Electronic, Magnetic and Transport Properties in Low Dimensional Materials: A First Principles Study

Abstract

Coexistence of remarkable physical, chemical, mechanical and electrical properties in low dimensional materials had made these materials very attractive for innovative devise applications. Here in this thesis, the first-principle methodology is used to investigate the two-dimensional (2D) materials and their one-dimensional (1D) counterparts. Various physical properties like structural, electronic, mechanical, electron transport and electron-phonon interaction are investigated. The section 3.1 is about intrinsic defects in monolayer phosphorene, where the electronic band structure, defect formation energy and bias dependent transport property of various defective phosphorene systems are presented. The defect formation energy is found to be much less than that in graphene. The defect configuration strongly affects the electronic structure. Interestingly, a single vacancy defect behaves like a p-type impurity for transport property. Unlike the common belief, we observe that the vacancy defect can contribute to greatly increase the current. Along the zigzag direction, the current in the most stable single vacancy structure was significantly increased as compared with that found in the pristine layer. We observed a strong anisotropic current ratio of armchair to zigzag direction even in the defective system. The section 4.1 is about the width-dependent magnetic properties of armchair black phosphorene nanoribbons (APNRs) by controlling the electron charge doping. Interestingly, the charge doping substantially altered the band and the magnetic moment was found in the charge-doped systems. Stoner condition could nicely explained the magnetic moment at the edge atoms. Moreover, we propose that the edge-to-edge magnetic coupling can be manipulated by charge doping because the transition from the antiferromagnetic to ferromagnetic state was achieved. As another possibility, we investigated the magnetic property of tilted black phosphorene nanoribbons (TPNRs) affected by an external electric field. In addition the edge passivation effect on the magnetism and thermal stability of the nanoribbons were studied. No edge magnetism was observed in hydrogen and fluorine passivated TPRNs. It was observed that the oxygen passivated TPNR was more stable than the pure TPNR and the edge-to-edge antiferromagnetic (AFM) ground state was obtained. We found that the magnetic ground state could be tuned by the electric field from antiferromagnetic (AFM) to ferromagnetic (FM) ground state. Interestingly, the oxygen passivated TPNR displayed a half-metallic state at a proper electric field in both FM and AFM states. The chapter 5 is about exploration of new 2D materials. We explored the electronic and magnetic properties of two-dimensional manganese di-halides (MnY2, Y = I, Br, Cl) and hydrogenated systems (MnHY2). The pristine MnY2 monolayers had a very weak magnetic exchange interaction and we found degenerated magnetic states between ferromagnetic and antiferromagnetic. However, it was found that the electronic band structure and magnetic properties could be significantly altered by functionalization with hydrogen atoms, the degeneracy was broken and the FM ground state was obtained in all MnHY2 systems. Using the universal structure predictor algorithm, we proposed that two-dimensional MnB structures with p4mmm (α-MnB) and pmma (β-MnB) symmetries could be synthesized. This finding was verified by calculating the dynamical stability, molecular dynamics, and mechanical properties. Both systems displayed a ferromagnetic ground state with metallic band structures. In the α-MnB, we found the unique feature of Kohn anomaly at q~2kF in the diagonal direction of the Brillouin zone. The calculated total electron-phonon coupling parameters were 1.20 and 0.89 in α-MnB and β-MnB systems. Overall, we predict that the α-MnB and β-MnB systems can display 2D ferromagnetic superconducting states with the estimated critical temperatures of Tc ≈ 10−13 K. The chapter 6 is about the heterostructures. In the section 6.1 the single layer and bilayer graphene on CrI3 (g-CrI3 and 2g-CrI3) are discussed. The induction of the valley polarization in these heterostructures is presented. In g-CrI3, we found a huge valley polarization with the majority gap difference of Δ1↑ - Δ2↑ =44 meV. Even in 2g-CrI3 system, we also found the valley polarization of Δ1↑ - Δ2↑ =21 meV. Moreover, we also investigated the electric field effect on the valley polarization. In both systems, we obtained that the valley polarization could be switched in the majority spin band. For instance, the sign of gap difference at ±K changed from Δ1↑ > Δ2↑ at zero field to Δ1↑ < Δ2↑ at a small applied electric field of 0.1 V/Å. With further increase of the electric field to 0.2 V/Å, the valley polarization disappeared. Thus, we propose that a large value of valley polarization can be achieved and the sign of polarization can also be switched with electric field instead of magnetic field.