Imaging exotic dimers

20 March 2023

Photo: AG Hemmerich

The figure shows nearly homogeneous occupation of the second Brillouin zone in an optical square lattice of atoms (left), atoms & molecules (middle), or molecules only (right).

Researchers from the Institute of Laser Physics at Universität Hamburg have for the first time succeeded in forming and imaging bosonic Feshbach molecules of two fermionic atoms in the second Bloch band of an optical lattice. The scientists report on the study of these exotic dimers in the journal Nature Physics.

Feshbach molecules are pairs of atoms in the last bound state below the dissociation limit. They can be formed with the help of a Feshbach resonance by a fast adiabatic change of the scattering length. To distinguish Feshbach molecules and unpaired atoms in the second Bloch band of an optical lattice, the scientists used an amazingly simple imaging method that has similarities to the technique of mass spectrometry. This enabled the team consisting of Yann Kiefer, Max Hachmann and Andreas Hemmerich, who also conducts research in the Cluster of Excellence "CUI: Advanced Imaging of Matter", to precisely measure the lifetimes and binding energies of the exotic dimers.

New forms of superfluidity are expected

These results can serve as a starting point for a series of exciting investigations into unconventional many-body scenarios. It has already been shown that condensed bosonic atoms in the second Bloch band have chiral character and can even possess topologic excitations. If Feshbach molecules composed of fermionic atoms are used instead of bosonic atoms, an extremely interesting new possibility arises the scientists say: if the initially positive scattering length is adiabatically detuned to negative values with the help of a Feshbach resonance, attractively bound Cooper pairs arise from repulsively bound molecules. New forms of superfluidity with exotic properties beyond conventional BCS theory are then expected.