Isotope effect on the E2g phonon and mesoscopic phase separation near the electronic topological transition in Mg1-xAlxB2

TL;DR

This study investigates the isotope effect on the E2g phonon mode in Mg1-xAlxB2, revealing insights into electron-phonon interactions and phase separation near the electronic topological transition, with implications for understanding high-temperature superconductivity.

Contribution

It provides new experimental evidence on isotope effects and phonon behavior near the ETT in Mg1-xAlxB2, highlighting the limited role of intraband phonon-mediated pairing in high Tc.

Findings

Isotope coefficient near 0.5 across the range

E2g phonon splitting near the ETT

Minor role of intraband phonon pairing in high Tc

Abstract

We report the boron isotope effect on the E2g phonon mode by micro-Raman spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes 10B and 11B. The isotope coefficient on the phonon frequency is near 0.5 in the full range decreasing near x = 0. The intraband electron-phonon (e-ph) coupling, for the electrons in the sigma band, has been extracted from the E2g line-width and frequency softening. Tuning the Fermi energy near the electronic topological transition (ETT), where the sigma Fermi surface changes from 2D to 3D topology the E2g mode, shows the known Kohn anomaly on the 2D side of the ETT and a splitting of the E2g phonon frequency into a hard and soft component from x = 0 to x = 0.28. The results suggest a minor role of the intraband phonon mediated pairing in the control of the high critical temperature in Mg1-xAlxB2. The common physical features of diborides with the novel multigap FeAs-based superconductors and cuprates is discussed.