\begin{tikzpicture}[scale=1]
  \draw [thick, black, ->] (-2,0) -- (4, 0);
  \draw [thick, red, ->] (0,0) -- (0.866,0.5)
  node [above, black] {$u_1$};
  \draw [thick, black, ->] (0,-0.1) -- (0, 3);
  \draw [thick, red, ->] (0,0) -- (-0.4,0.6928);
  \draw (-0.6, 0.75)
  node [above] {$u_2$};
  \draw [thick, red, ->] (0,0) -- (1,1.732);
  \draw (1.5,2)
  node [above] {$
      z = \left[ \begin{array}{c}
          a \\
          b
        \end{array} \right]
    $};
  \draw [dotted, black] (0.866,0.5) -- (1.5,0.866);
  \draw [dotted, black] (-0.25,0.433) -- (-0.5, 0.866);
  \draw [dotted, black] (1,1.732) -- (1.5,0.866);
  \draw [dotted, black] (-0.5, 0.866) -- (1,1.732);
  \draw (0.25,.144) arc (30:60:.32cm);
  \draw (0.35,.144)
  node [above, black] {$\theta$};
  \node [rotate=60] at (0.3,1.25) {$ |\overset{\rightarrow}{z}| $};
  \node [rotate=30] at (1,0.166) {$\alpha_1$};
  \node [rotate=120] at (-0.4, 0.066) {$\alpha_2$};
  \draw [decorate, decoration={brace, raise=3pt, mirror}] (0,0) -- (1.5, 0.866);
  \draw [decorate, decoration={brace, raise=3pt}] (0,0) -- (-0.5, 0.866);
\end{tikzpicture}
