Measurement of the interaction strength in a Bose-Fermi mixture with 87Rb and 40K

TL;DR

This study measures the interaction strength between bosonic 87Rb and fermionic 40K in a quantum degenerate mixture, providing a more precise scattering length value that challenges existing theoretical predictions about mixture stability.

Contribution

The paper presents a precise measurement of the interspecies scattering length in a Bose-Fermi mixture, refining previous estimates and impacting theoretical models of mixture stability.

Findings

Measured |a_RbK| = 250 +/- 30 a_0 with reduced uncertainty.

Found the measured value is lower than previous reports.

Results suggest a discrepancy with existing stability theories.

Abstract

A quantum degenerate, dilute gas mixture of bosonic and fermionic atoms was produced using 87Rb and 40K. The onset of degeneracy was confirmed by observing the spatial distribution of the gases after time-of-flight expansion. Further, the magnitude of the interspecies scattering length between the doubly spin polarized states of 87Rb and 40K, |a_RbK|, was determined from cross-dimensional thermal relaxation. The uncertainty in this collision measurement was greatly reduced by taking the ratio of interspecies and intraspecies relaxation rates, yielding |a_RbK| = 250 +/- 30 a_0, which is a lower value than what was reported in [M. Modugno et al., Phys. Rev. A 68, 043626 (2003)]. Using the value for |a_RbK| reported here, current T=0 theory would predict a threshold for mechanical instability that is inconsistent with the experimentally observed onset for sudden loss of fermions in [G. Modugno et al., Science 297, 2240 (2002)].