Metal-insulator transition with Gutzwiller-Jastrow wave functions

Transcription

1 Metal-insulator transition with Gutzwiller-Jastrow wave functions Odenwald, July 20, 2010 Andrea Di Ciolo Institut für Theoretische Physik, Goethe Universität Frankfurt Metal-insulator transition withgutzwiller-jastrow wave functions p. 1/31

2 Outline Metal-insulator transition for strongly correlated materials Variational approach with Gutzwiller-projected wave functions Gutzwiller-projected + Jastrow-correlated wave functions Metal-insulator transition withgutzwiller-jastrow wave functions p. 2/31

3 Strongly correlated Materials Phase Diagram of the Cuprates Phase Diagram of (V 1 x M x ) 2 O 3 Metal-insulator transition withgutzwiller-jastrow wave functions p. 3/31

4 Charge Transfer Salts κ (BEDT T T F) 2 Cu 2 (CN) 3 [Kandpal 2009] Geometrical Frustration = Suppression of Long-Range Magnetic Order Metal-insulator transition withgutzwiller-jastrow wave functions p. 4/31

5 Charge Transfer Salts Experimental Phase Diagram of κ (ET) 2 Cu 2 (CN) 3 [Kanoda 2003, 2005] Mott Insulator Fermi Liquid Metal Metal-insulator transition withgutzwiller-jastrow wave functions p. 5/31

6 Why the paramagnetic solution? The Hubbard model: H = i jσ t i j (c iσ c jσ+ c jσ c iσ)+ i U ˆn i ˆn i At T = 0 the half-filled solution on unfrustrated square lattices is an AFM-ordered insulator for any U/t > 0. BUT What happens for highly frustrated lattices? What happens for room-temperature paramagnetic materials? Strong correlations freeze charge degrees of freedom MOTT-INSULATOR for finite U/t. Metal-insulator transition withgutzwiller-jastrow wave functions p. 6/31

7 Strategy: variational approach The variational principle T = 0 E = Ψ H Ψ Ψ Ψ E gs Ψ is a variational wave function with variational parameters. The best variational wave function gives the lower energy. Metal-insulator transition withgutzwiller-jastrow wave functions p. 7/31

8 Why Projected wave functions? Ψ =P Ψ 0 No double occupancies: P= i (1 n i n i ): 3 He almost localized Fermi Liquid [Vollhardt 1984] Applications to the t-j model H t J = t i jσ (c iσ c jσ+ c jσ c iσ)+ i j JŜ i Ŝ j Applications to the Hubbard model H t J = t i jσ (c iσ c jσ+ c jσ c iσ)+ i Un i n i High T c superconductors (cuprates, cobaltates...) Metal-insulator transition withgutzwiller-jastrow wave functions p. 8/31

9 Computations with Gutzwiller wave functions Ψ G =P Ψ 0 =e g i n i n i Ψ 0 For g>0, P suppresses the double occupancies d i = n i n i. Variational Monte Carlo (VMC) Ψ Ô Ψ Ψ Ψ = x O x P x Electronic configurations x are generated according to the probability distribution P x = Ψ2 (x) x Ψ 2 (x ). Gutzwiller Approximation (GA) Ψ Ô Ψ Ψ Ψ = g Ψ 0 Ô Ψ 0 Ψ 0 Ψ 0 Metal-insulator transition withgutzwiller-jastrow wave functions p. 9/31

10 The Gutzwiller Approximation (GA) projected wave functions ( ) Ψ = P Ψ 0 = i 1 ni n i Ψ0 projected Hilbert space : Ψ pre-projected Hilbert space : Ψ 0 renormalization scheme Ψ Ô Ψ Ψ Ψ g Ψ 0 Ô Ψ 0 Ψ 0 Ψ 0 renormalization factors Hilbert-space arguments Metal-insulator transition withgutzwiller-jastrow wave functions p. 10/31

11 Gutzwiller renormalization factors kinetic energy Ψ T e Ψ Ψ Ψ = Ψ T Ψ Ψ Ψ Ψ 0 T Ψ 0 Ψ 0 Ψ 0 n σ (1 n) n σ (1 n σ ) Ψ T Ψ Ψ Ψ g t Ψ 0 T Ψ 0 Ψ 0 Ψ 0 g t = 1 n 1 n σ = 1 n 1 n/2 renormalized Hamiltonian in pre-projected Hilbert space Metal-insulator transition withgutzwiller-jastrow wave functions p. 11/31

12 The Gutzwiller Approximation Electronic energy: E e0 /N = g t e 0 +UD with g t = 8D(1 2D). E e0 = 0= Metal-Insulator Transition (MIT) for d U = U c = 8 e 0. Double occupancy d + compressibility κ q=0 vanish for U U c. METALLIC STATE (U < U c ) d = 1 ) ( (1 UUc U g t = 1 4 INSULATING STATE (U > U c ) U c d = 0 g t = 0 E e0 = 0 κ q=0 = 0 ) 2 E ) 2 e0 (1 N = e0 UUc The Gutzwiller Approximation finds the MIT but describes the insulating state trivially. Metal-insulator transition withgutzwiller-jastrow wave functions p. 12/31

13 Analytical results in the Gutzwiller Approximation First studied by [Brinkman-Rice 1970]. Ψ = P Ψ 0 = e g i n i n i [ k c kσ c k, σ 0 ]. En d U t U t Energy and double occupancy as a function of U/t in the GA. Metal-insulator transition at U c = 12.96t. In the insulator E = 0 and d = 0 d order parameter for a 2nd-order phase transition. Metal-insulator transition withgutzwiller-jastrow wave functions p. 13/31

14 VMC results with Gutzwiller wave functions First studied by [Yokoyama-Shiba 1987]. Ψ = P Ψ 0 = e g i n i n i [ k c kσ c k, σ 0 ]. Results for 242-site lattice in 2D. En d U t U t Energy and double occupancy as a function of U/t with VMC (dots) and in the GA (solid line). VMC results show no evidence of an insulator for finite U/t! Metal-insulator transition withgutzwiller-jastrow wave functions p. 14/31

15 Gutzwiller-Jastrow variational wave functions Charge fluctuations exist in realistic insulators: Ψ =PJ Ψ 0 P=e g i n i n i J = e 1/2 i j v i j n i n j The charge-charge Jastrow term J correlates empty sites (holons, h) and doubly occupied sites (doublons, d). Holons and doublons play a crucial role for the conduction: they must be correlated otherwise an applied electric field would induce a current. The long-range Jastrow factor J correlates h and d and can induce a Metal-Insulator Transition for a finite value of U/t (v i j > 0): n i n j = d i d j + h i h j h i d j d i h j + n i + n j 1 [n i = 1+d i h i ]. Metal-insulator transition withgutzwiller-jastrow wave functions p. 15/31

16 metallic vs. insulating states Basic property: An insulator has zero d.c. electrical conductivity (at T=0). Other properties charge carriers in Mott-Hubbard insulators: bound doublon-holons Charge density-density correlation function (factor structure): N q = Ψ n qn q Ψ Ψ Ψ N q q N q q 2 metal insulator Jastrow correlator [Fourier Transform] V q 1/q metal V q 1/q 2 insulator (1D) V q log(q)/q 2 insulator (2D) Metal-insulator transition withgutzwiller-jastrow wave functions p. 16/31

17 VMC results with Gutzwiller + Jastrow wave functions First studied by [Capello-Becca 2005]. Ψ = PJ Ψ 0 = e g i n i n i e 1 2 i j v i j n i n j [ k c kσ c k, σ 0 ]. v i j for i j =1. En d U t U t No evidence of an insulator for finite U/t also in this case. Metal-insulator transition withgutzwiller-jastrow wave functions p. 17/31

18 VMC results with Gutzwiller + Jastrow wave functions Ψ = PJ Ψ 0 = e g i n i n i e 1 2 i j v i j n i n j [ k c kσ c k, σ 0 ]. v i j for i j =2. En d U t U t No evidence of an insulator for finite U/t also in this case. but something is changing... Metal-insulator transition withgutzwiller-jastrow wave functions p. 18/31

19 VMC results with Gutzwiller + Jastrow wave functions Ψ = PJ Ψ 0 = e g i n i n i e 1 2 i j v i j n i n j [ k c kσ c k, σ 0 ]. v i j for i j =10. En d U t U t Transition at U c 8.5t! KINK in the Energy KINK in the Double Occupancy insulator (zero d.c. conductivity) Metal-insulator transition withgutzwiller-jastrow wave functions p. 19/31

20 VMC results with Gutzwiller + Jastrow wave functions Ψ = PJ Ψ 0 = e g i n i n i e 1 2 i j v i j n i n j [ k c kσ c k, σ 0 ]. v i j up to the longest possible distance. En d U t U t Transition at U c 8.5t. KINK in the Energy KINK in the Double Occupancy insulator (zero d.c. conductivity) Metal-insulator transition withgutzwiller-jastrow wave functions p. 20/31

21 SUMMARY! En d U t U t Energy and double occupancy as a function of U/t. Gutzwiller wave functions in GA (solid black line) Gutzwiller wave functions in VMC (red dots) Gutzwiller + very short-range Jastrow wave functions (green dots) Gutzwiller + short-range Jastrow wave functions (brown dots) Gutzwiller + medium-range Jastrow wave functions (blue dots) Gutzwiller + long-range Jastrow wave functions (black dots) Metal-insulator transition withgutzwiller-jastrow wave functions p. 21/31

22 Outlook Metal-Insulator transition with Gutzwiller wave functions is an artifact of the Gutzwiller Approximation! The transition is obtained correctly using variational wave functions which include a long-range charge-charge correlation [Jastrow correlators]. Metal-insulator transition withgutzwiller-jastrow wave functions p. 22/31

23 Basic References M. C. Gutzwiller, Phys. Rev. 137, A1726 (1965). W. F. Brinkman and T. M. Rice, Phys. Rev. B 2, 4302 (1970). H. Yokoyama and H. Shiba, J. Phys. Soc. Jpn. 56, 1490 (1987). M. Capello, F. Becca, M. Fabrizio, S. Sorella and E. Tosatti, 94, (2005). Metal-insulator transition withgutzwiller-jastrow wave functions p. 23/31

24 Gutzwiller Method The Hubbard model: H = i jσ t i j (c iσ c jσ+ c jσ c iσ)+ i U ˆn i ˆn i GZW TRIAL STATE: ψ G =P Ψ 0 Ψ 0 uncorrelated state P=e g i n i n i For g>0, P suppresses the double occupancies d i = n i n i. GUTZWILLER APPROXIMATION (GA) = E e = ψ G H e ψ G i jσ t i j g ti jσ Ψ 0 c iσ c jσ Ψ 0 + i Ud i g ti jσ [n iσ,n i, σ,d i ] GA renormalized hopping factors Metal-insulator transition withgutzwiller-jastrow wave functions p. 24/31

25 Gutzwiller Projected wave functions Totally projected wave function: Ψ =P Ψ 0 P= i (1 n i n i ) Ψ 0 uncorrelated state For single particle (hole) excitations with respect to Ψ = generalized GA. Partially projected wave function: Ψ γ = P γ Ψ 0 P γ = i γ (1 n i n i ) Double occupancy is projected out on all sites, but for the reservoir site γ. In computations with projected excited states partially projected states arise: for example, Pc γσ Ψ 0 =c γσ P γ Ψ 0 =c γσ Ψ γ. Metal-insulator transition withgutzwiller-jastrow wave functions p. 25/31

26 Renormalization scheme un-projected Hilbert space Hubbard Hamiltonian canonical transformation e is He is projected Hilbert space t-j Hamiltonian Gutzwiller renormalization g pre-projected Hilbert space renormalized Hamiltonian Ψ H t J Ψ Ψ Ψ Ψ 0 H (renor) t J Ψ 0 Ψ 0 Ψ 0 H (renor) t J = g t T e + g s J <i, j> S i S j g t = 1 n 1 n/2, g s = 1 (1 n/2) 2 Metal-insulator transition withgutzwiller-jastrow wave functions p. 26/31

27 RMFT scenario - Renormalization scheme Hubbard Hamiltonian: H = t i jσ (c iσ c jσ+ c jσ c iσ)+ i U ˆn i ˆn i = canonical transformation e is He is = t-j Hamiltonian: H t J = ˆP G [ t i jσ (c iσ c jσ+ c jσ c iσ)+ i j JŜ i Ŝ j ] ˆP G = GA renormalization factors g t and g S = t-j renormalized Hamiltonian: H t J = g t t i jσ (c iσ c jσ+ c jσ c iσ)+ i j g S JŜ i Ŝ j with ψ H t J ψ ψ ψ ψ 0 H t J ψ 0 ψ 0 ψ 0 Decoupling Scheme = BCS ψ 0 = k (u k + v k c k c k 0. Metal-insulator transition withgutzwiller-jastrow wave functions p. 27/31

28 The Gutzwiller Approximation (GA) Expectation values: Ψ γ Ô Ψ γ Ψ γ Ψ γ Gutzwiller Approximation (GA) Ψ γ Ô Ψ γ Ψ γ Ψ γ = g γ Ψ 0 Ô Ψ 0 Ψ 0 Ψ 0 Variational Monte Carlo (VMC) Ψ γ Ô Ψ γ Ψ γ Ψ γ = x O x P x Metal-insulator transition withgutzwiller-jastrow wave functions p. 28/31

29 Variational Monte Carlo (VMC) method Variational Monte Carlo (VMC): numerical approach to compute exact expectation values over correlated wave functions: Ψ γ Ô Ψ γ Ψ γ Ψ γ = x O x P x Electronic configurations x are generated according to the probability distribution P x = Ψ2 (x), using the x Ψ 2 (x ) Metropolis algorithm: [ P x x = min 1, Ψ(x ) Ψ(x) 2] Metal-insulator transition withgutzwiller-jastrow wave functions p. 29/31

30 Variational Monte Carlo (VMC) method Monte Carlo sampling of the configuration x shall fulfil two conditions. Does a stationary P x exist? Yes thanks to the Detailed Balance: The number of processes corresponding to the transition x x shall be compensated by the number of processes in the reverse sense x x : P x P x x =P x x P x Starting from an arbitrary P x, will the sampling converge to the stationary P x? Yes thanks to the Ergodicity: Any configuration can be reached from the initial one via a sufficiently large number of steps [iterations of the Markov chain]. [Unicity of the P x ] Metal-insulator transition withgutzwiller-jastrow wave functions p. 30/31

31 Variational Monte Carlo (VMC) method Standard Detailed Balance: P x P x x =P x x P x P x P x x p i j =P x x p ji P x p i j = p ji : probability to select two sites i and j for hopping or spin-flip processes in the transition x x. p i j = p: equiprobability to select two sites i and j. Our Detailed Balance for large lattices with a reservoir site γ: P x P x x p i j =P x x p ji P x p i j = θ(r α)p i j +[1 θ(r α)]p γ j ; r,α [0,1]. r: random number extracted in each step of the stochastic process. This will allow to accumulate more statistics for the computations of intensive quantities for the reservoir site, such as its double occupancy d γ. Metal-insulator transition withgutzwiller-jastrow wave functions p. 31/31