Softening of Roton and Phonon Modes in a Bose-Einstein Condensate with Spin-Orbit Coupling

Understand

Roton-type excitations usually emerge from strong correlations or long-range interactions, as in superfluid helium or dipolar ultracold atoms.

• However, in weakly short-range interacting quantum gas, the recently synthesized spin-orbit (SO) coupling can lead to various unconventional phases of superfluidity, and give rise to an excitation spectrum of roton-maxon character.
• Using Bragg spectroscopy we study a SO coupled Bose-Einstein condensate of $^{87}$Rb atoms, and show that the excitation spectrum in a "magnetized" phase clearly possesses a two-branch and roton-maxon structure.
• As Raman coupling strength $\Omega$ is decreased, a roton-mode softening is observed, as a precursor of the phase transition to a stripe phase that spontaneously breaks spatially translational symmetry.