Unraveling the role of disorder in the electronic structure of high entropy alloys

Abstract: Disorder in high entropy alloys, arising from the random distribution of multiple elements, plays a crucial role in their novel properties desirable for various advanced engineering applications. We investigate the role of compositional and structural disorder on the electronic structure of osmium-based superconducting high entropy alloys, (Ru/Re)${0.35}$Os${0.35}$Mo${0.10}$W${0.10}$Zr$_{0.10}$, using photoemission spectroscopy and density functional theory (DFT). Elemental and cumulative core level shifts are found to be commensurate with elemental electronegativities and valence electron counts (VEC), respectively. Valence band spectra together with DFT results indicate that the crystal structure plays an important role in deciding the electronic structure of these high entropy alloys. Through temperature dependent high-resolution spectra, we unveil strongly suppressed spectral density of states (SDOS) in the close vicinity of Fermi level. Energy and temperature dependence of the SDOS in accordance with Altshuler-Aronov theory confirms localization of charge carriers in the presence of strong intrinsic disorder. Computed electron-phonon coupling strength and superconducting transition temperature aligning reasonably well with experiments further shed light on phonon-mediated pairing mechanism and role of disorder in these systems. Our results provide a way forward to the understanding of superconducting high entropy alloys through strategic control of disorder, VEC and crystal structure.

• The paper demonstrates that electron redistribution, driven by differences in electronegativity and valence electron count, underpins core level shifts observed in spectroscopy and DFT analyses.
• The paper finds that intrinsic disorder suppresses the spectral density at the Fermi level, revealing signifiers of localized electron states across different crystallographic structures.
• The paper reveals that computed electron-phonon coupling constants and superconducting transition temperatures align with experiments, highlighting limitations in theoretical modeling of disorder.

Analysis of Disorder in High Entropy Alloys

Introduction

High Entropy Alloys (HEAs) have gained substantial attention due to their unique properties that stem from their complex compositions and high degrees of disorder. The paper "Unraveling the role of disorder in the electronic structure of high entropy alloys" investigates the impact of disorder in osmium-based superconducting HEAs. Employing methods such as photoemission spectroscopy and density functional theory (DFT), the study zeros in on the electron redistribution and structural complexities that influence the electronic properties and superconducting behaviors of these materials.

Methods and Results

Photoemission Spectroscopy and Core Level Shifts

The core level shifts observed using photoemission spectroscopy provide a substantial insight into electron transfer behaviors. Elements like Os, Mo, and Ru show shifts towards lower binding energy (BE), depicting their role as electron acceptors, while other constituents like Zr and W portray donor-like characteristics with shifts towards higher BE. This behavior highlights the local charge redistribution driven by differences in electronegativity and valence electron count (VEC).

Electronic Structure and Density Functional Theory

Using DFT, the study elaborates on the Total Density of States (TDOS) and atom-projected Partial Density of States (PDOS), capturing the nuanced role of crystallographic disorder in influencing the electronic structure of HEAs. The predominance of $d$-states near the Fermi level signifies metallic characteristics, corroborated by significantly high TDOS at $E_F$. Structural differences between Ru-HEA (hexagonal close-packed) and Re-HEA ($\alpha$-Mn structure) manifest via distinctive electronic structures despite similar compositions.

Electron-Phonon Coupling and Superconductivity

The computed electron-phonon coupling constants and superconducting transition temperatures ($T_C$) align well with experimental data, albeit slightly higher. For Ru-HEA, theoretical $T_C$ was higher due to the absence of consideration for intrinsic disorder in calculations, key to understanding the limitations and needs for real-world applications. The findings emphasize the profound impact of defect structures, intrinsic disorder, and crystal configuration on superconductivity.

Spectral Density of States and Localization

Temperature-dependent spectral analysis near the Fermi level revealed a suppression of the Spectral Density of States (SDOS), indicative of localized electron states in disordered systems. The observations align with the Altshuler-Aronov theory, reinforcing the correlation between structural disorder and electron localization. The degree of SDOS suppression further distinguishes the structural influence, with Re-HEA ($\alpha$-Mn) showing stronger disorder effects than Ru-HEA (hcp).

Conclusion

This study offers profound insights into how compositional and intrinsic crystallographic disorder of high entropy alloys modulate their electronic properties and superconducting functionalities. By combining spectral and theoretical analyses, the research underscores the need for a strategic approach in manipulating disorder and enhancing understanding for potential engineering optimizations. This research enriches the foundational knowledge essential for advancing HEAs tailored for specific applications, especially in superconductivity. Future investigations may explore disorder-induced phenomena and their exploitability in material performance enhancements.