Diploma thesis - Fachbereich Physik

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40 CHAPTER 3. RECURSION RELATIONS 3.2 Ground-State Energy without External Current For the potential the time-independent Schrödinger equation reads − ¯h2 2M ψ′′ (x) + V (x) = M 2 ω2 x 2 + gAx 3 + g 2 Bx 4 , (3.4) ( M 2 ω2 x 2 + gAx 3 + g 2 Bx 4 ) ψ(x) = E ψ(x) . (3.5) Inserting the ansatz (3.2) into the Schrödinger equation (3.5) yields an ordinary differential equation for the correction φ(x): − ¯h2 2M φ′′ (x) + ¯hω x φ ′ (x) − ¯h2 2M [φ′ (x)] 2 + gAx 3 + g 2 Bx 4 = ǫ , (3.6) where ǫ denotes the correction to the ground-state energy E = ¯hω 2 + ǫ . (3.7) Expanding the ground-state energy of the oscillator in powers of the coupling constant g, E = ¯hω ∞ 2 + ∑ g k ǫ k , (3.8) k=1 inserting the expansion (3.3) into (3.6), and performing a coefficients comparison, one obtains a differential equation for the φ k (x): − ¯h2 2M φ′′ k (x) + ¯hω x ¯h2 φ′ k (x) − 2M ∑k−1 φ ′ k−l (x) φ′ l (x) + δ k,1Ax 3 + δ k,2 Bx 4 = ǫ k . (3.9) l=1 Assuming that φ k (x) is a polynomial, one can show by induction that its degree cannot be greater than k + 2. Thus, the φ k (x) can be written in the form ∑k+2 φ k (x) = m=1 c (k) m xm . (3.10) Here, the constant terms c (k) 0 have been omitted as they can only be determined later by normalization of the wave function ψ(x). By inserting (3.10) into (3.9) for k = 1, one finds c (1) 1 = − A Mω 2 , For k = 2, the solution reads c (2) 1 = 0 , c (2) 2 = 7A2 8M 2 ω − 3B 4 4Mω , 2 c(1) 2 = 0 , c (1) 3 = − A 3¯hω , and ǫ 1 = 0 . (3.11) c(2) 3 = 0 , c (2) 4 = A2 8M¯hω − B 3 4¯hω , (3.12) and ǫ 2 = − 11A2¯h 2 8M 3 ω 4 + 3B¯h2 4M 2 ω 2 . (3.13)
3.2. GROUND-STATE ENERGY WITHOUT EXTERNAL CURRENT 41 For the general case k ≥ 3, it is helpful to rewrite (3.10) in the form ∞∑ φ k (x) = c (k) m xm , with c (k) m ≡ 0 for m > k + 2 , (3.14) m=1 as this allows the application of the Cauchy product rule to the product of the derivatives φ ′ k−l (x) and φ′ l (x) in (3.9). The recursively determinable solution for the φ k(x) and the energy corrections ǫ k is then obtained as c (k) m ǫ k = (m + 2)(m + 1)¯h 2mMω = −¯h2 M c(k) 2 − ¯h2 2M c (k) ¯h m+2 + 2mMω ∑k−1 l=1 ∑ ∑ k−1 m+1 l=1 n=1 with c (k) m n(m + 2 − n) c (l) n c (k−l) m+2−n , ≡ 0 for m > k + 2 , (3.15) c (l) 1 c(k−l) 1 . (3.16) Applying this result leads to the expansion coefficients and the energy correction in the third order of the coupling constant: c (3) 1 = − 5A3¯h M 4 ω 7 + 6AB¯h M 3 ω 5 , And for the fourth order one obtains c(3) 2 = 0 , c (3) 3 = − 13A3 12M 3 ω 6 + 3AB 2M 2 ω 4 , c (3) 4 = 0 , c (3) 5 = − A3 10M 2¯hω 5 + c (4) 1 = 0 , c (4) 2 = 305A4¯h 32M 5 ω − 123A2 B¯h + 21B2¯h 9 8M 4 ω 7 8M 3 ω , 5 c (4) 4 = 99A4 64M 4 ω 8 − 47A2 B 16Mω 6 + 11B2 16M 2 ω 4 , c (4) 6 = 5A 4 48M 3¯hω 7 − AB 5M¯hω 3 , (3.17) and ǫ 3 = 0 . (3.18) c(4) 3 = 0 , c(4) 5 = 0 , A2 B 4M 2¯hω 5 + B 2 12M¯hω 3 , (3.19) and ǫ 4 = − 465A4¯h 3 32M 6 ω 9 + 171A2 B¯h 3 8M 5 ω 7 − 21B2¯h 3 8M 4 ω 5 . (3.20) Comparing (3.13), (3.18), and (3.20) with (2.188), one sees that the result obtained by Bender-Wu recursion is indeed identical to the one obtained by explicitly evaluating Feynman diagrams. Since one wants to drive the expansion to higher orders, it is important to optimize the recursion formula with regard to its evaluation by a computer. To this end, it is helpful to introduce natural units, in which one has ¯h = 1, M = 1. Furthermore, since calculations on a computer can be performed more effectively when dealing with mere rational numbers, which are not afflicted with parameters like A, B, or ω, it is desirable to expand the coefficients and the energy corrections ǫ k in products of powers of these parameters. Dimensional considerations lead to the following approaches for the coefficients c (k) c (k) m m : ⌊k/2⌋ c (k) m = ∑ A k−2λ B λ c(k) ω5k/2−m/2−2λ m,λ , λ=0 with c(k) m,λ ≡ 0 for m > k + 2 , (3.21)
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Appendix A Harmonic Generating Func
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93 Evaluating the x 0 -, Re x m -,
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95 and that the eigenvalues of G
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98 APPENDIX B. STANDARD INTEGRALS R
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100 APPENDIX B. STANDARD INTEGRALS
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Appendix C Shooting Method In this
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x → −∞, or vice-versa. Thus,
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List of Figures 4.1 Schematic compa
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Bibliography [1] Physik Journal 3 (
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BIBLIOGRAPHY 115 [29] W. Janke and
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Diploma thesis - Fachbereich Physik

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