First Principles Study Of High-temperature Superconductivity In Two-dimensional Li-intercalated Boron-carbon Compounds

The exploration and discovery of new high-temperature superconductors is a constant goal and an important driving force for superconductivity development.In 1986,the discovery of cuprate high-temperature superconductors by Swiss physicists Bednorz and Müller set off a boom in high-temperature superconductivity research and promoted the study of strongly correlated quantum systems.In 2008,the discovery of iron-based superconductors by Japanese physicists Hosono et al,further expanded the field of high-temperature superconductivity research.Many new phenomena and effects usually emerge along with the discovery of high-temperature superconductors.The in-depth study of these novel effects not only enables the exploration of the microscopic mechanism of high-temperature superconductivity,but also helps to explore and develop the quantum theory of strongly correlated electron systems,Furthermore,it can provide a concrete research paradigm for the study of quantum many-body theory.However,the experimental discovery of high-temperature superconductors relies more on the growth process of materials.It will bring the material close to but not beyond the antiferromagnetic quantum instability point.Thus,the discovery of new high-temperature superconductors by such method is highly contingent,as is the case with the discovery of cuprate superconductors and iron-based superconductors.So,is there a general rule that can be applied in the discovery of new high-temperature superconductors?That is,if theσ-bonded electrons in a material can be metallized and become conducting electrons,the material will appear as a high-temperature superconductor.σ-bonds are spin singlet states formed by two electrons with opposite spins.They are formed by two identical or different atomic orbitals overlapping in a head-to-head manner.σ-bonds are the strong covalent bonds due to their large bond energy.σ-bonded electrons are very sensitive to ionic lattice vibrations and are strongly coupled to phonons.It is an important factor in stabilizing the crystal structure.However,theσband formed by hybridization is usually full and far from the Fermi energy level,thus does not participate in the conductivity.Therefore,to achieve superconductivity by metallizingσ-bonded electrons,it is necessary to make them conduction electrons by raising theσband above the Fermi energy level.It can be achieved by doping or other regulatory means to turnσ-bonded electrons into conducting electrons.Then,they will form Cooper pairs by coupling with phonons.The corresponding superconducting transition temperature will be higher.Based on the above research background,this thesis presents an in-depth and detailed study of the superconducting properties of the two-dimensional Li-intercalated boron-carbon compounds system.We combine the first-principles calculation package Quantum Espresso（QE）with the superconducting properties calculation package Electron-phonon Wannier（EPW）in our study.We have theoretically predicted two two-dimensional BCS-type high-temperature superconducting materials and demonstrated their dynamical and thermodynamic stability.We have also microscopically explained the origin for the high-temperature superconductivity of the system.To further increase the superconducting transition temperature,we apply biaxial tensile strain and carrier doping to our systems.Our research provides a specific research routine for the experimental search of two-dimensional high-temperature superconducting materials.The specific research of this thesis is as follows.1.Prediction ofπ-electron mediated high temperature superconductivity in monolayer Li C₁₂The prediction and synthesis of two-dimensional high-temperature superconductors is an extensively studied area.Based on calculations of the electronic band structure and lattice dynamics,we predict that graphene-like monolayer Li C₁₂is a BCS-type superconductor mediated byπelectrons.It is demonstrated that monolayer Li C₁₂is dynamically stable and thermodynamically stable,thus is expected to be synthesized experimentally.From the band structure and phonon spectra,we find that the saddle point of theπ-bonding band of the carbon atom produces a large density of states at the Fermi energy level.We have also identified a strong coupling between the vibrational modes in the in-plane direction of the lithium atom and theπ-electrons of the carbon atom.It leads to the superconductivity with high T_Cin Li C₁₂.Based on the calculation of the anisotropic Eliashberg equation,the material can reach a superconducting transition temperature T_Cof 41 K under an applied biaxial tensile strain of 10%.Our results suggest that monolayer Li C₁₂is a prospective candidate forπ-electron mediated electron-phonon coupled high T_Csuperconductor.2.Prediction ofσ-electron mediated high-temperature superconductivity in monolayer Li B₄C₈with carrier doping modulationPrevious studies have shown that high-temperature superconductivity can be generated if theσ-bonded electrons metallization in the material can be achieved.Based on such physical picture,We predict that two-dimensional monolayer Li B₄C₈can serve as a BCS-type high-temperature superconductor mediated byσ-bonded electrons.Through electronic band structure calculations,we discovered that there is aσband passing through the Fermi level.Furthermore,there is no imaginary frequency in the phonon spectrum of this material,indicating its dynamic stability.We have also calculated the formation energy of this material.The negative value suggests a high probability of experimental synthesis.Based on the calculation of the anisotropic Eliashberg equation,the electron-phonon coupling constant of this material under its intrinsic condition is 1.34,and its superconducting transition temperature reaches 43 K.Considering the Van Hove singularity of the boron-carbonσband near the Fermi level of the monolayer Li B₄C₈,we applied hole doping to enhance the system’s electron-phonon coupling.The electron–phonon coupling constant is found to be 5.70 at the optimum doping concentration,and its superconducting transition temperature is increased to 148 K by calculation.Our study provides a concrete research example for experimental discovery of high-temperature superconductors.We look forward to witness the experimental synthesis and verification of the superconducting properties of monolayer LiB₄C₈.