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On the formulation of a functional theory for pairing with particle number restoration. Guillaume Hupin GANIL, Caen FRANCE. Collaborators : M. Bender (CENBG) D. Lacroix (GANIL) D. Gambacurta (GANIL). Brief summary of the SC-EDF functional. SR-EDF and MR-EDF. SC-EDF - PowerPoint PPT Presentation
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On the formulation of a functional theory for pairing with particle number restoration
Guillaume HupinGANIL, Caen FRANCE
Collaborators : • M. Bender (CENBG) • D. Lacroix (GANIL)• D. Gambacurta (GANIL)
2
Brief summary of the SC-EDF functional
The SC-EDF functional
• SC-EDF• MR-EDFnon-regularized
No regularizationpossible
SC-EDF is
SR-EDF and MR-EDF
3
Practice
Expressing the 2-body densities a function of xi
with
withBCS
occupation probabilities
Or recurrence relation using
4
Variation After Projection in EDF (VAP)E
(MeV
)
SR-EDF broken symmetry minimum
SC-EDF minimum
PAV
VAPBCS
Exact
Coupling strength
Cor
rela
tion
ener
gy
Pairing Hamiltonian
VAP
Threshold effects
Sin
gle
parti
cle
ener
gy
Δε
Motivations
Better reproduction of the energy.
Correct finite size effects (no threshold).
Optimization of the auxiliary state .
Flexibility of EDF.
5
Variational principle
With the SC-EDF functional
Applied using the parameters of auxiliary state
K. Dietrich et al. Phys. Rev. 135 (1964)MV Stoitsov et al. PRC (2007)…
6
Numerical methods
76Kr
Preliminary : simplification of F(vk) to reduce the numeric to a minimum search.
1 - Imaginary time step method to diagonalized MF Hamiltonian.
2 - Gradient method to solve the secular equations with respect to vi.
Solve minimization with gradient method
New set of vi and MF potentials
Evolve imaginary time
New set of φi
ConvergenceVAP
BCS BCSVAP
7
Functional Theory : flexibility of SC-EDF
Solves the BCS threshold problem.
Avoids complex numeric.
Easy to implement in existing Mean-Field codes.
→ 1 Mean-Field.
Functional theory allows to modify the expression of the energy.
VAP
BCS
VAP
BCS
Original VAP VAP with ni nj
8
Achievements of this work
1. MR-EDF when regularized can be viewed as a DFT.
2. A modification of the regularization makes possible to associate MR-EDF with a correlated auxiliary state.
3. SC-EDF formalism allows to use density dependent interaction.
DFT
Here
Question : Is it possible to express the energy as a function of ρ1 [N] ?
It is already the case of the BCS theory :
9
Density Matrix Functional Theory (DMFT)- alternative path
Exact
DMFT
Focus on one body observables
DFT
Describes at the minimum of the functional energy
T.L. Gilbert PRB 12 (1975)
Cor
rela
tion
ener
gy
Full one body observables
N. N. Lathiotakis et al. PRB 75 (2007)
Recently applied in electronic systems
Example : Homogenous Electron Gas (HEG)
10
Sin
gle
parti
cle
ener
gy
DMFT from a projected BCS state
BCS PBCS
?
with
Δε
11
Applications and benchmarks of the new functional
?A new systematic 1/N expansion beyond BCS:
BCS
Lacroix and Hupin, PRB 82 (2011)
EH
F - E
Coupling
Objective : invert into
Finite size effectsOK when all terms
are included
Exact
Exact
BCS
12
Resummation into a compact functional
All contributions can be approximately summed to give:
with BCS
EH
F - E
Coupling
EH
F - E
EH
F - E
Coupling Coupling
4 particles 16 particles 44 particles
BCS
Exact
New func.
13
Applications : more insights
Hupin et al. PRC83 (2011)
What is required for realistic situations in Condensed Matter and Nuclear physics ?
1. Can be applied to odd system.
2. Functional applicable to small and large systems while reproducing the desired physics (here the finiteness of systems).
3. The single particle spectra upon which is applied the functional should not be constrained.
Richardson model Any spectra
14
Applications : odd systems
Great improvement over BCS
+Energy of odd systems is better
reproduced
Odd systems have been described in terms of a blocked state – the last occupied state (i) of the Fermi sea.
PBCS
Richardson
Particle number
We define the mean gap (BCS gap in thermo. limit)
BCS Functional
Exact
15
Applications : thermodynamic limit
Lacks some correlations at small
number of particles
+The functional does as good as
the PBCS ansatz
G. Sierra et al. PRB 61 (2000)
Cor
rela
tion
ener
gy
~1/A
Sin
gle
parti
cle
ener
gy
Δε=dFiniteness of physical systems is also of interest in condensed matter
SuperconductingNanoscale grains
Parameterization of the SP energy splitting and particle number (A) :
BCSFunctional
Exact
Dot = odd systems
16
Applications : random single particle spectra
This functional is efficient with
any SP spectra can be used
with self consistent methods
?Generate SP energy levels
Normalized to unity the average SP splitting
Solve minimization with gradient method
New set of vi and MF potentials
Evolve imaginary time
New set of φi
For instance
In a SC scheme
BCS
Functional
17
Extension : functional for finite temperature
DMFT : information reduced to one body observables
Functional build from Hamiltonian finite temperature
Entropy reduced to a set of one body observables
Balian, Amer. J. Phys. (1999)
D. Gambacurta (GANIL)
Esebbag, NPA 552 (1993)
Gibbs free energy
18
Conclusions and Perspectives
Restoration of particle number in MR-EDF
Reanalyzed the MR-EDF method with its regularization.
Proposed and alternative method that is a functional of the projected state SC-EDF.
• PAV : direct use of the SC-EDF functional
• VAP : variation of the functional
E (M
eV)
PAV
VAPSC-EDF
19
Conclusions and Perspectives
SC-EDF (↔ MR-EDF regularized) is a framework for the restoration of particle number in functional theory.
However, the SC-EDF restores the functional flexibility (ρα ).
Refitting of the pairing functional.
Application to others symmetries ?
Neutron / Proton pairing with finite size correction.
Pai
ring
ener
gy
N/Z
ExpBCS/HFB
20
Conclusions and Perspectives
New DMFT functional for finite size systems with pairing
Proposition.
Benchmark with exact solution of Richardson model.
Check the applicability in realistic cases.
Thermodynamics and dynamics of finite systems.
Quantum phase transition exploration.
• Large N• Odd even• Random spectra