Raman Spectroscopy Investigation of Phonon Behavior in ZnO-Buffered MgB₂ Tapes: Exploring Lattice Dynamics and Anharmonicity

We investigated the phonon behavior of ZnO-buffered MgB₂ tapes with varying ZnO buffer layer thicknesses using polarized Raman spectroscopy at room and cryogenic temperatures. Polar plots from integrated angle-resolved polarized Raman spectroscopy (ARPRS) at room temperature revealed substantial distortion in the boron plane geometry due to lattice mismatch among the MgB₂ film, ZnO buffer layer, and Hastelloy substrate. This distortion significantly affects the electron-phonon coupling (EPC) constant, λ, which we calculated using the modified McMillan equation by Allen-Dynes in relation to the superconducting transition temperature (<i>T</i>_c) of the sample. At cryogenic temperatures, our investigation of the <i>E</i>_2<i>g</i> mode exhibited a notable phonon hardening effect of up to ∼4.1%, correlated with the ZnO buffer layer thickness. Furthermore, analysis of the anharmonic <i>E</i>_2<i>g</i> phonon mechanism through line width (full width at half maximum) revealed damping behavior, indicating an additional coupling mechanism within the sample that varies with the temperature. This unique Raman scattering behavior potentially elucidates the high <i>T</i>_c mechanism of MgB₂, which is underestimated by traditional EPC calculations. Additionally, increasing the thickness of the ZnO layer is predicted to alleviate the distortion in the boron plane geometry, thereby promoting MgB₂ toward its inherent electron-phonon superconducting nature by mitigating the additional coupling mechanisms. Understanding how the ZnO buffer layer influences the phonon dynamics and EPC in MgB₂ will provide critical insights into optimizing its superconducting properties and advancing its practical applications in high-performance superconducting devices.