Millimeter-scale growth of ${\mathrm{YCrB}}_4$ single crystals and observation of the metallic surface state

Millimeter-scale growth of ${\mathrm{YCrB}}_4$ crystals was achieved by postannealing, followed by conventional arc melting. The bulk crystal was semiconducting, whereas the surface was metallic, as observed by photoemission spectroscopy. First-principles calculations consistently reproduced the metallicity in a model of the boron-terminated surface of the ${\mathrm{YCrB}}_4$ crystal. These results support further investigations of different states on the surface of metal boride bulk materials.

Figure 1

(a) Illustration of the ${\mathrm{YCrB}}_4$ atomic structure, showing a two-dimensional boron network featuring pentagons (five-membered rings) and heptagons (seven-membered rings) sandwiched between rare-earth metal (Y) and transition-metal (Cr) layers. (b), (c) Optical microscopy images of the ${\mathrm{YCrB}}_4$ samples prepared by (b) arc melting and (c) arc melting with the postannealing process. (d) Comparison of the powder x-ray diffraction patterns for ${\mathrm{YCrB}}_4$ crystals after arc melting (purple) and postannealing (yellow), and the calculated pattern for the ${\mathrm{YCrB}}_4$ crystal (red).

Figure 2

Reciprocal space sections reconstructed from the single-crystal XRD results: (a) ${\left(hk0\right)}^{*}$ and (b) ${\left(h0l\right)}^{*}$ reciprocal planes. The twin-related variants exist on the atomic arrangement level. The rectangle reciprocal lattices represented in red and blue are based on the unit cells of the original and twin-related variants, respectively.

Figure 3

(a) A collection of B $1s$ core-level photoemission spectra for the ${\mathrm{YCrB}}_4$ crystal, obtained before and after cleaning treatment of the sample surface. Spectra labeled points 1 and 2 were taken at the positions indicated in (c). The spectra were taken at $h\nu =285$ eV. (b) Optical microscopy image of the ${\mathrm{YCrB}}_4$ crystal after surface treatment for the PES measurement. (c) Spatial mapping of the photoemission intensity integrated over the $E_B$ range of 186–190 eV, measured with $h\nu =650$ eV. The step size was 100 $\mu \mathrm{m}$ along the $x$ and $y$ directions. The region corresponds to the area indicated by the red square in (a). All measurements were performed at room temperature

Figure 4

(a) Photoemission spectra of the ${\mathrm{YCrB}}_4$ crystal after surface treatment at point 1 and 2 indicated in Fig. 3, obtained in the binding energy range of 40–540 eV with $h\nu =650$ eV. (b), (c) Core-level spectra of the (b) Y $3d$ and (c) Cr $2p$ orbitals, measured at the point 1 with $h\nu =650$ eV.

Figure 5

EDCs with photon energy $h\nu =187$ eV at point 1, before (black dashed line) and after (purple line) cleaning the surface. EDCs of the spectra were calibrated using the Fermi edge of an Au reference sample at the corresponding incident photon energy and normalized to the photon flux. The measurement was performed at 50 K.

Figure 6

(a) A model of the B-terminated $\left(001\right)$ surface of ${\mathrm{YCrB}}_4$. (b) Two-dimensional Brillouin zone of the $\left(001\right)$ surface. (c) ${\mathrm{YCrB}}_4\left(001\right)$ surface band calculation based on the model depicted in (a).