Observation of five-fold local symmetry in liquid lead

The local point symmetry of the short-range order in simple monatomic liqui ds remains a fundamental open question in condensed-matter science. For mor e than 40 years it has been conjectured(1-4) that liquids with centrosymmet ric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentall y inaccessible owing to the unavoidable averaging involved in scattering ex periments, which can therefore determine only the isotropic radial distribu tion function. Here we overcome this limitation by capturing liquid fragmen ts at a solid-liquid interface, and observing the scattering of totally int ernally reflected (evanescent) X-rays, which are sensitive only to the liqu id structure at the interface. Using this method, we observe five-fold loca l symmetry in liquid lead adjacent to a silicon wall, and obtain an experim ental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond ord er in disordered structures such as liquids and glasses, these results shou ld lead to a better microscopic understanding of melting, freezing and supe rcooling.