Ruthenium Doping Alters Electronic and Magnetic Properties in Iron-Based Superconductor, Study Finds

Electronic Structure Transformations in Ruthenium-Doped Superconductors

A detailed theoretical investigation has revealed how ruthenium doping systematically modifies the electronic structure and magnetic properties of the iron-based superconductor LiFeAs, according to recent findings published in Scientific Reports. The study employed advanced computational methods to analyze how substituting ruthenium for iron at various concentrations affects lattice parameters, band structures, and magnetic characteristics of the material.

Electronic Structure Transformations in Ruthenium-Doped Superconductors
Lattice Expansion and Electron Correlation Effects
Band Structure Modifications and Superconductivity Implications
Projected Density of States Evolution
Fermi Energy Shifts and Thermodynamic Stability
Magnetic Property Modifications

Lattice Expansion and Electron Correlation Effects

Sources indicate that electron correlation effects significantly influence the structural properties of ruthenium-doped LiFeAs. When researchers incorporated Hubbard corrections (DFT+U) into their calculations, they observed systematic expansion of lattice parameters across all systems studied. For pristine LiFeAs, the lattice constants increased from a = 3.767 Å and c = 6.230 Å to a = 3.779 Å and c = 6.382 Å under DFT+U, representing expansions of approximately 0.3% along the a-axis and 0.08% along the c-axis., according to recent studies

The report states that at 50% ruthenium substitution, the expansion became more pronounced, with lattice constants increasing from a = 3.896 Å and c = 6.430 Å to a = 4.023 Å and c = 6.455 Å. Analysts suggest this anisotropic expansion, most evident along the c-axis, reflects enhanced localization of Fe 3d electrons where electron correlation weakens bonding interactions and drives lattice expansion. Interestingly, the study found that ruthenium incorporation reduces the sensitivity of the lattice to correlation effects compared to iron-rich compositions, attributed to ruthenium’s more delocalized 4d orbitals.

Band Structure Modifications and Superconductivity Implications

According to the analysis, substituting ruthenium at iron sites introduces substantial modifications to the electronic structure that progress with increasing doping concentration. The calculated band structures demonstrate progressive band shifts and redistribution of electronic states as ruthenium content increases. In pristine LiFeAs, multiple Fe-3d derived bands cross the Fermi level, creating electron and hole pockets considered essential for superconductivity in iron-based materials.

With 25% ruthenium doping, researchers observed that the overall band topology remains preserved but shows modest upward shifts of the Fermi level and band broadening. At higher doping levels, more pronounced modifications appear as bands become increasingly dispersive due to the greater delocalization of Ru-4d orbitals. The report indicates that some bands near the Fermi level are pushed below it, reducing the density of states at the Fermi energy. This reduction suggests suppression of electronic correlations and magnetic fluctuations, which analysts consider critical to unconventional superconductivity in these systems.

Projected Density of States Evolution

The evolution of projected density of states reveals how ruthenium doping influences orbital contributions near the Fermi level. In the pristine compound, Fe 3d orbitals dominate near the Fermi level with significant hybridization from As 4p states. With 25% ruthenium doping, Ru 4d states begin to emerge and overlap with Fe 3d orbitals, altering the density of states around the Fermi level. The Fe-3d peak becomes broader and less intense, indicating reduced electronic localization and changes in magnetic character., according to industry news

At 50% ruthenium doping, the Ru 4d contribution increases significantly, becoming comparable to Fe 3d and resulting in wider PDOS distribution with reduced peak sharpness near the Fermi level. In fully substituted LiRuAs, Fe 3d states are completely replaced by Ru 4d orbitals, creating flatter and broader PDOS near the Fermi level that reflects highly delocalized electronic behavior. According to reports, this reduced density of states may correlate with diminished magnetic exchange and potential suppression of superconductivity.

Fermi Energy Shifts and Thermodynamic Stability

The study calculated Fermi energies and binding energies across various doping levels, revealing clear trends with increasing ruthenium content. Standard DFT calculations show the Fermi energy increases from 9.35 eV for pristine LiFeAs to 10.03 eV at full substitution, representing an overall increase of approximately 7.3%. This upward shift reflects systematic modification of the electronic structure caused by ruthenium doping.

Binding energy analysis indicates that ruthenium incorporation strengthens the cohesive energy of the crystal. The total binding energy becomes more negative with higher ruthenium substitution, improving from -33.74 eV for the undoped system to -37.96 eV at full substitution, corresponding to a 12.5% increase in binding strength. The binding energy per atom follows a similar trend, becoming more negative with doping from -5.58 eV to -6.26 eV. Analysts suggest these results indicate that moderate to high ruthenium substitution not only modifies the electronic density of states near the Fermi level but also improves the thermodynamic stability of the host system.

Magnetic Property Modifications

The magnetic properties analysis examined the total energy difference between ferromagnetic and antiferromagnetic configurations along with the magnetic moments of the systems. Researchers report that ruthenium substitution progressively modifies the magnetic character of LiFeAs. In the pristine compound, Fe 3d orbitals dominate near the Fermi level with strong spin polarization, while hybridization between Fe 3d and As 4p states contributes to metallic behavior and magnetic interactions.

With 25% ruthenium substitution, Ru 4d states begin to appear near the Fermi level and mix with Fe 3d states, causing reduction in Fe 3d peak intensity and spin asymmetry that indicates partial suppression of magnetism. At 50% doping, the ruthenium contribution becomes more pronounced, and Fe-related peaks are further suppressed, showing broader, more delocalized states with weakened spin polarization. In fully ruthenium-substituted LiRuAs, the PDOS is dominated by Ru 4d orbitals that display minimal spin splitting. The study concludes that while ruthenium doping enhances metallicity through broader band dispersion, it simultaneously weakens superconductivity by diminishing the electronic interactions necessary for Cooper pairing.