Observation of Resonant Effects in Ultracold Collisions between Heteronuclear Feshbach Molecules*

Magnetic field dependent dimer-dimer collisional losses are studied with ultracold $^{23}$Na$^{87}$Rb Feshbach molecules. By ramping the magnetic field across the 347.8 G inter-species Feshbach resonance and removing residual atoms with a magnetic field gradient, $\sim$8000 pure NaRb Feshbach molecules with a temperature below 1 $\mu$K are produced. By holding the pure molecule sample in a crossed optical dipole trap and measuring the time-dependent loss curves under different magnetic fields near the Feshbach resonance, the dimer-dimer loss rates with respect to the atomic scattering length a are mapped out. We observe a resonant feature at around $a = 600 a_{0}$ and a rising tail at above $a = 1600 a_{0}$. This behavior resembles previous theoretical works on homonuclear Feshbach molecule, where resonant effects between dimer-dimer collisions tied to tetramer bound states were predicted. Our work shows the possibility of exploring four-body physics within a heteronuclear system.