Brief historical introduction. 50th anniversary of the BCS paper. Richardson exact solution (1963) Ultrasmall superconducting grains (1999).

Transcription

1 Brief historical introduction. 50th anniversary of the BCS paper. Richardson exact solution (1963) Ultrasmall superconducting grains (1999). Cooper pairs and pairing correlations from the exact solution in BCS- BEC crossover (2005) and in atomic nuclei (2007). Generalized Richardson-Gaudin Models for r>1 ( ). Exact solution of the T=0,1 p-n pairing model.

2 The Cooper Problem Problem : A pair of electrons with an attractive interaction on top of an inert Fermi sea. φ = c c FS, = k k 2ε E G 2ε E k> k k k> k F F k

3 Bound pair for arbitrary small attractive interaction. The FS is unstable against the formation of these pairs

4 Bardeen-Cooper-Schrieffer + vk Ψ e 0, Γ = u c c Γ k k k k

5 BCS in Nuclear Structure

6 Richardson s Exact Solution

7 Exact Solution of the BCS Model H = ε n + g c c c c P k k k Eigenvalue equation: H P k, k' Ψ = k k k' k' Ansatz for the eigenstates (generalized Cooper ansatz) E Ψ M Ψ = Γ 0, Γ = α = 1 α α k 1 2ε k E α c c + + k k

8 Richardson equations M M g + 2g = 0, E = E 2ε E E E Properties: ( ) k = 0 k α β α = 1 α β α= 1 This is a set of M nonlinear coupled equations with M unknowns (E α ). The first and second terms correspond to the equations for the one pair system. The third term contains the many body correlations and the exchange symmetry. The pair energies are either real or complex conjugated pairs. There are as many independent solutions as states in the Hilbert space. The solutions can be classified in the weak coupling limit (g 0). α

9 What is an exactly solvable model?.- A model is exactly solvable if we can write explicit expressions for the complete set of eigenstates in terms of a set of parameters, which are in turn solutions of an algebraic problem..- The exponential complexity of the many body problem is reduced to a polynomial complexity..- Simplest examples of ESM are dynamical symmetry models. The Hamiltonian is a combination of Casimir operators of a Lie algebra. Analytically solvable. Elliot SU(3), IBM SU(3), O(6), U(5). Etc Why are ESM important?.- They can unveil physical properties that cannot described with existing many-body theories..- They could constitute stringent test for many-body theories. Benchmark models.

10 Recovery of the Richardson solution: Ultrasmall superconducting grains A fundamental question posed by P.W. Anderson in J. Phys. Chem. Solids 11 (1959) 26 : at what particle size will superconductivity actually disappear? Since d~vol -1 Anderson argued that for a sufficiently small metallic particle, there will be a critical size d ~Δ bulk at which superconductivity must disappear. This condition arises for grains at the nanometer scale. Main motivation from the revival of this old question came from the works: D.C. Ralph, C. T. Black y M. Tinkham, PRL s 74 (1995) 3421 ; 76 (1996) 688 ; 78 (1997) 4087.

11 The model used to study metallic grains is the reduced BCS Hamiltonian in a discrete basis: ( ) jσ H = ε μ c c λd c c c c jσ Single particles are assumed equally spaced jσ jσ j+ j j j+ jj' ε jσ = jd, j = 1,, Ω where Ω is the total number of levels given by the Debye frequency ω D and the level spacing d as dω = 2 ω D

12 PBCS study of ultrasmall grains: cond D. Braun y J. von Delft. PRL 81 (1998)47 E = ψ H ψ ψ H ψ 0 0

13 Condensation energy for even and odd grains PBCS versus Exact J. Dukelsky and G. Sierra, PRL 83, 172 (1999)

14 Thermodynamic limit of the Richardson equations M. Gaudin (unpublished). J.M. Roman, G. Sierra and JD, Nucl. Phys. B 634 (2002) 483. V, N, G=g / V and ρ=n / V By using electrostatic techniques and assuming that extremes of the arc are 2μ+2 i Δ, Gaudin derived the BCS equations: Gap Number The equation for the arc Γ is, ( E μ ) ( ε ) 1 g 1 dε + = 0 2 G ( ) 2 2 ε μ +Δ ρ = dε g( ε) 1 ε μ ( ) 2 2 ε μ +Δ ( ) 2 2 z = ε μ +Δ z ( ε μ) ln + iδ Δ + ( E μ)( ε μ) + z ( ε μ) +Δ 0= Re dε ε ln ( ε μ) +Δ iδ( E ε ) 2 2 2

15 Pair energies E for a system of 200 equidistant levels at half filling

16 For a uniform 3D system in the thermodynamic limit the Gap equation is singular. Leggett (1980) proposed a regularization based on the subtraction the scattering length equation. m π 1 1 g = + dε ε 2 4 as G 2 ( ε ) Scattering length The Leggett model describes the BCS-BEC crossover in terms of a single parameter η = 1/ k. The resulting equations can be Fas integrated (Papenbrock and Bertsch PRC 59, 2052 (1999)) 2 2 μ 4 η = μ +Δ P1/2 2 2 μ +Δ 4 ( 2 2) 3/4 = ημ + μ +Δ P3/2 3π μ μ + Δ 2 2

17 Evolution of the chemical potential and the gap along the crossover μ -1-2 Δ η η

18 WhatCooper pair in the superfluid is medium? Cooper pair wavefunction From MF BCS: G. Ortiz and JD, Phys. Rev. A 72, (2005) ( ) ( ) ( ) Ψ= A ϕ r ϕ r ϕ r N/2 N/2 1 ik r ϕ( r) = ϕk e V k ϕ = BCS k C BCS v u k k From pair correlations: From Exact wavefuction: ϕ E ( r) = C E e r E/2 r P ϕk = BCS c c BCS = C k k Pukv k ϕ E k ( E) CE = 2ε E k E real and <0, bound eigenstate of a zero range interaction parametrized by a. E complex and R (E) < 0, quasibound molecule. E complex and R (E) > 0, molecular resonance. E Real and >0 free two particle state.

19 BCS-BEC Crossover diagram f pairs with Re(E) >0 f=1 Re(E)<0 1-f unpaired, E real >0 η= -1, f = 0.35 (BCS) η = 0, f = 0.87 (BCS) η = 0.37, f = 1 (BCS-P) η = 0.55, f = 1 (P-BEC) η = 1,2, f=1 (BEC) f=1 some Re(E)>0 others Re(E) <0

20 Cooper pair wave function Weak coupling BCS Strong coupling BCS BEC r 2 ϕ(r) x 10-2 η = ϕ E 6 ϕ P η = r ϕ BCS η =0

21 ODLRO and the fraction of the condensate For fermions ODLRO occurs in the two-body density matrix + + ( r ', r ', r, r ) = ( r ') ( r ') ( r ) ( r ) ρ ψ ψ ψ ψ if it has a macroscopic eigenvalue ρ r ', r ', r, r = ψ r ' ψ r ' ψ r ψ r + ρ ' ( ) + ( ) + ( ) ( ) ( ) For a homogeneous system in the thermodynamic limit ( ) ( ) ( ) r ' r, r ' r, ψ r ψ r = λmϕ r r λ 1 1 M M ( ) = dr1dr2 ϕ r1 r2 = ukvk k

22 Sizes and Fraction of the condensate ξe = πξ0 / 2 ξ = 2 π / Δ 2 0 ξ = ϕ r ϕ ξp πξ0 /2 2 η = 3 r2 = ξ 1/Im( ) E = E ξ BCS = 21/ 2 9 π /4 2 λ = dr dr N 2 3π Δ = 16 Im ϕ ( μ +Δ i ) ( r, r ) 1 2 P 1 2 2

23 Application to Samarium isotopes G.G. Dussel, s. Pittel, J. Dukelsky and P. Sarriguren, PRC 76, (2007) Z = 62, 80 N 96 Selfconsistent Skyrme (SLy4) Hartree-Fock plus BCS in 11 harmonic oscillator shells (40 to 48 pairs in 286 double degenerate levels). The strength of the pairing force is chosen to reproduce the experimental pairing gaps in 154 Sm (Δ n =0.98 MeV, Δ p = 0.94 MeV) g n =0.106 MeV and g p =0.117 MeV. A dependence g=g 0 /A is assumed for the isotope chain.

24 Correlations Energies Mass Ec(Exact) Ec(PBCS Ec(BCS+H) Ec(BCS)

25 Fraction of the condensate in mesoscopic systems λ = cccc α α cc α α cc α α α α M ( 1 M / L ) α λ BCS = 1 M M L ( 1 / ) α uv 2 2 α α From the exact solution f is the fraction of pair energies whose distance in the complex plane to nearest single particle energy is larger than the mean level spacing.

26 1,0 Exact Fraction 0,8 0,6 0,4 BCS 154 Sm BEC 0,2 ε 1 =μ 0,0 0,0 0,2 0,4 0,6 0,8 1,0 g 0 g

27 154 Sm Real Part Real Part C 2 C 5 C 4 C C C C C 1 G= ,0-0,5 0,0 0,5 1,0 G= Imaginary Part G= G= Imaginary Part

28 ψ(k) 2 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0, k BCS C 1 C 2 C 3 C 4 C 5

29 Some models derived from rank 1 RG BCS Hamiltonian (Fermion and Boson) Generalized Pairing Hamiltonians (Fermion and Bosons) Central Spin Model Gaudin magnets Lipkin Model Two-level boson models (IBM, molecular, etc..) Atom-molecule Hamiltonians (Feshbach resonances) Generalized Jaynes-Cummings models Breached superconductivity. LOFF and breached LOFF states. Reviews: J.Dukelsky, S. Pittel and G. Sierra, Rev. Mod. Phys. 76, 643 (2004); G. Ortiz, R. Somma, J. Dukelsky y S. Rombouts. Nucl. Phys. B 7070 (2005) 401

30 Exactly Solvable RG models for simple Lie algebras Cartan classification of Lie algebras rank A n su(n+1) B n so(2n+1) C n sp(2n) D n so(2n) 1 su(2), su(1,1) pairing so(3)~su(2) sp(2) ~su(2) so(2) ~u(1) 2 su(3) Three level Lipkins so(5), so(3,2) pn-pairing sp(4) ~so(5) so(4) ~su(2)xsu(2) 3 su(4) Wigner so(7) FDSM sp(6) FDSM so(6)~su(4) 4 su(5) so(9) sp(8) so(8) pairing T=0,1. Ginnocchio. 3/2 fermions

31 Exactly Solvable Pairing Hamiltonians 1) SU(2), Rank 1 algebra H = ε n g P P i + i i i j ij M M g + 2g = 0, E = E 2ε E E E 2) SO(5), Rank 2 algebra ( ) k = 0 k α β α = 1 α β α= 1 H = ε n g P P + i i i τ j τ i ijτ J. Dukelsky, V. G. Gueorguiev, P. Van Isacker, S. Dimitrova, B. Errea y S. Lerma H. PRL 96 (2006) ) SO(8), Rank 4 algebra H = ε n g P P g D D + + i i T iτ jτ S iσ jσ i ijτ ijσ S. Lerma H., B. Errea, J. Dukeslky and W. Satula. PRL 99, (2007). α

32 The exact solution M 1 E = e + εiu L α α = 1 i= 1 i ( ) 1 2 ( ) ( 2l + 1) 2 1 i 1 + = 0 e e ω e 2ε e g M M L α α' α' α α α' α i i α M = ω ω e ω η ω γ ω 2ε ω M M M M α' α α' α α' α' α α' α' α α' α' α α' i α ( ) = 0 η η ω η 2ε η M M L α' α α' α α' α' α i i α 4 2 ( ) = 0 γ γ ω γ 2ε γ M M L α' α α' α α' α' α i i α

33 80 Nucleons in L=50 equidistant levels Quartet n-n Cooper pair p-p Cooper pair

34 G= Even T 40 Odd T 40 T=0 T=1 T=0,1 30 T=0 T=1 T=0,1 30 E T T e 1 o 1 ET = T( T + λ), ET = T( T + λ) +ΔE 2J 2J T T T J T : iso-moi, λ: Wigner energy, ΔE: 2qp excitation (ν=2)

35

36 Summary From the analysis of the exact BCS wavefunction we proposed a new pictorial view to the nature of the Cooper pairs Alternative definition of the fraction of the condensate. Consistent with change of sign of the chemical potential. For finite system, PBCS improves significantly over BCS but it is still far from the exact solution. Typically, PBCS misses of order 1 MeV in binding energy. The T=0,1 pairing model could be a benchmark to study different approximations like the isocranking model or approximations dealing with alpha correlations and alpha condensation. It can be also describe the Ginnocchio model with non-degenerate orbits or spin 3/2 cold atom models. SP(6) RG model: A deformed-superfluid benchmark model?