Early stage of graphite-diamond structural phase transition induced by inter-layer charge transfer excitations in the visible region

Transcription

1 Early stage of graphite-diamond structural phase transition induced by inter-layer charge transfer excitations in the visible region Keiichiro Nasu Solid state theory division Institute of materials structure science High energy accelerator research organization (KEK) Graduate university for advanced study and CREST JST 1-1, Oho, Tsukuba, Japan

2 High energy accelerator research organization IMSS, KEK, Tsukuba, Japan

3 Lattice relaxation of optical excitations in solids

4 Lattice relaxation of optical excitation can finally induce structural phase transition K.Nasu, World scientific publisher ( Singapore, 2004 )

5 Photoinduced structural phase transition There discovered a new class of solids, which, being shone only by a few visible photons, become pregnant with an excited nano ~ micron domain, that has new structural and electronic orders (, charge, spin and gauge ), quite different from the starting ground state. Purpose Clarify, 1) conditions of its occurrence (, hidden multi-stability ), 2) its mechanism (, criticality, initial condition sensitivity ), 3) how different from thermally excited phases, or how it goes beyond the hysteresis loop (S.Miyashita). K.Nasu, Rep.Prog. Phys. ( IOP, London ) 67(2004)1607

6 E x c i t e d Hidden multistability and photoinduced phase transitions hν hν Franck- Condon state Lattice relaxation Photoinduced structural change Excited nano~micron domain New lattice structure and electronic order True ground state Microscopic lattice distortion Thermal energy Proliferation False ground state Macroscopic order parameter of phase transition

7 Photoinduced neutral nano domain in ionic phase of organic charge transfer crystal TTF-CA TTF CA + TTF + CA Ionic Phase (Dimer) True ground state hν e - Neutral Phase (Monomer) False ground state Charge Transfer Exciton Neutral Nano Domain Ionic Phase Neutral Nano Domain Ionic Phase By Y.Tokura

8 Photoinduced paramagnetic domain in diamagnetic phase of organo-metallic complex crystal [ Fe(2- pic) 3 ]Cl 2 EtOH (3d) 6 electrons of Fe 2+ Paramagnetic phase yello ( S = 2 ), T > 120K e g e g t 2g t 2g Diamagnetic phase red ( S = 0 ), T < 120K e g e g t 2g t 2g They are two equilibrium phases of this material.

9 Yellow paramagnetic domain can also be generated as a nonequilibrium phase by light, even at low temperatures. hν 1.8 ev Red equilibrium diamagnetic phase (S=0) Yellow nonequilibrium paramagnetic phase (S=2) By S.Koshihara

10 Early stage of graphite-diamond structural phase transition induced by inter-layer charge transfer excitations in the visible region hν Quantum iterative transition without high temperature 3000K nor high pressure 15Gpa sp 2 Graphite 0.3 e - sp 3 Diamond Inter - carbon distance( 10-8 cm )

11 4 Experimental discoveries by Tanimura and Kanazaki 1. Exciting light (1.6 ev) should be polarized perpendicular to layer. 2. Nonlinear and multi-photon process, less than 10 photons. 3. Too strong excitation is rather harmful, resulting in two layers abrasion. 4. Femto-second pulse excitation, not nano-second 5. Resultant domain size includes about 1000 carbons 6. Structural phase transition due to inter-layer bond formation ( STM image ) 7. Resultant domain is quite stable(, more than 10 days )

12 STM image of graphite before and after the illumination by photons (1.6 ev) STM e V s

13 Vs = - 20 mv First layer 2 nd layer

14 Vs = - 40 mv

15 Exciting light should be polarized perpendicular to the layer, parallel component gives no effect.

16 Too strong irradiation, Simultaneous two layers abrasion without residual structural change of third layer, neither partial melting nor ablation.

17 Photoinducerd semimetal - insulator transition by Tanimura p- Gap ps ps

18 Spontaneous broken symmetry

19 Inferred inter-layer bond structure

20 Local shear type displacement ΔZ t ΔZ t ΔZ t Shirotani, Inokuchi, Mol.Cryst.Liq.Cryst. 461(2007)93.

21 Hexagonal graphite - [ABAB] stacking

22 Graphite and diamond structures B= 1.42 Å B = 1.54 Å R = 1.54 Å R = 3.35 Å θ = 90.0 θ = LDF calculations by L. Cohen, S. Fahy and S.G. Louie

23 LDF calculation for graphite-diamond transition S. Fahy et al, Phys. Rev. B35(1987)7623. Tateyama et al, Phys Rev.B54(1996) Energy barrier ~ 0.3 ev However, it is a hypothetical uniform transformation. All the macroscopic number of carbons are assumed to move simultaneously? Real process is an iterative local domain formation, never be simultaneous. More high barrier Local domain formation is the essential process.

24 Local distortion of graphite layers Trial distortion pattern [ { ( )} ] 2 2 tanh θ x + y (L / 2) 1 Δq 0 L 0 /2 θ Layer no.1 Δq δ 1 *2 Inter-layer σbond δ 1 *2 Δq Layer no.2

25 Brenner s 3-, 4-body potential E = i, j( > i ) [ ] V (r ) B (r, θ ) V (r ) R ij ij ij i jk A ij j Hard core repulsion Attraction with chemical bond angle θ ijk i θ ijk k D.W. Brenner, Phys. Rev. B 42(1990)9458.

26 Comparison Brenner s potential Lenard-Jones potential Semi-empirical empirical inter-atomic potential,, 3-3 or 4-4 body force, Direction of chemical bonds, Gives proper data for both graphite and diamond structures, Short range (2.0 ) No o information about interalyer coupling o A Two body potential, Depends only on bond length No o information for chemical bond structure

27 Energy of uniform graphite-diamond transition by modified Brenner s potential B = 1.45 Å 0.33 R = 2.11 Å Energy(eV)/ atom Diamond B = 1.54 Å R = 1.54 Å B = 1.42 Å θ = ? 0.03 R = 3.35 Å Graphite θ = θ = 90.0 [ A o ]

28 Local distortion of graphite layers Trial distortion pattern [ { ( )} ] 2 2 tanh θ x + y (L / 2) 1 Δq 0 L 0 /2 θ Layer no.1 Δq δ 1 *2 Inter-layer σbond δ 1 *2 Δq Layer no.2

29 Cohesive energy with respect to modulation s s amplitude Size of the system: 200x200x2 atoms Total Energy [ev] Estimated parameters: dq= 0.78 Å L 0 = 94 atoms θ = 0.82 δ = 0.07 Å Å [ A] o

30 Local shear type displacement ΔZ t ΔZ t ΔZ t Shirotani, Inokuchi, Mol.Cryst.Liq.Cryst. 461(2007)93.

31 Cohesive energy with respect to modulation s s amplitude after applying shea ear displacement Z 3,5 Size of the system: 200x200x2 atoms Total Energy [ev] 3,0 2,5 2,0 1,5 1, ev 1.1 ev Estimated parameters: dq= 0.78 Å L 0 = 96 atoms θ = 0.82 δ = 0.2 Å Z=0.72 Å 0, A 0,0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 Modulation+Intrusion [A]

32 Diaphite structure [Å][

33 Inferred domain structure

34 Early time critical dynamics Competition between Electron-hole dimerization v.s. Charge separation Free carrier Attraction Dimerized e-h pair Free carrier

35

36 E ( k ) = k x k x e F ( kx, kx) ( k x,0) e F k x π Density of states arbi. unit

37 Inter-layer phonon Q H p = ω 2 l 2 Q 2 l + Q 2 l Inter-layer Coulomb interaction, triggers dimerization and depend on Q self-localization of exciton H = U(Q ) (n < n > )(n < n c l lσ11 lσ11 lσ 22 lσ 22 l,σ,σ > ) + < n > )(n < n > ) (nlσ21 lσ21 lσ 12 lσ 12

38 Theory for dynamics in electron, hole and multi-phonon coupled system with translational symmetry and a total momentum k(= 0) Attraction depends on phonon Phonons only at center Translational motion by k(=0) Distance ( l x, l y)

39 Dimerizes 10%, although mostly separates Distant electron hole pair Distant electron hole pair Separation Dimerization Time Time ( 2 π/ω) (2π / Time ω ) (2π / ω) Time(2 π/ ω)