Kavli Affiliate: Feng Yuan
| First 5 Authors: Huaisong Zhao, Peng Zou, Feng Yuan, ,
| Summary:
By calculating the dynamical structure factor along the high symmetry
directions in the Brillouin zone, the dynamical excitations of attractive
Fermi-Hubbard model in a two-dimensional square optical lattice are studied
with random phase approximation. {Two kinds of collective modes are
investigated, including a Goldstone phonon mode at transferred momentum ${bf
q}=left[0,0right]$ and a roton mode at ${bf q}=left[pi,piright]$. The
phonon origins from the spontaneously U(1) symmetry breaking of pairing gap,
and its speed is suppressed by the interaction strength. The collective roton
mode origins from the breaking of a global pseudospin SU(2) symmetry.}
Dynamical excitations at ${bf q}=left[pi,piright]$ consist of a sharp
roton molecular peak in the low-energy region and a broad atomic excitation
band in the higher energy region. Furthermore, the weight of the roton
molecular peak decreases monotonically with increasing the hopping strength,
while the weight of the atomic excitations increases quickly. Interestingly we
check that the area covered by the roton molecular peak scales with the square
of the pairing gap, which is also true in the system with spin-orbit coupling.
This conclusion paves a potential way to measure the pairing gap of lattice
system experimentally by measuring the dynamical structure factor at ${bf
q}=left[pi,piright]$.
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