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Hello everyone,

I have this weirdo potential for homework

\begin{equation}

U(x) = \frac{U_1}{ \left( 1+e^{x/a}\right)^2 } - \frac{U_2}{ \left( 1+e^{x/a}\right)}

\end{equation}

where U

and I need to find the energies for the bound states and also the wave functions for them. I got stuck on one part for like 2 or 3 hours, tried tons of ways, but still nothing. So I'll post the most relevant way by far and I'm hoping to get some help from you. By the way, If anyone knows the name of this potential or some kind of a book (I prefer English, Russian or German, but any other language would do :D ) where I can get some help, please tell me. I looked up in Landau, Flugge and Greiner but couldnt find anything that could help...

So here it goes:

The Schrodinger equation looks like this:

\begin{equation}

-\frac{\hbar^2}{2m}\frac{d^2\Psi(x)}{dx^2} + \left(\frac{U_1}{ \left( 1+e^{x/a}\right)^2 } - \frac{U_2}{ \left( 1+e^{x/a}\right)}\right)\Psi(x)=E\Psi(x),

\end{equation}

For the Bound states we have E=-|E|. Using these following notations:

\begin{equation}w=1+e^{x/a},\end{equation}

\begin{equation}\frac{2ma^2U_1}{\hbar^2} = \alpha^2, \end{equation}

\begin{equation} \frac{2ma^2U_2}{\hbar^2} = \beta^2, \end{equation}

and

\begin{equation} \frac{2ma^2|E|}{\hbar^2} = k^2\end{equation}

I get

\begin{equation}

(w-1)^2 \Psi''(w) + (w-1) \Psi'(w) -\left(\frac{\alpha^2}{w^2} - \frac{\beta^2}{w} + k^2 \right) \Psi(w)

\end{equation}

and \begin{equation} 1<w< \infty \end{equation}.

When \begin{equation} w \to \infty \end{equation}, then the equation is

\begin{equation}

(w-1)^2 \Psi''(w) + (w-1) \Psi'(w) - k^2 \Psi(w)=0

\end{equation}

This is Eulers differential equation and the solution is \begin{equation} \Psi(w) = (w-1)^{-k} \end{equation}.

So here is the problem... When

\begin{equation} w \to 1 \end{equation}

then I can clearly say that

\begin{equation}

\frac{\alpha^2}{w^2} - \frac{\beta^2}{w} = \alpha^2 - \beta^2

\end{equation},

but I have no idea what should I do about the first and the second derivative. Any ideas?

I have this weirdo potential for homework

\begin{equation}

U(x) = \frac{U_1}{ \left( 1+e^{x/a}\right)^2 } - \frac{U_2}{ \left( 1+e^{x/a}\right)}

\end{equation}

where U

_{1},U_{2}and "a" are positiveand I need to find the energies for the bound states and also the wave functions for them. I got stuck on one part for like 2 or 3 hours, tried tons of ways, but still nothing. So I'll post the most relevant way by far and I'm hoping to get some help from you. By the way, If anyone knows the name of this potential or some kind of a book (I prefer English, Russian or German, but any other language would do :D ) where I can get some help, please tell me. I looked up in Landau, Flugge and Greiner but couldnt find anything that could help...

So here it goes:

The Schrodinger equation looks like this:

\begin{equation}

-\frac{\hbar^2}{2m}\frac{d^2\Psi(x)}{dx^2} + \left(\frac{U_1}{ \left( 1+e^{x/a}\right)^2 } - \frac{U_2}{ \left( 1+e^{x/a}\right)}\right)\Psi(x)=E\Psi(x),

\end{equation}

For the Bound states we have E=-|E|. Using these following notations:

\begin{equation}w=1+e^{x/a},\end{equation}

\begin{equation}\frac{2ma^2U_1}{\hbar^2} = \alpha^2, \end{equation}

\begin{equation} \frac{2ma^2U_2}{\hbar^2} = \beta^2, \end{equation}

and

\begin{equation} \frac{2ma^2|E|}{\hbar^2} = k^2\end{equation}

I get

\begin{equation}

(w-1)^2 \Psi''(w) + (w-1) \Psi'(w) -\left(\frac{\alpha^2}{w^2} - \frac{\beta^2}{w} + k^2 \right) \Psi(w)

\end{equation}

and \begin{equation} 1<w< \infty \end{equation}.

When \begin{equation} w \to \infty \end{equation}, then the equation is

\begin{equation}

(w-1)^2 \Psi''(w) + (w-1) \Psi'(w) - k^2 \Psi(w)=0

\end{equation}

This is Eulers differential equation and the solution is \begin{equation} \Psi(w) = (w-1)^{-k} \end{equation}.

So here is the problem... When

\begin{equation} w \to 1 \end{equation}

then I can clearly say that

\begin{equation}

\frac{\alpha^2}{w^2} - \frac{\beta^2}{w} = \alpha^2 - \beta^2

\end{equation},

but I have no idea what should I do about the first and the second derivative. Any ideas?

Last edited: