科技论文英语演讲稿Bismuth_关于科技的英语演讲稿

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Good morning, my dear teacher and clamates, my name is ****, from***** I am here today to talk about the topic: ‘ Strong Spin-Orbit Splitting on Bi Surfaces’.Before the topic ,I would like to introduce that ***** is located in Chinese economic and financial center of Shanghai, being one of the most prestigious universities in China and having two beautiful campuses.Welcome to visit our school.Now I will use the following four parts to introduce my topic: Firstly, The Background of Rashba Effect , secondly, Surface Geometric Structure of Bismuth, then ,Surface States on Low-index Surfaces of Bismuth, finally, we will have the Conclusion.Rashba effect is a momentum-dependent splitting of spin bands in two-dimensional condensed matter systems similar to the splitting of particles and anti-particles in the Dirac Hamiltonian.The splitting is a combined effect of atomic spin-orbit coupling and asymmetry of the potential in the direction perpendicular to the two-dimensional plane.The poible use of this effect is for spintronics applications, spin-filter devices and quantum computing.Bi is a striking example of the poible difference between surface and bulk material properties.Most importantly, the surfaces are much better metals than the bulk due to the existence of electronic surface states croing the Fermi level.Here the Bi surfaces were simulated by a 22 layer film embedded in vacuum., being confirmed by angleresolved photoemiion(ARPES)investigations and first-principles calculations.Here we explain the Bi(111)surface in details.This figure shows surface states of Bi(111)calculated without and with spin-orbit splitting included.The shaded areas show the projection of the bulk bands obtained without and with SOC and their superposition.In the case without SOC, a parabolic surface state is located around Γ in the non-relativistic energy gap.This surface state band gives rise to a hexagonal electron-like Fermi surface element.When the SOC term is included in the calculation it results in a spin-splitting of the surface state in all directions and leaves it degenerate only at Γ and at M.Very close to Γ the relativistic surface state bands are degenerate with bulk states and show le clear surface character.A comparison to the experimental dispersion can therefore immediately reveal if the spin–orbit splitting picture is correct.Such a comparison is shown in Fig.5.The figure shows the calculated and measured electronic structure in the vicinity of two high symmetry points on surfaces of Bi(111), which is in good agreenment.We can see that the intensity of both surface states strongly decreases close to Γ.This is most likely due to the overlap with the projected bulk band structure.The surface states are no longer genuine surface states but surface resonances, which penetrate much more deeply into the crystal and give a lower photoemiion intensity.Having inversion symmetry, this calculation yields two spin-degenerate bands near the Fermi energy that do not cro at Γ.In this picture, we can also see that both calculations of Bi(110)and Bi(100)surfaces show the very steep dispersion of the bands close to the symmetry point which is characteristic for the strong SOC.In conclusion, strong Spin-Orbit Splitting on Bi Surfaces is studied by first-principles calculations.The SOC term leads to a strong and anisotropic splitting of the surface-state bands that profoundly modifies the dispersion of the surface states and the surface FS.Thank you for listening!