Chemical bonding dictates drastic critical temperature difference in two seemingly identical superconductors

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• YB₆ and LaB₆ share similar structures but show vastly different superconducting properties.
• YB₆ exhibits a high critical temperature (8.4 K), while LaB₆ superconducts below 0.45 K.
• Previous research identified differences in density of states and electron-phonon coupling (EPC) but lacked detailed electronic structure analysis.

Purpose of the Study:

• To elucidate the fundamental electronic structure differences driving the contrasting superconductivity in YB₆ and LaB₆.
• To identify the specific electronic interactions responsible for YB₆'s enhanced superconductivity compared to LaB₆.

Main Methods:

• Analysis of chemical bonding and electronic structure.
• Investigation of electron-phonon coupling (EPC) through phonon modes and electronic state interactions.
• Supercell calculations at the M k-point to study Peierls-like effects.

Main Results:

• Unoccupied 4f atomic orbitals in lanthanum are identified as the key differentiator.
• These 4f orbitals hybridize with π B-B bonds in LaB₆, lowering their energy relative to σ B-B bonds.
• Optical phonon modes significantly alter σ-orbital overlap in YB₆, indicating strong EPC, but couple weakly to LaB₆'s π-orbitals.
• YB₆ exhibits electronic state crossings due to phonons, signifying strong EPC, absent in LaB₆.
• Peierls-like effects at the M k-point in YB₆ contribute additional EPC.

Conclusions:

• LaB₆ and YB₆ possess fundamentally different superconductivity mechanisms despite structural similarities.
• The presence and energy of unoccupied 4f orbitals in LaB₆ critically influence its superconducting behavior.
• Electronic structure, particularly the role of 4f orbitals and their interaction with phonon modes, dictates the observed differences in superconductivity.