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We measured basic parameters such as the lifetime, mobility, and diffusionconstant of trapped paraexcitons in cuprous oxide at very low temperatures(below 1 K) using a dilution refrigerator. To obtain these parameters, weobserved the space- and time-resolved luminescence spectrum of paraexcitons instrain-induced trap potential. We extracted the lifetime of 410 ns from themeasurements of the decay of the luminescence intensity. By comparing theexperimental results and numerical calculations, we found that the mobility andthe diffusion constant increase as the temperature of the paraexcitonsdecreases below 1 K. In particular, we obtained a mobility of 5.1e7 cm^2/V*s atthe corresponding temperature of 280 mK. To the best of our knowledge, thisvalue is the highest exciton mobility that has been measured. These resultsshow that the mean free path of the paraexcitons reaches a size (~300 um)comparable to that of the cloud of trapped paraexcitons (~100 um). From ouranalyses, we found that the spatial distribution of the paraexcitons can reacha distribution that is defined by the statistical distribution function and theshape of the three-dimensional trap potential at ultra-low temperatures (wellbelow 1 K). Our survey shows that the ultra-low temperature ensures that theBose--Einstein condensation transition in a trap potential can be investigatedby examining the spatial distribution of the density of 1s paraexcitons.

Observation of ultra-high mobility excitons in a strain field by space- and time-resolved spectroscopy at sub-Kelvin temperatures