The Application Of The Generalized Bloch Theorem In The Spintronics Of 2D Materials

The discovery of quasi-one dimensional and two-dimensional(2D)layered systems,opens a new leading research field for condensed matter physics,and accelerates the develop-ment of fundamental researches on materials science at nanoscale.Comparing the traditional bulk materials,low-dimensional systems have the unparalleled advantages in respect of de-signing and fabricating new generation of electronic devices and its miniaturization.Due to their novel properties,the designation and production of micro-semiconductor devices will be renovated that has fundamental and positive impact on the advancement of the modern semiconductor technology.Strain engineering is serving as an effective method to tune the electronic and phononic properties via introducing structural deformation,which is of great importance for low-dimensional systems.In general,the flexible 2D materials always have high-level threshold limits of elastic deformation,i.e.,low-dimensional materials can bear a wide range of structural deformation.Therefore,we can apply various strains,including in-homogeneous strain,to modulate the electronic properties of 2D systems.On the other hand,the response of strain for most low-dimensional systems,such as nanotubes,nanowires and nanoribbons,is very sensitive and has a significant influence on their electronic properties.Accordingly,strain engineering is an effective method to tune the electronic properties in 2D materials.Our thesis mainly focus on the modulation of electronic properties of silicene based materials,such as 2D-Xenes(X=Si,Ge,Sn)nanoribbons and silicene bilayer,via strain en-gineering.The associated strain patterns involve inhomogeneous strain,such as in-plane bending and out-plane bending.It is well known that density functional theory(DFT)based first-principles calculations are used widely in the research field for condensed matter physics,however,the associated calculations is becoming severely difficult when materials consider-ing inhomogeneous strain.Due to the conventional calculation method(such as VASP)de-pendent transition symmetry is removed when the system applied inhomogeneous strain,such as bending,the standard quantum mechanical calculation is not suitable any more.Therefore,our proposed theoretical calculations in the thesis based on our group developed generalized Bloch theorem,which is uniquely suitable for calculating band structure for this system.Firstly,we propose a new mechanism based on strain engineering to induce half-metallicity(HM)in 2D-Xenes(X=Si,Ge,Sn)nanoribbons.Specifically,we find the zigzag silicene nanoribbons have the localized and spin-polarized edge states,which are different from the conventional bonding and anti-bonding states.Moreover,the conduction band minimum(CBM)and valence band maximum(VBM)states of zigzag silicene nanoribbons have un usual deformation potential,i.e.,under compressive strain,the energy of CBM and VBM states all shift upward.Combination of the localized spin-polarized edge states and the un-usual deformation potential of CBM and VBM states of zigzag silicene nanoribbons,we propose an new method based on stain engineering,such as an in-plane bending,to induce the spin-splitting and even the HM.Based on the generalized Bloch theorem,our SCC-DFTB results show that the spin degeneracy of the spin-polarized edge states is lifted under bend-ing,inducing spin-splitting for both VBM and CBM states.Importantly,the bending angle at?=2.0°,the HM state of zigzag silicene nanoribbons is realizedSimilarly,we also find the AB-stacking bilayer silicene has the antiferromagnetic(AFM)ground state,and the electronic states are spin-polarized.In accordance to the AFM config-uration,the spin orientations of VBM(CBM)states on top Si atoms and that on bottom Si atoms are opposite.Moreover,we also find bilayer silicene has unusual deformation po-tential,i.e.,as strain ? increases(i.e.,from tension to compression),both CBM and VBM shifts upward.With the unusual deformation potential of CBM and VBM,and the revealed spatial separation of spin charge in bilayer silicene,it is possible to induce spin-splitting by inhomogeneous strains,thus we consider an out-of-plane bending deformation.Based on the generalized Bloch theorem,both SCC-DFTB and DFT results demonstrate that the spin-splitting at CBM and VBM states of bilayer silicene appears under bending,and at bending angle around 6.5°,the HM state can be approachedOur investigations of realization HM in zigzag silicene nanoribbons and bilayer silicene via bending method are of great importance.This is because most of recent reported 2D mate-rials with HM behavior are all based on external conditions,such as strong electric/magnetic field,selective passivation,the precisely control positions of functional groups and carefully selective doping,are still formidable challenge for experimental realizationThe method we proposed in this work provides an practical way to realize half-metallicity in other massive 2D materials and paves way to design nanospintronics devices in the near future.The efforts we devoted to is seeking for stable half-metallic 2D systems with excellent performances and developing more practical and operable experimental methods,to fulfill the demanding requirements for the design of spintronic devices at nanoscale.