Observation Of Ultrahigh Mobility Excitons In A Strain Field By Space- And Time-Resolved Spectroscopy At Subkelvin Temperatures

We measured basic parameters such as the lifetime, mobility, and diffusion constant of trapped paraexcitons in Cu2O at very low temperatures (below 1 K) using a dilution refrigerator. To obtain these parameters, we observed the space- and time-resolved luminescence spectrum of paraexcitons in a strain-induced trap potential. We extracted the lifetime of 410 ns from the measurements of the decay of the luminescence intensity. By comparing the experimental results and numerical calculations, we found that the mobility and the diffusion constant increase as the temperature of the paraexcitons decreases below 1 K. In particular, we obtained a mobility of 5.1 x 10(7) cm(2)/V s at the corresponding temperature of 280 mK. To the best of our knowledge, this value is the highest exciton mobility that has been measured. These results show that the mean-free path of the paraexcitons reaches a size (similar to 300 mu m) comparable to that of the cloud of trapped paraexcitons (similar to 100 mu m). From our analyses, we found that the spatial distribution of the paraexcitons can reach a distribution that is defined by the statistical distribution function and the shape of the three-dimensional trap potential at ultralow temperatures (well below 1 K). Our survey shows that the ultralow temperature ensures that the Bose-Einstein condensation transition in a trap potential can be investigated by examining the spatial distribution of the density of 1s paraexcitons.