Semimetallicity of free-standing hydrogenated monolayer boron from MgB2

Electronic states of a free-standing hydrogenated monolayer boron (HB) sheet were studied via soft x-ray spectroscopies at the B $K$-shell absorption edge and first-principles calculations. The HB sheet is semimetallic with electron and hole pockets at the $Y$ and $\mathrm{\ensuremath{\Gamma}}$ points, respectively. The electron band results from the B-H-B bonds formed during synthesis from a ${\mathrm{MgB}}₂$ crystal, while the hole band is kept through the process and originates from a honeycomb lattice boron layer or borophene in ${\mathrm{MgB}}₂$. Our results suggest that the HB sheet is a promising two-dimensional material for realizing new boron-based or superconducting nanodevices.