Renormalization of microscopic Hamiltonians. Renormalization Group without Field Theory
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1 Renormalization of microscopic Hamiltonians Renormalization Group without Field Theory Alberto Parola Università dell Insubria (Como - Italy)
2 Renormalization Group Universality Only dimensionality and symmetry matter Effective Field Theory (long wavelengths) Statistical Model Renormalization Group HRT Hierarchical Reference Theory Keeps track of higher order operators Provides information on non universal properties (critical temperature, crossovers etc.) A.P. & L.Reatto: PRL, 53, 2417 (1984); PRA 31, 3309 (1985); Adv. Phys. 44, 211, (1995).
3 Outline Wilson s momentum-shell integration RG for microscopic models Sharp & Smooth cut-off formulations Approximate non perturbative closures Relation to the Local Potential Approximation First order transition and the convexity requirement Extension to Quantum Hamiltonians Open problems
4 HRT vs. RG: A short story (scalar order parameter) Φ 4 Field Theory Statistical Model S = dr [ 1 2 Φ 2 + r 2 Φ2 + u ] 4! Φ4 H = 1 2 i j v(r i r j ) Z = DΦ(r) e S Z = dr 1 dr N e βh Perturbation theory in u Perturbation theory in β
5 Formal Perturbation Theory Split the potential into a Hard Sphere part and an Attractive tail v(r) = v HS (r) + w(r) Expand the free energy (ln Z) in powers of w(r) Order the diagrams according to the number of loops
6 Zero loops ln Z 0 = V 2 Φ 4 Field Theory dq (2π) D ln [ r + q 2 2π ] Statistical Model ln Z MF = ln Z HS One loop = V 2 dq (2π) D ln[1 + βρ S HS(q) w(q)] Two loops 1 8
7 Correspondence Field Theory Statistical Model Propagator: 1 r + q 2 [ β w(q)] [ρs HS (q)] βρs HS (q) w(q) Vertex: u n (r 1, r n ) c n (r 1,,r n ) δn ln Z HS [ρ(r)] δρ(r 1 ) δρ(r n ) Momentum Shell Integration RG Cut-off on the propagator Cut-off on the interaction w(q)
8 Sharp cut-off implementation Sequence of intermediate Q-systems w Q (k) = { w(k) for k > Q 0 for k < Q Q : 0 lim w Q(k) = 0 Q lim Q 0 w Q(k) = w(k) The cut-off Q limits the range of density fluctuations included in the Q-system Liquid-vapor transition inhibited at every Q 0
9 Evolution of the free energy with Q da Q dq = d 2 Ω d Q d 1 ln [ 1 F Q (Q) β w(q) ] = d 2 Ω d Q d 1 ln [ 1 + β w(q) C Q (Q) ] kt A Q = Free energy density of the Q-system + Mean field contribution [Γ k ] C Q (k) = Direct correlation function of the Q-system [ Γ (2) k (q) ] + Mean field contribution = C Q (r 1 r 2 ) = δ 2 A Q δρ(r 1 )δρ(r 2 ) Ω d = volume of the unit sphere in d-dimension Exact hierarchy of differential equations for C Q and c n Q (n = 3, )
10 Smooth cut-off implementation Sequence of intermediate t-systems w t (r) = w(r) e (d+2 η)t w(r/e t ) w t (k) = w(k) e (2 η)t w(k e t ) t : 0 Q e t lim w t(k) = 0 t 0 lim t w t(k) = w(k) Phase transitions are suppressed at all finite t s A.P: J. Phys. C 26, 5071 (1986) A.P., D.Pini and L.Reatto: PRL 100, (2008)
11 Evolution of the free energy with t da t dt = β 2 d d k d w t (k) (2π) d dt [ Ct (k) + e (2 η)t β w(k e t ) ] 1 kt A t = Free energy density of the t-system + Mean field contribution [Γ k ] C t (k) = Direct correlation function of the t-system + Mean field contribution = C t (r 1 r 2 ) = [ Γ (2) k (q) ] δ 2 A t δρ(r 1 )δρ(r 2 ) Exact hierarchy of differential equations for C t and c n t (n = 3, )
12 The Dictionary NPRG k φ Smooth cut-off HRT Q = e t ρ R k (q) e (2 η)t βw(q e t ) Γ k [φ] A t (ρ) Γ (2) k (q) C t(q) Γ (n) k (q 1 q n ) c n t (q 1 q n )
13 Highlights of the exact HRT equations For Q 0 and in the critical region the HRT equations simplify and, through a simple rescaling, reduce to the standard RG hierarchy of equations for a scalar field theory. The HRT smooth cut-off prescription depends on the interparticle potential w(r) and corresponds to the smooth cut-off RG with a suitable choice of smearing function The full HRT equations retain information on non-universal properties and short range correlations (i.e. the full UV behavior of the statistical model is preserved in the RG procedure) The HRT strategy can be trivially generalized to O(n) spin models on a lattice
14 An approximate closure λ(ρ) = 1 = Mean Field C t (k) = c HS (k) λ t (ρ) β w(k) λ t (ρ) defined by the compressibility sum rule C t (k = 0) = 2 A t ρ 2 Local Potential Approximation at long wavelengths C t (k) k 0 2 A t ρ 2 b k2 Closed (approximate) partial differential equation for the thermodynamics of the t-system To be solved with initial condition A t = βa MF /V for t = 0 In the t limit A t Physical free energy density of the fully interacting model
15 Critical Properties Formal structure of the HRT evolution equation at large t ( ) da t dt = e dt Φ e 2t 2 A t ρ 2 where Φ(x) is a non-linear function depending on the choice of sharp/smooth cut-off Rescaling: z = (ρ ρ c ) e d 2 2 t H t (z) = e dt [A t (ρ) A t (ρ c )] Fixed point equation: the standard RG structure in LPA is recovered d 2 2 z H (z) d H (z) = Φ(H (z)) Φ(H (0))
16 HRT flow of the inverse susceptibility χ 1 t = 2 A t (ρ) ρ 2 ρc Approach to the fixed point value in the critical region Flow to the low temperature fixed point in the two-phase region
17 Critical exponents and amplitudes χ 1 = 2 A (ρ) ρ 2 ρc 1 C ± T T c T c γ ρ ρ c (T c T) β ǫ-expansion d = 3 γ = 1 + ǫ 6 + β = 1 2 ǫ 6 + HRT sharp HRT smooth Exact γ β η U 2 = C + /C PRL, 53, 2417 (1984); PRL 100, (2008)
18 Numerical integration of the full HRT equation Smooth cut-off Inverse susceptibility Coexistence curve
19 Phase diagram of a fluid HRT-smooth MC T c (2) ρ c (2)
20 First order transition Fluctuations restore the convexity of the free energy For t : In one-phase regions the susceptibility is always positive At coexistence the susceptibility identically vanishes Phys. Rev. E 48, 3321 (1993); Phys. Rev. E 76, (2007)
21 First order transition Sharp vs Smooth cut-off 5 Sharp cut-off Smooth cut-off χ -1 χ ρ ρ c At coexistence lim r ρ2 g(r) = ρ(ρ v + ρ l ) ρ v ρ l
22 Quantum HRT General interacting quantum system of bosons/fermions/spins Ĥ = Ĥ R + ˆV = Ĥ R dx dy ˆρ(x)w(x y)ˆρ(y) Order parameter < ˆρ(x) > = Perturbative expansion in w(x) Ξ Ξ R = Tr e βĥ Tr e βĥ R = n=0 ( 1) n 2 n n! ds 1 ds 1...ds n ds n ρ R (s 1,...s n) φ(s 1, s 1 )... φ(s n, s n) φ(s, s ) = w(x x )δ(τ τ ) τ (0, β) s = (x, τ) ρ R (x 1, τ 1 ;...x n, τ n ) = 1 Ξ R [J] δ n Ξ R [J] δj(x 1, τ 1 )...δj(x n, τ n ) J=0 A.P & P.Gianinetti PRB 63, (2001)
23 Formal analogy with a classical model in d + 1 dimension Sharp cut-off on the Fourier components of w(x) Exact evolution equation for the Helmholtz free energy density da Q dq = Qd 1 d 2 Ω d ω n ln[1 F Q (Q, ω n ) w(q)] Connected two point function in imaginary time: F(x 1, τ 1 ;x 2, τ 2 ) = δ 2 lnξ[j] δj(x 1, τ 1 )δj(x 2, τ 2 ) J=0 Matsubara frequencies at finite temperature: ω n = 2π β n
24 The antiferromagnetic Heisenberg model Ĥ = J <R,R >Ŝ R Ŝ R h R e i π R Ŝ z R Order parameter: staggered magnetization < Ŝ z R >= m ei π R Parametrization of the two point functions (α Q, α Q ) FQ xx α (k, ω) = Q w(k) m 2 ω 2 + α Q 2 w(k) 2 F xy m Q (k, ω) = F zz Q (k, ω) = δ ω,0 α Q + w(k) 1 ω m 2 ω 2 + α Q 2 w(k) 2
25 Physical response functions Single mode approximation Im S xx (k, ω) = πω 1 exp( βω) δ(ω ǫ k ) + δ(ω + ǫ k ) α + w(k) Im S zz (k, ω) = 2π β δ(ω) α + w(k) Spin wave dispersion: ǫ k = m α 2 w(k) 2
26 LPA-like closure ( χ s h 1 ( ) A 1 m) = = βm = F xx ( π,0) = (α 2Jd) 1 m ( ) ( χ s h 1 2 ) 1 A = = β m m 2 = F zz ( π,0) = (α 2Jd) 1 QHRT Equation d A Q dq = D d(q) 2 4 ln ( ) sinh 12 βmα Q ) sinh( 12 βm α 2 Q 4J 2 (d 2 Q 2 ) + ln [ α Q 2 α Q 2 4J 2 (d 2 Q 2 ) ] Q (0, d) m (0,1)
27 Numerical results in two dimensions Spin-1 model Spin- 1 2 model log(ξ) 4 1 ρ s = /T ξ(t) ξ 0 e 2π ρ s T The Heisenberg model always falls in the renormalized classical regime
28 Non Linear σ-model S[Ω] = ρ s 2 d d r β 0 dτ Ω c 2 s ( Ω τ ) 2 Ω = 1 One loop RG equations Q dg dq = (d 1)g K d 2 g2 coth(g/2t) d dq ( ) g T = g T Effective coupling constant g = ( Q d ) d 1 4d m Q QHRT evolution of the spontaneous magnetization ( ) dm d 2 Q Q dq = K d coth(βqm Q ) d g(q) 20 g c 10 * T=0.2 T=0.4 T= Q
29 Outlook Closures Beyond LPA: the second equation of the hierarchy η at two loop order Better representation of short range physics Repulsive interactions Classical and Quantum Heisenberg models Order parameters Beyond the O(n) model: inhomogeneous systems Competing interactions: more complex order parameters The Fermi surface in QHRT The Coulomb gas (primitive model)
30 BMW approach PRL, 53, 2417 (1984) = Equation for Γ (2) (k) = BMW approximation
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