Electron-phonon coupling superconductivity in 2D orthorhombic MB6 (M=Mg, Ca) and hexagonal MB6 (M=Mg, Ca, Sc, Ti, Sr, Y)

TL;DR

This study predicts that various 2D metal borides, including orthorhombic and hexagonal structures, are stable phonon-mediated superconductors with transition temperatures up to 28 K, and their properties can be enhanced by strain.

Contribution

It introduces a series of 2D MB6 structures with detailed analysis of their stability, electronic, and superconducting properties, expanding the database of 2D superconductors.

Findings

All MB6 are thermally stable up to 700-1000 K.

They are intrinsic phonon-mediated superconductors with Tc up to 21.3 K.

Strain can significantly enhance Tc, reaching 28 K under compression.

Abstract

Combining crystal structure search and first-principles calculations, we report a series of two-dimensional (2D) metal borides including orthorhombic (ort-) MB6 (M=Mg, Ca) and hexagonal (hex-) MB6 (M=Mg, Ca, Sc, Ti, Sr, Y). Then, we investigate their geometrical structures, bonding properties, electronic structures, mechanical properties, phonon dispersions, thermal stability, dynamic stability, electron-phonon coupling (EPC), superconducting properties and so on. Our ab initio molecular dynamics simulation results show that these MB6 can maintain their original configurations up to 700/1000 K, indicating their excellent thermal stability. All their elastic constants satisfy the Born mechanically stable criteria and no visible imaginary frequencies are observed in their phonon dispersions. The EPC results show that these 2D MB6 are all intrinsic phonon-mediated superconductors with the superconducting transition temperature (Tc??) in the range of 2.2-21.3 K. Among them, the highest Tc (21.3 K) appears in hex-CaB6, whose EPC constant () is 0.94. By applying tensile/compressive strains on ort-/hex-CaB6, we find that the compressive strain can obviously soften the acoustic phonon branch and enhance the EPC as well as Tc. The Tc of the hex-CaB6 can be increased from 21.3 K to 28 K under compressive strain of 3%. These findings enrich the database of 2D superconductors and should stimulate experimental synthesizing and characterizing of 2D superconducting metal borides.