Alignat issue - extra space in one line

Question

My code looks like this:

\begin{alignat}{2}
\theta^2&\equiv\frac{2m a^2\epsilon}{\hbar^2},\ \lambda&\equiv&\theta^{\frac{2p}{p+1}}\frac{E}{\epsilon},\\
E&=\lambda\epsilon\theta^{-\frac{2p}{p+1}},\ u&=&\theta^{\frac{1}{p+1}}\frac{x}{a}
\end{alignat}

And the picture looks like this:

Why is there extra space after lambda, and not after u? The first pair of equations in upper and lower row look good, why is second ones making an issue?

Thank you for help.

Answer 1

You should position the ampersands slightly different.

\documentclass{article}
\usepackage{mathtools}  % loads »amsmath«

\begin{document}
  \begin{alignat}{2}
    \theta^2 &\equiv \frac{2m a^2\epsilon}{\hbar^2}, & \lambda &\equiv \theta^{\frac{2p}{p+1}}\frac{E}{\epsilon},\\
    E &= \lambda\epsilon\theta^{-\frac{2p}{p+1}},\quad & u &= \theta^{\frac{1}{p+1}}\frac{x}{a} 
  \end{alignat}
\end{document}