Search for Superfluidity in Solid Hydrogen

A torsional oscillator study of solid para-hydrogen has been carried out down to 20 mK in a search for evidence of superfluidity. We found evidence of a possible phase transition, marked by an abrupt increase in the resonant period of oscillation and onset of extremely long relaxation times as the temperature was raised above 60 mK. In contrast to solid ${}^4\mathrm{He}$, the change in the period for para-hydrogen is not a consequence of irrotational superflow. The long relaxation times observed suggest the effect is related to the motion of residual ortho-hydrogen molecules in the solid.

Figure 1

Schematic of the torsion cell. The ${\mathrm{H}}_2$ space in the cell is comprised of an open annulus and an axial fill line, which are connected by three channels that span the diameter of the cell. The annulus has an average radius, width, and height of 6.5, 2.3, and 3.8 mm, respectively. The radial channels and fill line have a diameter of 1.0 mm, a combined volume that constitutes 7% of the total cell volume, and contribute about 2% of the rotational inertia of all the ${\mathrm{H}}_2$ in the cell. The “blocked cell” has the same geometry, with the addition of a barrier at one point in the annulus.

Figure 2

(a) $T$ dependence of ${\tau }_0$ for both the open and blocked cells. All of the data shown were obtained at maximum rim velocities less than $40\mu \mathrm{m}/\mathrm{s}$. For the open annulus, data is plotted for the empty cell, cell containing HD, and cell containing ${\mathrm{H}}_2$. ${\mathrm{H}}_2$ data taken using the blocked cell exhibits the same qualitative features, including the long relaxation times. (b) $T$ dependence of $\Delta {\tau }_0$, normalized by the total ${\mathrm{H}}_2$ mass loading (NCRIF, the apparent NCRI fraction). The $T$ dependence is similar to that observed in solid ${}^4\mathrm{He}$. However, the same effect is seen in the blocked cell.

Figure 3

$T$ dependence of measured relaxation times, including NMR and thermal conductivity data from the literature [18]. ○ signify the growth ($T<1\mathrm{K}$) or decay ($T>1\mathrm{K}$) rates of isolated orho-${\mathrm{H}}_2$ molecules. ▵ and ▿ are, respectively, the exponential decay and growth rates of the thermal conductivity. In the present study, -⋄- are extracted from ${\tau }_0$ versus time data. The sharp increase in $t$ coincides with the observed ${\tau }_0$ shift. The largest $t$ values are observed near 80 mK. Near 180 mK, a minimum in $t$ exists for both the oscillator and conductivity measurements. Inset: ${\tau }_0$ versus time curves for different $T$. The lower $T=57\mathrm{mK}$ curve, from a previous sample, clearly demonstrates the time independence at low $T$. The curves are shifted vertically for clarity.