Fig. 2: Direction and fluence dependencies of exciton transport in a 10L sample.

a, Top: spatial PL profile recorded at T_N = 132 K under a fluence of 3,100 μJ cm⁻². Broadening along the a and b directions is indicated by the blue and red arrows, respectively. The dashed circles mark the σ values extracted from the Gaussian fits of the PL and laser profiles. Bottom: spatial profile of the excitation laser. b, Time and fluence dependencies of Δσ²(t) recorded at T_N along the a and b directions (Extended Data Fig. 8 and Supplementary Fig. 2). The lines are linear fits to the data during the first 25 ps. Insets: the axis of transport measurement. c, Position dependence of X₀ emission under an excitation fluence of 260 μJ cm⁻² (top) and 3,100 μJ cm⁻² (bottom), corresponding to the estimated exciton densities between 1.1 × 10¹² and 1.3 × 10¹³cm⁻² per layer, respectively. The sample temperature was nominally 4 K, but spectral shifts in the region of laser excitation locally indicate an effective increase in temperature due to excitation (Extended Data Fig. 5). The laser profile is shown by the grey line. d, Schematic illustrating an incoherent magnon flux j_mag (blue) propagating away from the excitation region, dragging excitons (red and blue circles) along. Pulsed laser excitation is indicated by the red line. e, Calculated magnon dispersion. Compared with exciton masses from ref. ²⁶, magnons are substantially heavier than excitons; magnon-to-exciton mass ratios are 38 and 7 along the b and a directions. M_a and M_b denote the masses of magnons along the a- and b-directions, respectively; m₀ is the free electron mass. The solid and dashed lines represent two branches that are very close in energy (Supplementary Section 6).