Y tick scientific notation only

Question

So I tried to follow this solution on this link.

Code:

\documentclass[12pt]{article}
\usepackage{tikz}
\usepackage{float}
\usepackage{pgfplots}
\begin{document}
    \begin{figure}[H]\centering
        \begin{tikzpicture}
            \begin{axis}[%
            width=1.952in,
            height=1.493in,
            at={(3.5in,2.6in)},
            scale only axis,
            xmin=0,
            xmax=3.5,
            xlabel style={font=\color{white!15!black}},
            xlabel={$t$},
            ymin=-5000,
            ymax=500,
            ylabel style={font=\color{white!15!black}},
            ylabel={$u$},
            y tick label style={scaled ticks=base 10:-3},
            axis background/.style={fill=white}
            ]
                \addplot [color=blue,domain = 0:3.5]{-4000*x/3};
            \end{axis}
        \end{tikzpicture}
        \caption{Sample plot.}
    \end{figure}
\end{document}

Output:

While I get the y axis as desired, the x axis gets labeled as scientific notation when I don't want to. How do I only set the y axis tick marks to be displayed in scientific notation only, but not the x axis? I want to disply the scientific notation such that the exponent and mantissa is on top of the y-axis.