In an article recently published in the Journal of Applied Physics, researchers investigated the influence of strain and pressure on the superconductivity and electron-phonon coupling in magnesium diboride (MgB₂)_. They also discussed the corresponding theoretical approaches and the future of nanostructure design.

Background

The discovery of MgB₂ as a high-temperature superconductor with a superconducting transition temperature (T_c) = 39 K led to a flurry of experimental and theoretical studies on the compound's various properties. Understanding superconductivity requires knowledge of both the phonon and electrical band structures. Inelastic X-ray scattering measurements of phonon band structure reveal Kohn anomalies. The depths of the Kohn anomalies can be directly linked to the superconducting transition temperature, according to theoretical investigations.

More research is needed, however, to compare approaches for predicting T_c based on the Kohn anomaly strength to estimate the electron-phonon coupling (EPC). The impact of positive hydrostatic pressure suppresses T_c in MgB₂. Anisotropic strain's effect on MgB₂ superconductivity has also been studied experimentally and theoretically. The Migdal–Éliashberg formalism of superconductivity is known to perform well in calculating the transition of superconductivity in MgB₂, in both anisotropic and isotropic approximations, under ambient circumstances.

About the Study

In this study, the authors investigated the effect of hydrostatic pressure, strain, and anisotropic tension on the T_c of MgB₂ by using a variety of theoretical approaches. This was accomplished by looking at Kohn anomalies in phonon dispersions alone, as well as by calculating the electron-phonon coupling explicitly. An alternative method was offered via co-deposition using ternary diborides that thermodynamically avoided mixing with MgB₂. This method encouraged columnar growth, which caused a strain in all directions.

The researchers compared alternative methods for predicting the T_c of MgB₂ under hydrostatic pressure, as well as anisotropic stress and strain. On the optical E₂g branch, the first method was based on the pressure-derived evolution of the Kohn anomaly and phonon dispersions. The discrepancies in the detected Kohn anomalies along Γ–q (q = K, M, H, L) across the procedures were quantified. Phonons obtained from both finite displacements and linear response methods were compared. The Migdal–Éliashberg formalism was used to calculate EPC as a function of pressure. T_c was calculated by using both isotropic and anisotropic approximations.

The team utilized two approaches to determine electronic and phonon characteristics in MgB₂ from first-principles computations. Two phonon-based finite displacement and linear response approaches were used to characterize the response of Kohn anomaly in the optical E₂g branch to the in-plane stretching modes of the B–B bond. The Kohn anomaly approach calculated T_c by utilizing finite displacements at ambient pressure when averaged across all directions. A 5 x 5 (a, c) grid was used to investigate the lattice parameter space around the MB₂ (M = V, Ti, Hf, Zr, Y) candidates.

Image 1: The effect of strain and pressure on the electron-phonon coupling and superconductivity in MgB₂—Benchmark of theoretical methodologies and outlook for nanostructure design. Image Credit: Gorodenkoff/Shutterstock.com

Image 2: The hexagonal unit cell of MgB₂. Each unit cell consists of one magnesium atom (orange) and two boron atoms (purple). Image Credit: Johansson, E et al., Journal of Applied Physics

*Original article by Surbhi Jain and reviewed by Susha Cheriyedath, M.Sc. Published in azom.com from Journal of Applied Physics.