Real space investigation of local field effects on surfaces

Transcription

1 Real space investigation of local field effects on surfaces Nicolas Tancogne-Dejean, Valérie Véniard Laboratoire des Solides Irradiés,Ecole Polytechnique, CNRS, CEA/DSM European Theoretical Spectroscopy Facility 1

2 Outline Surfaces, Super-cells and Local Field Effects Effect of the vacuum on spectra 1D Real Space treatment Local Field Effects on Surfaces 2

3 Outline Surfaces, Super-cells and Local Field Effects Effect of the vacuum on the spectra 1D Real Space treatment Local Field Effects on Surfaces 3

4 Surfaces Different surfaces for the same material (e.g. Silicon) Si(111) 7x7 Si(001) 2x1 Si(001) 4x2 4

5 Example of surface reconstruction : Si(001)2x1 Dangling bonds Asymmetric dimers 5

6 Model of surface Super-cells Ground state calculation What we want Plane waves Best representation of a surface Surface Supercell System with 2 surfaces (slab) 6

7 Model of surface Super-cells Ground state calculation Asymmetric dimers Construction of super-cell (atoms + vacuum) Reconstruction of the surface System with 2 surfaces (slab) 7

8 Optical properties in TDDFT Independent Particles (IPA) Random Phase Approximation (RPA) + (No Local Field Effects) (Local Field Effects included) 8

9 Outline Surfaces, Super-cells and Local Field Effects Effect of the vacuum on the spectra 1D Real Space treatment Local Field Effects on Surfaces 9

10 Effect of the vacuum on the spectra Analysis of the effect of the vacuum on the IPA and RPA spectra for 3 systems : z-axis Vacuum 1/2-1/2 1/3 2/3 1/4 3/4 10

11 In-plane calculations : ε xx IPA calculations 11

12 IPA calculations Renormalized in-plane calculations : ε xx 12

13 Out-of-plane : ε zz IPA calculations 13

14 IPA calculations Renormalized out-of-plane : ε zz 14

15 IPA calculations Renormalized spectra (to slab volume) Include : Surface states Reconstruction 15

16 RPA calculations in TDDFT In-plane calculations : ε xx 16

17 RPA calculations in TDDFT Renormalized in-plane calculations : ε xx 17

18 RPA calculations in TDDFT Out-of-plane : ε zz Position of the peak depends on the vacuum! 18

19 Result of the analysis IPA : Can be renormalized to the volume of the slab RPA in-plane out-of-plane Small LFE Behave as IPA Strong LFE Position of the peak depends on the size of the vacuum Response of the slab supercell 19

20 Abs. Vs EELS Abs = v 0 Im{χ 00 } EELS = v 0 Im χ 00 1 v 0 χ 00 v 0 χ 00 α 1 V When V, Abs EELS 1 ε zz converges to the plasmon of silicon 20

21 RPA calculations in TDDFT Out-of-plane : ε zz Plasmon position (bulk silicon) 21

22 Where does normalization come from? In momentum space, χ (0) is explicitly normalized to the volume Explains the behavior of IPA calculations 22

23 Where does normalization come from? In momentum space, χ (0) is explicitly normalized to the volume In real space, χ (0) is not normalized to the volume Computing Local Field Effects in real space could solve the problem of normalization 23

24 Outline Surfaces, Super-cells and Local Field Effects Effect of the vacuum on the spectra 1D Real Space treatment Local Field Effects on Surfaces 24

25 Optical properties in Real Space Independent Particles (IPA) Random Phase Approximation (RPA) + (No Local Field Effects) Tiago, et al. PRB 73, (2006) Ogut, et al. PRL 90, (2003) (Local Field Effects included) 25

26 Real Space and supercell The slab is periodic in x and y-directions. We can define a 2D Fourier Transform x, y, z q x + G x, q y + G y, z = (q + G, z) Approximation : One can neglect in-plane LFE G = 0 x, y, z (q, z) This eases calculation for surfaces Silkin, et al. PRL 93, (2004) 26

27 Real Space and supercell Dyson-like equation becomes with v, 2D-Fourier transform of the Coulomb potential given by 27

28 Outline Surfaces, Super-cells and Local Field Effects Effect of the vacuum on the spectra 1D Real Space treatment Local Field Effects on Surfaces 28

29 Roadmap for computing ε Real Space code Macroscopic Average 1D Dyson-like equation DP code 29

30 Computational Details 32 atoms 8x16x1 shifted k-point grid 800 G z vectors z 0.2 Bohr q = 10 3 [reduced coordinates] 30

31 Application to surfaces Validation : In-plane RPA calculation Neglecting in-plane LFE 31

32 Local Field effects from real space Out-of-plane IPA/RPA calculations 32

33 Local Field effects from real space Out-of-plane IPA/RPA comparison 33

34 Out-of-plane component and vacuum Independence from the amount of vacuum 34

35 Out-of-plane component and vacuum Independence from the amount of vacuum 35

36 Conclusion and Future work Conclusion 1D real space treatment of LFE Out-of plane RPA response Work in progress Formulation in momentum space and without approximation Second Harmonic Generation from surfaces in RPA 36

37 Thank you for your attention 37

38 38

39 Application to surfaces Coulomb potential in our case of interest : Divergent at q = 0 Question : How to compute ε zz with a q? 39

40 ε is a tensor The dielectric function is a tensor, independently of the level of approximation used in its calculation For instance, choosing q = qe x, qεq = q 2 ε xx. And if q = q x e x + q z e z, qεq = q 2 x ε xx + q 2 z ε zz + 2q x q z ε xz. 40

41 ε is a tensor The dielectric function is a tensor, independently of the level of approximation used in its calculation For instance, choosing q = qe x, qεq = q 2 ε xx. And if q = q x e x + q z e z, qεq = q x 2 ε xx + q z 2 ε zz + 2q x q z ε xz. Can be computed due to a non-zero q Contains ε zz 41

42 Obtaining ε zz in our model From simple algebra one can obtain ε zz from the combination of 3 calculations : q = qe x q = q x e x + q z e z q = q x e x q z e z 42