Abstract
Solid hydrogen, a simple system consisting only of protons and electrons, exhibits a variety of structural phase transitions at high pressures. Experimental studies1 based on static compression up to about 230 GPa revealed three relevant phases of solid molecular hydrogen: phase I (high-temperature, low-pressure phase), phase II (low-temperature phase) and phase III (high-pressure phase). Spectroscopic data suggest that symmetry breaking, possibly related to orientational ordering1,2, accompanies the transition into phases II and III. The boundaries dividing the three phases exhibit a strong isotope effect3, indicating that the quantum-mechanical properties of hydrogen nuclei are important. Here we report the quantum distributions of protons in the three phases of solid hydrogen, obtained by a first-principles path-integral molecular dynamics method. We show that quantum fluctuations of protons effectively hinder molecular rotation—that is, a quantum localization occurs. The obtained crystal structures have entirely different symmetries from those predicted by the conventional simulations which treat protons classically.
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Acknowledgements
We thank H. Nagara for discussions. H.K. was supported by the Japan Society for the Promotion of Science for Young Scientists. This work was partially supported by the Research for the Future Program of the Japan Society for the Promotion of Science, and by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.
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Kitamura, H., Tsuneyuki, S., Ogitsu, T. et al. Quantum distribution of protons in solid molecular hydrogen at megabar pressures. Nature 404, 259–262 (2000). https://doi.org/10.1038/35005027
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DOI: https://doi.org/10.1038/35005027
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