Evidence for melting of HD and ${\mathrm{D}}_2$ clusters in solid neon below 1 K

We report on an electron spin resonance study of H and D atoms stabilized in solid mixtures of neon, molecular deuterium, and hydrogen deuteride. We observed that H and D atoms can be stabilized in pure HD and ${\mathrm{D}}_2$ clusters formed in pores of solid Ne as well as in a Ne environment. Raising temperature from 0.1 to $1.3\mathrm{K}$ results in a rapid recombination of a significant fraction of both H and D atoms in HD and ${\mathrm{D}}_2$ clusters. The recombination rate appears to be five and seven orders of magnitude faster than in solid bulk samples of HD and ${\mathrm{D}}_2$, respectively. We explain this recombination rate enhancement by melting of clusters of molecular hydrogen isotopes, similar to what has been observed for atomic hydrogen in ${\mathrm{H}}_2$ clusters [Sheludiakov et al., Phys. Rev. B 97, 104108 (2018)]. Our observations do not provide evidence for a superfluid behavior of these clusters at temperatures of $0.1–1.3\mathrm{K}$.

Figure 1

(a) The sample cell schematic. (b) Energy levels for H (top row) and D atoms (bottom row) in a high magnetic field. The electronic and nuclear spin states are labeled as $|m_S,m_I\rangle$, respectively. The allowed ESR transitions are shown by solid lines.

Figure 2

(a) ESR spectrum of D atoms and trapped electrons in Ne:1%${\mathrm{D}}_2$ after two days of storage. The electron line appears as a shoulder on Component 2 of ${\mathrm{D}}_{\beta \epsilon }$ line. (b) ESR spectra for the same sample after one week of storage. The electron line becomes concealed by Component 3.

Figure 3

ESR spectra of D (a) and H atoms (b) in the Ne:1%HD sample.

Figure 4

Accumulation of H and D atoms in Ne:1%HD (blue squares) and Ne:1%${\mathrm{D}}_2$ (red circles) samples scaled to the estimated number of HD and ${\mathrm{D}}_2$ molecules in these samples. The accumulation rate of H atoms in a pure $2.5\mu \mathrm{m}{\mathrm{H}}_2$ film scaled to the number of ${\mathrm{H}}_2$ molecules there is shown by magenta diamonds for comparison.

Figure 5

Effect of temperature and a superfluid helium film on the central ${\mathrm{D}}_{\beta \epsilon }$ ESR lines of D atoms in the Ne:1%${\mathrm{D}}_2$ sample. The other ESR lines of D atoms in the Ne:1%${\mathrm{D}}_2$ sample behaved identically to the ${\mathrm{D}}_{\beta \epsilon }$ line and are not shown.

Figure 6

(a) Time evolution of C1 component amplitude for ESR spectrum of D atoms in Ne:1%${\mathrm{D}}_2$ sample at $T=0.9$ and (b) at $T=1.3\mathrm{K}$. No helium was condensed into the SC.

Figure 7

Normalized amplitudes of the ESR lines of H and D atoms (blue squares and red open circles, respectively) in HD clusters (C1 component) in Ne:1% HD sample. No helium was condensed into the SC.

Figure 8

(a) Time evolution of C1 component amplitude for the ESR spectrum of H atoms in Ne:1%HD sample at $T=0.9$ and (b) at $T=1.3\mathrm{K}$. No helium was condensed into the SC.