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DOE OSTI2020

Materials Data on BaAl4 by Materials Project

BaAl4 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Ba is bonded in a 8-coordinate geometry to sixteen Al atoms. There are eight shorter (3.51 Å) and eight longer (3.65 Å) Ba–Al bond lengths. There are two inequivalent Al sites. In the first Al site, Al is bonded in a 8-coordinate geometry to four equivalent Ba and four equivalent Al atoms. All Al–Al bond lengths are 2.73 Å. In the second Al site, Al is bonded in a 9-coordinate geometry to four equivalent Ba and five Al atoms. The Al–Al bond length is 2.70 Å.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Observation of a Flat and Extended Surface State in a Topological Semimetal

A flat band structure in momentum space is considered key for the realization of novel phenomena. A topological flat band, also known as a drumhead state, is an ideal platform to drive new exotic topological quantum phases. Using angle-resolved photoemission spectroscopy experiments, we reveal the emergence of a highly localized surface state in a topological semimetal BaAl4 and provide its full energy and momentum space topology. We find that the observed surface state is localized in momentum, inside a square-shaped bulk Dirac nodal loop, and in energy, leading to a flat band and a peak in the density of state. These results imply this class of materials as an experimental realization of drumhead surface states and provide an important reference for future studies of the fundamental physics of correlated quantum effects in topological materials.

36 MATERIALS SCIENCE↗