X-Ray Imaging of the Phase Separation in Mixed ³He/⁴He Nanodroplets

A mixture of ³He and ⁴He liquids separates into a ³He-rich and a ⁴He-rich phase below 0.87 K. Here, we present a study of this phase separation in freely-rotating, sub-micrometer-sized helium droplets under molecular beam conditions in vacuum at T ≈ 0.15 K. Density distributions within individual droplets are studied by ultrafast x-ray diffraction using the LCLS free electron laser. Strong indications for the separation of the two phases within the droplet are provided by pronounced azimuthal intensity modulations of the ring-shaped diffraction patterns, that the inner phase has a different shape than the outer one. We examine the possibilities that the ⁴He rich phase may consists of two lobes or a torus immersed in ³He. The results are also compared to recent theoretical work on phase separation in classical rotating droplets [1] and DFT calculations on mixed ³He/⁴He droplets [2].

[1] S. L. Butler, Phys Fluids 32, 012115 (2020).
[2] M. Pi et al., Phys Rev B 102, 060502 (2020).