Session W6: Alkaline Earth Atoms and SU(N) Magnetism

Two-orbital SU(N) magnetism with ultracold alkaline-earth atoms

Fermionic alkaline-earth atoms have unique properties that make them attractive candidates for the realization of atomic clocks and degenerate quantum gases. At the same time, they are attracting considerable theoretical attention in the context of quantum information processing. In this talk, we demonstrate that when such atoms are loaded in optical lattices, they can be used as quantum simulators of unique many-body phenomena. In particular, we show that the decoupling of the nuclear spin from the electronic angular momentum can be used to implement many-body systems with an unprecedented degree of symmetry, characterized by the SU(N) group with N as large as 10. Moreover, the interplay of the nuclear spin with the electronic degree of freedom provided by a stable optically excited state should enable the study of physics governed by the spin-orbital interaction. Such systems may provide valuable insights into the physics of strongly correlated transition-metal oxides, heavy-fermion materials and spin-liquid phases. [Reference: Nature Phys. 6, 289 (2010)]   

Double-degenerate Bose-Fermi mixture of strontium

We report on the attainment of a double-degenerate Bose-Fermi mixture of strontium. A sample of fermionic $^{87}$Sr atoms is spin-polarized and sympathetically cooled by interisotope collisions with the bosonic isotope $^{84}$Sr. A degeneracy with $T/T_F=0.30(5)$ is reached for a $^{87}$Sr Fermi sea of $2\times 10^4$ atoms together with an almost pure $^{84}$Sr BEC of $10^5$ atoms. The rich electronic structure and the large nuclear spin of $^{87}$Sr make it a promising candidate for quantum simulation of SU($N$) magnetism and quantum information processing.   

Degenerate Fermi Gas of Strontium-87

Degenerate Fermi gases of alkaline earth metal atoms such as strontium and ytterbium open new possibilities in the study of many-body physics because of the existence of isotopes with large nuclear spin I (e.g. I=9/2 in strontium-87). With the closed-shell electronic ground state in these atoms, the nuclear spin is decoupled from other degrees of freedom. Interactions between atoms are spin-independent, leading to a large SU(N=2I+1) symmetry of the Hamiltonian. This results in a large degeneracy of the ground state, which has been predicted to result in novel spin liquid and valence bond states. Strong attractive interactions would favor formation of N-particle singlets, in analogy to the formation of baryons in quantum chromodynamics. (For a short overview, see C. Wu, \textit{Physics} \textbf{3}, 92 (2010).) We will describe the experimental realization of a degenerate Fermi gas of strontium-87 and characterization of an optical Feshbach resonance in this system that would be needed to manipulate the atom-atom interactions.   

Realization of an SU(6) invariant Fermi system

We report the realization of a novel Fermi system with an enlarged spin symmetry of SU(6) in a cold atomic gas of ytterbium $^{173}$Yb with nuclear spin I=5/2, which will open up a new opportunity for exotic many-body physics . While the achievement of quantum degeneracy of $^{173}$Yb with 6 spin components was already reported three years ago, an important technique of the separate imaging of the nuclear spin components was not developed. Recently we have made this possible by exploiting an optical Stern-Gerlach effect using a spatially inhomogeneous laser beam. The metallic state to Mott insulator transition for SU(6) Fermi gas is also investigated by loading $^{173}$Yb atoms into a 3D optical lattice. We find some results suggesting the formation of SU(6) Mott state at low lattice temperatures expected for SU(N) systems. The similar adiabatic cooling effect is also observed in the Bose-Fermi mixture of spinless boson of $^{174}$Yb and the SU(6) Fermi system of $^{173}$Yb. In addition, an all-optical sympathetic evaporative cooling method is applied to the two fermionic isotopes of ytterbium $^{171}$Yb with the nuclear spin I=1/2 and $^{173}$Yb, and we successfully cool the mixture below the Fermi temperatures. The same scattering lengths for different spin components make this mixture featured with the novel SU(2) x SU(6) symmetry. The mixture is loaded into a 3D optical lattice to implement the SU(2) x SU(6) Hubbard model. In particular, we find interaction-induced suppression of Bloch oscillations for the mixture in the 3D lattice.   

Exotic magnetism and new states of matter with alkaline earth atoms

A crucial basic property of antiferromagnetic insulators with SU(2) symmetry is that adjacent spins can (and tend to) combine to form singlets, or valence bonds. The classical analog of this fact is that adjacent spins prefer to be antiparallel. These two facts underly much of our thinking about ground states of quantum antiferromagnets. Ultracold alkaline earth atoms can be used to realize magnetic insulators with SU(N) symmetry, where a minimum of N spins is required to form a singlet, and where N can be as large as 10. These systems belong to a largely unexplored class of quantum magnets. In this talk, I will discuss some of the remarkable new states of matter that are strong candidates to arise in these systems, including chiral spin liquids with fractional and non-Abelian statistics. I will also briefly discuss the issue of temperature, and point out an advantage of high-spin alkaline earths as compared to spin-1/2 magnetic systems.