A simple model of the electric field dependence of complex-dominated ultracold molecular collisions

Recent experiments on bosonic NaRb [1, 2], RbCs [3] and fermionic NaK [4] molecules have observed high losses of non-reactive ultracold ground state molecules undergoing two-body collisions. A lot of theoretical effort has been devoted to understand this kind of loss mechanism, believed to originate from the formation of a tetramer complex at short-range. Using a recent unified formalism [5] that compiles average cross sections, random matrix and quantum defect theories, these losses of molecules can be characterized by a specific coefficient x, which can be linked to a short-range apparent loss probability p. When p ≈ 1, the loss is said to be universal and when p < 1, the loss is said to be non-universal. Experimentally, it is found that, while at zero electric field the collisions are non-universal [1, 3, 4, 6], they quickly become universal upon applying a small electric field [2,4]. We present a simple model of this phenomenon [7], showing that p can go from a value different than 1 to a value near 1 as an electric field is turned on.

[1] Ye et al., Science Advances 4, eaaq0083 (2018)

[2] Guo et al., PRX 8, 041044 (2018)

[3] Gregory et al., Nat. Commun. 10, 3104 (2019)

[4] Bause et al., arXiv:2103.00889 (2021)

[5] Croft et al., PRA 102, 033306 (2020)

[6] Bai et al., PRA 100, 012705 (2019)

[7] Quéméner et al., in preparation.