Functions

Debye functions

The Debye functions are given by

\[ D_n(x)= \frac{n}{x^n} \int_0^x \frac{t^n}{\text{e}^t-1} \, \text{d}x\]

And

x = range(0,25,1000)
plot(x,Debye_function.(1,x),label=L"D_1(x)")
plot!(x,Debye_function.(2,x),label=L"D_2(x)")
plot!(x,Debye_function.(3,x), label=L"D_3(x)")
title!("Debye Functions")
xlabel!(L"x")

Coulomb wave functions

The Coulomb wave equation for a charged particle with arbitrary angular momentum and charge is given by

\[ \nabla^2\psi +\left( k^2-\frac{2\mu}{\hbar^2}V(r)\right)\psi = 0,\]

where $\mu$ is the reduced mass of the system. The radial wave function $u(r)$ satisfies the following differential equation

\[ \frac{\text{d}^2 u_\ell}{\text{d}r^2}+\left( k^2-\frac{\ell(\ell+1)}{r^2}-\frac{2\mu}{\hbar^2}\frac{Ze^2}{r}\right)u_\ell=0,\]

where $Z$ is the product of the charges. Two independent solutions can be found to this equation – these are called the regular and irregular Coulomb wave functions denoted $F_\ell(r)$ and $G_\ell(r)$ respectively. The regular Coulomb wave function $F_\ell(r)$ is a real function that vanishes at $r=0$ and the behaviour of the function is described using a parameter $\eta$ which describes how strongly the Coulomb interaction is

\[ \eta = \frac{Zmc\alpha }{\hbar k},\]

where $m$ is the mass of the particle, $k$ is the wave number and $\alpha$ is the fine structure constant. The solution to is given by

\[ F_\ell(\eta,kr) = C_\ell (\eta) (kr)^{\ell+1}\text{e}^{-ikr} {}_1 F_1(\ell+1-i\eta,2\ell+2,2ikr),\]

where ${}_1F_1(kr)$ is a confluent hypergeometric function and $C_\ell(\eta)$ is a normalization constant given by

\[ C_\ell(\eta) = \frac{2^\ell \text{e}^{-\pi\eta/2}|\Gamma(\ell+1+i\eta)|}{(2\ell+1)!},\]

where $\Gamma$ is the gamma function. For numerical purposes, it is useful to use the integral representation of the regular Coulomb wave function

\[ F_\ell(\eta,\rho) = \frac{\rho^{\ell+1}2^\ell e^{i\rho-(\pi\eta/2)}}{|\Gamma(\ell+1+i\eta)|} \int_0^1 e^{-2i\rho t}t^{\ell+i\eta}(1-t)^{\ell-i\eta} \, \text{d}t.\]

This implementation need the gamma function from SpecialFunctions.jl

x = range(0,25,1000)
plot(x,regular_Coulomb.(0,0.3,x), label=L"F_0(0.3,ρ)")
plot!(x,regular_Coulomb.(0,-0.3,x), label=L"F_0(-0.3,ρ)")
xlabel!(L"ρ")
title!("Regular Coulomb Wave Functions")

Use a similar approach to plot the regular Coulomb functions for different a $\ell$

x = range(0,25,1000)
plot(x,regular_Coulomb.(1e-5,5,x), label=L"F_0(5,ρ)")
plot!(x,regular_Coulomb.(1,5,x), label=L"F_1(5,ρ)")
plot!(x,regular_Coulomb.(2,5,x), label=L"F_2(5,ρ)")
plot!(x,regular_Coulomb.(3,5,x), label=L"F_3(5,ρ)")
title!("Regular Coulomb Wave Functions")
xlabel!(L"ρ")

Struve functions

The Struve functions are solutions of the non-homogeneous Bessel's differential equation

\[ x^2 \frac{\text{d}^2 y}{\text{d}x^2} + x \frac{\text{d}y}{\text{d}x}+(x^2-\alpha^2)y = \frac{4(x/2)^{\alpha+1}}{\sqrt{\pi}\Gamma(\alpha+1/2)}\]

The Struve functions are implemented using the following integral representation

\[ \mathbf{H}_\nu(z) = \frac{2(z/2)^\nu}{\sqrt{\pi}\Gamma(\nu+1/2)} \int_0^1 (1-t)^{{\nu-1/2}}\sin(zt) \, \text{d}t\]

And

\[ \mathbf{H}_\nu(z) = \frac{2(z/2)^\nu}{\sqrt{\pi}\Gamma(\nu+1/2)} \int_0^{\pi/2} \sin(z\cos(\theta)) \sin^{2\nu}(\theta) \, \text{d}\theta\]

Here is an example

x = range(-5,5,1000)
plot(x,Struve.(0,x),label=L"H_0(x)")
plot!(x,Struve.(1,x),label=L"H_1(x)")
plot!(x,Struve.(2,x),label=L"H_2(x)")
plot!(x,Struve.(3,x),label=L"H_3(x)")
plot!(x,Struve.(4,x),label=L"H_4(x)")
plot!(x,Struve.(5,x),label=L"H_5(x)")
xlabel!(L"x")
title!("Struve Functions")

Fresnel functions

The Fresnel functions are both implemented using the trigonometric functions and the error function.

\[ S(z) = \sqrt{\frac{\pi}{2}}\frac{1+i}{4} \bigg( \text{erf}\big(\frac{1+i}{\sqrt{2}}z \big) - i \text{erf}\big(\frac{1-i}{\sqrt{2}}z \big)\bigg)\]

And

\[ C(z) = \sqrt{\frac{\pi}{2}}\frac{1-i}{4} \bigg( \text{erf}\big(\frac{1+i}{\sqrt{2}}z \big) + i \text{erf}\big(\frac{1-i}{\sqrt{2}}z \big)\bigg)\]

The two implementations are shown in the examples below

x = range(-25,25,5000)
plot(x,Fresnel_C_integral.(x),label=L"C(x)")
plot!(x,Fresnel_C_erf.(x), ls=:dash, lw=1.5, label=L"\tilde{C}(x)")
title!("Fresnel Integral")
xlabel!(L"x")

and

x = range(-25,25,5000)
plot(x,Fresnel_S_integral.(x),label=L"S(x)")
plot!(x,Fresnel_S_erf.(x), ls=:dash, lw=1.5, label=L"\tilde{S}(x)")
title!("Fresnel Integral")
xlabel!(L"x")

Hypergeometric functions

The confluent hypergeometric functions are solutions of Kummer’s equation

\[ z \frac{d^2 F}{dz^2} +(b-z)\frac{dF}{dz}-a F = 0.\]

Kummer's equations has two linearly independent solutions given by

\[ {}_1 F_{1}(a;b;z) \]

and

\[ z^{1-b}{}_1F_1(a-b+1;2-b;z).\]

The numerical implementation in based on the following series expansion

\[ {}_1 F_1 (a,b,z) = 1+\frac{a}{b}z+\frac{a(a+1)}{b(b+1)2!}z^2 + \frac{a(a+1)(a+2)}{b(b+1)b(b+2)3!}z^3 + \cdots\]

Using this expansion the terms can be computed as

\[ t(n) = \frac{(a+n-1)z}{(b+n-1)n}t(n-1),\]

and a cumulative product is produced. The errors are quite small; approximately $10^{-12}-10^{-13}$. The figure below shows four examples of ${}_1F_1$.

x = range(0,3.5,100)
plot(x,confluent_hypergeometric_1F1.(2,0.5,x),label=L"{}_1F_1(2,0.5,x)")
plot!(x,confluent_hypergeometric_1F1.(3,0.7,x),label=L"{}_1F_1(3,0.7,x)")
plot!(x,confluent_hypergeometric_1F1.(2,1.1,x),label=L"{}_1F_1(2,1.1,x)")
plot!(x,confluent_hypergeometric_1F1.(3,1.3,x),label=L"{}_1F_1(3,1.3,x)")
title!("Confluent hypergeometric function")
xlabel!(L"x")

Fermi-Dirac integrals

In solid state physics the Fermi-Dirac integral is given by

\[ F_j(x) = \int_0^\infty \frac{t^j}{\exp(t-x)+1} \, dt.\]

Approximations to this and the normalized case for $j=-1/2$, $j=1/2$, $j=3/2$ and $j=5/2$ are implemented to varying accuacy. Most are of the order of $10^{-12}$.

Here is an example

x = range(0,100,10000)
plot(x,FermiDiracIntegralNorm.(-1/2,x),label=L"F_{-1/2}(x)")
plot!(x,FermiDiracIntegralNorm.(1/2,x),label=L"F_{1/2}(x)")
plot!(x,FermiDiracIntegralNorm.(3/2,x),label=L"F_{3/2}(x)")
plot!(x,FermiDiracIntegralNorm.(5/2,x),label=L"F_{5/2}(x)")
title!("Fermi-Dirac Integral")
xlabel!(L"x")