J. P. Draayer, J. G. Hirsch, Feng Pan, A. I. Gueorguieva C. Bahri, G. Popa, G. Stoitcheva
description
Transcript of J. P. Draayer, J. G. Hirsch, Feng Pan, A. I. Gueorguieva C. Bahri, G. Popa, G. Stoitcheva
Abstract: While large-scale shell-model calculations may be useful, perhaps even essential, for reproducing experimental data, insight into the physical underpinnings of many-body phenomena requires a deeper appreciation of the underlying symmetries or near-symmetries of a system. Our program is focused on standard as well as novel algebraic approaches to nuclear structure … many-body shell-model methods within a group theoretical framework.
J. P. Draayer, J. G. Hirsch, Feng Pan, A. I. Gueorguieva
C. Bahri, G. Popa, G. Stoitcheva
V. G. Gueorguiev, K. D. Sviracheva, K. Drumev
H. Ganev, H. Matevosyan, H. Grigoryan
Principals
Students
- End of the Day -
Q Space
Q SpaceP Space
xhyh
postulate: x(~2) + y(~12) = ~12
… sy
mplectic
…
- Ongoing Research Projects -
• Pseudo-SU(3) calculations for heavy nuclei• E2 / M1 (scissors) modes in deformed nuclei• Algebraic solution for non-degenerate pairing• Non-linear modes (surface solitons) of a drop• Boson and Fermion representations of sp(4)• Mixed-mode (oblique) shell-model methods• SU(3) calculations for lower fp-shell nuclei
good
more
WORKING
Protons
ψ ν
θψ π
Neutrons
Castaños / Draayer / HirschDirk Rompf (PhD-1998)
Thomas Beuschel (PhD-1999)Carlos Vargas (PhD-2000)Gabriela Popa (PhD-2001)
Study shell-model dynamics against the background of an extremely
elegant algebraic/geometric picture
Rot(3) SU(3) Macroscopic Microscopic
Reproduce experimental M1 spectra including observed fragmentation and
clustering in a microscopic model
Ann. Phys. 180 (1987) 290Z. Phys. A 354 (1996) 359PRC 57 (1998) 1233; 1703
... PRC 61 (2000) 031301; 62 (2000) 064313 ...
… For the record ...
Enhanced M1 strengths first predicted in 1978 by Lo Iudice and Palumbo within the framework of the so-called phenomenological two-rotor model (TRM) …
[N. Lo Iudice and F. Palumbo, PRL 41 (1978) 1532]
Enhanced M1 strengths first observed in 1984 in 156Gd …[D. Bohle, A. Richter, W. Steffen, A. E. L. Dieperink, N. Lo Iudice, F. Palumbo, and O. Scholten, PL 137 B (1984) 27 & U.E.P. Berg, C. Basing, J. Drexler, R. D. Heil, U. Kneisl, W. Naatz, R. Ratzek, S. Schennach, R, Stock, T. Weber, B. Fischer, H. Hollick, and D. Kollewe, PL 149 B (1984) 59]
Discussed within the context of the pseudo-SU(3) model … [O. Castaños, J. P. Draayer and Y. Leschber, Ann. Phys. 180 (1987) 290]
Considered within the framework of the interacting boson (proton-neutron) model (IBM-2) …
[K. Heyde and C. De Coster, PRC 44 (1991) 2262]
0
0.2
0.4
0.6
0.8
1
1.2
2.00 2.50 3.00 3.50 4.00
M1 transition strength spectrum
Energy [MeV]
B(M1, 0
+
-> 1+) [μ n2 ]
160 Gd ( )Experiment
Typical M1 Spectrum
Clustering
Fragmentation
M1
GS 0+ 1+s2 MeV
4 MeV
*
*Adiabatic representation mixing ...
Basis States: Pseudo-SU(3) (spherical cartesian)
n[f]( μ()
particle number
permutation symmetry
multiplicity label ( μ)
deformation [SU(3)]
K-band [SU(3)]
total orbitaltotal spin
total a.m.a.m. proj.
n[f]( μSMs
Q0 -> z-deformation
xy - orbital a.m.xy - a.m. proj.
total spinspin proj.
cartesian system(Slater Determinants)
sphericalsystem
transform between these yields a bit representation of basis states …
Tricks of the Trade(config geometry algebraic shape)
Invariants InvariantsRot(3) SU(3)Tr(Q2) C2
Tr(Q3) C3
2 ~ 2 + μ + μ + + μ1 = tan1 μ 2 + μ
π μπ ν μν
π + ν μπ + μν π + ν - 2 μπ + μν + 1 π + ν 1 μπ + μν - 2 π + ν - 1 μπ + μν - 12
m,l π+ ν-2m+l μπ+μν+m-2l)k
Direct Product Coupling
Coupling proton and neutron irreps to total (coupled) SU(3):
TWIST
SCISSORS
SCISSORS + TWIST
… Orientation of the π-ν system is quantized with the multiplicity denoted by k = k(m,l)
… multiplicity
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GROUND STATE
Hamiltonian: Symmetry Preserving
SU(3) preserving Hamiltonian:
H = c1(Qπ • Qπ + Qν • Qν) + c2Qπ • Qν
+ c3 L2 + c4 K2 + c5 (Lπ2 + Lν
2)
Using the identities ... Q • Q = C2 - 3 L
2 with π or ν Qπ • Qν = Q • Q - Qπ • Qπ - Qν • Qν]/2
L = Lπ + Lν
l = Lπ - Lν
the Hamiltonian becomes ...H = Hrot + Hint
with rotor & interactions terms … Hrot = a L2 + b K2
Hint = c l2 + d C2 + e (C2π + C2ν)
Hint = c l2 + dC2 + e(C2π + C2ν
kinetic potential
System DynamicsHrot = a L2 + b K2
quantum rotor
It can be shown that:
Hint = cθ lθ2 + dθ θ2
+ c-l-
2 + d--
2
where: - π ν
so that: Eint = hθ (nθ + 1/2)
+ h- (n-
+ 1/2)
where: nθ = m, n- = l
Scissors Mode
Twist Mode
…requires triaxialshape distributions
…always present, axial and triaxial
Recall: dynamics driven by interplay between the interaction and statistics
M1 Operator
M1 = (3/4)1/2 μN gorbit L gspin S
gorbit(proton) 1
gorbit(neutron) 0
gspin(proton)
gspin(neutron) 26
Actual
Values
0
0.5
1
1.5
2
M1 transition strength spectrum
B(M1, 0
+
-> 1+) [μ
n2]16 Gd ( )Experiment
0
0.
1
1.
2.00 2.0 .00 .0 .00
[ ]Energy MeV
conserved ()pseudo SU scissors
+ scissors twist(2,)
(26,)
( 1, 0B M
+
-> 1+) [μ
n2]
Scissors and Twist Mode Examples
0
0.2
0.4
0.6
0.8
1
1.2
M1 transition strength spectrum
�
B(M1, 0
+
-> 1+) [μ
n2]160 Gd ( )Experiment
0
0.
1
1.
2
1.60 2.00 2.0 2.0 .20 .60 .00
[ ]Energy MeV
( 1, 0B M
+
-> 1+) [μ
n2]
(29,6)
(26,9) (2,)1,2
twist
+ scissors twistscissors
()conserved pseudo SU
Axial+
Triaxial
Triaxial+
Triaxial
Example: M1 Modes in 156-160Gd
Particle distributions:
total normal unique μ
π 156-160Gd 14 8 6 (10,4)
ν 156Gd 10 6 4 (18,0) 158Gd 12 6 6 (18,0) 160Gd 14 8 6 (18,4)
Valence space: U(π) U(ν)
total normal unique
π = 32 nπ = 20 (pf-shell) u
π = 12 (h11/2)
ν = 44 nν = 30 (sdg-shell) u
ν = 14 ( i13/2 )
~~
~~~
Hamiltonian: Realistic (Version 1)
H = H0 Oscillator
… mixed irrep shell-model basis with 5 proton and 5 neutron irreps plus all products having S = 0 and J 8
SymmetryPreserving
SymmetryBreaking
+ dπ l2π Proton single-particle l2
+ dν l2ν Neutron single-particle l2
+ gπ Hpπ Proton Pairing
+ gν HpνNeutron Pairing
- a2 C2 Q•Q symmetric
+ a3 C3 Q•(QxQ) asymmetric
+ b K2 K-Band splitting
+ c L2Rotational bands
Results for 156-160 Gd Nuclei
160Gd: Excitation Spectrum
160Gd: M1 Strength Distribution
Fragmentation of M1 Strength in 160Gd
Results for 160-164 Dy Nuclei
M1 Sumrule vs E2 Strength
Triaxial Case: 196 Pt
Sumrule ~ 1.5 μ2 or about one-halfthat of the strongly deformed case
Shape (m,l) Mode B(M1) [μ2N]
a + a (1,0) s
π
2 π
π
+
− 1
a + t (1,0) s
π
μπ
+ μ
+ 2
μπ
+ μ
+ 1
π
π
+
− 1
(1,1) s + t
π
1
μπ
+ μ
+ π
+
μπ
μν
( π
+
)
π
+
+ 1
+
π
ν
( μπ
+ μ
)
μπ
+ μ
+ 1
+ ν
μπ
+ π
μν
⎡
⎣
⎢
⎤
⎦
⎥
t + t (1,0) s
π
μπ
+ μ
+ 2
μπ
+ μ
+ 1
π
π
+
− 1
(0,1) t
π
π
+
+ 2
π
+
+ 1
μπ
μ
μπ
+ μ
− 1
(1,1) (s + ) + (t t + s)
π
1
μπ
+ μ
+ π
+
μπ
μν
( π
+
)
π
+
+ 1
+
π
ν
( μπ
+ μ
)
μπ
+ μ
+ 1
+ ν
μπ
+ π
μν
⎡
⎣
⎢
⎤
⎦
⎥
Analytic Results
Castaños ‘87
Rompf ‘96
Beuschel ‘98
Castaños ‘87
Rompf ‘97
Note: t s … (μ) (μ)
Odd-A Case: 163 Dy
0
0.2
0.4
0.6
0.8
1
Energy Spectrum 163 Dy
Energy [Mev]
exp. theo. exp. theo. exp. theo.
Ground StateBand
7/2-
9/2-
11/2-
13/2-
15/2-
5/2-
17/2-
1/2-
5/2-
3/2-
7/2-
1/2-
3/2-
7/2-
5/2-
5/2-
3/2-
5/2-
3/2-
7/2-
9/2-
11/2-
13/2-
15/2-
5/2-
17/2-
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
1.50 2.00 2.50 3.00 3.50 4.00 4.50
M1 transition strength 163 Dy
TheoryExperiment
Energy [MeV]
B(M1, 0
+
-> 1+) [μ
n2]
Recent Results: Beuschel/Hirsch
Hamiltonian: Realistic (Version 2)
H = H0 Oscillator
… mixed irrep shell-model basis with ~5 proton and ~5 neutron irreps & all products having S = 0 and J 8
SymmetryPreserving
SymmetryBreaking
+ dπ l2π Proton single-particle l2
+ dν l2ν Neutron single-particle l2
+ gπ Hpπ Proton Pairing
+ gν HpνNeutron Pairing
- (2 Q • Q Q•Q symmetric
+ a3 C3 Q•(QxQ) asymmetric
+ b K2 K-Band splitting
+ c L2Rotational bands
+ asym C2 Intrinsic symmetry
FIXED
FINETUNE
Nucleus Gπ Gν HO a b a asym16Gd 0.00 0.1 0.109 .616 0.00 0.16 9. 10- 0.01Gd 0.006 0.109 0.10 .9 0.001 0.10 1.010- 0.10-
160Gd 0.00 0.106 0.106 .2 0.002 0.10 1.10- 1.910-
160Dy 0.00 0.106 0.106 .2 0.00 0.000 1.10- 0.10-
Hamiltonia n ParametersHamiltonian Parameters
FIXED FINE TUNE
- (2 Q • Q
+ dπ l2π
+ dν l2ν
+ gπ Hpπ
+ gν Hpν
+ a3 C3
+ b K2
+ c L2
+ asym C2
-0.5
0
0.5
1
1.5
2
0+
8+
6+
4+
2+
1+
0+
2+
4+
2+
4+
5+
3+
6+
Exp Th Exp Th Exp Th Exp Th
158 Gd
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
2.00 2.50 3.00 3.50 4.00 [ ]Energy MeV
1 Gd
158Gd
160Gd
-0.5
0
0.5
1
1.5
2
0+
8+
6+
4+
2+
2+
4+
6+
8+
0+
2+
4+7+
5+
3+
1+
Exp Th Exp Th Exp Th
Ene
rgy
[MeV
]
160Gd
0
0.2
0.4
0.6
0.8
1
2.0 2.5 3.0 3.5 4.0
ExpTh
[ ]Energy MeV
160 Gd
Some Observations
• Fragmentation can be interpreted as effect of non-collective residual interactions
• Gross structure reproduced by adding a new “twist” to the “scissors” system
• Triaxial proton-neutron configuration gives a physical interpretation of multiplicity
• Analytic results for scissors as well as twist modes and combination now available
• Future research with S=1 mode for odd-A as well as even-even nuclei underway
Scissors
Twist
• Scissors mode (proton-neutron) oscillation always exists in deformed systems
• Extra “twist” mode if triaxial proton or neutron distributions are included
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Conclusions - Part 1: E2 / M1 modes ...
EEXXCCIITTIINNGG(simple)(simple)
PPHHYYSSIICCSS!!
EEXXCCIITTIINNGG(simple)(simple)
PPHHYYSSIICCSS!!
• Scissors mode always present …
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• Twist mode(s) requires triaxiality …• Fragmentation via symmetry mixing ...
• B(E2) values good (effective charge) …• Near perfect excitation spectra …
Mixed-Mode SMC … a new shell-model code (SMC) that integrates the best shell
model methods available:m-scheme spherical shell-model
SU(3) symmetry based shell-model
----- Developers -----Vesselin Gueorguiev
Jerry DraayerErich Ormand Calvin Johnson
(H - Eg)0
The Challenge ...Nuclei display uniqueand distinguishable characteristics:
• Single-particle Featues• Pairing Correlations• Deformation/Rotations
ClosedShell
ClosedShell
Single
Single
RotationsSU(3)
PairingSUQ(2)
PairingSUQ(2)
.
Usual Shell-Model Approach• Select a model space based on a simple approximation (e.g.
Nilsson scheme) for the selection of basis states.
• Diagonalize a realistic Hamiltonian in the model space to obtain the the energy spectrum and eigenstates.
• Evaluate electromagnetic transition strengths (E2, M1, etc.) and compare the results with experimental data.
Dual Basis “Mixed Mode” Scheme• Select a dual (non-orthonormal) basis (e.g. pairing plus
quadrupole scheme) for the interaction Hamiltonian.
• Diagonalize a realistic Hamiltonian in the model space to obtain the the energy spectrum and eigenstates.
• Evaluate electromagnetic transition strengths (E2, M1, etc.) and compare the results with experimental data.
Hamiltonian Matrix
SU(3)
standard jj-couplingshell-model scheme
single-particle___________
pairing driven
o v e r l a p
overlap
Shell-Model Hamiltonian
€
H = εiai+ai
i
∑ + Vijkli, j,k,l
∑ ai+aj
+akal
where ai+ and ai are fermion creation and annihilation operators,
εi and Vijkl are real and Vijkl =Vklij =−Vjikl =−Vijlk
Spherical shell-model basis states are eigenstates of the one-body part of the Hamiltonian - single-particle states.
The two-body part of the Hamiltonian H is dominated by the quadrupole-quadrupole interaction Q·Q ~ C2 of SU(3).
SU(3) basis states - collective states - are eigenstates of H for degenerate single particle energies and a pure Q·Q interaction.
Eigenvalue Problem in an Oblique Basis
Spherical basis states ei SU(3) basis states E
Overlap matrix
€
ˆ g =ei ej ei Eβ
Eα ej Eα Eβ
⎛
⎝ ⎜
⎞
⎠ ⎟ =
1 μ
μ+ 1
⎛
⎝ ⎜ ⎞
⎠ ⎟
€
Hψ =Eψ ⇒ ˆ H ⋅ ˆ ψ =E ˆ g ⋅ ˆ ψ
The eigenvalue problem
Current Evaluation Steps
Matrix elements
( H and g )
i and <j1j2J'T'|V|j3j4J''T''>
m-scheme
SM basis(spherical)
SU(3) basis(cylindrical)
H and gg=UUT (Cholesky)
(U-1)THU-1 (U)=E (U)Eigenstates (Lanczos)
Expectation values and matrix elements of
operators<O> and <E1| O |E2>
O and g |E>
We use the Wildenthal USD interaction and denote the spherical basis by SM(#) where # is the number of nucleons outside the d5/2 shell, the SU(3) basis consists of the leading irrep (8,4) and the next to the leading irrep, (9,2).
Example of an Oblique Basis Calculation: 24Mg
Visualizing the SU(3) space with respect to the SM space using the naturally induced basis in the SU(3) space.
SM(4) & SU3+
SU(3) basis space
SM spaceSM space
SU(3) basis space
SM(2) & SU3+
10064.179.921.570.101.750.450.08%
285031829028294492950012823Dimension(m-scheme)
FullSM(4)SM(2)SM(1)SM(0)GT100SU3+
(8,4) & (9,2)
SU3
(8,4)
Model Space
Ground State Convergence for 24Mg
-94
-91
-88
-85
-82
-79
-76
-2000 4000 10000 16000 22000 28000
Number of Basis States
Ground State Energy (MeV)
SM ground state
SM+1 SU(3) irrep
SM+2 SU(3) irreps
SM(0)
SM(1)
SM(2)
SM(3)
SM(4)SM(5) SM(6)
FULL
3.3 MeV (0.5%) 4.2 MeV(54%)
Better Dimensional Convergence!
-95
-90
-85
-80
-75
-70En
erg
y (
MeV
)
SM(0)
(8,4)
(8,4)(9,2)More
irreps
FULLSM(4)
SM(2)
SM(1)
0
2
2434
Level Structure
-95
-90
-85
-80
-75
-70
Energy (MeV)
SM(0)(8,4)
SM(1)(8,4)
SM(0)(8,4)(9,2)
SM(1)(8,4)(9,2)
SM(2)(8,4)
SM(4)(8,4)
SM(4)
SM(4)(8,4)(9,2)
FULL
SM(2)(8,4)(9,2)
0
2
24
43
Oblique Basis Spectral Results
Oblique basis calculations have the levels in the correct order!
• Spherical basis needs 64.2% of the total space - SM(4)!• SU(3) basis needs 0.4% of the total space - (8,4)&(9,2)!• Oblique basis 1.6% of the total space for SM(1)+(8,4)! • SM(0)+(8,4) is almost right… 0.2%
Overlaps With The Exact Eigenvectors For 24Mg
0
20
40
60
80
100
120
Eigenvectors
Mai
n C
ontr
ibu
tion
%
SM(2) 57.77 53.02 39.78 42.50 42.99 35.92
SU(3)+ 63.02 63.77 71.49 59.46 70.15 54.14
SM(2)+SU(3)+ 91.58 90.95 87.72 89.06 87.35 82.23
SM(4) 93.25 92.81 89.98 92.47 91.10 88.33
SM(4)+SU(3)+ 98.57 98.73 97.92 98.41 98.55 96.59
1 2 3 4 5 6
4 35 45 3 8 4
Oblique basis calculations have better overlap with the exact states!
• Using 10% of the total space in oblique basis SM(2)+(8,4)+(9,2)
as good as
• using 64% of the total space in spherical basis SM(4)
Mixed mode (oblique basis) calculations lead to:
better dimensional convergence
correct level order of the low-lying states
significant overlap with the exact states
Conclusions - Part 2: Mixed modes ...
Further Considerations #1
Traditional configuration shell-model scheme(s) versus
SU(3) and symplectic shell-model approaches
Common “bit arithmetic” (Slater determinants) Both use harmonic oscillator basis functions … Translation invariance okay for both approaches Both use effective reduced matrix element logic One single-particle the other quadrupole driven
Complementary … ???
Further Considerations #2
4h 0h2h
ConfigurationShell-Model (multi-h
horizontalslices
SymplecticShell-Model (multi-h
verticalslices
6hsymplectic
slice
translation invarianceokay for both
SU(3) limit (0h
Further Considerations #3… mixed-mode shell-model …
Q Space
Q SpaceP Space
xhyh
postulate: x(~2) + y(~12) = ~12
… sy
mplectic
…
Abstract: … consider standard as well as novel algebraic approaches to nuclear structure, including use of the Bethe ansatz and quantum groups, that are being used to explore special features of nuclei: pairing correlations, quadrupole collectivity, scissors modes, etc. In each case the underlying physics is linked to symmetries of the system and their group theoretical representation.
• E2 / M1 (scissors) modes in deformed nuclei• Mixed-mode shell-model methods feasible• Possible configuration + symplectic extension
?