抄録
Metal borides have been investigated over the past decades since the material group shows varieties of physical phenomena, such as superconductivity and quantum criticality. Recently, two-dimensional boron networks in the crystals have paid attention among researchers to examine the topology physics and to utilize them as mother materials for synthesis of the functional atomic sheets. An example is YCrB4, which is one of the XYB4-type ternary borides with X= rare-earth metal and Y= transition metal. The boron layer consists of pentagons (5-membered rings) and heptagons (7-membered rings). The tiling arrangement belongs to the non-symmorphic symmetry group and our theoretical work has successfully made a topological classification of the boron sheet depending on chemical environments [1,2]. Recently, we synthesized a wide-area atomic sheet of HB by the ion-exchange reaction and the novel material has captured interests for real-world applications [3-5]. On the other hand, the as melt samples through arc-melting (without subsequent post-annealing), are essentially polycrystalline, inherently exhibit the YCrB4 phase as a major phase and minor ones with amorphous phases arising from oxides and with other metal borides. To make the better understanding of bulk and surface properties of YCrB4, a large size growth of the single crystal has been called for.
In this study, we have grown YCrB4 crystals using the arc-melting and post-annealing method at various conditions and we have succeeded in synthesizing 0.2~0.3 mm-sized single-crystals, as confirmed by SEM, EDX, and XRD. The two-dimensional (2D) flakes, prepared by mechanical peeling of the crystal, have allowed us to prepare the 2D platform and to make the detailed surface analysis. After the surface cleaning by Ar-sputtering and post-annealing, we have discovered formation of the boron-terminated surface and found the metallicity by micro-focused photoelectron spectroscopy at synchrotron radiation beamline BL-28A at KEK-PF. In the presentation, bulk/surface electronic states of the YCrB4 crystal are described in detailed with the experimental results and theoretical calculations. The surface electronic structure is discussed by comparisons with borophene layers, discovered on crystal surfaces [6-8].
Reference:
[1] I. Tateishi, X. Zhang et al., Molecules 27, 1808 (2022).
[2] Y. Ando, X. Zhang et al., Phys. Rev. B 106, 195106 (2022).
[3] X. Zhang et al., Molecules 28, 2985 (2023).
[4] X. Zhang et al., J. Phys. Chem. C, 126, 12802 (2022).
[5] M. Niibe, X. Zhang et al., Phys. Rev. Mater. 5, 084007 (2021).
[6] Y. Tsujikawa, X. Zhang et al., Surf. Sci. 732, 122282 (2023).
[7] Y. Tsujikawa, M. Horio, X. Zhang et al., Phys. Rev. B 106, 205406 (2022). [8] I. Matsuda, K. Wu ed. 2D Boron: Boraphene, Borophene, Boronene (Springer, 2021).
