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Conductive dense hydrogen

Abstract

Molecular hydrogen is expected to exhibit metallic properties under megabar pressures. This metal is predicted to be superconducting with a very high critical temperature, Tc, of 200–400 K (ref. 1), and it may acquire a new quantum state as a metallic superfluid and a superconducting superfluid2. It may potentially be recovered metastably at ambient pressures3. However, experiments carried out at low temperatures, T<100 K (refs 4, 5), showed that at record pressures of 300 GPa, hydrogen remains in the molecular insulating state. Here we report on the transformation of normal molecular hydrogen at room temperature (295 K) to a conductive and metallic state. At 200 GPa the Raman frequency of the molecular vibron strongly decreased and the spectral width increased, evidencing a strong interaction between molecules. Deuterium behaved similarly. Above 220 GPa, hydrogen became opaque and electrically conductive. At 260–270 GPa, hydrogen transformed into a metal as the conductance of hydrogen sharply increased and changed little on further pressurizing up to 300 GPa or cooling to at least 30 K; and the sample reflected light well. The metallic phase transformed back at 295 K into molecular hydrogen at 200 GPa. This significant hysteresis indicates that the transformation of molecular hydrogen into a metal is accompanied by a first-order structural transition presumably into a monatomic liquid state. Our findings open an avenue for detailed and comprehensive studies of metallic hydrogen.

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Figure 1: Pressure shift and broadening of the Raman H2 and D2 vibrons at 295 K.
Figure 2: Raman spectra of hydrogen at high pressures and room temperature.
Figure 3: Electrical measurements of the hydrogen samples.
Figure 4: Phase diagram of hydrogen.

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Acknowledgements

Support provided by the Max Planck Society, DFG under the grant SPP 1236, and the European Research Council under the 2010-Advanced Grant 267777 is gratefully acknowledged. We appreciate the valuable comments and support of M. O. Andreae.

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Correspondence to M. I. Eremets or I. A. Troyan.

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Eremets, M., Troyan, I. Conductive dense hydrogen. Nature Mater 10, 927–931 (2011). https://doi.org/10.1038/nmat3175

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