\hbar not displayed with fontspec on XeLaTeX/LuaLaTeX

Question

With \usepackage{fontspec}, \hbar does not show up correctly. Tested on LaTeX2e <2018-04-01> patch level 5 and fontspec.sty 2018/07/30 vv2.6h.

\documentclass{article}
\usepackage{fontspec}
\begin{document}
[$\hbar$]
\end{document}

With LuaLaTeX:

With XeLaTeX:

I found that

• Adding \usepackage{unicode-math}

or

• \usepackage[no-math]{fontspec}

can resolve this problem, but I'm not sure which is better.

Answer 1

The definition of hbar is quite encoding dependant. As fontspec changes the operator font it breaks.

You could redefine it if you don't want to switch to unicode-math:

\documentclass{article}
\usepackage{fontspec}

\begin{document}\pagestyle{empty}

\def\hbar{{\mathchar175\mkern-9muh}}
[$\hbar$]

\def\hbar{\mathit{^^^^0127}}

[$\hbar$]

\end{document}

Answer 2

The character used for the bar in legacy math fonts has a different code in Unicode.

\documentclass{article}
\usepackage{fontspec}

\renewcommand{\hbar}{{\mathchar"AF\mkern-9mu h}}

\begin{document}

[$h$]

[$\hbar$]

\sbox0{$h$}\the\wd0

\sbox0{$\hbar$}\the\wd0

\end{document}

The difference in width is slightly more than 0.00004 millimeters, less than the difference with pdflatex. The bar is a bit higher, though.

The standard definition for \hbar is

\def\hbar{{\mathchar'26\mkern-9muh}}

and \mathchar'26 refers to the character in slot octal 26 (decimal 22) in math group 0 (the standard text font). This is the same character used for the macron accent, which in OpenType fonts is at slot 0xAF. Depending on the font it can be higher or lower.

Note that Unicode has a special slot for the Planck constants

U+201E PLANCK CONSTANT
U+201F PLANCK CONSTANT OVER TWO PI

and unicode-math will define \hbar to select the latter.