Paper 4, Section I, D

Methods
Part IB, 2014

Consider the ordinary differential equation

d2ψdz2[15k24(kz+1)23kδ(z)]ψ=0\frac{d^{2} \psi}{d z^{2}}-\left[\frac{15 k^{2}}{4(k|z|+1)^{2}}-3 k \delta(z)\right] \psi=0

where kk is a positive constant and δ\delta denotes the Dirac delta function. Physically relevant solutions for ψ\psi are bounded over the entire range zRz \in \mathbb{R}.

(i) Find piecewise bounded solutions to this differential equations in the ranges z>0z>0 and z<0z<0, respectively. [Hint: The equation d2ydx2cx2y=0\frac{d^{2} y}{d x^{2}}-\frac{c}{x^{2}} y=0 for a constant cc may be solved using the Ansatz y=xαy=x^{\alpha}.]

(ii) Derive a matching condition at z=0z=0 by integrating ( \dagger ) over the interval (ϵ,ϵ)(-\epsilon, \epsilon) with ϵ0\epsilon \rightarrow 0 and use this condition together with the requirement that ψ\psi be continuous at z=0z=0 to determine the solution over the entire range zRz \in \mathbb{R}.