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 $\mathrm{\Gamma }$ 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.

The existence of a topological Dirac nodal loop is predicted for hydrogenated monolayer boron sheets with a nonsymmorphic symmetry. The three-center two-electron bonds in boron compounds restrict the electronic system to be insulating or semimetallic with a Dirac nodal loop. Two types of electronic structures are distinguished by the ${\mathbb{Z}}_2$ topological index and confirmed by first-principles total-energy calculations. The topological taxonomy, developed in this research, can be applied to other two-dimensional materials and to seek novel Dirac fermions.

Fermi surfaces and band dispersion curves of a ${\mathrm{Cu}}_2\mathrm{Si}$ layer on Si(111), quasi-“$5\times 5$” were mapped by angle-resolved photoemission spectroscopy using synchrotron radiation. Two metallic bands were observed within the Si bulk band gap, which are likely assigned to the electronic bands of the ${\mathrm{Cu}}_2\mathrm{Si}$ layer. Additional bands were found in the Si bulk band gap that originate from the interactions between the substrate state and the ${\mathrm{Cu}}_2\mathrm{Si}$ states that have the $p_z$ character. The present research aims at investigating the changes in the electronic structure of an atomic layer when prepared on a substrate.

We investigate THz metamaterial resonators which provide highly subwavelength confinement of the electromagnetic field. In these structures the capacitive part is reduced into nano-scale dimensions and allows strong local electric field enhancement, useful for infrared detectors and sensors. A crucial issue for these structures is the fraction of the incident THz radiation that couples into the capacitive part of the metamaterial. To this end we develop a strategy to enhance the harvesting of the incoming free space radiation into volumes as small as 5x10⁻⁸λ³ for a free space wavelength λ. We also provide an analytical model in order to evaluate propagation effects in the inductive part of the structures.

The electronic structure of the ($3\times 3$)-ordered phase of a silicon (Si) layer on Al(111) has been studied by angle-resolved photoemission spectroscopy using synchrotron radiation and modeled by a trial atomic model. A closed Fermi surface originating from a linearly dispersing band is identified. A band structure calculation of a trial atomic model of the honeycomb silicene on Al(111) implies that the metallic band originates from the Al-Si hybrid state that has the Dirac-cone-like dispersion curve. The Si layer on Al(111) can be a model system of Xenes to realize the massless electronic system through the overlayer-substrate interaction.