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Šoškić, Božidar
Magnetism and superconductivity in two-dimensional (2D) boron crystal structures
2025

https://phaidra.ucg.ac.me/detail_object/o:1925?tab=0#mda

Autorstvo-Nekomercijalno-Bez prerade 3.0 Srbija (CC BY-NC-ND 3.0)
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Phd. theses

Prirodno-matematičke nauke

doktor nauka - fizičke nauke
Univerzitet Crne Gore
Prirodno-matematički fakultet
Studijski program Fizika

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Due to intrinsic electron deficiency, boron is renowned for its complex bonding nature and exhibits remarkable structural versatility, with 16 predicted bulk phases, though only a few have been experimentally confirmed. This adaptability, along with its intriguing physical and chemical properties, extends to the two-dimensional (2D) boron (B) sheet known as borophene, which exists in multiple polymorphic forms due to its flexible bonding arrangements. Theoretical studies have predicted exceptional physical properties for borophene, including high electrical conductivity, mechanical strength, metallicity, and potential for hosting superconductivity, positioning it as a more promising candidate than graphene and other emerging 2D elemental materials. However, its synthesis and large-scale production remain a significant challenge, primarily because it is relying on methods such as molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), which require metallic substrates, high temperatures, and ultra-high vacuum (UHV) conditions. Alternative top-down strategies, like low-temperature liquid-phase exfoliation (LTLPE), have been explored but often suffer from residual contamination, insufficient domain sizes, and uncontrolled thickness, thereby limiting their scalability and practical implementation. Additionally, achieving true atomic resolution with scanning tunneling microscopy (STM) or other characterization techniques remains challenging for borophene, even with enhanced tip modifications. In this context, density functional theory (DFT) calculations are indispensable for gaining deeper insights into the true structures of the synthesized borophenes. However, one of the main challenges is its high chemical reactivity, which leads to rapid oxidation under ambient conditions, posing a fundamental obstacle to its integration into practical devices. To mitigate these challenges, passivation strategies, including surface hydrogenation of monolayer borophene, and functionalization of bilayer configurations, have been employed, rendering borophene inert under ambient conditions while preserving its tunable mechanical, electronic and other physical properties. This thesis investigates β12 borophene – by using spin-dependent density functional theory (SDFT) and other theoretical models – as a promising platform for stabilizing artificial Fe-based 2D magnets, leveraging its unique periodic lattice and high reactivity to prevent the clustering of adsorbed adatoms, in contrast to graphene. While these systems exhibit well-ordered 2D Fe magnetic structures, the Curie temperature remains below room temperature, suggesting that further optimization, such as substituting Fe with Co, could enhance it due to higher anisotropy. However, the material’s susceptibility to oxidation limits its practical applications. We propose therefore Mn-intercalated bilayer β12 borophene as a solution to this oxidation problem, as it is also the only dynamically stable structure among the considered intercalants that maintains some magnetic moment. We investigated the microscopic origin of magnetism in this system, revealing a dominant direct dyz–dyz interaction of Mn atoms and superexchange or super-superexchange interaction mediated by B-pz orbitals, leading to stable long-range magnetic order, positioning therefore this material as a strong candidate for spintronic applications. In addition to magnetism, we investigated the potential of boron-based 2D materials for creation of superconducting nano-devices by employing density functional perturbation theory (DFPT) and by solving the anisotropic Migdal-Eliashberg equations. While previous studies suggested that the bare β12 borophene is an intrinsic superconductor, experimental confirmation has remained elusive. Our findings challenge these reports, showing that the bare β12 borophene is dynamically unstable in its freestanding form and can only be stabilized under biaxial tensile strain (substrate effect) but with absence of measurable superconductivity. This discrepancy with previous reports is explained by the reduced contribution of B-σ states to the total density of states (DOS) at the Fermi level (EF ) under influence of this strain effect. However, we demonstrate that hydrogenation actually stabilizes β12 borophene and enhances its superconducting properties. Strain engineering or hole doping further optimize these properties, achieving critical temperatures (Tc) up to 30 K, primarily due to the increased contributions of B-pz and B-σ states to the DOS at the EF. Our work opens a pathway for experimentally probing superconductivity in this system, knowing that the transfer of borophene from metallic to non-metallic substrates is nowadays possible. Additionally, we explore intercalated bilayer borophenes, including β12, χ3, δ4, and kagome phases, identifying seventeen potential superconducting systems by using alkali, alkaline-earth, and transition metals as intercalants, which are exhibiting intriguing nesting effects, high Fermi surface anisotropy, multi-gap behavior, and more. For their superconducting properties it is important to find a good balance between partial occupation of B-pz and B-σ states. Among considered materials, Ca-intercalated kagome borophene emerges as the best candidate, with a Tc approaching 58 K. These findings underscore the dual potential of borophene-based materials for both magnetic and superconducting applications, providing valuable insights for further development of advanced 2D devices for spintronics and superconducting technologies.