Abstract

Using the exciton-mediated photovoltaic effect, we examine exciton transport over large distances in Cu2O as a function of temperature and particle density. Evidence for a phase transition at low temperatures and high densities is attributed to the onset of excitonic superfluidity.

We have performed exciton transport measurements over a range of temperatures and exciton densities in ultrapure, oriented large Cu2O single crystals. A sketch of the experimental method is shown in Fig. 1. The crystal is illuminated on the back surface by 10 ns pulses from a frequency-doubled YAG laser (λ = 532 nm). The initial exciton density created over an absorption depth (about one micron at λ = 532 nm) can be varied by inserting calibrated neutral density filters in the laser beam, reaching values of up to 1019 cm−3. The excitons which have migrated to the opposite face of the crystal are dissociated into free carriers by the high electric field near the Cu Schottky contact [1] deposited in a comb configuration together with an ohmic Au electrode, resulting in an external current. A time-resolved measurement of that current will give the velocity distribution of the excitons migrating through the crystal. This method of detection – as opposed to photoluminescence – is particularly well suited to the study of optically inactive paraexcitons in Cu2O; moreover, since the migration time is of the order of one microsecond as compared to the lifetime of 13 μs [2] for paraexcitons, recombination processes have little influence on the measurements.