In this approach atoms are cooled to just a few billionths of a degree above absolute zero using new techniques involving lasers and magnets. At these temperatures, atoms lose their individuality and behave more as quantum mechanical waves than as single particles. This is precisely the conditions required to create the collective many-body phenomena of interest. However, the artificial materials that we create with the ultra-cold atoms in isolated vacuum are far purer and more controllable than the conventional electronic materials. The cold atom materials can be made to conform to specific models, so that physicists can test whether that model has the crucial ingredient to show a particular collective effect. In our work, we have explored several materials that probe several attributes that physicists feel are important in determining a material property. These include the effects of random disorder, like impurities in an otherwise perfect crystal lattice, one-dimensional confinement, and the application of magnetic fields. We have learned that a superconductor can survive a certain amount of disorder, but not too much, that one-dimensional wires are able to superconduct, and that even a magnetic field can be accommodated by atoms that form bonds with other atoms moving by at certain speeds. All of these results add to our growing fundamental understanding of quantum collective effects.
