\documentclass{standalone}
\input{palette}
\usepackage{pgfplots}
\pgfplotsset{compat=default}
% With this compatability setting, pgfplots runs in backwards compatability mode using pgfplots version pre-1.3. It's safe to use a more current version, but axis and tick labels may move and need to be repositioned.

%\usepackage{txfonts}
%\usepackage[mathscr]{euscript}
% The above font packages were used to generate figures for the article, but are here commented out to ensure better compability.

\usetikzlibrary{pgfplots.groupplots}

\newcommand{\mytick}[2]{\draw[thick,#2] (#1, 0.01) -- (#1, -0.15);}

\begin{document}
\begin{tikzpicture}
\def\xclas{1.4966}
\begin{groupplot}[
    group style={
        group name=my plots,
        group size=1 by 2,
        xlabels at=edge bottom,
        xticklabels at=edge bottom,
        vertical sep=0pt
    },
    width=0.8\textwidth,
    height=0.4\textwidth,
    xlabel=$h_x/J$,
    xmin=0,
    xmax=2.6,
    ymin=0,
    xticklabel style = {/pgf/number format/fixed, /pgf/number format/zerofill, /pgf/number format/precision=1},
    xtick = {0.0, 0.2, 0.4, ..., 2.2},
    yticklabel style = {/pgf/number format/fixed, /pgf/number format/zerofill, /pgf/number format/precision=1},
    tickpos=left,
    ytick align=inside,
    xtick align=outside,
    ylabel style = {yshift=-0.3cm} % <--- this is yshift which makes no sense!
]


\nextgroupplot[ylabel=$m^x$, ytick = {0.0, 0.1, 0.2, ..., 0.6},legend style = {anchor = south east,at={(axis description cs: 1, 0)}},
title = {(a)}, title style = {at = {(axis description cs:0.05,0.75)}}]
\addplot[virmiddle, mark=*, dashed, mark options={solid}] table [x=<h>, y=<Mx>, col sep=space, y expr = \thisrowno{2}*(-1)] {DMRG_Data/Magnetization_Delta012_h.dat};

\nextgroupplot[legend style = {anchor = south east,at={(axis description cs: 1, 0)}}, ylabel=$\mathrm{S}_\mathrm{vN}/\ln 2$, ytick={0,0.5, 1.0, ...,1.5},
ymax=2.0,
title = {(b)}, title style = {at = {(axis description cs:0.05,0.75)}}, xlabel style = {at = {(axis description cs: 0.96,0.13)}}]
\addplot[virmiddle, mark=*, dashed, mark options={solid}] table [x=<h>, y=<SvN>, y expr=\thisrowno{2}/ln(2), col sep=space] {EntanglementData/EntanglementEntropy_Delta012_L100_h.dat};
\addplot [domain=\xclas:3.5, virdark]{ {ln(2)/ln(2))} };
\addplot +[mark=none,virdark] coordinates {(\xclas, 0) (\xclas, 2)};
\end{groupplot}
\end{tikzpicture}

\end{document}