According to research published in Physical Review Research, a research team led by Prof. LUO Xuan from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences discovered the existence of stacking faults in the lattice structure of topological phononic β-MoB₂ single crystal.

The metal diboride compounds with a boron hexagonal ring structure have attracted significant attention in the field recently due to their nontrivial topology and abnormal superconductivity (SC) under pressure and doping. β-MoB₂ has been identified as a topological phonon material, and performs a structural transition and two SC transition under pressure. Specifically, SC I corresponds to β-MoB₂, while SC II corresponds to α-MoB₂. Currently, the physical origin of SC II has been clarified, while SC I remains incomprehensible. Therefore, further investigations of the phonon properties of β-MoB₂ corresponding to SC I are necessary.

In this study, the researchers obtained high-quality β-MoB₂ single crystals by improving the crystal growth process, and further investigated its phonon properties through systematic Raman spectroscopy measurements and theoretical calculations. The study revealed the presence of two anomalous Raman modes on the edge plane of β-MoB₂ single crystal, and they persist even at low temperatures of 4.2 K.

Subsequently, they employed theoretical calculations to elucidate the origin of these two anomalous Raman modes. Based on the theoretical results, the surface phonon on the edge plane of β-MoB₂ were ruled out as the origin of new modes. Additionally, isotopic splitting of boron and higher-order Raman scattering origins were also eliminated based on the peak intensity ratios.

Ultimately, according to the symmetry analysis, the emergence of these anomalous modes was attributed to the distortion of the B atomic layer in the lattice structure.

This work provides crucial structural information for understanding the rich superconducting behaviors of topological phonon materials under external conditions.

The single crystal exhibits seven vibration modes on the ac plane, including two Eg modes and five A1g modes. (Image by CHENG Ming)