\begin{tikzpicture}
% lambda = 1/4
  \begin{axis}[
    width=1.8in,
    height=2.5in,
    axis lines=center,
    xlabel=\empty,
    ylabel=\empty,
    % xticklabel=\empty,
    % yticklabel=\empty,
    ylabel style={above right},
    xlabel style={below right},
    xtick={0,-3,3},
    ytick={0,-11,11},        
    xmin=-3.7,xmax=3.7,ymin=-12,ymax=12]
    % x equation
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(6):sqrt(6), variable=\t, line width=1.3, color = cb-green]
    ({t}, {(2*sqrt(6-t^2)/sqrt(3)});
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(6):sqrt(6), variable=\t, line width=1.3, color = cb-green]
    ({t}, {-(2*sqrt(6-t^2)/sqrt(3)});
    \draw[cb-green, line width = 1.3] (0,-11.8) -- (0, 11.6);

    % y equation
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(8):sqrt(8), variable=\t, line width=1.3, color = cb-purple]
    ({t}, {2*sqrt(3)*sqrt(8 - t^2)});
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(8):sqrt(8), variable=\t, line width=1.3, color = cb-purple]
    ({t}, {-2*sqrt(3)*sqrt(8 - t^2)});
    \draw[cb-purple, line width = 1.3] (-3.6,0) -- (3.4,0);

    % highlight intersections
    \addplot [only marks, mark=x, line width = 1.3, mark size = 3pt] table {
    0  0
    % 2.44949  0
    % -2.44949  0
    0  9.79796
    0  -9.79796
    };
    \addplot [only marks, mark=square*, mark size = 2pt] table {
    2.44949  0
    -2.44949  0
    };

    \node at (2.3,-10.5) [rectangle,draw,fill = cb-palegrey,inner sep=0.8pt] {$\lambda = \frac{1}{4}$};
    \end{axis}
\end{tikzpicture}
\,
\begin{tikzpicture}
% lambda = 1/2
  \begin{axis}[
    width=1.8in,
    height=2.5in,
    axis lines=center,
    xlabel=\empty,
    ylabel=\empty,
    % xticklabel=\empty,
    % yticklabel=\empty,
    ylabel style={above right},
    xlabel style={below right},
    xtick={0,-3,3},
    ytick={0,-6,6},        
    xmin=-3.7,xmax=3.7,ymin=-7,ymax=7]
    % x equation
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(6):sqrt(6), variable=\t, line width=1.3, color = cb-green]
    ({t}, {sqrt(2/3)*sqrt(6 - t^2)});
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(6):sqrt(6), variable=\t, line width=1.3, color = cb-green]
    ({t}, {-(sqrt(6-t^2)*sqrt(2/3)});
    \draw[cb-green, line width = 1.3] (0,-6.9) -- (0, 6.7);

    % y equation
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(4):sqrt(4), variable=\t, line width=1.3, color = cb-purple]
    ({t}, {sqrt(6*(4 - t^2))});
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(4):sqrt(4), variable=\t, line width=1.3, color = cb-purple]
    ({t}, {-sqrt(6*(4 - t^2))});
    \draw[cb-purple, line width = 1.3] (-3.6,0) -- (3.4,0);

    % highlight intersections
    \addplot [only marks, mark=x, line width = 1.3, mark size = 3pt] table {
    0  0
    2.44949  0
    -2.44949  0
    0  4.89898
    0  -4.89898
    };
    \addplot [only marks, mark=square*, mark size = 2pt] table {
    1.93649  1.22474
    -1.93649  1.22474
    1.93649  -1.22474
    -1.93649  -1.22474
    };

    \node at (2.3,-5.92) [rectangle,draw,fill = cb-palegrey,inner sep=0.8pt] {$\lambda = \frac{1}{2}$};
    \end{axis}
\end{tikzpicture}
\,
\begin{tikzpicture}
% lambda = 4
  \begin{axis}[
    width=1.8in,
    height=2.5in,
    axis lines=center,
    xlabel=\empty,
    ylabel=\empty,
    % xticklabel=\empty,
    % yticklabel=\empty,
    ylabel style={above right},
    xlabel style={below right},
    xtick={0,-3,3},
    ytick={0,-1,1},        
    xmin=-3.7,xmax=3.7,ymin=-1.5,ymax=1.5]
    % x equation
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(6):sqrt(6), variable=\t, line width=1.3, color = cb-green]
    ({t}, {sqrt(6 - t^2)/sqrt(12)});
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(6):sqrt(6), variable=\t, line width=1.3, color = cb-green]
    ({t}, {-(sqrt(6-t^2)/sqrt(12)});
    \draw[cb-green, line width = 1.3] (0,-1.48) -- (0, 1.44);

    % y equation
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(1/2):sqrt(1/2), variable=\t, line width=1.3, color = cb-purple]
    ({t}, {1/2*sqrt(3/2)*sqrt(1 - 2*t^2)});
    \addplot+[no markers, samples=200, samples y=0, domain=-sqrt(1/2):sqrt(1/2), variable=\t, line width=1.3, color = cb-purple]
    ({t}, {-1/2*sqrt(3/2)*sqrt(1 - 2*t^2)});
    \draw[cb-purple, line width = 1.3] (-3.6,0) -- (3.4,0);
    
    % highlight intersections
    \addplot [only marks, mark=x, line width = 1.3, mark size = 3pt] table {
    0  0
    2.44949  0
    -2.44949  0
    };
    \addplot [only marks, mark=square*, mark size = 2pt] table {
    0  0.612372
    0  -0.612372
    };

    \node at (2.3,-1.28) [rectangle, draw, fill = cb-palegrey, inner sep=1.5pt] {$\lambda = 4$};
    \end{axis}
\end{tikzpicture}

    