Electronic structure of a borophene layer in rare-earth aluminum/chromium boride and its hydrogenated derivative borophane

We studied electronic states of borophene in a crystal of rare-earth aluminum/chromium boride and the hydrogenated sheet, borophane, successfully prepared by ion exchange and liquid exfoliation of the crystal. The layer has a boron network with five-membered and seven-membered rings. Electronic structures of all the boron-derived layers are found to be gapless by soft x-ray absorption and emission spectroscopy measurements at the B K-shell absorption edge and density functional theory calculations. The present results support the existence of a Dirac nodal loop at the Fermi level in the borophane layer with five- and seven-membered rings, predicted recently by topological calculations.