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arXiv · 2610.17812

Orthoexciton-to-paraexciton conversion rate in Cu$_2$O at sub-Kelvin temperatures

Yusuke Morita·Yukihiro Akiyama·Yuki Haita·Yuga Hirota·Kosuke Yoshioka
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Abstract

We measured the conversion rate from 1s orthoexcitons to 1s paraexcitons in Cu$_2$O below 1 K using a dilution refrigerator. To determine the conversion rate, we performed time-resolved luminescence spectroscopy of 1s orthoexcitons and 1s paraexcitons confined in a strain-induced trap potential. From the time evolution of the $Γ$$_3$$^{-}$-LO-phonon-assisted luminescence of 1s orthoexcitons, we obtained a conversion rate of $5.9 \times 10^7$ s${^{-1}}$ at a mixing-chamber temperature of 64 mK under an applied stress of 2.2 kbar. Between 64 and 800 mK, the conversion rate depends only weakly on temperature, and it is less than half of the value of $1.6 \times 10^8$ s${^{-1}}$ reported at 2 K under the same applied stress. The conversion rate therefore continues to decrease below 2 K, contrary to the previous expectation that it becomes nearly constant below about 1.7 K. We discuss these results in terms of the momentum distribution of orthoexcitons, which does not reach equilibrium with the lattice during the conversion at these low temperatures. These findings call for a theory that accounts for both the small magnitude of the conversion rate and its weak temperature dependence below 1 K.

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Yusuke Morita, Yukihiro Akiyama, Yuki Haita, Yuga Hirota, Kosuke Yoshioka. 2026-10-13. Orthoexciton-to-paraexciton conversion rate in Cu$_2$O at sub-Kelvin temperatures. https://arxiv.org/abs/2610.17812

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