Atomic hydrogen (deuterium) impurities stabilized in molecular H2 (D2) crystals

Illustrating H in H2 crystal and zero-point motionSolid hydrogen is one of the best examples of quantum solids which have large zero-point motion. In case of H2 it is 18% of the lattice constant. Atomic hydrogen and its heavier isotope, deuterium, stabilized in solid H2 and D2 matrices provide a system where a number of fascinating quantum phenomena could be reached.

Already in 1969 Andreev and Lifshitz [1] hypothesized that any impurity in a solid cooled to sufficiently low temperature and having large enough tunnelling probability becomes delocalized and can move freely along the crystal like electrons in conductors. A gas of such impurities or vacancies may exhibit quantum degeneracy phenomena at sufficiently large density, and like the atoms of the ideal gas undergo Bose-Einstein condensation. Chester and Leggett pointed out that phenomenon similar to the superfluidity of 4He may also be reached in its solid phase, so-called “supersolidity”[2,3].

However, if very high concentrations of atoms of the order of a few percents can be reached, the exchange interaction will lead to correlations between their electrons and appearance of electrical conductivity in such system. This would create hydrogen in a metallic state, the “holy grail” of high-pressure physics at zero pressure.Schematic drawing of the sample cell

The H,D:H2,D2  samples are prepared by in-situ electron-impact cryogenic dissociation of solid H2 (D2) and studied by magnetic resonance methods (ESR,NMR,ENDOR) in high magnetic field of 4.6T below 1K using a unique cryogenic ESR spectrometer developed in our lab. Recently we have succeeded in reaching record high densities of H atoms, 6x1019cm-3 using exchange quantum reactions D+H2=HD+H and HD+D=H+D2.

At present we are modifying the setup for studies of layered H2,D2 films. A long term goal is to implement atomic and molecular beam epitaxy below 1 K to create samples. It will improve the sample quality and is expected to lead to higher impurity atom densities.

 

  1. A.F. Andreev and I.M. Lifshits, Sov. Phys. JETP 29, 1107 (1969) 
  2. G.V. Chester, Phys. Rev. A 2, 256 (1970)
  3. A. Leggett, Phys. Rev. Lett. 25, 1543.

 

 

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