Neutron-diffraction study of the static structure factor and pair correlations in liquid<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow></mml:math>

An investigation of the structure of liquid $^{4}\mathrm{He}$ at saturated vapor pressure by means of neutron diffraction is reported for 11 temperatures in the range 1.00 to 4.27 K. At each temperature, the static structure factor $S(Q)$ is obtained for $0.8\ensuremath{\le}Q\ensuremath{\le}10.8$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$ with an average statistical precision of 0.8% and with a residual systematic error which is estimated to be 1%. Our results for $S(Q)$ are in good agreement with the very precise x-ray results of Hallock which cover the range $0.133\ensuremath{\le}Q\ensuremath{\le}1.125$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$ and with the requirement that the corresponding pair correlation functions $g(r)$, which we obtain from $S(Q)$ by Fourier inversion, vanish inside the core of the interatomic potential. The temperature variations of both $S(Q)$ and $g(r)$ show clearly that the degree of spatial order in liquid $^{4}\mathrm{He}$ increases with decreasing temperature above the $\ensuremath{\lambda}$ point, 2.17 K, but then decreases with decreasing temperature in the superfluid phase. The latter behavior is believed to be a consequence of the Bose-Einstein condensation. A comparison of our results for $S(Q)$ and $g(r)$ at 1.00 K with the results of theoretical calculations for $T=0$ reveals significant discrepancies which indicate that there is a greater degree of spatial order in liquid $^{4}\mathrm{He}$ than would be the case if the atoms interacted via additive forces of the Lennard-Jones type.

Not yet translated