pgfplot Problem zwei x-Achsen benötigt mit vertikaler Verschiebung

\documentclass{article}
\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}
\begin{document}

\begin{tikzpicture}
  \begin{groupplot}[
    group style={
      group size=1 by 2,
      x descriptions at=edge bottom,
      y descriptions at=edge left,
      vertical sep=0pt,
    },
    width=8cm,
    height=4cm,
    domain=0:2*pi,
    no markers,
    ]

    \nextgroupplot
    \addplot {sin(deg(x))};

    \nextgroupplot
    \addplot {cos(deg(x))};

  \end{groupplot}
\end{tikzpicture}

\end{document}

\documentclass[margin=20mm]{standalone}
%\documentclass{article}
\usepackage[ngerman]{babel}

\usepackage{pgfplots}
    \pgfplotsset{compat=1.9}

\pgfkeys{/pgfplots/AxisStyle/.style={
width=13.5cm, height=5cm,
axis x line=center, 
axis y line=middle, 
samples=100,
ymin=-1, ymax=1.5,
xmin=0.0, xmax=7.0,
ytick={1},
%enlargelimits,
%enlarge x limits=0.05,
enlarge x limits={abs value=0.5,lower},
enlarge y limits=0.05,
axis line style = {-latex},
xlabel=$\varphi$[rad],xlabel style={anchor=north},
ylabel=$y$[\%],ylabel style={anchor=east},
}}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
AxisStyle,
xtick={1.5708, 3.14159, 4.7123889, 6.28318},
xticklabels={$\frac{\pi}{2}$, $\pi$, $\frac{3\pi}{2}$, $2\pi$}
]
\addplot [mark=none,thick, domain=0:2*pi] {sin(deg(x))^2};

% Zusatzachse
\draw[yshift=-0.75cm, -latex] (axis cs: 0,0) -- (axis cs: 7,0) node[below, xshift=-5pt]{$t$[s]};
%
\pgfplotsinvokeforeach{1.5708, 3.14159, 4.7123889, 6.28318}{
\draw [yshift=-0.75cm] (axis cs: #1, 0.07) -- (axis cs: #1,-0.07); 
}
%
\foreach \x/\text in {1.5708/$\frac{T}{4}$, 3.14159/$\frac{T}{2}$, 4.7123889/$\frac{3T}{4}$, 6.28318/$T$}{%
\edef\temp{%
\noexpand
\node[yshift=-1.1cm] at (axis cs:\x,-0.07) {\text};
}
\temp
}%
\end{axis}
\end{tikzpicture}

\end{document}