Analysis of Raman modes of YNi2B2C by two-dimensional correlation spectroscopy

In this study, we have applied two-dimensional (2D) correlation analysis to the temperature-dependent Raman spectra of superconducting YNi2B2C single crystal. The 2D correlation analysis on apparently single peak B1g phonon mode near 200 cm−1 shows that the peak is essentially composed of two modes at 197 and 203 cm−1, and only the weight of the two modes changes as the temperature changes. This finding is an example of the power of the 2D correlation analysis on resolving highly overlapped peaks in Raman spectra. The finding of two overlapped peaks' nature of B1g phonon mode suggests that there are two distinctive environments of the Ni-B bonds in YNi2B2C system. Therefore, we extended 2D correlation analysis to the boron A1g mode in the range of 800-950 cm−1 in a hope to see multiple boron modes like the Ni mode. The 2D correlation analysis of the boron A1g indeed shows that the peak is composed of multiple (three) peaks at 840, 900, and 930 cm−1. Furthermore, 2D hetero-spectral correlation analysis was employed to relate the origins of the multiple phonon modes of YNi2B2C. Synchronous 2D hetero correlation spectrum obtained from the temperature-dependent Raman spectra of the Ni B1g and B A1g modes shows that a Ni B1g phonon mode at 197 cm−1 and B A1g mode at 840 cm−1 have one origin, while a Ni B1g phonon mode at 203 cm−1 and B A1g modes at 900 and 930 cm−1 arise from the other origin.